山地森林的碳捕獲潛力：從全球角度看地方景觀的韌性

著：（美）托馬斯·A·克拉克 譯：李正

在當(dāng)今世界，森林是捕獲碳以減少溫室氣體（GHG）凈排放的主要場(chǎng)所，其通過(guò)這種方式減緩或扭轉(zhuǎn)了全球變暖，從而緩和了變暖的負(fù)面影響。但是，這種能力在多大程度上來(lái)自那些位于山地上的森林？相對(duì)于與海洋、濕地、農(nóng)業(yè)實(shí)踐、技術(shù)相關(guān)的其他封存手段，或者相對(duì)于主要由化石燃料燃燒產(chǎn)生的溫室氣體的持續(xù)釋放，山地森林的封存潛力是否具有影響力？在對(duì)山地景觀的補(bǔ)償方面，這種封存是否優(yōu)于其他碳封存方式，或者優(yōu)于與全球供暖/制冷、制造和運(yùn)輸相關(guān)的能源生產(chǎn)減排？提高韌性是為了在面對(duì)系統(tǒng)性沖擊和壓力時(shí)促使功能延續(xù)，就山林而言，這種沖擊和壓力來(lái)自氣候變化本身以及人類侵占和資源開(kāi)采[1]。山地始終是容易引起爭(zhēng)議的景觀，其本地化管理將傾向于照顧當(dāng)?shù)厝说睦?，無(wú)論是居民還是商業(yè)企業(yè)。碳封存可以帶來(lái)超出本地范圍的全球性益處，如欲將碳封存的重要性凌駕于其他土地利用活動(dòng)之上，則需要區(qū)域或國(guó)家層面的介入，特別是在土地利用競(jìng)爭(zhēng)最激烈的低海拔地區(qū)。

山地森林為碳封存提供了獨(dú)特的機(jī)會(huì)，但同時(shí)也提出了挑戰(zhàn)。它們的上游植被稀少，位置偏遠(yuǎn)，因而得到保護(hù)；它們的下游植被更為豐富，但受到的威脅也更大[2]。此外，森林的碳密度在時(shí)間和空間上都有很大差異，其目前的碳封存能力如何？由于加速排放、森林退化、土地使用競(jìng)爭(zhēng)和氣候變化本身，這種能力可能會(huì)隨著時(shí)間的推移而發(fā)生什么變化？是否有更好的、成本更低的方法來(lái)實(shí)現(xiàn)同等的碳捕獲效果，從而減緩地球變暖？筆者將回應(yīng)上述問(wèn)題，首先識(shí)別現(xiàn)有相關(guān)研究的空白，同時(shí)嘗試將這些問(wèn)題置于更大范圍的山地景觀韌性主題之中進(jìn)行探討[3-4]。

目前，各個(gè)山地區(qū)域正受到威脅，其完整性受到內(nèi)源性和外源性的自然及人為干擾的損害。區(qū)分這2類干擾將有助于形成當(dāng)今最迫切需要的政策干預(yù)：內(nèi)源性干擾來(lái)自山地景觀本身，因此可以進(jìn)行本地化的政策處理；外源性干擾來(lái)自山地景觀之外，其在影響這些景觀的同時(shí)本身不會(huì)因此而改變。從這個(gè)角度來(lái)說(shuō)，碳捕獲可被視為山地韌性特征之一。調(diào)整治理結(jié)構(gòu)以涵蓋與山地韌性相關(guān)的主要因果關(guān)系，理當(dāng)是一個(gè)必要的目標(biāo)。但是，管理韌性的多個(gè)維度需要進(jìn)行優(yōu)先級(jí)排序，因?yàn)椴⒎撬芯S度都能同時(shí)實(shí)現(xiàn)，有些甚或是相互排斥的。

1 溫室氣體、反射率和蓄熱

從古氣候?qū)W家使用古希臘人的粗略觀察方法開(kāi)始，對(duì)于極端天氣事件和長(zhǎng)期氣候變遷的研究已經(jīng)持續(xù)了數(shù)個(gè)世紀(jì)。當(dāng)然，現(xiàn)在我們的觀察、診斷和行動(dòng)能力有了很大的提高，且這些能力正以前所未有的方式被應(yīng)用于理解和解決全球變暖帶來(lái)的挑戰(zhàn)。20世紀(jì)60年代，對(duì)于因地球大氣反射能力增加而產(chǎn)生的蓄熱效應(yīng)（即反射率）的認(rèn)識(shí)，揭示了CO2和其他溫室氣體導(dǎo)致大氣整體變暖的潛在作用，以及這種整體變暖產(chǎn)生的極端后果[5]。在所有的溫室氣體中，CO2是截至目前變暖效應(yīng)最大和持續(xù)時(shí)間最長(zhǎng)的。工業(yè)化之前的大氣二氧化碳濃度為2.8×10-4，如今這個(gè)值超過(guò)了4×10-4。

目前大多數(shù)知情分析家認(rèn)為，氣候變暖所造成的全球社會(huì)成本將會(huì)非常高，以至于需要做出重大努力來(lái)阻止這種變暖趨勢(shì)[6]。他們宣稱，考慮到潛在破壞的規(guī)模，有理由進(jìn)行大量投資以抑制乃至扭轉(zhuǎn)這一趨勢(shì)[7]。除了默許之外，我們還有2種可能的行動(dòng)方案：1）從源頭上減排，主要涉及化石燃料的燃燒[8]；2）通過(guò)在海洋、土壤、濕地和森林等主要碳匯中進(jìn)行技術(shù)性捕獲和吸收，減少大氣中的溫室氣體。最近的研究表明，通過(guò)機(jī)械過(guò)程將碳從大氣中移除并儲(chǔ)存在地下是有一定前景的，但這一工序的可擴(kuò)展性仍未得到檢驗(yàn)。海洋儲(chǔ)存了大量的碳，但其吸收能力正因氣候變暖和酸化而受到損害，其中酸化會(huì)對(duì)可吸收CO2的海藻造成傷害。濕地提供了額外的碳捕獲場(chǎng)地——特別是沿海濕地，但它們的效果有限且難以人為操控，不過(guò)保護(hù)濕地仍然是一個(gè)高度優(yōu)先事項(xiàng)。鑒于這些情況，森林是幫助我們捕獲CO2（主要的溫室氣體）和儲(chǔ)存碳的最佳途徑。被剝離了氧氣的碳將會(huì)存留在森林、草原和牧場(chǎng)之下以及海洋深處，3.67 t的CO2等于1 t的碳儲(chǔ)存。

可能除熱帶以外，北方森林（泰加）是世界上最大的森林生物群落[9]，因而其碳封存潛力很大。該區(qū)域沒(méi)有發(fā)生過(guò)嚴(yán)重的侵略性森林砍伐，盡管受到病蟲害和火災(zāi)的威脅，它仍有可能借助全球變暖而恢復(fù)和擴(kuò)展森林，從而擴(kuò)大自己的碳吸收能力。在全球范圍內(nèi)，每年可能有13萬(wàn)km2的林地消失，主要位于熱帶，大多是由于畜牧業(yè)和農(nóng)業(yè)擴(kuò)張而導(dǎo)致的[10]。全球溫室氣體排放的1/4來(lái)自熱帶森林的砍伐，超過(guò)了全球交通方面的總排放量[11]。

2 全球變暖的后果

CO2只是4種主要的溫室氣體之一，但其體積卻在所有的溫室氣體中是最大的，甚至其影響也是最大的。其他3種主要溫室氣體分別是甲烷（CH4）、一氧化二氮（N2O）和臭氧（O3），其中甲烷是天然氣的主要成分，其危害在短期內(nèi)至少是CO2的80倍以上，限制它的排放可以有效緩解全球變暖。如果沒(méi)有溫室氣體，地球?qū)⒆兊煤洹．?dāng)大氣向下的反射率超過(guò)向上的反射率時(shí)，地球的表面就會(huì)變暖，這對(duì)許多地方或許是不利的，但對(duì)某些地方卻是有利的。事實(shí)上，隨著谷物帶向北轉(zhuǎn)移、曾經(jīng)冰封的北方海路被打開(kāi)以及干旱區(qū)在水文方面獲益，一些國(guó)家從氣候變暖中發(fā)現(xiàn)了機(jī)會(huì)。然而，隨著海平面上升，沿海平原和城市被淹沒(méi)在不斷加深的海洋中，其他一些地方將遭受痛苦。如果氣候變暖不受控制，有些地方將變得不適合居住，所有物種的地理分布將發(fā)生變化。這些負(fù)效應(yīng)有助于我們估算氣候變暖的惡果，并證明我們?yōu)榛謴?fù)更適宜的氣候而必須承受的代價(jià)是合理的。對(duì)每個(gè)國(guó)家來(lái)說(shuō)，這種計(jì)算方法是不同的，因此在集體行動(dòng)上達(dá)成一致仍然是困難的。

每個(gè)國(guó)家都會(huì)受氣候變暖的影響，但這些影響的組合方式會(huì)有所不同，隨之而來(lái)的抵制進(jìn)一步變暖的決心也將有程度上的不同[12]。對(duì)一些國(guó)家來(lái)說(shuō)，變暖可能是有利的；對(duì)其他國(guó)家來(lái)說(shuō)，氣候變暖的后果將是災(zāi)難性的，如不適宜居住的溫度。后者如果缺乏緩解氣候變暖負(fù)面影響的財(cái)政支持或技術(shù)能力，則其通過(guò)與其他國(guó)家達(dá)成交易來(lái)對(duì)抗氣候變暖的決心將增強(qiáng)。面積小和欠發(fā)達(dá)的國(guó)家一般排放溫室氣體較少，因而無(wú)法在源頭減排方面采取行動(dòng)，也無(wú)法就此進(jìn)行交易。一些易受氣候變暖影響的國(guó)家可能在森林碳封存方面有極大的潛力，所以他們可能希望將這種潛力提供給其他受困于氣候變暖趨勢(shì)的國(guó)家。這種交易可能會(huì)變得越來(lái)越普遍，事實(shí)上碳市場(chǎng)正被作為一種爭(zhēng)取多國(guó)參與溫室氣體排放管理的手段[7]。

3 碳封存與能源行業(yè)去碳化

源于化石燃料使用和水泥生產(chǎn)等工業(yè)過(guò)程中的溫室氣體減排仍然是許多國(guó)家的首要任務(wù)，碳封存對(duì)大多數(shù)國(guó)家來(lái)說(shuō)不是一種替代方法，而更多是一種與溫室氣體減排配套使用的策略。這種相互作用是視情況而定的，任何一方的進(jìn)步都會(huì)改變另一方的投資計(jì)算。這種互動(dòng)是不對(duì)稱，每一方還會(huì)受到其所在環(huán)境特有的其他因素的影響。

碳封存有生物、地質(zhì)和化學(xué)作用3種形式。在生物形式中，森林是最重要的，其能力遠(yuǎn)遠(yuǎn)超過(guò)草原、牧場(chǎng)和濕地。人類可以提高每種區(qū)域的吸收能力，樹木、草本、藻類等都能通過(guò)光合作用從CO2和H2O中獲得碳水化合物并釋放出O2。伐木、耕作、放牧、燃料采集等人類活動(dòng)都是改善碳吸收的機(jī)會(huì)，伐木產(chǎn)生的木材可以在建筑中封存碳，林業(yè)和農(nóng)業(yè)生態(tài)系統(tǒng)可以培育土壤以提高碳封存[13-14]。太陽(yáng)能、風(fēng)能、水能及其他替代性能源可以取代燃料采集，特別是在發(fā)展中國(guó)家。停止排干濕地，可以提高濕地的碳封存能力。給海洋施肥會(huì)產(chǎn)生消耗碳的藻類水華，后者最終將死亡并下沉。雖然海洋是一個(gè)巨大的碳匯，但進(jìn)一步吸收CO2會(huì)繼續(xù)降低海洋的pH值（即增加酸化），這將損害作為海洋中吸收碳的主要媒介的藻類生態(tài)系統(tǒng)，削弱其進(jìn)一步吸收碳的能力。在地質(zhì)形式中，我們有多種方法可以將碳導(dǎo)入地下，使碳封存具有必要的持久性。最后，還有一些技術(shù)性作用的可能性，其大多被歸于“直接空氣捕獲”（Direct Air Capture, DAC）類別下。無(wú)機(jī)土壤也可以在干旱的沙漠地貌中以鈣鈦礦（即沉積巖）的形式儲(chǔ)存碳元素。在上述形式中，森林的碳封存潛力最大。

聯(lián)合國(guó)環(huán)境規(guī)劃署《2019年排放差距報(bào)告》[15]對(duì)目前上升進(jìn)入地球大氣層的溫室氣體凈排放量與實(shí)現(xiàn)2015年12月《巴黎協(xié)定》[16]目標(biāo)所需的排放水平之間的差距進(jìn)行了估算，其中2018年全球溫室氣體排放總量為55.3 GtCO2e①。該報(bào)告還注意到，1 t的C相當(dāng)于3 667 t的CO2，因?yàn)闅鈶B(tài)的CO2一旦被封存并脫去O2就成了C。上述測(cè)算主要考慮了地球上的森林、濕地和海洋中的凈碳封存。2018年，幾乎70%的溫室氣體排放來(lái)自工業(yè)和化石燃料的燃燒。正如后來(lái)所觀察到的，每年凈釋放的溫室氣體一部分來(lái)源于自然過(guò)程，其中一些可能是當(dāng)森林中的凈平衡為負(fù)值時(shí)被釋放的，即未被吸收的剩余物在大氣中的釋放。為實(shí)現(xiàn)2015年《巴黎協(xié)定》的目標(biāo)，2018—2030年間的年溫室氣體凈排放量必須減少25%～50%。雖然沒(méi)有絕對(duì)永久性的碳封存手段，但今天的挑戰(zhàn)是如何減少溫室氣體在大氣中的瞬間釋放，同時(shí)推遲已封存的碳的最終釋放，直到可再生能源能夠取代化石燃料。此后，化石燃料的替代可能會(huì)降低碳封存的緊迫性，盡管自然發(fā)生的碳儲(chǔ)量釋放仍然會(huì)引起某種程度的變暖。

迄今為止，已有100多個(gè)國(guó)家承諾在21世紀(jì)中期實(shí)現(xiàn)零排放，但如何在各國(guó)之間分配這項(xiàng)任務(wù)卻被證明是有爭(zhēng)議的。中國(guó)在2018年的化石燃料相關(guān)排放總量接近14 GtCO2e，是美國(guó)的2倍以上。在新冠肺炎疫情之前，中國(guó)的凈排放一直在穩(wěn)步上升，而除印度之外的其他大多數(shù)國(guó)家都正趨于平穩(wěn)。然而，雖然在穩(wěn)步下降，美國(guó)的人均二氧化碳排放量是全球最高的，超過(guò)了排在第二位的俄羅斯。2018年中國(guó)的人均二氧化碳排放量還不到美國(guó)的1/2，也低于俄羅斯和日本。值得注意的是，中國(guó)在20世紀(jì)90年代開(kāi)始大規(guī)模植樹造林，并對(duì)原有森林進(jìn)行培育以使其恢復(fù)健康。隨著中國(guó)的森林成熟到了吸收高峰期，上述努力可能已經(jīng)抵消了該國(guó)近40年來(lái)超過(guò)20%的化石燃料排放量，這一點(diǎn)有待更多文獻(xiàn)資料予以證實(shí)。

在減排與吸收這2種方法中，假如有一種更為便宜、更有成本效益、更容易實(shí)施且有足夠的可擴(kuò)展性，那么它將成為我們唯一的關(guān)注點(diǎn)。然而，這2種方法單獨(dú)使用都是不夠的，因而已經(jīng)達(dá)成的共識(shí)傾向于將2種方法進(jìn)行組合。碳封存（即碳捕獲）可能是更便宜的選擇，因而一些國(guó)家會(huì)為其森林景觀爭(zhēng)取碳信用，作為降低源頭減排率的理由。減排計(jì)劃通常是在國(guó)家層面上制定，在國(guó)家以下的行政層級(jí)以及非營(yíng)利組織和企業(yè)實(shí)體范圍內(nèi)實(shí)施，有時(shí)也在地方層級(jí)共同發(fā)起。國(guó)際合作幾乎是必不可少的，因?yàn)槿魏蔚貐^(qū)——也許除了前面提到的美國(guó)和中國(guó)等大國(guó)——單獨(dú)行動(dòng)都難以改變?nèi)蚩諝赓|(zhì)量變化的進(jìn)程，而且也沒(méi)有任何機(jī)制可以讓單獨(dú)行動(dòng)的大國(guó)從不愿意或根本無(wú)法行動(dòng)的“搭順風(fēng)車”的國(guó)家那里收回成本。晚近實(shí)現(xiàn)工業(yè)化的國(guó)家認(rèn)為那些較早實(shí)現(xiàn)工業(yè)化的國(guó)家應(yīng)該承擔(dān)更大的責(zé)任，因?yàn)樗麄兊呐欧攀谴髿庵谐掷m(xù)存在的溫室氣體的主要來(lái)源。

目前碳封存與其說(shuō)是一種選擇，不如說(shuō)是一種與基于源頭減排相互配套的一個(gè)策略。這一情況體現(xiàn)在《聯(lián)合國(guó)氣候變化框架公約》的準(zhǔn)則中，這些準(zhǔn)則最早在1997年的《京都議定書》中開(kāi)始實(shí)施（2005年生效，2013年到期）。2012年的《多哈修正案》雖然遇到了阻力，但最終在2020年底獲得了必要數(shù)量簽字國(guó)的批準(zhǔn)。在上述努力中，2個(gè)表示行動(dòng)的縮寫詞出現(xiàn)了，即A/R（造林/再造林）以及REDD+（減少因毀林和退化而導(dǎo)致的排放量，包括各種森林管理工作）。貧窮的國(guó)家呼吁和堅(jiān)持要求更大、更富有的國(guó)家?guī)ь^，但在公平分配減排份額時(shí)，顯然并非所有國(guó)家都贊成允許減免碳封存，盡管這種態(tài)度可能正在改變[17-18]。

森林景觀是上述舉措的主要對(duì)象，因?yàn)樯志坝^在溫室氣體減排方面被認(rèn)為比其他植物生境更有用。目前存在4種基于森林封存的不同策略，它們的功用在各國(guó)之間以及在平地和山地之間存在差異。這些策略包括：1）停止砍伐森林，禁止將林地永久地轉(zhuǎn)變?yōu)槠渌猛荆?）減緩森林退化，以免降低其作為碳儲(chǔ)存庫(kù)的功效；3）提高森林的健康水平，提高其碳密度；4）植樹造林，在邊緣土地或荒地上有選擇地重新種植最有效的樹種和其他地被植物。

雖然森林砍伐仍在繼續(xù)，但我們?nèi)杂泻艽髾C(jī)會(huì)可以阻止這種衰退趨勢(shì)。全球森林面積在20世紀(jì)90年代每年減少8.3萬(wàn)km2，在隨后10年中降為每年減少5.2萬(wàn)km2，證明了小幅改善的存在。每個(gè)數(shù)字都是自然和人為增加值的總和，說(shuō)明因耕作、放牧、伐木和其他土地使用而造成的森林破壞正在減少[19]。據(jù)估計(jì)，地球上可能有多達(dá)2 000萬(wàn)km2的土地適合恢復(fù)為森林，其中一部分位于山地[20]。

4 作為凈碳匯的森林

由于地球上的森林景觀既能吸收碳，又能釋放碳，所以它們與氣候變化過(guò)程之間存在內(nèi)在聯(lián)系[21]。碳攝入量（+）減去輸出量（–）的凈值就是碳平衡，即森林碳循環(huán)的結(jié)果。只有當(dāng)攝入量超過(guò)輸出量（凈值為正）時(shí)，森林才是一個(gè)碳匯。這種平衡由3個(gè)共同發(fā)生的過(guò)程組成：涉及光合作用的呼吸（利用陽(yáng)光將CO2和其他成分轉(zhuǎn)化為生物質(zhì)中的碳水化合物和糖）；地上（葉子、枝干和樹干產(chǎn)生木制品）和地下（根部）的生物量生產(chǎn)；以及枯木和垃圾的腐爛產(chǎn)生土壤[22-23]。上述過(guò)程既影響氣候變化，也受到氣候變化的影響。碳封存——相當(dāng)于陽(yáng)極碳的凈攝入——是一個(gè)短暫的、瞬間的系統(tǒng)狀態(tài)，因?yàn)樗蟹獯娴奶甲罱K都會(huì)逃到大氣中，除非被轉(zhuǎn)化或燒毀。當(dāng)然，延遲釋放可以為減少來(lái)自化石燃料燃燒的碳排放爭(zhēng)取時(shí)間，這些化石燃料（煤、石油、天然氣）本身在燃燒之前就處于地下封存的狀態(tài)。

此外，變暖過(guò)程在某種程度上可能被“偏差放大”，因?yàn)檫@個(gè)過(guò)程本身可能是一種加速劑或抑制劑。例如，隨著極地冰雪的融化，長(zhǎng)期隱藏的泥炭沼澤會(huì)被暴露而釋放出CO2。氣候變暖在提高山地森林樹線的海拔高程的同時(shí)，也可以改善樹線以下的森林生境，這2種方式都會(huì)擴(kuò)大森林面積。另一方面，如果伐木、火災(zāi)、物種滅絕、腐爛和耕種所釋放的CO2超過(guò)了森林呼吸的攝入量，可能會(huì)促使世界上的植物群加快生長(zhǎng)，從而提高了未來(lái)的碳吸收。更為普遍的是，全球地面覆蓋物將隨著難以預(yù)測(cè)的氣候變化而發(fā)生變化。近年來(lái)，亞馬孫流域可能有1/5的平原森林被砍伐，這主要是畜牧和農(nóng)業(yè)擴(kuò)張的結(jié)果，事實(shí)上全球變暖可能增強(qiáng)了該流域?qū)π竽恋奈?，從而加速了森林砍伐?；馂?zāi)已經(jīng)進(jìn)入內(nèi)陸地區(qū)，其中不少是由于農(nóng)民為了種地而清除亞馬孫森林所引燃的，同時(shí)，由于巴西在20世紀(jì)70年代修建大范圍公路網(wǎng)絡(luò)及推動(dòng)內(nèi)陸城市發(fā)展，森林被進(jìn)一步開(kāi)辟，導(dǎo)致火災(zāi)更為易發(fā)。在亞馬孫流域，我們見(jiàn)證了一個(gè)因土地利用競(jìng)爭(zhēng)而引發(fā)惡果的典型示例，在那里商業(yè)活力是砍伐森林的一種重要誘因。包括山林在內(nèi)的其他森林是地球的最后防線之一，即使雨林正在被修復(fù)。下文聚焦2個(gè)問(wèn)題：山地森林在未來(lái)的碳封存中可能會(huì)起到什么作用？在提升這種作用時(shí)有哪些潛在的挑戰(zhàn)？筆者將從土地利用競(jìng)爭(zhēng)的角度來(lái)看待這些挑戰(zhàn)。

5 山地森林的碳封存效果

為了評(píng)估那些促進(jìn)山地森林碳封存的工作的效果，筆者首先建立了一個(gè)數(shù)量級(jí)來(lái)衡量山地森林的數(shù)量、與地球森林總量的對(duì)比、碳足跡和碳密度。根據(jù)這些數(shù)據(jù)，筆者將試圖判斷山地森林在處理全球溫室氣體排放方面的潛力，并將把這種潛力歸結(jié)為土地利用競(jìng)爭(zhēng)的產(chǎn)物，其中相互沖突的利用模式——但不一定是競(jìng)爭(zhēng)對(duì)手——都在爭(zhēng)奪空間。當(dāng)不同用途的支持者爭(zhēng)奪土地使用權(quán)的時(shí)候，這種競(jìng)爭(zhēng)將呈現(xiàn)出各種不同的形式。

筆者采納通行的山地定義，即海拔2 500 m以上的地區(qū)，加上海拔300～2 500 m的表面崎嶇不平的地區(qū)[24-25]。在這些山地中，只有約1/4（1 150萬(wàn)km2）是森林[24]。正如瑞士發(fā)展與合作署（Swiss Agency for Development and Cooperation）所指出的：“除了那些常年特別干燥或寒冷的地區(qū)，森林在大多數(shù)山地都占有相當(dāng)大的比例。以歐洲為例，森林覆蓋了總山地面積的41%，其中森林占比超過(guò)1/2的山地有阿爾卑斯山、巴爾干山、喀爾巴阡山、派勒斯山等，其他森林覆蓋率特別高的山地包括阿巴拉契亞山脈、澳大利亞阿爾卑斯山、圭亞那高地以及中非、東南亞、婆羅洲和新幾內(nèi)亞的山地?！盵26]除此以外，還有北美洲的落基山脈、非洲中部和中國(guó)的山地以及中美洲和南美洲的安第斯山脈。但總的來(lái)說(shuō)，山林只占地球陸地森林總面積（4 590萬(wàn)km2）的約1/4。

森林，特別是山地森林，是否適合作為封存碳的選項(xiàng)？首先考慮一下林地的總體規(guī)模和范圍：地球上的陸地和水面面積之和為5.1億 km2，其中陸地面積接近30%（1.53億 km2），陸地面積中大約30%是森林（4 590萬(wàn) km2），而這些森林中可能有25%生長(zhǎng)在山地（1150萬(wàn)km2）。山地森林的確切面積和品質(zhì)尚不明確，如森林的碳密度肯定會(huì)隨著海拔的升高而下降。北方森林無(wú)疑代表了山地森林的最大份額，如果我們確切知道這種森林的組成及其樹下和土壤的碳密度，就有可能明確其碳封存的總體潛力[27]，但目前這還不可能。

讓我們考慮一下全球碳封存的所有選項(xiàng)，包括發(fā)生在濕地、海洋和森林等主要儲(chǔ)存地的碳封存。森林和海洋是每年地球上的主要碳吸收者，盡管海洋中儲(chǔ)存的碳的累積量遠(yuǎn)遠(yuǎn)超過(guò)森林生物群落中的。據(jù)IPPC[28]估計(jì)，森林及濕地每年總共吸收約10 GtCO2，而海洋每年吸收約8 GtCO2[29]，這個(gè)數(shù)字可能正在下降。全球化石燃料的燃燒和水泥生產(chǎn)每年向大氣中釋放近29 GtCO2，而土壤退化的耕作方式每年又增加了4 GtCO2。近1 500萬(wàn)噸的CO2仍然懸浮在大氣中，沒(méi)有被吸收。每年的二氧化碳總排放量中，幾乎有1/2仍漂浮在大氣中。

當(dāng)然，森林對(duì)碳的年捕獲率和總儲(chǔ)存能力有所不同。森林碳密度最高的是北方森林生物群落，熱帶森林次之，再次是溫帶森林。由針葉樹、樺樹和楊樹構(gòu)成的北方森林在北極以南的寒溫帶地區(qū)占主導(dǎo)地位，與主要由針葉樹森林和濕地覆蓋的泰加林帶相鄰。濕地生物群落的平均碳密度（700 t/hm2）超過(guò)了北方森林（400 t/hm2）[30]。

因此，森林不僅在全球碳捕獲工作中占有重要份額，而且是在所有選項(xiàng)中最可能通過(guò)人類干預(yù)而提高碳捕獲作用的一個(gè)。事實(shí)上，地球表面有20億hm2（2 000萬(wàn)km2）的土地可能適合用于上述基于森林的環(huán)境修復(fù)活動(dòng)。今天，森林可能吸收了1/3的化石燃料燃燒產(chǎn)生的CO2（每年近30億噸，或33億短噸）。在所有自然吸收（封存）的手段中，森林似乎擁有最大的潛力，實(shí)現(xiàn)方式包括在以前沒(méi)有森林的地方植樹造林、在曾經(jīng)有森林的地方恢復(fù)森林以及扭轉(zhuǎn)森林衰退，恢復(fù)性策略是通過(guò)施肥、推廣韌性樹種、保護(hù)現(xiàn)有林分免受火災(zāi)和蟲害來(lái)加速林分的建立。

6 估算全球山地森林的凈碳封存量

理論上，全球山林最大碳封存潛力的粗略值可以按照總面積與單位面積年吸收能力的乘積進(jìn)行計(jì)算，但并不是所有的山地都有森林，而且目前的記錄也不充分。如前所述，地球上有1 150萬(wàn)km2的森林在山地，在全球范圍內(nèi)，樹齡大于200年的北方森林每年封存約700 t/km2CO2[31]，1 150萬(wàn)km2乘以700 t/km2，得到每年73億噸（80.5億短噸）的CO2。

有許多原因?qū)е律降厣值幕盍捌涮紳摿Υ嬖诤艽蟛町悺＿@些山地的主要部分遠(yuǎn)遠(yuǎn)高于樹線，而且樹線會(huì)隨著氣溫降低而下移。北方森林等森林生物群落將與巖石上的貧瘠土壤交織在一起，而在低海拔地區(qū)伐木和某些娛樂(lè)活動(dòng)將進(jìn)一步削弱森林碳封存能力。陡峭的地形將遭受侵蝕和森林退化的影響，導(dǎo)致碳密度降低。在低海拔地區(qū)存在競(jìng)爭(zhēng)關(guān)系的活動(dòng)將爭(zhēng)奪空間，這種對(duì)土地的競(jìng)爭(zhēng)——土地使用的競(jìng)爭(zhēng)——在溫帶和熱帶森林生物群落中將更有可能被尋求森林產(chǎn)品貨幣化或拾取柴火的貧困人口所包圍。在海拔較低、碳含量較高的茂密森林中發(fā)生的火災(zāi)可能會(huì)更嚴(yán)重，在釋放大量碳的同時(shí)也為未來(lái)幾十年后長(zhǎng)出新的森林做好了準(zhǔn)備。坡向也同樣會(huì)影響森林，使山坡免受全日照，并在一定程度上阻礙了植物生長(zhǎng)。同時(shí)，較為茂密的林地不僅會(huì)減緩水土流失，還會(huì)調(diào)節(jié)當(dāng)?shù)氐乃模瑸樯较碌木用窈蜕鐓^(qū)提供更穩(wěn)定的水源。但是，假如我們提供資金將當(dāng)?shù)鼐用窦邪仓迷跍\山區(qū)，也會(huì)導(dǎo)致與農(nóng)業(yè)（用于商業(yè)及維持生計(jì)）、采礦、伐木和燃料采伐有關(guān)的森林破壞性行為。

看起來(lái)，隨著平地森林的增加，山地森林在碳封存方面可能會(huì)被認(rèn)為作用不大。正如聯(lián)合國(guó)糧食及農(nóng)業(yè)組織的全球森林資源評(píng)估報(bào)告（Forest Resources Assessment, FAO）每5年1次記錄的，世界各地的森林都在遭受威脅。在拉丁美洲（亞馬孫、大西洋森林、大查科、塞拉多和喬科–達(dá)連）、東南亞（大湄公河）、非洲（剛果盆地和東非）和南太平洋（婆羅洲、澳大利亞?wèn)|部、新幾內(nèi)亞和蘇門答臘），森林消失和退化最為迅速。此外，商業(yè)化農(nóng)業(yè)繼續(xù)占用大片較平坦的林地，對(duì)熱帶生物群落中的森林砍伐和退化負(fù)有主要責(zé)任。在所有大規(guī)模的商業(yè)化農(nóng)業(yè)中，大豆、棕櫚種植以及畜牧是最不適合較平坦的林地的。伐木是森林退化的另一個(gè)主要原因。目前人們現(xiàn)在正在研究如何在上述情況下提高森林恢復(fù)率。氣候變暖是森林退化和衰弱的一個(gè)獨(dú)立原因，同時(shí)也是火災(zāi)的助燃劑。

7 山地森林的能力：要點(diǎn)重述

回到本研究開(kāi)始時(shí)提到的與山地森林有關(guān)的問(wèn)題。首先，目前世界上的山林在碳捕獲方面的能力是什么？回顧之前引用的數(shù)據(jù)，地球上總共有大約4 590萬(wàn)km2的林地，其中1 150萬(wàn)km2（約25%）位于山地，據(jù)稱全球森林每年可吸收10 GtCO2?？紤]到前面提到的森林碳平衡中的所有綜合效應(yīng)，山地森林的最大額外凈碳吸收能力將是2.5 GtCO2。如果全球化石燃料的使用每年產(chǎn)生約29 GtCO2，那么其中約9%的產(chǎn)量將被山林所吸收。這一數(shù)字可能偏高，因?yàn)樵谌蚩偣浪懔恐校瑹釒в炅值淖饔靡驯黄骄?，雖然其作用相對(duì)較大。同時(shí)，海洋、山地及濕地一起也未能捕獲近10 Gt的CO2，其尚漂浮在大氣中的。雖然現(xiàn)在海洋的碳捕獲能力受到酸化的限制，但由于極地融化，海洋的容量可能略有增加，但這不足以消除每年10 GtCO2的赤字。

包括山林在內(nèi)的森林是否可以發(fā)揮更大功效，以吸收這一額外量？也許可以。如前所述，地球上可能有2 000萬(wàn)km2的土地被認(rèn)為適合恢復(fù)森林。按照全球平均森林碳凈封存率10 Gt CO2每4 590萬(wàn)km2或218 tCO2/km2計(jì)算，山地森林可以凈封存額外的1.1 GtCO2，這是假設(shè)適合開(kāi)墾的額外山地與全球林地中的山林比例成正比。這一碳封存量將相當(dāng)于目前每年飄入大氣層的二氧化碳量的11%左右。這樣的估計(jì)必然是初步的，因?yàn)樗韪鼮槎嘣臄?shù)據(jù)尚未被獲取。結(jié)論則是，山林已經(jīng)在碳封存中發(fā)揮了重要作用，如果適當(dāng)?shù)嘏嘤搅?，這種作用可能會(huì)擴(kuò)大。

其次，由于土地利用競(jìng)爭(zhēng)和氣候變化本身，這種能力將如何隨著時(shí)間推移而發(fā)生變化？是否有更好的、成本更低的方法來(lái)實(shí)現(xiàn)碳捕獲的同等效果，從而減緩地球變暖？如前所述，由于缺乏替代方案，任何進(jìn)一項(xiàng)減少全球溫室氣體排放的措施都必須來(lái)自2個(gè)主要策略：1）從源頭減排；2）在森林中進(jìn)行封存。目前必須探討下列問(wèn)題：是通過(guò)降低化石燃料的碳密度或轉(zhuǎn)向可再生能源來(lái)減少源頭排放的成本更低，還是進(jìn)行進(jìn)一步封存的成本更低？如果主張后者，那么在各種封存策略選項(xiàng)中，哪些是最可行和最具成本效益的？哪些森林封存的投資方案是最好的？

從3個(gè)方面進(jìn)行考慮有助于我們回答上述問(wèn)題：1）哪種方法可以在政治上被接受；2）考慮到相關(guān)政府部門和非政府部門的能力，哪種方法最有可能被實(shí)施；3）哪種方法最具成本效益？具有封存潛力但缺乏行動(dòng)資金的窮國(guó)將不得不利用較富裕國(guó)家的資源，這使全球政治計(jì)算變得復(fù)雜。這種計(jì)算可分為兩部分：較富裕的國(guó)家會(huì)不會(huì)對(duì)較貧窮的國(guó)家進(jìn)行交叉補(bǔ)貼，而貧窮國(guó)家是否會(huì)接受這樣的外部參與并為此付出多大的國(guó)內(nèi)成本？森林封存的方案根據(jù)地點(diǎn)、土地質(zhì)量、對(duì)附近居民的影響、所選樹種的適當(dāng)性、當(dāng)?shù)卣O(jiān)督森林管理的能力、現(xiàn)在和未來(lái)的氣候條件等因素而有所不同。對(duì)于森林改良的倡導(dǎo)者來(lái)說(shuō)，這些因素可被結(jié)合在一起對(duì)植樹造林投資選址方案的可取性進(jìn)行打分。這種打分被視為一種評(píng)價(jià)功能，其評(píng)價(jià)結(jié)果是關(guān)于某一用途或使用者的森林投資選擇的比較權(quán)重。這種評(píng)價(jià)使單一用途的使用者能夠判斷不同土地對(duì)特定用途的相對(duì)吸引力。當(dāng)相互競(jìng)爭(zhēng)的使用者或用途爭(zhēng)奪指定空間時(shí)，該評(píng)價(jià)也會(huì)起作用。這種相互比較構(gòu)成了土地功能區(qū)劃的基礎(chǔ)，這就是土地使用的競(jìng)爭(zhēng)[32]。

這種競(jìng)爭(zhēng)基于3種分歧：1）平地和山地之間；2）土地使用的不平等競(jìng)爭(zhēng)者之間；3）發(fā)達(dá)國(guó)家和發(fā)展中國(guó)家之間。如果將這些差異囊括在一個(gè)類似維恩圖解的關(guān)系中，當(dāng)然會(huì)有一些缺失的單元。首先，平地森林在可達(dá)性和森林豐富度方面具有吸引力，但其土地的潛在用途或使用者之間的競(jìng)爭(zhēng)可能很激烈。山地的偏遠(yuǎn)性和不可滲透性將減少潛在使用者的數(shù)量和類型，因此競(jìng)爭(zhēng)可能不那么激烈，這使得山地可能更適合于需要大片土地的碳捕獲。大多數(shù)山林是在沒(méi)有人類作用的情況下自行發(fā)展起來(lái)的。但是擴(kuò)大其范圍可能需要積極的管理。

爭(zhēng)奪林地的第2種分歧存在于強(qiáng)大利益集團(tuán)與窮人之間。在這場(chǎng)競(jìng)爭(zhēng)中，競(jìng)爭(zhēng)者擁有不均等的能力來(lái)確保競(jìng)爭(zhēng)優(yōu)勢(shì)。未摻入溫室氣體的“清潔”空氣是一種公共資源，其價(jià)值幾乎完全不屬于當(dāng)?shù)?，因此在?dāng)?shù)馗?jìng)爭(zhēng)者的決策考量中沒(méi)有得到重視。在發(fā)展中國(guó)家的山地及其周邊地區(qū)，生活著大約7.2億人。其中7/10的人生活在山地的最為鄉(xiāng)野部分，在較小的山坡上勉強(qiáng)維持生計(jì)，同時(shí)從那些在有著更多工作機(jī)會(huì)和收入保證的遙遠(yuǎn)城市打工的居民獲得匯款。對(duì)這些人來(lái)說(shuō)，森林提供了木材燃料、水、自給自足的農(nóng)業(yè)和放牧空間，以及采礦、農(nóng)業(yè)、放牧、旅游和伐木的商業(yè)性工作[33]。森林與貧困的并存已經(jīng)導(dǎo)致這些森林被破壞，事實(shí)上自19世紀(jì)中期以來(lái)世界上的森林面積已經(jīng)減少了30%以上，而這一損失大部分發(fā)生在發(fā)展中國(guó)家。森林砍伐在20世紀(jì)90年代達(dá)到頂峰，每年的凈損失率約為8.3萬(wàn)km2。自2000年以來(lái)，這個(gè)年凈損失率已經(jīng)下降，可能約為40%，這是開(kāi)墾活動(dòng)、山地經(jīng)濟(jì)城市化和氣候變化本身的結(jié)果。

國(guó)家背景構(gòu)成了爭(zhēng)奪山地及其森林的第3個(gè)分歧。一些國(guó)家缺乏足夠的國(guó)內(nèi)資源和組織能力來(lái)促進(jìn)森林發(fā)展和加強(qiáng)碳匯，這對(duì)于提高全球森林碳匯能力是一個(gè)特殊挑戰(zhàn)。越來(lái)越多的人認(rèn)為，爭(zhēng)取他們的參與需要在管理和資金方面予以援助[34]。對(duì)于不少國(guó)家需要給予誘導(dǎo)條件，因?yàn)樘挤獯娴暮锰幵诤艽蟪潭壬媳灰暈槭欠潜镜氐?，在?guó)內(nèi)的回報(bào)率不足以吸引大量投資。

在發(fā)達(dá)國(guó)家與發(fā)展中國(guó)家的山地中，相當(dāng)一部分區(qū)域位于國(guó)家或區(qū)域?qū)蛹?jí)政府的管轄范圍內(nèi)。在這些情況下，政府本身就是所有者，因此在這些土地使用的任何談判中均是本國(guó)最為主導(dǎo)的實(shí)體。許多國(guó)家政府簽署了《巴黎協(xié)定》或最近的《波恩挑戰(zhàn)》[35]②，協(xié)定要求簽署國(guó)承諾在2030年前造林3.5億hm2并將其用于碳封存。為此，這些國(guó)家將根據(jù)這一目標(biāo)來(lái)利用山地，使該目的凌駕于任何來(lái)自政府以外的訴求及相關(guān)投標(biāo)之上。

世界銀行已經(jīng)成為推動(dòng)對(duì)包括碳封存在內(nèi)的環(huán)境服務(wù)進(jìn)行支付的主要機(jī)構(gòu)之一，同時(shí)其還通過(guò)發(fā)展拉丁美洲和非洲各國(guó)經(jīng)濟(jì)來(lái)阻止森林破壞[36]。然而，與土地利用競(jìng)爭(zhēng)的標(biāo)準(zhǔn)概念不同，因?qū)で髨?chǎng)地而進(jìn)行競(jìng)爭(zhēng)的森林使用者將通過(guò)一系列政治和法律手段來(lái)提出空間主張，這些手段是參與競(jìng)爭(zhēng)的資本。土地所有者之間的狹義價(jià)格戰(zhàn)顯然不是筆者所考慮的。對(duì)林地（特別對(duì)山林地）的爭(zhēng)奪涉及一些索賠者，這些索賠者的數(shù)量隨著海拔高度的增加而減少。

8 結(jié)論性意見(jiàn)

包括山林在內(nèi)的森林是碳封存的主要背景，也是應(yīng)對(duì)全球變暖的部分“解藥”。事實(shí)上，目前有人認(rèn)為地球自身有能力減少溫室氣體排放，使地表溫度在未來(lái)幾十年內(nèi)開(kāi)始穩(wěn)定，這比以前預(yù)期的要早得多，是一個(gè)最為樂(lè)觀的前景[37]。然而全球溫室氣體零排放仍然是一個(gè)挑戰(zhàn)，排放量增加的前景將使基于源頭的減排和碳封存成為一個(gè)更加緊迫的目標(biāo)，特別是在發(fā)展中國(guó)家。

在更好的管理方法和更大范圍的跨國(guó)合作的輔助下，山地森林應(yīng)該被視為任何未來(lái)解決方案的一個(gè)重要因素，但還要更多的信息才能采取行動(dòng)。對(duì)山地森林的全部碳封存潛力的估算仍然只是一個(gè)猜想，亟須更多關(guān)于山地森林組成的時(shí)空變化的分類數(shù)據(jù)，從而衡量現(xiàn)有績(jī)效和未來(lái)潛力。鑒于其困難的地形、不尋常的土壤特性、多變的陽(yáng)光照射以及土地所有權(quán)和控制權(quán)的變化無(wú)常，山地森林是一個(gè)特別的挑戰(zhàn)。假如凈零碳可以實(shí)現(xiàn)，一定不能缺少包括山林在內(nèi)的森林的重要貢獻(xiàn)。

了解山林以及所有其他海洋和陸地碳匯的封存潛力至關(guān)重要，因?yàn)檫@些碳匯的總封存潛力越大，就越?jīng)]有必要減少來(lái)自主要能源使用行業(yè)的碳排放。相反，所有碳匯的總封存潛力越低，就必須更加努力地減少基于源頭的碳排放，這主要是在能源部門，但不限于此。這個(gè)話題是在與韌性山地景觀維護(hù)、規(guī)劃和設(shè)計(jì)相關(guān)的更大范圍事務(wù)中提出的。提升韌性是為了持續(xù)保持能力，以面對(duì)系統(tǒng)沖擊和壓力。雖然碳封存只是該目標(biāo)的一個(gè)部分。然而，許多相互競(jìng)爭(zhēng)的土地所有者在尋求從山地韌性中獲益，他們的訴求并不容易調(diào)和。我們?cè)谔岣呱降厣值奶挤獯鏉摿Ψ矫娴臎Q心取決于：1）實(shí)現(xiàn)該目標(biāo)的難易程度；2）實(shí)現(xiàn)該目標(biāo)所產(chǎn)生的機(jī)會(huì)成本；3）在山地景觀上可以進(jìn)行其他用途和活動(dòng)的有效性和相互關(guān)系；4）相對(duì)于替代性的海洋和陸地碳匯的功效；5）能源行業(yè)去碳化方法的成本效益。

與運(yùn)輸、制造和建筑供暖/制冷的去碳化相比，或者與提高海洋、濕地和土壤的封存能力相比，包括山林在內(nèi)的森林的潛在貢獻(xiàn)可能是一種更為便宜和快速的方式[38]。從長(zhǎng)遠(yuǎn)來(lái)看，能源行業(yè)的去碳化幾乎總是必不可少的。零碳排放幾乎肯定需要通過(guò)電氣化（主要由核能和可再生能源提供燃料）和封存來(lái)實(shí)現(xiàn)。中國(guó)預(yù)計(jì)在未來(lái)15年內(nèi)出售的大多數(shù)新車將是電動(dòng)的。通用汽車公司承諾，到2035年只銷售零排放的汽車。這些話題之所以被放在“山地韌性”這一題目之下，是因?yàn)榫S持和提高山地森林的碳封存能力必然要與山地景觀規(guī)劃和管理中所追求的其他目標(biāo)相競(jìng)爭(zhēng)，其中包括農(nóng)業(yè)、資源開(kāi)采、燃料采伐、旅游等。

筆者將山地森林的碳匯潛力視為一個(gè)問(wèn)題。我們需要獲取更多關(guān)于山地森林組成的時(shí)空變化的分類數(shù)據(jù)，以衡量現(xiàn)有表現(xiàn)和未來(lái)潛力。為了判斷山地森林生物群落作為碳匯的功效，我們還需要進(jìn)一步研究3個(gè)不同的森林過(guò)程，包括呼吸作用、生物質(zhì)生產(chǎn)和分配，以及生物質(zhì)腐爛和土壤生成的化學(xué)計(jì)量。在上述狀況下，“只見(jiàn)樹木不見(jiàn)森林”的諺語(yǔ)顯得尤為貼切。我們?cè)诹私鈧€(gè)別樹木的同時(shí)，也需要了解整個(gè)山地森林生物群落或生態(tài)系統(tǒng)的所有協(xié)同復(fù)雜性。由于山地森林及地球上所有其他地球碳匯的封存潛力隨著時(shí)間的推移而變化，我們必須考慮到每個(gè)碳匯的韌性。如果某些碳匯出現(xiàn)問(wèn)題，其他碳匯可能要承擔(dān)更大的責(zé)任。如果化石燃料可以被其他能源以更具成本效益的方式所取代，那么碳匯可能會(huì)承擔(dān)較少的責(zé)任。能源行業(yè)的碳封存和去碳化都受制于變化無(wú)常的人類意志、技術(shù)、氣候、環(huán)境容量和其他因素的影響，因此它們也許是人類面臨的倒數(shù)第二大挑戰(zhàn)，因?yàn)槊總€(gè)方面的進(jìn)展都將不可避免地遭受系統(tǒng)沖擊，威脅到任何特定“瞬間”解決方案的韌性?；镜默F(xiàn)實(shí)是，為防止全球變暖而必須采取的行動(dòng)和條件在本質(zhì)上是沒(méi)有韌性的。在沒(méi)有替代品的情況下，有權(quán)勢(shì)的社會(huì)群體將繼續(xù)挖掘和燃燒化石燃料。森林、海洋、土壤和濕地遲早會(huì)釋放其所封存的碳，因?yàn)槠涮純?chǔ)量的封存并不完全。在能源、氣候、生物群落構(gòu)成的“系統(tǒng)”中，很少有元素會(huì)永久保持不變。韌性——包括其位于山地的部分——將需要多種補(bǔ)救措施來(lái)維持系統(tǒng)性績(jī)效，包括修復(fù)或替換系統(tǒng)中失效的元素，或用新的手段來(lái)實(shí)現(xiàn)以前的做法或條件所不能解決的目的。包括山林在內(nèi)的全球森林保護(hù)地也許遲早會(huì)在這個(gè)由脆弱部分組成的復(fù)雜系統(tǒng)中出現(xiàn)，成為一種穩(wěn)定的力量。在森林和氣候科學(xué)的指導(dǎo)下，森林管理者、規(guī)劃者和風(fēng)景園林師應(yīng)該引領(lǐng)我們共同努力，充分挖掘包括山林碳封存在內(nèi)的森林潛力。

注釋：

① 該度量單位中的“e”表示所有溫室氣體成分的等效性，以CO2的重量表示，以千兆噸（Gt）為單位，其中1 Gt為10億（1×109）t。CO2e一詞是指所有溫室氣體類型的排放總量，以與這些氣體等同的二氧化碳當(dāng)量表示。

②《波恩挑戰(zhàn)》是由森林管理委員會(huì)和私營(yíng)企業(yè)推動(dòng)的，這是一個(gè)致力于森林保護(hù)和重新造林的聯(lián)盟。

（編輯/王一蘭）

The Potential of Mountain Forests in Carbon Capture: A Global Perspective on Local Landscape Resilience

Author: (USA) Thomas A. Clark Translator: LI Zheng

The world’s forests are today a prime venue for carbon capture to reduce net greenhouse gas(GHG) emissions and by this means to slow or reverse global warming and thereby to moderate warming’s negative consequences. But does this capacity extend to forests on mountainous terrain? Is the sequestration potential of mountain forests impactful vis-à-vis all other means for sequestration — associated with oceans, wetlands,agricultural practices, technological means —or in relation to the continual release of GHG’s resulting largely from the combustion of fossil fuels? Is the sequestration pay-off on mountain landscapes sufficient to warrant its prioritization over either other means of carbon sequestration,or over reduction of emissions arising from energy production associated with heating/cooling (HVAC), manufacturing and transport the world over? To promote resilience is to engender perpetual functionality in the face of system shocks and stresses. In the case of mountain forests these originate in climate change itself as well as human occupation and resource extraction[1]. Mountains are invariably contentious landscapes. Localized management of mountain landscapes will tend to favor localized interests whether residents or commercial enterprises. Carbon sequestration confers a non-local, global benefit. The assertion of the primacy of carbon sequestration over other land-using activities will require regional or national intervention particularly at lower elevations where land use rivalries are most pronounced.

Mountain forests present unique opportunities but also challenges. Their higher reaches are sparcely vegetated yet remote, hence protected;their lower reaches, are more abundantly vegetated yet more at risk[2]. Forest carbon densities moreover are highly variant over both time and space. What is their current sequestration capacity? How may this capacity change over time, the result of accelerating emissions, forest degradation, land use rivalries, and climate change itself? Are there better less costly means for achieving an equivalent effect in carbon capture, hence the slowing of planetary warming?This brief commentary begins to answer these questions. In doing so it begins to identify lapses of research coverage while attempting to situate these questions within the larger rubrics of resilience in mountain landscapes[3-4].

Mountain terrain is today everywhere under duress, its integrity compromised by both natural and anthropogenic disturbances, both endogenous and exogenous. Establishing the divide between these two classes of effects will go a long way to shape the policy interventions most desperately required today. The endogenous are those originating within mountain landscapes themselves hence amenable to localized policy treatments.The exogenous originate beyond these landscapes,impacting them but not being affected as a result of these impacts. In this light, carbon capture is seen to be interposed among co-existent features of mountain resilience. Aligning governance structures so as to encompass principal causal relations regarding any of the multiple dimensions of mountain resilience is of course a necessary objective. But managing multiple dimensions of resilience requires prioritization since not all will be simultaneously achievable. Some are mutually exclusive.

1 GHG’s, Albedo, and the Heat Trap

Extreme weather events and prolonged climatic shifts have been the object of study for centuries, beginning with the work of paleoclimatologists deploying the crude observational methods of the ancient Greeks. Today our powers of observation, diagnosis and action are of course substantially advanced and they are now being applied as never before to understand and address the challenges posed by global warming. Awareness of the heat trapping effect of increase in the reflective capacity of the earth’s atmosphere — its albedo — pointed by the 1960’s to the potential role of the accumulation of carbon dioxide (CO2)and other GHG’s in the planet’s atmosphere leading to its overall warming and to the extreme outcomes that this condition is believed to engender[5].Of all GHG’s, CO2has had by far the greatest warming effect over the greatest duration. Prior to industrialization CO2’s atmospheric density was 280 ppm. Today it exceeds 400 ppm.

Warming’s projected global societal costs are now deemed by most informed analysts to be so severe as to warrant a major effort to arrest this warming trend[6]. The magnitude of potential damage is said to justify large investments to dampen, then reverse the trend[7]. We have currently just two potential courses of action beyond mere acquiescence: cut emissions at the source, principally the combustion of fossil fuels[8], or reduce their presence in the atmosphere,through technological capture and by absorption in our principal carbon sinks — our oceans, soils,wetlands and forests. Recent efforts show some promise in removing carbon from the atmosphere via mechanical processes, then storing it below ground. Scalability of this procedure though,remains untested. While oceans store vast quantities of carbon their absorptive capacity is being compromised due to warming and to acidification which harms the very algae that absorb CO2.Wetlands, coastal wetlands in particular, offer additional capability, but their effect is limited and difficult to manipulate through human agency.Their preservation remains a high priority however.Given these conditions, forests represent our best opportunity to capture carbon dioxide (CO2), the principal GHG, and to store carbon. CO2is the object of capture. Carbon, stripped of oxygen,is what remains on the forest floor, under our grasslands and rangelands, and in the depths of our oceans. 3.67 tons of CO2equal one ton of stored carbon.

The Boreal Forest (taiga), is the world’s largest such forest biome[9], possibly excepting the tropical, so its sequestration potential is great. It is not the principal nexus of the most deleteriously aggressive deforestation. At the same time the Boreal may offer an opportunity to expand its carbon absorbing capacity through forest recovery and expansion made in part possible by global warming itself, despite threats of pestilence and fire. Globally, perhaps 130,000 km2of forest land,mainly tropical, is lost annually, mostly to cattle ranching and agricultural expansion[10]. As much as one-fourth of the earth’s GHG emissions result from tropical deforestation exceeding the total emission from the planet’s transport sector[11].

2 Consequences Of Global Warming

Carbon dioxide is but one of four major GHG’s, but of all it is the most voluminous if not the most potent. Joining it in this nefarious process of global warming are Methane (CH4), Nitrous Oxide (N2O) and Ozone (O3). Methane, the main component of natural gas is over 80 times as harmful as CO2at least in the short term. Limiting it remains a very fruitful enterprise. Absent GHG’s the earth would grow cold. But when downward atmospheric reflectivity exceeds upward reflectivity the earth’s surface warms and uncomfortably so for many places but not all. Indeed some nations find opportunity in warming as grain bands shift north, northerly sea routes once frozen open, and arid places realize hydrologic benefits. Other places though, will suffer as oceans rise, and coastal plains and cities submerge in deepening oceans. Some will become uninhabitable if warming is unchecked.The geography of all living species will change. It is these net negative effects that help us to value or price the ill-effects of warming and to justify the expenses we will have to endure to restore a more hospitable climate. For each nation the calculus differs, so attaining agreement on collective action remains difficult.

Every nation will experience warming’s impacts but the mix of impacts will vary as will the attendant degree of determination to resist further warming[12]. For some warming might appear advantageous. For others warming’s consequences will be catastrophic and could include temperatures above the habitable. The lack of domestic financial or technological capacity in these places with which to mitigate warming’s negative effect will only heighten the determination to confront warming through bargains struck with other nations. Smaller and less prosperous nations generally have the lesser rates of aggregate GHG production, and so cutting emissions at the source is not an action they can take, with which to bargain. A few of the more vulnerable may have inordinate potential,principally in forest carbon sequestration, so they may aspire to make this potential available to those other nations that are also burdened by warming trends. Such trades may become more commonplace. Carbon markets, indeed, are being pursued as one way to enlist multi-national involvement in emissions management[7].

3 Carbon Sequestration Versus Decarbonization of the Energy Sector

Reduction of GHG emissions, originating in the use of fossil fuels and in industrial processes like cement production, remains the priority of many nations, and for most sequestration is considered less as an alternative and more as a paired strategy in emissions reduction. The interplay is contingent.Advance on either side alters the investment calculus on the other though each is influenced by additional factors that are unique to its circumstance, so the interaction is asymmetric.

Sequestration takes three forms: biologic,geologic and chemo-mechanical. Of the biologic,forests are preeminent, offering capacity well in excess of grasslands, range lands and wetlands.Humans can improve the absorptive capacities in each area. Trees, plants, grasses, algae et al. all photosynthesize carbohydrates from CO2and H2O releasing O2as a by-product. Such human activities as logging, farming, ranching, fuel harvesting and more all represent opportunities to improve carbon absorption. Logging yields lumber which sequesters carbon in buildings. Forestry and agro-ecosystems can nurture soils to improve carbon retention[13-14].Alternate energy regimes — solar, wind, hydro,and others — can displace fuel harvesting, largely in the Global South. Ceasing to drain wetlands can improve their efficacy. And fertilizing the oceans could produce carbon-consuming algae blooms that eventually die and sink. While oceans are a massive carbon sink, the further absorption of CO2continues to lower the ocean pH (i.e. increases acidification), and this damages algae ecosystems —the principal agent of carbon absorption in oceans— weakening their capacity for further carbon absorption. Of the geologic option, there are various means for channeling carbon below ground into geologic formations having requisite longevity.And lastly there are technological-mechanical possibilities largely subsumed under the name of Direct Air Capture (DAC). Inorganic soils can also store carbon as carbonates in the form of caliche(i.e. sedimentary rock) in arid desert landscapes.None of the forest-alternatives noted here, as yet offers the sequestration potential of forests.

The UN Environmental Programme’s Emissions Gap Report for 2019[15]estimates the difference — the “gap” — between current net GHG emissions rising into the earth atmosphere and the level of emissions that would be needed to achieve the goals of the Paris Agreement of December, 2015[16]. Total global GHG emissions were found to be 55.3 GtCO2e①in 2018. Note further that one metric tonne of carbon (C) is equivalent to 3,667 metric tonnes of CO2. Gaseous CO2once sequestered and stripped of oxygen is carbon. The measure takes into account net sequestration principally in the earth’s forests,wetlands and oceans. Almost 70% of GHG emissions originated in industry and the burning of fossil fuels in 2018. As observed later, a portion of the annual net release of GHG originates in natural processes. Some indeed may be released when the net balance in forests is negative — denoting the atmospheric release of the unabsorbed surplus.Annual net emissions would have to be cut after 2018, from between 25 to 50% by 2030 to attain the goal of the Paris Agreement of 2015. While there is no means for carbon sequestration that can be said to be absolutely permanent, the challenge today is to diminish the momentary release of GHG’s into the atmosphere while deferring the eventual release of stored carbon until renewables can displace fossil fuels. Thereafter the displacement of fossil fuels may lessen the urgency of sequestration though the naturally occurring release of carbon stocks could still induce some degree of warming.

Over one-hundred nations to date have pledged to achieve net zero emissions by midcentury. Apportioning this task among nations however is proving contentious. China’s aggregate fossil fuel-related emission in 2018 approached 14 GtCO2e, more than double that of the United States. China’s net emission had been, prepandemic, rising steadily whereas that of most other nations, save India had been levelling off. US per capita CO2emissions, however, lead the world but are in steady decline, topping Russia, next in line. China’s per capita CO2emissions, however,were less than half those of the US in 2018, at a level also exceeded by Russia and Japan. China,of note, in the 1990’s began a large scale effort to plant new forests while nurturing older forests back to health. Such efforts may have offset upwards of 20% of its fossil fuel emissions four decades later as its forests matured into peak absorption. Further documentation will be useful.

If either approach — reduction versus absorption — were to be cheaper, more cost effective, easier to implement, and sufficiently scalable, it alone might be our sole focus.Neither approach alone though will be sufficient.Consequently, the emerged consensus favors the combination. Sequestration — carbon capture— may be the cheaper option so some nations seek credit for their forested landscapes when seeking to justify lower rates of source reduction.Emission reduction efforts are generally forged at the national level, implemented down the subnational governing hierarchy and across a spectrum of non-profits and corporate entities, and at times co-originated at the local scale. International cooperation is almost essential because any region acting alone is unlikely to have the capacity to deflect the course of global change in air quality,possibly excepting the very largest nations including the United States and China, as noted previously. And there is no mechanism for large nations acting alone to recover costs from freeriding nations disinclined, or often simply unable to act. Late industrializers argue early industrializers should bear a larger responsibility due to the length of time in which their emissions were the principal source of persisting atmospheric GHG’s.

Sequestration stands now not so much as an alternative as it is a strategic partner therefore, with source-based reduction. This reality is expressed in the aspirational precepts of the UN Framework Convention on Climate Change (UNFCCC)operationalized first in the Kyoto Protocol of 1997 (effectuated in 2005, expiring in 2013). The Doha Amendments of 2012 though met with resistance but eventually secured the requisite national signatories in late 2020. From these efforts emerged two distinct acronyms denoting action:Afforestation/Reforestation (“A/R”), and Reducing Emissions From Deforestation and Degradation(“REDD+” including various forest management practices). Poorer nations were vocal in their insistence that the larger, richer nations take the lead.

Clearly, not all nations have favored the allowance of sequestration off-sets when tabulating fair shares of emissions reduction though this attitude might be dissipating[17-18].

Forested landscapes are a principal target in these efforts because of the perceived utility of treed landscapes for this function, over other herbaceous habitats. There are four distinct forest-based sequestration strategies, and their utility will vary among nations and between flatland and mountainous terrain. These include:1) halting deforestation — ceasing the permanent transformation of forest lands to other uses,2) slowing forest degradation that renders them less efficient as carbon repositories, 3) enhancing the health of forests elevating their carbon densities, and 4) engaging in afforestation whereby marginal lands or wastelands are selectively replanted in the most efficacious tree species and other ground covers.

While the decimation of forests continues,there is considerable space available with which to halt this decline. Global forest area declined by 83,000 km2per annum during the 1990’s. During the ensuing decade this figure fell to 52,000 km2per annum, evidencing marginal improvement.Each figure is the net of natural and human-assisted increase less losses due to forest destruction for farming, grazing, logging and other land uses[19]. It is estimated that there may be as much as 20 million km2of land on earth suited for forest restoration,some portion of which resides in mountains[20].

4 Forests as Net Carbon Sinks

Because the earths’ forested landscapes both absorb and release carbon they are intrinsic to the processes of climate change[21]. The net of carbon intake (+) less output (–) is the carbon balance,the upshot of the forest carbon cycle. Only when intake exceeds output (net positive) is a forest a carbon sink. This balance is composed of three co-occurring processes: respiration involving photosynthesis, using sunlight to convert C02and other ingredients into carbohydrates and sugar in biomass, biomass production both above ground(leaves, limbs, and trunks yielding wood products)and below ground (roots), and decay of deadwood and litter, yielding soils[22-23]. The processes noted both affect and are affected by climate change.Carbon sequestration — equivalent to net positive carbon intake — is an ephemeral, and momentary system state condition insofar as all sequestered carbon will eventually escape into the atmosphere unless transformed or cauterized. Delayed release of course, buys time to draw down source-based emissions emanating largely from the combustion of fossil fuels. Prior to combustion, of course,these fossil fuels (coal, oil, natural gas) themselves were in an underground sequestered state.

The process of warming moreover may in part be “deviation amplifying” inasmuch as the process itself can be an accelerant or a depressant.For example the exposure of long concealed peat bogs with the melt of polar ice will release CO2.Warming could also elevate the demarcation of mountain forest tree-lines while also improving forest habitats below the tree-line, thereby in both ways expanding the forested area. On the other hand the release of CO2due to logging, fire,species loss, decay and cultivation in excess of that ingested in forest respiration could prompt faster growth of the world’s flora thereby elevating future carbon absorption. More generally global ground cover will change, with climatic changes that will be difficult to predict. Perhaps one-fifth of the flat forested plains of the Amazon Basin has been deforested in recent years, mainly the result of cattle ranching and agricultural expansion. It is possible that global warming, in fact, enhanced the attractiveness of the Basin principally for livestock grazing, accelerating deforestation. Fires, many ignited by farmers clearing the Amazon forest for planting, have ploughed their way into the interior,made easier by Brazil’s major effort to open up the forest in the 1970’s, by building an extensive road network and fostering the growth of urban settlements in the interior. In the Amazon Basin we have a classic demonstration of the ill-effects of land use competition. There commercial viability is a substantial inducement to deforest the landscape.Forests elsewhere, including those on mountainous terrain are one of the planet’s last lines of defense even as efforts to restore the rainforest are being mounted. Two questions are the focus of the remainder of this commentary. What may be the potential role of mountain forests in future carbon sequestration? What are the potential challenges in elevating this generalized forest potential in mountain forest biomes? These challenges are viewed through the lens of land use competition.

5 Efficacy of Carbon Sequestration in Mountain Forests

To evaluate the efficacy of efforts to promote mountain forest sequestration I first establish an order of magnitudes to gauge the quantity of forested lands in mountains, their size relative to the totality of the earth’s forests, the carbon footprint of these mountain forests, and their carbon density. From these data I will seek to judge the potential of mountain forests to address global GHG production. And I will frame this potential as being a product of land use competition wherein rival uses, but not necessarily rival users, compete for space. Such competition will take widely different forms as the proponents of rival uses vie for the right to use land in particular ways.

I accept the prevailing definition of mountainous areas to be those whose elevation exceeds 2,500 meters above sea level or higher,plus those areas whose elevation is between 300 and 2,500 meters and whose surface is rugged and irregular[24-25]. Of this mountainous area, just one-quarter (11.5 million km2) is forested[24]. As noted by the Swiss Agency for Development and Cooperation, “Forests cover a significant proportion of most mountain regions except those that are particularly dry or cold year-round. In Europe for instance, forests cover 41% of the total mountain area — over half of the Alps, Balkans,Carpathians, and Pyreness … . Other mountain regions with particularly high proportions of forest cover include the Appalachians, the Australian Alps, the Guiana Highlands, and the mountains of Central Africa, Southeast Asia, Borneo , and New Guinea”[26].

To these I would add the Rocky Mountains of North America, the mountain domains of central Africa and of China, and the Andes of Central and South America. But overall, mountain forests constitute just one-quarter of the total forested area set upon the earth’s land mass (45.9 million km2).

Are forests, and mountain forests in particular,suitable candidates for emissions sequestration?Consider first the overall size and scope of forested lands. The earth’s surface over land and water is 510 million km2. Close to 30% of this surface is land mass, or 153 million km2. Of this land mass,around 30%, is forested, or 45.9 million km2.And perhaps 25% of the earth’s forests reside in mountains, or 11.5 million km2. The exact area is not definitively known, nor are the qualities of forested areas in mountains. Forest carbon densities,for example, surely decline with altitude. The Boreal forest undoubtedly represents the largest single share of the mountainous forest, and if we reliably knew the composition of this forest and the carbon density of its undergrowth and soils it would be possible to specify its overall potential in carbon sequestration[27]. This, however, is not yet possible.

Consider the full scope of global sequestration options including that occurring in its principal repositories: wetlands, oceans and forests. Forests and oceans are the earth’s principal carbon absorbers per annum, though the cumulative amount of carbon stored in oceans far surpasses that in forest biomes. The IPPC[28]estimates that forests, plus wetlands to a much lesser degree,absorb combined together around 10 GtCO2per year while oceans absorb perhaps 8 GtCO2per year[29], a figure that may be in decline. Global combustion of fossil fuels plus cement production release close to 29 Gt per year into the atmosphere,an amount augmented by another 4 Gt per year from soil-degrading farming practices. Close to 15 GtCO2remains suspended in the atmosphere,unabsorbed. Almost half of the total CO2emitted each year that is, remains adrift.

Forests of course vary both in annual capture rates and aggregate storage capacity. The highest forest carbon densities are found in Boreal forest biomes, with the tropical next and temperate forests lagging behind. Boreal forests of conifers,birch and popular predominate in the cold temperate region south of the Arctic, coterminous with the taiga which is largely covered in coniferous forests and wetlands. The carbon densities of wetland biomes (700 metric tonnes of carbon per hectare on average) exceed those of Boreal forests(400 metric tonnes per hectare)[30].

Forests then not only do a major share of the global work of carbon capture, but of all the options they offer the greatest opportunity for human intervention to elevate their role in carbon capture.Indeed, two billion hectares (20 million km2) of the earth’s land surface might be suited for these forest-restorative activities. Forests today absorb perhaps one third (almost 3 billion metric tonnes per annum, or 3.3 billion non-metric (i.e. short tons) of the CO2produced by the burning of fossil fuels. Of all means for natural absorption(sequestration) forests appear to possess the greatest potential achieved through afforestation where no forest previously existed, reforestation to reclaim old forests areas, and the reversal of degradation.Restorative strategies include accelerating stand establishment through nutrient provision,promotion of resilient tree species, and protection of extant stands from fire and pestilence.

6 Estimating Net Carbon Sequestration in Global Mountain Forests

A rough approximation of the maximal sequestration potential of global mountain forests could in theory be computed as a product of area multiplied by annual absorptive capacity per unit of area, but not all mountain area is forested and current documentation is inadequate. There are, as previously noted, 11.5 million km2of forested land in the earth’s mountains. Further it is estimated that global boreal forests that are 200 years old or older,sequester around seven tons of CO2per hectare per year, or 700 tons per km2[31]. Eleven and onehalf million km2times 700 tons per km2yields 8.05 billion U.S. tons of CO2or 7.3 billion metric tonnes of CO2per annum.

There are many reasons why there is high variability in viability of mountain forests and their carbon potential. Major portions of these mountains will rise well above tree-line. And the tree-line itself will fall to lower altitudes as temperatures fall. Forested biomes such as the Boreal will be interspersed with patches of poor soils resting atop rocky outcrops, and at lower elevations logging and certain recreational pursuits will further erode sequestration capacity. Steeply sloped terrain will suffer erosion and forest degradation leading to lesser carbon densities. At lower altitudes rival pursuits will vie for space. This competition for land — land use competition —in both temperate and tropical forest biomes will be more likely to be surrounded by impoverished populations seeking to monetize forest products or scavenge for firewood. Fires in the denser forests at lesser altitudes having higher carbon content may burn hotter, releasing droves of carbon while preparing the ground for new growth in ensuing decades. Slope aspect will similarly shape forest outcomes, shielding the slopes from full sun and impeding growth somewhat. At the same time denser wooded areas will not only slow erosion but also modulate the local hydrology,furnishing steadier water supplies to residents and communities below. But furnishing the wherewithal for local populations to cling to the lower slopes will also promote destructive forest practices associated with both commercial and subsistence farming, mining, logging, and fuel harvesting.

It might seem that with the abundance of flatland forests, the work of mountain forests in carbon sequestration might be considered to be of less utility. Forests everywhere are under duress as documented every five years in the UN’s Food and Agriculture Organization’s Global Forest Resources Assessment (FAO). Areas of most rapid forest loss and deterioration are found in Latin America(Amazon, the Atlantic Forest, Gran Chaco, the Cerrado, and Choco-Darien), SE Asia (Greater Mekong), Africa (the Congo Basin, and East Africa), and the South Pacific (Borneo, Eastern Australia, New Guinea, and Sumatra). Moreover,commercial agriculture continues to claim large tracts of the flatter forested lands. It bears primary responsibility for deforestation and degradation in the tropical biome. Of all large-tract commercial agriculture, soybean, palm, and cattle grazing are most inimical in this area. Logging short of clear-cutting is another principal cause of forest degradation. More is being learned now about how to improve recovery rates in such settings.Warming operates as an independent source of forest degradation and decay while also serving as a fire accelerant.

7 Mountain Forest Capacity: Recapitulation

At the outset of this commentary several questions related to mountain forests were put forward. I return to those now. Firstly, what is the current capacity of the world’s mountain forests in carbon capture? Recall from previously cited data that there are a total of about 45.9 million km2of forested land on earth at large, and of this amount 11.5 million km2(25%) resides in mountainous terrain. Global forests are said to absorb 10 GtCO2per annum. The absolute maximum additional capacity of mountain forests for net carbon ingestion, allowing for all the combined effects in the forest carbon balance noted earlier, would be 0.25 × 10GtCO2, or 2.5 GtCO2. If the global usage of fossil fuels produces around 29 Gt per annum, then around 9% of this production would be absorbed in mountain forests. This figure, moreover, may be on the high side given the greater role of tropical forests that has been averaged into these global aggregate estimates. At the same time, the combined sequestration of both oceans and mountains plus wetlands still leaves almost 10 GtCO2uncaptured and adrift in the global atmosphere. While the capacity of oceans at large, now limited by acidification, may increase marginally owing to polar melting, this would not be sufficient to erase the 10 GtCO2annual deficit.

Can forests including mountain forests do more to take up the slack? Perhaps they can. This is because, as already noted, possibly 20 million km2of the earth’s land mass has been deemed suited for forest restoration. At the average global forest net sequestration rate of 10 Gt CO2/ 45.9 × 106km2or 218t CO2/ km2mountain forests could net sequester an additional 1.1 Gt CO2. This assumes the additional mountain land suited for reclamation is proportional to the ratio of mountain forest land to global forest land. This would amount to around 11% of the amount of CO2currently drifting into the atmosphere each year. Such an estimate is necessarily preliminary since the needed data— more disaggregated — are not yet available. It is concluded that mountain forests already play a vital role in carbon sequestration, and that this role might be expanded with proper nurturing of mountain forests.

Secondly, how may this capacity change over time as a result of land use rivalries and of climate change itself? And may there be better, less costly means for achieving an equivalent effect in carbon capture, hence the slowing of planetary warming?As noted already, given the lack of alternatives any further reduction in global GHG emissions will have to originate in just two principal strategies:1) reduction at the source of emissions, and 2) sequestration, largely in forests. A succession of questions must now be asked. Is it less costly to reduce emissions at their source by reducing the carbon intensity of fossil fuels or by switching to renewables, or to engage in further sequestration?If the latter is advocated, then which among the options for sequestration are most feasible and cost-effective? And which of the forest sequestration investment options are best?

Three considerations help us to answer these questions: 1) which approach can gain political acceptance, 2) which is most likely to be implemented in light of capacities in both the involved governmental and non-governmental sectors, and 3) which is most cost-effective? Poor nations having sequestration potential but lacking the wherewithal to act, will necessarily have to draw upon the resources of wealthier nations complicating the global political calculus. This calculus is bifurcated. Will wealthier nations crosssubsidize the poorer? And will the poorer accept such external involvement and at what domestic cost? Forest sequestration options are differentiated in accord with location, land quality, impact on nearby resident populations, appropriateness of chosen tree species, capacity of local governments to oversee forest management, climatic conditions both now and in the future, and more. For the advocates of forest enhancements such factors as these combine together to score the desirability of alternate locational options for investment in afforestation and reforestation, and in arresting forest degradation. Think of this scoring function as an evaluation function whose output is a comparative weighting of forest investment options for a single use or user. Such evaluations allow single-use users to judge the relative attractiveness of different lands for a given use. They also come into play as rival uses or users “compete” for given spaces. Such comparisons constitute the basis upon which land is allocated to various functions. This is land use competition[32].

Three different divides define this competition: 1) between flatland and mountainous terrain, 2) among unequal contestants for the use of land, and 3) between the contexts of the Global North and South. Combined in a Vennlike diagram of course there will be some missing cells. First, flatland forests offer the attraction of both accessibility and forest abundance, but there the competition amongst potential uses or users of land can be intense. The remoteness and impenetrability of mountain terrain will tend to reduce the number and type of potential users and hence competition may be less fierce, making such places perhaps more suited for carbon capture which requires huge tracts of land. Most mountain forests have developed of their own accord,without human agency. But expanding these domains could require active management.

A second divide in the competition for forested lands is between large and powerful interests and the poor. In this competition contestants have unequal capacities to secure a competitive edge. And “clean” air, unburdened by an infusion of GHG’s, is a common pool resource whose value is almost entirely non-local hence unappreciated in the decision calculus of local contestants.

In and on the perimeters of the mountainous regions of the Global South live perhaps 720 million people. Seven in ten of these live in the most rural portions of mountain terrain eking out an existence on the lesser slopes while securing remittances from residents who travel to distant cities where jobs are more plentiful and income more easily secured. For these persons forests provide wood fuel, water, space for subsistence farming and grazing, and commercial employments in mining, farming, grazing, tourism, and logging[33].The juxtaposition of forests and poverty has taken its toll on such forest lands. Indeed, the area devoted to the world’s forests has declined by over 30% since the mid 19thCentury, and much but not all of this loss has been registered in the Global South. Deforestation peaked in the 1990’s at a net loss rate of perhaps 83,000 km2per annum. Since 2000 this annual net rate has fallen, possibly by as much as 40%, the result of both reclamation activities, urbanization of mountain economies,and climate change itself.

National context constitutes a third divide in the competition for mountain land and the forests set upon it. Those nations lacking sufficient domestic resources and organizational capacity with which to promote forest development and enhance sequestration represent a particular challenge in elevating global capacities in forest carbon sequestration. Assistance, both management and financial, is increasingly seen to be needed to enlist their participation[34]. Inducements are needed in many instances in which sequestration’s benefits are largely seen to be non-local, and in which the domestic payoff is therefore insufficient to engender substantial investment.

Considerable fractions of the terrain in mountains in both the Global North and South are in the province of national or regional governments. In these instances government per se is the owner hence the lead domestic entity in any negotiation over the uses of such lands.Many national governments, having signed on to the Paris Agreement or to the more recentBonn Challenge[35]②— which seeks national commitments to reforest 350 million hectares and to purpose them for carbon sequestration by 2030— will dedicate their own mountain lands to this purpose, over-riding any claims and associated bids originating outside of government.

The World Bank has become one principal agent in promoting payments for environmental services including carbon sequestration while building domestic economies to forestall forest destruction in Latin America and Africa[36]. Unlike the standard notion of land use competition however, competing forest users seeking sites will muster an array of political and legal capacities with which to claim space. These attributes are the coin of competition. Narrowly defined price-competition among rival land claimants is clearly not contemplated in this discussion. The competition for forested lands, particularly on mountainous terrain, entails few claimants, their number diminishing with altitude.

8 Concluding Observations

Forests, including mountain forests, are a principal context in carbon sequestration and a partial antidote for global warming. Indeed some now believe that it is within the earth’s reach to reduce GHG emissions to the point that surface temperatures may commence to stabilize in coming decades — far sooner than had previously been anticipated — , a most hopeful prospect[37]. Net zero global GHG emissions though remains a challenge and the prospect of rising emissions particularly in the Global South will make the pursuit of source-based emissions reduction and carbon sequestration an even more urgent but tandem objective.

Mountain forests, aided by both better management practices and greater cross-national collaboration, must be considered an essential element of any future solution. But to act more information is needed. Estimation of the full sequestration potential of mountain forests remains a matter of conjecture. More disaggregate data regarding the temporal and spatial variability of mountain forest composition will be required not only to gauge current performance but also future potential. Forests set upon mountains represent a particular challenge given difficult topography,unusual soil properties, variable sun exposure, and the vagaries of land ownership and control. Net zero carbon may be attainable but not absent the essential contributions of our forests including our mountain forests.

Knowledge of the sequestration potential of mountain forests and that of all other oceanic and terrestrial carbon sinks is of vital importance since the greater the collective sequestration potential of these sinks the less may be the need to reduce carbon emissions associated with the major energy-using sectors. On the other hand, the lower the aggregate sequestration potential across all sinks, the greater must be the effort to reduce source-based carbon emissions, primarily but not exclusively in the energy sector. This subject is set within the broader concerns associated with the maintenance, planning and design of resilient mountain landscapes. To promote resilience is to engender perpetual functionality in the face of system shocks and stresses. Carbon sequestration though is but one element of this pursuit. There are, however, many rival claimants seeking to benefit from mountain resilience and their quests are not easily reconciled. How determined must we be to elevate carbon sequestration potentials in mountain forests will depend on 1) the ease with which such an end can be achieved, 2) opportunity costs incurred in achieving this end, 3) the efficacy and mutuality of alternative uses and activities that could be pursued on mountain landscapes,4) the comparative efficacy of alternate oceanic and terrestrial carbon sinks, and 5) the costeffectiveness of decarbonization approaches in the energy sector.

The potential contributions of our forests including our mountain forests could represent a cheaper, faster way forward compared to the decarbonization of transport, manufacturing, and heating/cooling of buildings, or to elevating the sequestration capacities of the oceans, wetlands and soils[38]. In the longer term decarbonization of the energy sector will almost invariably be essential.Net zero will almost certainly require both decarbonization through electrification, largely fueled by nuclear and renewable energy sources,and sequestration. China anticipates that most new cars sold there in 15 years will be electric. General Motors pledges to sell only zero-emission vehicles by 2035. These topics are set within the rubrics of“mountain resilience” inasmuch as the maintenance and enhancement of the capacity for carbon sequestration in mountain forests necessarily must compete with rival purposes to be pursued in the planning and management of mountain landscapes.Among these are agriculture, resource extraction,fuel harvesting, tourism, and the like.

This commentary is a problematization of the carbon sequestration potential of mountain forests. More disaggregate data regarding the temporal and spatial variability of mountain forest composition will be required not only to gauge current performance but also future potential.Judging the efficacy of mountain forest biomes as carbon sinks will require further research into three distinct forest processes: respiration,biomass production and disbursement, and the stoichiometry of biomass decay and soil generation. The idiom that “one can’t see the forest for the trees” is especially apt in this context.While we require knowledge of the individual tree we also need to understand the entire mountain forest biome or ecosystem in its full synergetic complexity. Because the sequestration potential of mountain forests and indeed all other planetary carbon sinks varies over time, the resilience of each must be taken into account. If some falter, other sinks may bear a greater responsibility. If fossil fuels can be supplanted by other energy sources in a cost-effective manner then carbon sinks may carry a lesser responsibility. Both sequestration and decarbonization of the energy sector are subject to the vagaries of the human will, technology,climate, environmental capacity and more. As such they represent perhaps the penultimate challenge facing humankind since progress in each facet will inevitably suffer system shocks threatening the resilience of any particular “momentary”resolution. The fundamental reality is that the actions and conditions that must be in place to forestall global warming are not intrinsically resilient. Powerful societal forces will continue, in the absence of alternatives, to unearth and burn fossil fuels. Forests, oceans, soils and wetlands will give up their sequestered carbon in time since the cauterization of their carbon stores will be imperfect. Few elements of the energy-climatebiome “system” are going to be held constant in perpetuity. Resilience, including that small piece of this system lodged on mountainous terrain,will require a multitude of remedies to maintain system performance. These include the repair or replacement of failing elements of the system, or the substitution of new means to achieve purposes that can no longer be addressed by prior practices or conditions. The protected domains of the earth’s forests including its mountain forests might perhaps emerge in time as a steadying force within this complex system of vulnerable parts. Forest managers, planners, and landscape architects,informed by both forest and climate science,should be at the forefront of our collective effort to extract the full potential of forests, including mountain forests in carbon sequestration.

Notes:

① The “e” in this measure denoted equivalency across all the GHG components expressed in terms of the weight of carbon dioxide (CO2), given in gigaton (Gt) metric, wherein one Gt is one billion (1×109) metric tonnes. The term CO2e signifies aggregate emission across all GHG types,expressed in terms of their CO2equivalent.

② The Bonn Challenge is driven by the Forest Stewardship Council and private industry, an alliance bent on forest preservation and reforestation.

(Editor / WANG Yilan)