“Benefits of planting trees are grossly overestimated”

Early in 2019, a sci­entif­ic paper entitled “The glob­al tree res­tor­a­tion poten­tial” was pub­lished by Bastin et al. ¹ in the pres­ti­gi­ous journ­al Sci­ence, imme­di­ately mak­ing waves around the world. In it, the authors present an argu­ment that massive tree-plant­ing across the globe could be enough to mit­ig­ate cli­mate change much more than had pre­vi­ously been assumed. They con­cluded that plant­ing an extra 0.9 bil­lion hec­tares of trees would store 205 bil­lion tonnes of car­bon – thus, claim­ing that glob­al tree res­tor­a­tion is “our most effect­ive cli­mate change solu­tion to date”.

As such, the study prom­ised to offer hope of an action­able and reli­able solu­tion to reduce atmo­spher­ic CO₂ levels. By the end of the year, it became the second most cited cli­mate change paper in the press, gen­er­at­ing nearly 600 media art­icles. Nev­er­the­less, this strik­ing claim has since pro­voked massive push­back from sci­ent­ists, includ­ing five tech­nic­al com­ments² in response stat­ing that the study greatly over­es­tim­ated the mit­ig­a­tion poten­tial of mass tree-plant­ing. We talked to two experts rep­res­ent­ing the nat­ur­al and social sci­ence per­spect­ives to under­stand why this top­ic is so crit­ic­al for net zero com­mit­ments, for policy and for the sci­entif­ic community.

Much can be said around the sci­entif­ic con­tro­versy that the 2019 paper gen­er­ated. Why are we still inter­ested in this, two years on?

Julia Pon­gratz. The research­ers in the study used a large amount of photo inter­pret­a­tion meas­ure­ments and com­bined those with inform­a­tion on cli­mate and soils to cal­cu­late glob­al tree cov­er poten­tial. Such high-res­ol­u­tion estim­ates of poten­tial tree cov­er are much needed, in fields such as cli­mate research to assess how much veget­a­tion exists in the absence of human inter­fer­ence. In this respect, the study is a laud­able achieve­ment. Based on this new estim­ate the authors went on to draw con­clu­sions con­cern­ing how much car­bon could be taken up if the tree cov­er were restored every­where [apart from present-day cro­p­lands]. In com­par­is­on to pre­vi­ous estim­ates, the paper was at the very high end of the scale for CO₂   reduc­tion poten­tial. Most of the fol­low­ing tech­nic­al com­ments cri­ti­cised this point, claim­ing that Bastin et al. drastic­ally over­es­tim­ated the envir­on­ment­al bene­fits of tree-planting.

Steph­en Woroniecki. Sci­ent­ists are wor­ried that we will con­tin­ue to dra­mat­ic­ally over­es­tim­ate how much car­bon can be drawn down by ter­restri­al eco­sys­tems. And that, as a res­ult, the glob­al eco­nomy will con­tin­ue to pump out green­house gases only to real­ise fur­ther down the line that the car­bon sink cre­ated by trees is less effect­ive than we thought – or that it is more vul­ner­able to cli­mate change itself. Des­pite warn­ings, plant­ing trees are form­ing a cent­ral part of many policy pro­pos­als. So, even though many sci­ent­ists pushed back on the ori­gin­al paper, their con­cerns had impact on policy or pub­lic debate. Plant­ing trees or let­ting them grow back nat­ur­ally is an import­ant part of the cli­mate chal­lenge, but it is not a sil­ver bul­let, and can­not com­pensate for emis­sions else­where in the economy.

There also seems to be some con­fu­sion about the dif­fer­ent ter­min­o­logy being used, namely the dif­fer­ence between refor­est­a­tion and affor­est­a­tion. Could you explain?

SW. Affor­est­a­tion is plant­ing trees where they have not pre­vi­ously grown – or in any recent time sig­ni­fic­ant to us, at least. This includes tree-plant­ing in grass­land or oth­er eco­sys­tem types which his­tor­ic­ally have had few trees. Where­as refor­est­a­tion is replant­ing trees where they pre­vi­ously grew. This includes parts of the Amazon basin, for example, where refor­est­a­tion aims to re-estab­lish pre-exist­ing forests.

JP. Sci­ent­ists are con­cerned that affor­est­a­tion can dam­age eco­sys­tems that are vital for both biod­iversity and car­bon sequest­ra­tion. Plant­ing trees on grass­lands or drain­ing peat lands to plant forest plant­a­tions could have neg­at­ive con­sequences for the cli­mate and for loc­al fauna and flora. The estim­ates by Bastin et al. include using all cur­rent pas­tures and ran­ge­lands (grass­lands, shrub­lands etc), which would require massive changes in our cur­rent agri­cul­tur­al man­age­ment and/or diets.

Sci­ent­ists fear that affor­est­a­tion will dam­age eco­sys­tems essen­tial for biodiversity.

There were five tech­nic­al com­ments in total. Can you talk about the sci­entif­ic backlash?

JP. A key cri­ti­cism of the ori­gin­al manu­script was that it seemed that zero car­bon was assumed for the exist­ing eco­sys­tems, which clearly would be a flawed assump­tion; even if grass­lands do not have a lot of bio­mass, soil car­bon stocks are sub­stan­tial. As a res­ult, the meth­od sec­tion was changed to cre­ate very sim­il­ar num­bers to the ori­gin­al while now account­ing for car­bon in the pre-exist­ing eco­sys­tems. This is an unsat­is­fy­ing res­ult for the pro­cess of sci­entif­ic pub­lish­ing, giv­en that with such a major meth­od­o­lo­gic­al change the study should have gone through form­al peer-review again, which it did not.

I think harm has been done to the trust in sci­ence. In Ger­many, one of the largest news­pa­pers repor­ted on a full page about this new pan­acea against cli­mate change, only to acknow­ledge the very next day that exactly those state­ments of the study were heav­ily cri­ti­cised by oth­er renowned research­ers. This sug­ges­ted to the pub­lic that there was a dis­pute in the sci­entif­ic com­munity that does not really exist. The state­ment about reforestation/afforestation being “our most effect­ive cli­mate solu­tion to date” was sub­sequently removed from the paper because it is not true – obvi­ously, redu­cing anthro­po­gen­ic emis­sions from fossil fuels and car­bon­ates as well as from land use are the solu­tions. But the dam­age in pub­lic per­cep­tion of research in this field had already been done. 

Are there oth­er import­ant concerns?

SW. Yes, there are two addi­tion­al issues to con­sider when plant­ing trees as a car­bon solu­tion. One con­cerns biod­iversity like: what effects will there be on biod­iversity of large-scale tree plant­ing? And the oth­er is social: how will these pro­jects affect people who live in the area? The IPCC spe­cial report on cli­mate change and land³ is an excel­lent resource on these neg­at­ive con­sequences. It also provides ana­lys­is of refor­est­a­tion show­ing that neg­at­ive food secur­ity effects are likely to be less pro­nounced for refor­est­a­tion than affor­est­a­tion. A lot depends on which trees are planted, where, why, and do they provide for loc­al needs. There are some ter­rible examples of cut­ting down primary forests only to plant trees to claim the car­bon benefit.

It also troubles me that land­scapes can be seen as these empty spaces that are ripe for res­tor­a­tion. When you make a map of res­tor­a­tion poten­tial, and you don’t take people who live there into account you run the risk of walk­ing over peoples’ rights – espe­cially in coun­tries with poor gov­ernance and sys­tem­at­ic neg­lect of human rights. The changes of res­tor­a­tion or affor­est­a­tion con­trib­ut­ing to pos­it­ive devel­op­ment out­comes in those places is much lower.

JP. We must not also for­get that when we talk about cli­mate impact of grow­ing trees, more than CO₂ mat­ters. We also need to con­sider what we call bio-geo­phys­ic­al effects such as changes in energy and water fluxes. If you plant a forest, you may eas­ily change loc­al tem­per­at­ures by sev­er­al degrees, depend­ing on the region. We are on the way to under­stand­ing which forests cre­ate an addi­tion­al cool­ing effect, which ones may lead to addi­tion­al warm­ing if it is pos­sible to mit­ig­ate heat extremes or drought con­di­tions. Ideally, we would cre­ate win-win situ­ations of mit­ig­a­tion of with adapt­a­tion to cli­mate change when the planted forest coun­ter­acts the effect of glob­al warm­ing locally.

Some say that the part of the issue is try­ing to man­age tree-plant­ing at the glob­al scale. Is loc­al man­age­ment a bet­ter way?

SW. In the research com­munity there is debate between those who favour a com­munity-based approach, and those who favour pro­jects at lar­ger scale, focused on glob­al tar­gets where the com­munity may be an after­thought. Find­ing a bal­ance between the two is why some organ­isa­tions like IUCN and Oxford University’s Nature-based Solu­tions Ini­ti­at­ive have developed guidelines for nature-based solu­tions to build the guard­rails for large-scale imple­ment­a­tion of tree plant­ing. Involving people from the begin­ning, listen­ing to their his­tor­ies and pri­or­it­ies, can get you so much fur­ther than march­ing in and fen­cing off a new plantation. 

JP. All land-based car­bon diox­ide remov­al meth­ods are imple­men­ted loc­ally. It is great if estab­lish­ing new forest cov­er sucks up CO₂, but if it is cut down five years later because loc­al farm­ers have no oth­er option than move into this land, little has been gained. To assess social accept­ab­il­ity of meas­ures like affor­est­a­tion it is import­ant to bring in dir­ectly the stake­hold­ers on the ground. We are thus now mov­ing towards a more inter­dis­cip­lin­ary approach as well, where the nat­ur­al sci­ent­ists and eco­nom­ists that have always been inter­ested in forests’ inter­ac­tions with cli­mate change col­lab­or­ate intens­ively with the social sci­ences and humanities.

Interview by Denise Young