The effect of clear-cutting forests on the environment

Reg­u­larly con­demned by cit­izens’ groups in the Mor­van and Landes regions, do clear-cut­ting oper­a­tions threaten forest eco­sys­tems? At the end of 2022, 70 experts presen­ted the res­ults of a col­lect­ive study¹ answer­ing this question.

Clear-cuts are hard to miss: this type of sil­vi­cul­ture con­sists in felling an entire forest zone at once, then replant­ing it. In 2010, clear-cuts accoun­ted for 0.4% of forest area in main­land France, mainly mari­time pines, chest­nuts, spruces and pop­lars. But there are region­al dis­par­it­ies: the fig­ure rises to 2.1% for the Landes mas­sif, for example. “This equates to a peri­od of less than 50 years between two clear-cuts,” explains Jérôme Ogée. “This is equi­val­ent to the rota­tion recom­men­ded for mari­time pine, and there­fore means that clear-cut­ting is the most com­mon prac­tice across the whole region.”

While some mem­bers of the pub­lic con­demn the neg­at­ive impact of clear-cut­ting on biod­iversity and land­scapes, for­est­ers argue that it “optim­ises har­vest­ing from a tech­nic­al, logist­ic­al and eco­nom­ic point of view.” How­ever, as Laurent Ber­gès points out, “clear-cut­ting is syn­onym­ous with increased mech­an­isa­tion in forestry.” Log­ging and felling machines, for­ward­ers and skid­ders are used to har­vest the wood, fol­lowed by stump remov­al and scar­i­fic­a­tion equip­ment, and plant­ers for regen­er­a­tion. “For reas­ons of eco­nom­ic prof­it­ab­il­ity and the labor intens­ive nature of the work, we are cur­rently exper­i­en­cing a revolu­tion sim­il­ar to the agri­cul­tur­al revolu­tion of the 1950s, with increased mech­an­isa­tion in forestry,” notes Laurent Ber­gès. “And even if clear-cut­ting is still not widely prac­tised today on a region­al scale, its effects are not negligible.”

First and fore­most, it affects the envir­on­ment itself. The water cycle is altered. Water con­tent in the soil rises from 18% to 66%: as there are no longer any trees draw­ing water from the soil or pre­vent­ing rain­fall from reach­ing it! The ground is heav­ily com­pacted by the pas­sage of increas­ingly heavy machinery – par­tic­u­larly on clay soils. As a res­ult, the soil’s infilt­ra­tion capa­city is reduced, water runs off and the stream flow out of water­sheds increases by 30 to 100%. Large quant­it­ies of sed­i­ment (up by 700%) are washed into water­courses, along with nitrates and soil cations (cal­ci­um, potassi­um, alu­mini­um), some­times degrad­ing water qual­ity. The soils them­selves become less fer­tile, less rich in car­bon, less aer­ated… Most of the effects can be observed for sev­er­al years after the cut, or are almost irre­vers­ible, as in the case of erosion. “Clear-cut­ting also affects the sur­round­ing plots: for example, trees at the edge of the forest become highly vul­ner­able to storms,” explains Jérôme Ogée. “As for the impact on the micro­cli­mate, it can be meas­ured up to sev­er­al hun­dred meters away.”

Loc­al pop­u­la­tions are also affected. “Dur­ing heavy rains, the forest acts as a buf­fer zone, allow­ing for the infilt­ra­tion of the rain­wa­ter,” explains Jérôme Ogée. “Clear-cut­ting increases the occur­rence of peak flood­ing.” The sci­entif­ic com­munity has recently been study­ing anoth­er less­er-known effect. The banks of water­courses are pop­u­lated by spe­cial trees known as ripari­an veget­a­tion. “It appears that ripari­an forests, whose decidu­ous spe­cies are less flam­mable than con­i­fer­ous spe­cies, act as nat­ur­al firebreaks dur­ing fires,” points out Jérôme Ogée. “These res­ults have yet to be con­firmed by research.” When ripari­an forests are razed, the nat­ur­al wild­fire bar­ri­er is lost, pos­ing a dir­ect threat to the sur­round­ing infra­struc­ture and population.

Finally, the forest eco­sys­tem is also dis­rup­ted. In the first two dec­ades fol­low­ing a clear-cut or pro­gress­ive cut, the total num­ber of spe­cies increases by more than 10% com­pared with a con­trol plot. It then decreases after 20 years. “These plots act as sub­sti­tute hab­it­ats for open, agri­cul­tur­al spe­cies, often birds and but­ter­flies, when they are threatened in the sur­round­ing area by intens­ive agri­cul­ture, for example,” explains Laurent Ber­gès. This sun­ni­er envir­on­ment is also home to a new flora… but this is not a pos­it­ive devel­op­ment. “This masks a decline in the pres­ence of spe­cies spe­cif­ic to forest areas,” warns Mari­on Gos­selin. “These forest spe­cies thrive in old trees: if they’re all cut down, they’ll even­tu­ally dis­ap­pear, as they have no altern­at­ive hab­it­at. This com­pletely dis­rupts the ecosystem.”

In their sum­mary, the experts also point to the neg­at­ive short-term effects (less than eight years after felling) of clear-cut­ting on birds and mosses, and a non-sig­ni­fic­ant effect on vas­cu­lar plants, lichens, fungi, arach­nids and insects. “Soil com­pac­tion and mech­an­ised pre­par­a­tion before plant­ing sig­ni­fic­antly alter biod­iversity: trees grow more slowly, flor­al com­munit­ies are altered, micro­bi­al bio­mass decreases and fungal com­munit­ies are changed”, adds Laurent Ber­gès. Finally, the intro­duc­tion of non-nat­ive spe­cies – at the time of plant­ing or in the tyres of machinery, for example – can endanger nat­ive species.

Behind these glob­al obser­va­tions lie geo­graph­ic­al dis­par­it­ies. Erosion is amp­li­fied on slop­ing ground, and com­pac­tion is great­er on clay soils. But in par­tic­u­lar, clear-cut­ting close to a water­course has even great­er neg­at­ive effects. “It changes the micro­cli­mate, includ­ing that of the water­course, and con­trib­utes to releas­ing a huge amount of nitrates into the water­course in the fol­low­ing months”, explains Jérôme Ogée. The group of experts recom­mends that clear-cut­ting should be strictly avoided with­in 30 metres of watercourses.

Cli­mate change is already likely to lead to more forest die­back due to drought

Oth­er recom­mend­a­tions include adopt­ing spe­cif­ic har­vest­ing meth­ods that reduce the neg­at­ive impact of clear-cut­ting. Recom­men­ded prac­tices include not stump strip­ping, leav­ing the remains of branches on the ground, car­ry­ing out very loc­al­ised till­age around the plants, keep­ing to road­ways to lim­it soil com­pac­tion, main­tain­ing at least 10–15% of hab­it­at trees to provide shel­ter for spe­cial­ist forest spe­cies and replant­ing with a vari­ety of nat­ive spe­cies. Mari­on Gos­selin adds: “Gen­er­ally speak­ing, it is also bene­fi­cial to main­tain integ­ral reserves and to regen­er­ate forests by pro­gress­ive cut­ting or by small gaps rather than by clear-cut­ting.” How­ever, put­ting this into prac­tice comes up against oper­a­tion­al and eco­nom­ic obstacles. “We need to strike a bal­ance between these recom­mend­a­tions and their imple­ment­a­tion: it’s much more com­plic­ated to carry out till­age at a spe­cif­ic area rather than on an entire plot,” explains Laurent Ber­gès. Jérôme Ogée adds: “These recom­mend­a­tions are not reg­u­lated, and we now need to train pro­fes­sion­als in the sec­tor in these practices.”

Com­pli­ance with these recom­mend­a­tions is all the more import­ant in the con­text of a chan­ging cli­mate. “Cli­mate change is already likely to lead to more forest die­back due to drought,” says Laurent Ber­gès. “Recent assess­ments have shown that the CO₂ stor­age capa­city of forest eco­sys­tems has been halved in ten years.” Jérôme Ogée adds, “Stud­ies in France have shown that heat­waves and drought cause plant­a­tion fail­ures fol­low­ing clear-cut­ting.” On such bare ground, daily tem­per­at­ure ranges are high­er, radi­at­ive trans­fer increases and the soil dries out at the sur­face. Con­versely, the pres­ence of tree cov­er tem­pers cli­mat­ic extremes, lim­it­ing their harm­ful effects on the sur­viv­al of young trees. “Cli­mate change calls into ques­tion sil­vi­cul­ture prac­tices, which are no longer appro­pri­ate today,” con­cludes Laurent Ber­gès. We need to think about new sil­vi­cul­tur­al meth­ods, tak­ing into account the con­text of the indi­vidu­al plots.”

Anaïs Marechal