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