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The effect of clear-cutting forests on the environment

Laurent Berges
Laurent Bergès
Research engineer at Ecosystems and Societies in Mountains Laboratory (LESSEM research unit) at INRAE Grenoble Centre
Jérôme Ogée
Jérôme Ogée
Researcher at INRAE specialising in interactions between climate and vegetation
Marion Gosselin
Marion Gosselin
Ponts, Eaux et Forêts engineer at INRAE's "Forest Ecosystems" research unit (Nogent-sur-Vernisson)
Key takeaways
  • Clear cutting is the felling of an entire forest, usually before replanting.
  • This method of forestry helps to optimise the harvest in technical, logistical and economic terms.
  • Many members of the public are critical of the long-term environmental impact of this practice, particularly on the water cycle and soil quality.
  • As well as affecting ecosystems, the repercussions are also felt by neighbouring communities, with flooding, the threat of fire, and a decline in flora and fauna.
  • Although scientists have issued recommendations, there are no regulations in force, so professionals need to be trained in good practice.

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.”

A practice with many spin-offs

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
1A sum­ma­ry : http://​www​.gip​-eco​for​.org/​e​x​p​e​r​t​i​s​e​-​c​r​r​e​f​-​c​o​u​p​e​s​-​r​a​s​e​s​-​e​t​-​r​e​n​o​u​v​e​l​l​e​m​e​n​t​-​d​e​s​-​p​e​u​p​l​e​m​e​n​t​s​-​f​o​r​e​s​t​iers/ the sum­ma­ry report: http://​www​.gip​-eco​for​.org/​c​r​r​e​f​-​s​y​n​t​h​e​s​e​-​d​e​-​l​e​x​p​e​r​tise/ and the expert report: http://​www​.gip​-eco​for​.org/​c​r​r​e​f​-​s​y​n​t​h​e​s​e​-​d​e​-​l​e​x​p​e​r​tise/

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