Mechanisation of forest operations drives long-term changes in plant communities

Using a permanent network of plots, we resurveyed vegetation in a broadleaved forest of northeastern France, tracking the effects of climate change, eutrophication and forest management between 1990 and 2022. To better detect the impact of mechanisation on plant communities, we established subplots based on microtopography (skid trails and wheel tracks) and recorded species separately in these subplots. We then tested changes in species’ frequency and used ordination to analyse changes in communities’ composition. Shifts were assessed using species’ indicator values in plots and subplots, as well as regression analysis of plot shifts on axis 1 of the correspondence analysis as a function of stand characteristics, vehicle disturbances, canopy openings, game pressure, storm impact and ownership. Results showed a progress of non-forest species, hygrophilous species and species preferring wheel tracks and skid trails. Compaction from forestry vehicles was associated with an increase in indicator values for soil moisture and nitrogen. The projection of subplot surveys onto the correspondence analysis confirmed wheel tracks and skid trails as the main driver of observed vegetation shifts. These, as well as canopy openings, were significant in the regression analysis. Although we observed thermophilisation of plant communities (+0.27 °C per bioindication) which lagged behind recorded temperature increase (+1.5 °C), this was only a minor cause of overall vegetation changes. Mechanisation is therefore becoming a driver of vegetation changes at the scale of entire forests. As the use of harvesters in temperate deciduous forests increases, the planning of forestry operations should better account for these impacts on vegetation.