Effects of different site preparation treatments on species diversity, composition, and plant traits in Pinus halepensis woodlands

International audienceBiodiversity maintenance is a key component of Mediterranean forest management, yet studies on the effects of silvicultural treatments on plant diversity are scarce. Our experiment assessed the impact of five different site preparation treatments on the composition, diversity, ecological traits (life-form, pollination mode, leaf morphology, seed dispersal mode), indicator values (shade tolerance, nutrients demand) of the understory vegetation in a mature thinned Pinus halepensis stand in southern France. The treatments—chopping, chopping followed by scarification in one or two directions, prescribed burning, control—were replicated four times and applied on a total of 40 plots. Vegetation relevés were performed on each plot in the first, second and fourth year following treatment applications. Plant diversity, measured by the species richness or Shannon's index, increased in the non-control treatment plots in the first year but then decreased through time. Vegetation composition differed between treatments, with the chopping treatment exhibiting composition and ecological trait values more comparable to those of control plots than the other treatments. The burning and scarification treatments led to higher abundance of therophytes, plants with malacophyllous leaves and insect-pollinated plants, and shade-intolerant and nutrient-demanding species. However, these changes were transient in time, the shade-intolerant species remained abundant but the ruderal species decreased while the ligneous species increased indicating a gradual return to a forest vegetation composition. For the Mediterranean area, most of the findings were similar to those in temperate forests subjected to the same site preparation treatments