Impact of active coppice management on microclimate and understory vegetation in a Mediterranean oak forest

Plant diversity of the forest understory contributes to forest functionality and provisioning of ecosystem services. In recent years, however, understory thermophilization processes caused by global warming have been detected especially in temperate regions with warm macroclimate, such as the Mediterranean one. Coppice-with-standards is still one of the most common types of management of broadleaved and evergreen forests in this region, aimed at the production of renewable energy, especially firewood. Trees are cut at regular time intervals, thus leading to cycles of vegetative regeneration. However, the strong modification of forest structure caused by coppicing could mine the capacity of forest canopy to buffer temperature extremes and to offer microclimatic refuges for the herb communities. Hence, it is crucial to assess the sustainability of this traditional management form under the current climatic stressors. We contributed to this topic by analyzing shifts in temperature buffering capacity, understory diversity (taxonomic, phylogenetic, functional), and productivity in an ancient forest of central Italy with Quercus cerris and Q. petraea (Bosco ai Frati, Tuscany). Here coppice-with-standards and high forest are next to each other and under homogeneous site conditions since long time. To this purpose, in 2021 we installed air and soil temperature dataloggers in three high forest sites and three coppice-with-standards sites. Following a nested sampling design, forest structural variables light availability (photosynthetic active radiation) and soil pH were determined before surveying understory vegetation in four 5 x 5 m randomly selected quadrats in each of the six sites (24 quadrats). Understory aboveground productivity (dry weight of herbaceous and woody biomass) was determined in 48 0.5 x 0.5 m plots (two per quadrat). Moreover, five functional traits associated with the acquisition and conservation of resources (vegetative traits) and reproductive efficiency were collected from the TRY database: Specific Leaf Area, Leaf Dry Matter Content, plant vegetative height, plant reproductive height, and seed mass. Regarding microclimate, we confirmed the strong effect of forests in limiting maximum temperatures. The mean offset values between forest and reference open areas in daily maximum temperatures were significantly larger in the high forest than in the coppice stands during all seasons. Our results supported that coppice management promotes understory species richness, although this is due to the presence of mostly generalist and light-demanding species with strong colonization capacity. Interestingly, coppicing led to a clustering in phylogenetic structure (measured with Mean Nearest Taxon Distance) and differed significantly from high forest in functional diversity for Leaf Dry Matter Content, highlighting the presence of ongoing acclimation processes in the understory. Coppicing also influenced the diversity-productivity relationship. In the light of these results, we emphasize the need to take into account different facets of plant diversity, to reach a more holistic understanding of the effects of coppicing on deciduous oak woodlands of the Mediterranean region, on especially plant diversity and temperature buffering capacity of the forest canopy.