Effects of climatic, soil, structural and management factors on plant functional traits in forest understory vegetation of Italy.

Understanding how vegetation responds to environmental variations is a fundamental goal in ecology, and the trait-based approach seems to be particularly suitable, since functional traits mediate the response of plants to environment (1). In this context, several studies demonstrated the relationship between plant functional traits (PFTs) and individual environmental drivers such as climate, disturbance regimes or land-use change across different scales (2, 3, 4, 5). However, the observed pattern of vegetation functional signatures cannot be attributed to a single explanation, but rather to a combination of environmental factors (6, 7). Unfortunately, only a small number of studies showed how PFTs may be influenced simultaneously by different environmental variables and explained the relative role of different drivers (8). Furthermore, such studies have been mostly conducted in non-forest ecosystems. We aim to quantify the relative contributions of 17 climatic, soil, structural and management factors in determining the community-level PFT values of the forest understory vegetation of Italy. The ICP-Forests Level I dataset has been used, where vascular species have been recorded within 201 sample areas (stands) using a probabilistic approach (thus, representing the Italian forests). Three PFTs have been selected being fundamental trade-offs controlling plant strategies: specific leaf area (SLA), canopy height and seed mass (9). Prior to the analysis, a stepwise ordination has been used to select factors with higher explanatory power. Then, a variance partitioning has been applied in order to assess the contribution of climate, management, soil and structural factors (alone and in combination) to explain the variation in attributes. Finally a Redundancy analysis has been used to evaluate the relation between PFTs and factors. For canopy height, climatic factors (mean temperature and rainfall/PET, i.e. aridity index) are responsible for 25% of the variation in understory community PFT values. However, also soil (pH, available K, effective soil volume) and structural factors (forest type, n. of tree layers) play a relevant role (13-14% of the variation). While SLA variation is explained mainly by management factors (9%, intensity of management) and seed mass mainly by soil factors (17%, pH) and secondly by climatic factors (12%, potential evapotranspiration). However, unexplained variation is very high for all three PFTs, suggesting that many other biotic and abiotic factors play a key role. In detail, warmer and drier stands (within Mediterranean forest types) seem to be associated to higher values of canopy height and seed mass, respectively; stands with lower management intensity lead to higher values of SLA; stands with higher level of pH are associated to higher values of seed mass, while stands with deeper soils and more available K are associated to higher canopy height. Our results could be useful to explore the effects of climate and land-use changes on the functional signature of forest understory communities.