Abstract : Tree species is one of the mains attributes of a forest. It is a factor that modifies resources levels (light, nutrients, water), and thus it potentially acts on understory vegetation composition and diversity. However, few studies deal on tree species effect on understory vegetation, at least in temperate forests. These studies show a lower vegetation diversity in coniferous stands compared to hardwoods, and generally a lower diversity in two-species mixed stands compared to pure ones. Even so, tree species richness and relative abundance of the main species (pureness) are used in France and Europe as biodiversity indicators (respectively positive and negative), without previous scientific validation.
We have developed statistical models to test these indicators for the richness of ecological groups of understory vegetation species. We have compared these models with other ones, based on tree species composition and abundance. We worked in lowland forests in the center of France (Bassin Parisien). Site type was either constant, either incorporated into models. According to our results, tree species richness is not a good indicator of understory vegetation species richness, and pureness of the main tree species has positive or null effects. Positive responses are probably due to tree species stratification in the forests studied: they are mainly oak forests managed as standards or coppice (hornbeam) with standards, in which the decrease of pureness often corresponds to an increase in coppice density. Coppice species have negative or null effects (or sometimes positive effects, for woodland species).
There are better indicators than tree species richness or pureness, such as main tree species identity and abundance of tree species groups. Main tree species identity is a useful indicator: it shows for example that coniferous stands are less rich in shad-tolerant species, ash stands have a higher richness for many understory groups, and sessile oak stands are less rich than common oak stands for most of understory groups. The abundance of tree species groups (oaks, pioneers, shad tolerant trees, other hardwoods, post-pioneer conifers and late-successional conifers) is the best descriptor for the richness of quite a few ecological groups. However, this multivariate model is not appropriated to be communicated as an indicator based on tree species.
To avoid site type bias, we have studied tree species effect at the individual tree scale. Soil water variations between tree pairs (pineoak; beech-oak) were rare, available light for vegetation sometimes highly varied with tree species (mainly near the trunk), but there were few understory differences between tree species at this scale (except for bryophytes). Stand structure seems to be more determining than characteristics peculiar to each individual tree. Tree species composition is a synthetic variable that partially informs about factors of understory diversity to which it is bound: site type, management practices, trees structure and historical factors. These factors are inherent and closely linked to tree species composition in managed forests. If future changes occurred in management practices (tree species choice according to site type, cutting frequency and intensity), relations between tree species and understory diversity may change; thus our indicators should be tested again.