Tree species diversity and abundance as indicators of understory diversity in French mountain forests: Variations of the relationship in geographical and ecological space

• We analyzed the relationship between dendrometric indicators and floristic diversity.
• Tree species richness and abundance indicated parts of floristic diversity.
• Dendrometric indicators interacted with ecological gradients.
• Better documentation of what and where biodiversity indicators indicate is needed.

Trees are one of the main components of forest ecosystems; they modify resource levels (light, nutrients, water) that affect understory vegetation composition and diversity. Tree species diversity is used as a biodiversity indicator in various European and French monitoring schemes for sustainable forest management. Moreover, tree species basal area has been found to better indicate floristic biodiversity than tree species richness or diversity. Herein we empirically check this finding by analyzing data from mountain spruce-fir forests in France with Bayesian statistical models. We insist on the magnitude of the relationship and its variation in geographical and ecological space. Our results indicate that both tree species abundance (based on cover or basal area) and tree species richness and dominance are good indicators of some parts of understory vascular plant species richness. The effect of dendrometric indicators on floristic biodiversity varied among ecological groups and along ecological gradients such as aspect, soil acidity, region and altitude. As a result, plots with north-facing and south-facing slopes exhibited opposite relationships of species richness with tree species abundance, and so did plots located on acidic and basic sites. We discuss these results in light of other empirical results relating positive interactions between species and abiotic stress. Our study supports evaluating biodiversity indicators to determine when they actually have non-negligible relationships with biodiversity, i.e. for which ecological groups and in which ecological contexts.