Abstract
Nitrogen (N) is a key nutrient elements for ecosystems which has been highly impacted by human development and activities since the early 20th century. Despite that changes in N-availability have been demonstrated to impact forests, we still miss evidence of its effect on species composition over the long-term.
Based on a large number of floristic observations (n = 45 604), the French forest status related to soil N-availability was reconstructed from herb species assemblage between 1910 and 2010 using both a bioindication approach and a spatiotemporal sampling aiming to pair past and recent floristic observations.
We showed that soil N content bioindicated from forest herb communities was higher at the start of the 20th century than over the 2005–2010 period. It decreased more or less continuously until 1975 and 2005 in coniferous (mean ΔC:N=+0.79) and broadleaved (mean ΔC:N=+0.74) forests, respectively, and then was lower than the most recent bioindicated N level observed over the 2005–2010 period (mean ΔC:N=−0.10 and −0.16, respectively). Spatial analysis confirmed the temporal trends with a decrease and increase in forest surface areas where soil N impoverishment and enrichment have been bioindicated over the time, respectively.
N bioindicated trends are opposite to changes in N atmospheric deposition compared the 2005–2010 period, while they follow temporal variation in mean N deposition until 1990.
Synthesis. Our results showed that forest herb communities have been reshuffled in regards of their soil N requirements over the 20th century highlighting that temporal changes in soil N supply have impacted the understory species composition of forest. We evidenced changes in communities towards less nitrophilous plant assemblage followed by a recent eutrophication since 2005. We propose that the nitrogen forest vegetation status is likely related to N atmospheric deposition trend, but also to both acidification, climate change and forestry management which impacted organic matter decomposition and soil N mineralization through effects on soil microbial and fauna activities. The current eutrophication observed in forest herb communities is worrisome for temperate forest ecosystem and its functioning in regards of biodiversity homogenization which often accompanied such a community reshuffling.
