Shade attenuates long‐term forest plant community responses to warming and nitrogen pollution

Forest disturbances are increasing in frequency and intensity worldwide, resulting in canopy opening across many wooded landscapes. Canopy opening increases exposure of below‐canopy organisms to external influences such as climate warming and nitrogen pollution. However, there is a lack of long‐term experimental data to test how forest understorey plant community responses to warming and nitrogen pollution are modulated by these novel light and thermal regimes, and to mechanistically quantify the effects of different drivers.

Here we report dynamics of understorey plant biodiversity in a long‐term, full‐factorial experiment in ancient, temperate forest after 14 years of warming, nitrogen addition and simulated canopy opening with fluorescent tubes.

Our results show an increasing dominance of warmth‐demanding species at the expense of cold‐adapted taxa over time (thermophilization), an effect accelerated by light addition even after 14 years. Warming alone led to species loss but not to thermophilization.

In spite of the relatively strong imprints of nitrogen enrichment on soil eutrophication and acidification, nitrogen addition left unexpectedly small direct effects on plant communities under closed canopies. Yet, in illuminated plots, nitrogen‐demanding species have increased during the last 8 years, likely owing to nitrogen excess from high background deposition levels.

Synthesis and applications . Our findings demonstrate that forest understorey plant biodiversity is surprisingly resistant to warming and nitrogen enrichment under the shade of trees. Hence, we advocate for management actions and policies to locally maintain closed canopies to alleviate climate‐change induced thermophilization and imprints of nitrogen pollution in forest‐floor plant communities, at least temporarily.