Rapid thermophilization of understorey plant communities in a 9 year-long temperate forest experiment

The vast majority of plant biodiversity associated with temperate forests is harboured by the understorey layer. This layer also plays crucial roles in ecosystem functions such as tree regeneration, nutrient cycling and carbon dynamics. Research using space-for-time substitutions and resurveys of vegetation plots has shown that climate warming, changes in forest management and resource availability are key determinants of forest understorey biodiversity change and functioning. However, long-term experiments are needed to better unravel their complex interactive effects. Here we study the influence of nearly a decade of experimental warming, light addition using fluorescent tubes (as a proxy for management-driven changes in forest-floor light levels) and nitrogen input on understorey plant communities of temperate broadleaved forest. Plant communities shifted towards a higher dominance of warm-adapted species, a process referred to as thermophilization. We detected a marked community shift in all treatments including the control plots, reflecting ongoing ambient environmental changes. This reordering over time was greater than the shift induced by the treatments. Thermophilization was, however, greatest when temperature and/or light availability were enhanced. Communities were also taller in response to warming and increased light availability. Synthesis. We studied the influence of nearly a decade of experimental warming, light addition and nitrogen input on understorey plant communities. Plant communities shifted towards a higher dominance of warm-adapted species, a process called thermophilization. We detected a marked community shift in all treatments including the control plots, reflecting ongoing ambient environmental changes (especially warming of air temperature, changing precipitation patterns and increasing droughts) . This reordering over time was greater than the shift induced by the treatments. Thermophilization was, however, greatest when temperature and/or light availability were enhanced. Communities were also taller in response to warming and increased light availability.