Understanding context dependency in the response of forest understorey plant communities to nitrogen deposition

Abstract

Understorey communities can dominate forest plant diversity and strongly affect forest ecosystem structure and function. Understoreys often respond sensitively but inconsistently to drivers of ecological change, including nitrogen (N) deposition. Nitrogen deposition effects, reflected in the concept of critical loads, vary greatly not only among species and guilds, but also among forest types. Here, we characterize such context dependency as driven by differences in the amounts and forms of deposited N, cumulative deposition, the filtering of N by overstoreys, and available plant species pools. Nitrogen effects on understorey trajectories can also vary due to differences in surrounding landscape conditions; ambient browsing pressure; soils and geology; other environmental factors controlling plant growth; and, historical and current disturbance/management regimes. The number of these factors and their potentially complex interactions complicate our efforts to make simple predictions about how N deposition affects forest understoreys. We review the literature to examine evidence for context dependency in N deposition effects on forest understoreys. We also use data from 1814 European temperate forest plots to test the ability of multi-level models to characterize context-dependent understorey responses across sites that differ in levels of N deposition, community composition, local conditions and management history. This analysis demonstrated that historical management, and plot location on light and pH-fertility gradients, significantly affect how understorey communities respond to N deposition. We conclude that species' and communities' responses to N deposition, and thus the determination of critical loads, vary greatly depending on environmental contexts. This complicates our efforts to predict how N deposition will affect forest understoreys and thus how best to conserve and restore understorey biodiversity. To reduce uncertainty and incorporate context dependency in critical load setting, we should assemble data on underlying environmental conditions, conduct globally distributed field experiments, and analyse a wider range of habitat types.

Light and warming drive forest understorey community development in different environments

“…This lack of interaction on the short term does not imply that such interactive effects are not important to understorey community development in response to global change. It rather shows the complementary of experimental research to long‐term vegetation resurveys (Perring, Bernhardt‐Römermann, et al, ; Perring, Diekmann, et al, ; Verstraeten et al, ) or mechanistic modelling approaches (Dirnböck et al, ; Landuyt et al, ). Long‐term experiments, vegetation resurveys and modelling are perhaps better suited to unravel such long‐term interactive effects between global change drivers on understorey communities (Luo et al, ; Verheyen et al, ).…”

Impacts of nitrogen addition on plant species richness and abundance: A global meta‐analysis

“…The responses of plant communities to N deposition vary depending on the environmental context (Perring, Diekmann, et al, ; Simkin et al, ; Vellend et al, ). Modifying factors include the amount and duration of N deposition, which determine the cumulative N input over time (Bernhardt‐Römermann et al, ; Duprè et al, ); soil pH and acid‐neutralizing capacity (Clark et al, ; Simkin et al, ); the chemical forms of N input (Stevens et al, ); environmental conditions, such as climate (Clark et al, ; Humbert, Dwyer, Andrey, & Arlettaz, ; Limpens et al, ); and vegetation types (Pardo et al, ; Simkin et al, ).…”

Plant functional trait response to environmental drivers across European temperate forest understorey communities

“…Furthermore, global change drivers can drive changes in plant communities through altering local resource availabilities, e.g . soil nitrogen or phosphorus; and local resources and conditions can, vice versa , determine whether and how global change drivers affect plant communities (Smith et al, ; Perring et al ). Studies evaluating the functional response of communities to global change drivers should therefore aim to incorporate effects of local resources and conditions (Bruelheide et al, ).…”