Excess Nitrogen in Temperate Forest Ecosystems Decreases Herbaceous Layer Diversity and Shifts Control from Soil to Canopy Structure

Abstract

Research Highlights: Excess N from atmospheric deposition has been shown to decrease plant biodiversity of impacted forests, especially in its effects on herbaceous layer communities. This work demonstrates that one of the mechanisms of such response is in N-mediated changes in the response of herb communities to soil resources and light availability. Background and Objectives: Numerous studies in a variety of forest types have shown that excess N can cause loss of biodiversity of herb layer communities, which are typically responsive to spatial patterns of soil resource and light availability. The objectives of this study were to examine (1) gradients of temporal change in herb composition over a quarter century, and (2) spatial patterns of herb cover and diversity and how they are influenced by soil resources and canopy structure. Materials and Methods: This study used two watersheds (WS) at the Fernow Experimental Forest, West Virginia, USA: WS4 as an untreated reference and WS3 as treatment, receiving 35 kg N/ha/yr via aerial application. Herb cover and composition was measured in seven permanent plots/WS from 1991 to 2014. In 2011, soil moisture and several metrics of soil N availability were measured in each plot, along with measurement of several canopy structural variables. Backwards stepwise regression was used to determine relationships between herb cover/diversity and soil/canopy measurements. Results: Herb diversity and composition varied only slightly over time on reference WS4, in contrast to substantial change on N-treated WS3. Herb layer diversity appeared to respond to neither soil nor canopy variables on either watershed. Herb cover varied spatially with soil resources on WS4, whereas cover varied spatially with canopy structure on WS3. Conclusions: Results support work in many forest types that excess N can decrease plant diversity in impacted stands. Much of this response is likely related to N-mediated changes in the response of the herb layer to soil N and light availability.

Drivers of above‐ground understorey biomass and nutrient stocks in temperate deciduous forests

“…For nitrogen deposition, we expected two counteracting effects: a positive effect on understorey biomass (as found, e.g., by Gilliam () for cover) as a result of increasing soil nitrogen availability (Falkengren‐Grerup, Brunet, & Diekmann, ) and a negative effect as a result of increasing overstorey biomass reducing light availability (Hyvönen et al, ; Reyer, ). Neither of those was detected in our analysis.…”

“…Most of these responses are, however, context‐dependent. Whether or not the understorey reacts to global change often depends on local site conditions, defined by the overstorey and the soil (Chen, Biswas, Sobey, Brassard, & Bartels, ; Gilliam, ; Kim et al, ; Perring, Diekmann, et al, ; Verheyen et al, ). Nitrogen deposition has, for example, the potential to decrease species richness in the understorey by stimulating competitive exclusion (Gilliam, ), but this response is often not generalizable (Perring, Diekmann, et al, ).…”

Plant communities on nitrogen‐rich soil are less sensitive to soil moisture than plant communities on nitrogen‐poor soil

“…While moisture and nitrogen appeared to be independent co‐limiting resources, we did not find any discernible role of canopy openness on the functional composition and diversity of understorey plant communities. Light tends to be a more important factor in plant community assembly when below‐ground resources are in greater supply (DeMalach, Zaady, & Kadmon, ; Gilliam, ).…”

“…Functional diversity also changes along resource gradients (Schellenberger Costa et al, ), but the nature of the relationship between functional diversity and resource availability is variable. Some studies have found that functional diversity decreases with increasing resource availability (trait convergence) as species that thrive in low resource conditions are out‐competed (Eskelinen & Harrison, ; Gilliam, ; Harpole & Suding, ). Others have found that functional diversity increases with increasing resource availability (trait divergence) as species with more diverse strategies can fill a larger niche space (Bernard‐Verdier et al, ; Jacobs et al, ).…”

Effects of Disturbance on Understory Vegetation across Slovenian Forest Ecosystems

“…A large body of research over the past decade demonstrates that the understory herb layer exhibits strong temporal dynamics [5]. These dynamics have typically been attributed to several different anthropogenic drivers, including changes in light conditions due to forest management [6], nitrogen deposition [7][8][9], climate change [10,11], and excessive deer browsing [12,13].…”