Abstract
Human‐induced changes of the environment and their possible impacts on temperate forest understory plant communities have been examined in many studies. However, the relative contribution of individual environmental factors to these changes in the herb layer is still unclear. In this study, we used vegetation survey data covering a time period of 21 years and collected from 143 permanent plots in the Northern Limestone Alps, Austria. Data on soil chemistry (49 plots), light condition (51 plots), soil temperature and moisture (four and six plots), disturbance (all plots), climate (one station in a clearing area), and airborne sulfur (S) and nitrogen (N) deposition (two forest stands) were available for analyses. We used these data together with plot mean Ellenberg indicator values in a path analysis to attribute their relative contributions to observed vegetation changes. Our analysis reveals a strong directional shift of the forest understory plant community. We found strong evidence for a recovery of the ground‐layer vegetation from acidification as response to decreased S deposition. We did not observe a community response to atmospheric N deposition, but we found a response to altered climatic conditions (thermophilization and drying). The path analysis revealed that changes in the light regime, which were related to small‐scale disturbances, had most influence on herb layer community shifts. Thermophilization and drying were identified as drivers of understory community changes independent of disturbance events.
Highlights
Human activities have led to fundamental alterations of the environment causing unprecedented and accelerating losses of biodiversity worldwide (MEA 2005; Tittensor et al, 2014)
We argue that disturbances are responsible for these contrary trends: One-third of the plots were affected by disturbances between 1993 and 2014, mainly by windthrow and the removal of bark beetle-infested trees
We identified direct and indirect effects of multiple environmental drivers on forest floor vegetation changes in a forest that has recently been affected by strong disturbances
Summary
Human activities have led to fundamental alterations of the environment causing unprecedented and accelerating losses of biodiversity worldwide (MEA 2005; Tittensor et al, 2014). Several studies observed responses to increased soil N availability due to N deposition (Bernhardt-Römermann et al, 2007; Brunet, Diekmann, & Falkengren-Grerup, 1998; Diekmann, Brunet, Rühling, & Falkengren-Grerup, 1999; Keith, Newton, Morecroft, Bealey, & Bullock, 2009; Naaf & Kolk, 2016; Reinecke et al, 2014; Thimonier, Dupouey, Bost, & Becker, 1994) and to acid rain (Diekmann et al, 1999; Hédl, 2004; Thimonier et al, 1994), while others report recovery from acidification in recent years (Reinecke et al, 2014; Vanhellemont, 2014) Local factors such as forest management, disturbance regime, and game density have been shown to be responsible for affecting the responses of the forest herb layer to deposition effects and climate warming (Bernhardt-Römermann et al, 2015; Hédl, 2004; Hopkins & Kirby, 2007; Müllerová, Hédl, & Szabó, 2015).
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