Initial site conditions and interactions between multiple drivers determine herb-layer changes over five decades in temperate forests

Abstract

The resurvey of forest plant communities some decades after the first survey has become a valued approach to determine long-term responses in species diversity and composition to various anthropogenic environmental drivers, such as management changes, excess nitrogen deposition, or overabundant deer populations. However, little is known about the effects of initial site conditions, such as soil acidity or soil moisture, on these long-term responses and how multiple drivers interact with each other in affecting herb layer diversity and composition. Moreover, the extent to which local changes in canopy cover and composition may mimic or counterbalance effects of regional or global drivers is largely unexplored. Here, we resurveyed herb layer vegetation plots in northeastern Germany after more than five decades. Our main objectives were (1) to determine the effects of initial site conditions, here soil pH buffer range, on herb layer and environmental changes; (2) to disentangle the effects of presumed regional and global drivers from local changes in canopy density and composition; and (3) to identify interactions between environmental drivers in affecting species diversity and composition. We observed significant changes in species diversity and environmental conditions, mostly in dependence on the initial site conditions. Species richness increased only at moderately acidic to base-rich sites, while floristic distinctiveness (beta diversity) generally decreased but decreased significantly stronger on base-rich than on acidic sites. The indicated environmental changes comprised increased shading (particularly on acidic sites), a decrease in soil moisture, acidification (only on initially moderately acidic to base-rich sites), eutrophication (only on acidic sites), an increase in browsing pressure and more continental climatic conditions, probably in terms of increased summer drought (on well-drained sites only). The presumed regional/global environmental changes could not be explained by local changes in canopy composition. The herb layer responses were affected by several important interactions among the drivers. The floristic homogenization was more pronounced the stronger the eutrophication signal, but only on (moderately) acidic sites, while the homogenization occurred independent of any eutrophication on base-rich sites. The environmental change indicators for soil acidity and nitrogen availability showed opposing effects on temporal species turnover and a positive interaction, pointing to atmospheric deposition as the most important driver. Our study demonstrates that both initial site conditions and interactions between different drivers should be taken into account in resurvey studies to broaden our understanding of plant community responses to environmental changes.