Given the significance of the herbaceous understorey layer for temperate forest ecosystem biodiversity and functioning, it is important to have a thorough understanding of its dynamics in response to environmental change. However, site-specific factors such as soil type may cause differential temporal herb layer changes within a single study area under comparable external pressure. Surprisingly, relatively few studies have examined herb layer changes on different site types within the same forest complex. The aim of this study is to determine the temporal differences in herb vegetation and to compare these for two differing site types in the same forest complex.
Ancient broad-leaved woodlands traditionally managed as coppice with standards in South Belgium, an area with secondary deposits and a mild temperate climate.
In 2008 we re-inventoried 43 vegetation recordings from 1953 to 1954 in forests on two site types (acid and neutral forests). First, we described differences in soil pH, litter and overstorey characteristics between the sites. Next, multivariate analysis, Ellenberg indicator value and CSR signature were used to determine differences in herb layer diversity and composition between both the two survey times and the two site types. Finally, we analysed temporal differences in individual species abundances and frequencies.
The two site types clearly differed with respect to soil, litter and overstorey characteristics. Temporal changes between the surveys were mostly highly significant on the neutral sites and mostly not significant on the acid sites. On both site types, the species pool size declined. On the neutral site, the mean plot species richness also declined. Based on the plant-derived characteristics (Ellenberg indicator values) of the plots, light availability decreased and nitrogen availability significantly increased on the neutral sites.
Input of eutrophying deposits and management changes are seen as key drivers of herb layer changes in the study area over the last half-century. However, our study suggests that site characteristics such as parent material and soil pH should also be considered. Together with the key drivers, incorporating differences in site characteristics may improve our understanding of temporal shifts in herb layer vegetation in response to environmental change.