Changes in species composition and richness in an alluvial hardwood forest over 52yrs

Questions(1) How has the species richness of an alluvial forest changed over the past 52yrs, and what are the main drivers of the observed temporal changes; (2) has the species composition changed over this period in response to changes in environmental variables; and (3) what are the main drivers of species richness (change) in this forest? LocationHaseder Holz, Innerste floodplain in lowland northern Germany. MethodsIn 2012 we resurveyed the plant species composition of 19 permanent plots analysed for the first time in 1960 and for a second time in 2002. At the most recent survey, several environmental variables (photosynthetically active radiation, soil water content, groundwater table, pH and content of soil P, Ca, Mg, K, C and N) were measured in five locations within each plot. GLM were used to examine the relationship between explanatory variables and species richness, while variation in species composition, in time and space, was assessed by DCA. ResultsThe number of species generally increased with decreasing soil nutrient (except Mg) content, with increasing variation in light availability and with increasing variation in K content. The significant increase in species richness from 1960 to 2012 was mainly attributed to the increase in true forest species, such as Paris quadrifolia and Mercurialis perennis, and woody species. The most pronounced increase in species richness was found in plots with lower soil phosphate content. Species typical for open habitats showed the most pronounced decrease from 1960 to 2012. A significant homogenization of the plots over time was observed. ConclusionsAgainst the common trend in European forests, we found a significant increase in mean species richness, especially in plots with relatively low nutrient content. While the total pool of species has not consistently increased, we found a strong increase in plot-scale species richness of woody and herbaceous species. This overall increase and the slight decrease in the proportion of species typical of open habitats were probably driven by a decrease in light availability caused by less intensive management. Our results demonstrate that fine-scale spatial environmental heterogeneity positively affects species richness. In contrast to recently reported findings, we found a decrease in the number of species with increasing nutrient content.