Effects of gap size, light and herbivory on the herb layer vegetation in European beech forest gaps

Abstract

Understanding the mechanisms that control vegetation development beneath canopy gaps is of major importance for a nature-based management of European beech forests. We studied the herb layer vegetation beneath 56 canopy gaps of various sizes (116–1410 m 2) in near-natural beech forest stands of the Kellerwald-Edersee National Park, Germany. Three objectives were pursued: (i) to compare the patterns of species number and species composition caused by different gap sizes and light conditions with those found in North America, (ii) to investigate the impact of herbivory by ungulates on gap vegetation and (iii) to assess establishment from a persistent seed bank and zoochorous seed dispersal as potential pathways for gap colonisation. Herbivory intensity was determined for each gap as the proportion of browsed terminal shoots of beech saplings. The mean Ellenberg indicator value for light was used as a measure for light availability in gaps. The data were analysed by a variation partitioning approach using multiple regression and redundancy analysis. Species number increased with gap size and light availability. Herbivory caused a reduction of competition by tree regeneration and was therefore responsible for higher species numbers, too. Species composition was relatively homogenous but significantly determined by gap size, light availability and herbivory. With more favourable light conditions, an increasing proportion of generalist species occurred within gaps. By means of two dispersal traits, it is shown that these species may use both pathways, a persistent seed bank or transportation by ungulates, to colonise gaps.