Abstract
Background and Objectives: The existence of common ash (Fraxinus excelsior) in Europe is severely endangered by ash dieback. To support its future sustainability, it is essential to improve the natural ash regeneration. The main aim of this study was to investigate the influence of light conditions, conceivably influenced by stand structure/ash dieback, on ash regeneration and the competition between ash seedlings and species growing in the understory. Materials and Methods: We selected 40 plots in a riparian forest located in Bavaria, Germany. Light-related variables (Leaf Area Index, gap fraction) were gathered with fish-eye photography, whereas other environmental factors were derived from vegetation surveys (Ellenberg indicator values). We assessed vegetation parameters such as species’ richness and coverage of the herb layer to account for competition with ash seedlings. Results: Our results indicate that ash regeneration is favoured under shady conditions. The majority of other abiotic factors were not statistically associated with the analysed ash metrics. In contrast, the coverage of grass was negatively related to LAI and positively to gap fraction. Higher herb and grass coverages were linked to a suppression of ash regeneration. A higher litter coverage was associated with a higher frequency of ash seedlings. Nonparametric partial correlation analyses demonstrated the influence of light and stressed that litter coverage is of particular importance. Conclusions: We conclude that gaps, inter alia induced by ash dieback, favour grass invasion. In turn, this invasion might suppress regeneration of ash. In this regard, rapid silvicultural management such as reforestation of gaps after dieback of mature trees is recommended. The influence of litter on interspecific competition during growth should be also considered. The pace of dieback might additionally influence the timing and quantity of litter accumulation; thus, further research should also focus on these interrelations.
Highlights
The broad-leaved common ash (Fraxinus excelsior L.) is a widespread tree species in Europe
The aim of this study was to investigate how light conditions and especially biotic factors influence the occurrence of the natural regeneration of ash
We conclude that crown gaps inter alia induced by mortality, leaf fall and damage of shoots caused by ash dieback favour grass invasion
Summary
The broad-leaved common ash (Fraxinus excelsior L.) is a widespread tree species in Europe. Due to its flood tolerance, it is one of the main tree species in floodplain forest ecosystems [1,2]. Many ashes have been planted in order to revalue mixed forest ecosystems economically and ecologically [4]. The importance of ash—besides its ecological relevance—results from valuable wood characteristics such as stability and elasticity, monopodial and rapid vertical growth [5,6]. Their roots dominate in the upper mineral soil horizon, forcing roots of other species such as beech downwards and allowing ashes to be the first acquiring infiltrated water [7,8]. Ash is characterized by a high drought tolerance and is able to withstand high temperatures, making it an optimal tree species under climate change conditions [9]
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