Browsing regime and growth response of naturally regenerated Abies alba saplings along light gradients

摘要

Tree saplings are part of the normal diet of ungulate species such as roe deer. From a forest managers point of view, silver fir (Abies alba) is a highly desirable tree species in mountain forests, but at the same time it is often browsed by ungulates, and its recent decline is attributed to its sensitivity to browsing. We investigated naturally regenerated fir saplings to determine: (i) under which light conditions saplings grow best; (ii) where saplings are browsed less often by ungulates; (iii) which saplings are browsed less often; and (iv) under which conditions saplings react best to browsing. At three sites in Switzerland (Napf, Russikon and Zollikerberg), fir regeneration along light transects from closed forest to cleared windthrow areas was assessed, individually measured height and height increment before and after browsing, and basal diameter, and the timing, type and extent of reaction after the last browsing event was recorded. On all three sites, fir saplings grew approximately linearly better with more light. However, on Zollikerberg less light (810% canopy openness) was needed for good height growth than on the other two sites. No direct light dependence of browsing intensity was found, but due to the improved growth in more light, the time until a tree grows out of the reach of ungulates is shorter under good site conditions. Vigorously growing trees tended to be browsed most strongly. In the first year after terminal shoot loss, half of the browsed trees formed only a new bud, and only in the second year produced a new shoot, whereas the other trees reacted during the year of browsing by producing new shoots mostly out of the uppermost whorl, i.e. not by flagging of twigs. Light had only an indirect effect on this reaction via improved growth, i.e. taller trees reacted more often directly by developing new shoots. Time delays of 14years were observed after browsing, but particularly if terminal shoots had been damaged by insects in closed stands. Thus, browsing impacts on fir saplings are likely to be mediated primarily by the higher consumption rate of ungulates and the slower tree response after browsing, resulting in lower growth performance compared to other species. While light may promote resistance to browsing via enhanced height increment, A. alba regeneration has a competitive advantage under somewhat shaded but otherwise good site conditions (e.g., in mixed fir forests where the soil is neither dry nor very acidic).