Light availability and ungulate browsing determine growth, height and mortality of Abies alba saplings

Abstract

Ungulate browsing is one of the many factors that affect structure and species composition of forests. Silver fir (Abies alba) is a highly desirable tree species in many European mountain regions, but at the same time one of the most preferred tree species by ungulates. Failure of natural fir regeneration is thus often attributed to browsing.We employ natural light gradients with planted fir saplings but natural browsing to analyze: (i) under which light conditions fir saplings grow and survive best; (ii) where and which ungulate species browse saplings; and (iii) which saplings react and survive best after browsing. The experiment was carried out in a Swiss forest in two fenced plots where parts extended from closed stands into gaps and an unfenced plot under closed canopy. On the three plots 803 fir saplings had been planted in 2008. On half of the total area, browsing had been allowed in at least the winter of 2009/2010.The more light available, the better was the growth of the saplings before and after browsing. For canopy openness⩾11%, some fir saplings had large height increments irrespective of the exact light level. Video surveillance demonstrated that chamois was the only browsing ungulate species. Mortality was positively correlated with light availability, probably due to site preparation, i.e. mainly where ground vegetation was abundant, and with mouse browsing in one plot. Mortality due to chamois or mice was 2.8–16.4% in plots unfenced for one winter and 52% in the plot unfenced for three winters. A small fraction (10%) of browsed saplings that survived used flagging of a twig to form a new leader, and fully compensated for browsing-related height loss. The other saplings showed a partial compensation in the 2nd year after browsing with equally long shoots, but they still remained shorter than unbrowsed saplings. Saplings under a very dense canopy often did not react (7%).The strong impacts of ungulates on fir recruitment are thus more due to the strong preference of the ungulates for this tree species rather than to a low tolerance of fir to browsing. We do not recommend planting firs in forest gaps due to higher mortality rates, since: (1) competition with ground vegetation is higher in gaps, (2) they risk becoming desiccated and (3) mouse browsing is more likely. Thus we provide evidence that A. alba regeneration benefits from moderately shaded conditions.