Ungulate browsing on silver fir ( Abies alba) in the Swiss Alps: beliefs in search of supporting data

Abstract

European silver fir ( Abies alba Mill.) is the most heavily browsed of the commercially important tree species in montane forests of central and southeastern Europe. There are widespread reports that, for decades, fir regeneration has been poor or has even failed completely. Almost unanimously, browsing ungulates are considered the major cause. We review the scientific evidence for the role of ungulate herbivory in silver fir population dynamics, and use silver fir as a model species to highlight shortcomings in understanding and handling of the problem, with an emphasis on the Swiss situation. (1) Abundance and distribution of mature silver fir today reflect mainly a variable history of the silvicultural value of the species, and do not necessarily provide a standard for target values in today’s close-to-nature silviculture. (2) Evidence for the strong impact of ungulate herbivory on silver fir is overwhelming but from a scientific view, it is largely circumstantial, and a quantitative understanding of how ungulates drive silver fir dynamics is lacking. (3) Browsing herbivores mainly interfere with silver fir development in the sapling stage, but impacts are spatially and temporally variable, and often little related to the size of the herbivore populations. Browsing is only one of a complicated interplay of factors affecting seedling establishment and subsequent growth and survival. Sapling mortality due to browsing has hardly ever been directly measured, let alone been considered in proportion to later mortality caused by inter-tree competition. (4) Most forestry services in central Europe run extensive survey schemes of browsing intensities, but in general, conclusions of ‘insufficient regeneration’ are unwarranted given the limited predictive value of the data for judging long-term stand dynamics. (5) Ultimately, only a sound understanding of tree population dynamics will help to solve this ‘forest-wildlife conflict’, but for the time being, it would help if foresters were more open to the idea that silver fir regeneration may be a highly variable and irregular process in both time and space. If silver fir is dependent on rare ‘windows of opportunity’, wildlife management would also have to abandon the view that strong fluctuations in ungulate populations are to be prevented.