Disentangling the causes of damage variation by deer browsing on youngThuja plicata

Abstract

Long‐lived trees experience different levels of damage due to mammalian herbivores. To untangle the mechanisms that underlie this variation, we combined chemical with dendrochronological analyses to study variation in browsing on Western redcedars (Thuja plicata) on Haida Gwaii (British Columbia, Canada). Since the last glaciation, Haida Gwaii forests had lacked large herbivore browser until Sitka black‐tailed deer (Odocoileus hemionus sitkensis) were introduced at the beginning of the 20th century. Dendrochronology yielded information on radial growth and plant annual responses to environmental stresses including herbivory. Secondary metabolite content and plant nutritional quality provided insights into proximate causes of food choices made by herbivores. We sampled lightly‐ and heavily‐browsed young trees at four sites: three clear‐cut sites with high browsing pressure and one old‐growth forest site where browsing pressure had, until recently, been lower. Heavily‐browsed young trees had lower concentrations of secondary metabolites and were of lower nutritive value than lightly‐browsed trees at all sites. Under high browsing pressure, tree growth patterns suggested that all young trees were initially severely browsed until some trees, currently scored as lightly‐browsed, started to escape deer. At the old‐growth site, both lightly‐ and heavily‐browsed trees tended to have lower overall average secondary metabolite concentrations than those of all other sites, a trend possibly related to greater canopy closure. Lightly‐browsed trees were older than heavily‐browsed ones which resulted, during the period of lower browsing pressure, in higher growth rate and a same pattern of change in growth from one year to the next year. This suggests that, under low browsing pressure, selection of young trees related to chemical defense was weak and that growth differences due to other factors than browsing could be expressed. Under strong browsing pressure, however, all young trees had equally low growth rates until trees with better genetic potential to produce effective defenses were able to escape deer. This suggests that selection by deer could occur on a long‐lived tree.

Growth change of young Picea sitchensis in response to deer browsing

“…Genetic differences in growth rate, tolerance to browsing as well as micro-site characteristics may explain this variability. Variations in tree palatability, visibility or shape have been shown to influence sensitivity to browsing (Chouinard and Filion, 2001;Vourc'h et al, 2001Vourc'h et al, , 2002. However, in the case of Sitka spruce Vila et al (2002) showed that there was no evidence for chemical nor nutritive variation coupled with variation in browsing impact.…”

Food Selection by Herbivores and Neighbourhood Effects in the Evolution of Plant Defences

“…Through selective foraging, herbivores can influence the composition of plant populations and thus contribute to the ecology and evolution of plant species (Bryant et al 1989, Vourc'h et al 2002, O'Reilly-Wabstra et al 2004, Andrew et al 2007, Bailey et al 2007. From the point of view of an individual plant, herbivory influences fitness by reducing growth or reproduction and increasing mortality (e.g.…”

Can we reconstruct deer browsing history and how? Lessons from Gaultheria shallon Pursh

“…They affect plant morphology [13,30], plant growth [8,51,52,55] and plant chemistry [5,55]. Disturbance of forest ecosystems by herbivory can be analysed by a posteriori approaches based on woody plant morphology [30] and on the analysis of ring-width series [45,46].…”