Moose Browsing on Scots Pine along a Gradient of Plant Productivity

Abstract

Pines growing in unproductive habitats experience more damage by moose than those growing in more productive habitats. This suggests that trees in unproductive habitats are qualitatively different, e.g., more palatable to moose, and/or less able to recover from browsing. Moose browsing was compared on four phenotypes of Scots pine taken from a gradient of forest productivity. The trees were exposed, side by side, at test stations during winter. Consumption of twig biomass was highest on pines from the most productive habitat and lowest on those from the moderately productive habitat with an overstory of mature pines. Consumption was intermediate on pines from the least productive habitat and from the moderately productive habitat without an overstory. Pines from the highly productive habitat had greater nitrogen, potassium, and phosphorus content and more readily degradable dry matter (when incubated in the rumen of a cow) than did pines from the less productive habitats. Even though they provided a lower quality food for moose, pines in the less productive habitats generally suffered most from moose browsing. The slender twigs and the slow growth rate of these pines allow the moose to browse the main part of the needle—bearing twigs, in contrast to pines from more productive habitats. We also recorded the mortality response to simulated winter browsing in living pines on this same forest productivity gradient. Pine mortality was greater in the less productive habitats than in the more productive ones. These findings largely explain why pines growing in habitats of low and medium productivity suffer more than pines in highly productive habitats, where the possibility for growth is better. Severe damage to stands growing on poor soils most probably arises where moose density is high and where moose have already harvested most of the available twigs from pines growing in the more productive habitats. The moose are then forced to utilize the suboptimal food resources available in less productive habitats.