Contrasting patterns and combined effects of moose and insect herbivory on striped maple ( Acer pensylvanicum)

Abstract

Although both mammalian and insect herbivores can strongly affect plant growth and biomass, interactions between these taxa feeding upon shared plant resources seldom have been studied experimentally in northern forests. In this study, we manipulated density of leaf-chewing insects and took advantage of a natural gradient of moose ( Alces alces) foraging activity to examine their separate and combined effects on sapling striped maple ( Acer pensylvanicum). Winter moose browse was patchy over space and time, but when present it resulted in a median bud loss of 72%. Saplings responded to moose browse by increasing leading shoot growth during the following summer as well as by developing leaves from epicormic buds. However, they did not fully compensate for the loss of biomass, and produced fewer leaves than in the prior year. Leaf-chewing insect herbivory occurred during the summer and was ubiquitous and more uniform than moose browse, but of a lesser magnitude (average 7% leaf area removed). Insect herbivory suppressed shoot growth and leaf production. Leaf damage was greater on browsed than unbrowsed trees, suggesting that browsing may improve leaf quality for insect herbivores, further constraining sapling growth. Although striped maple mortality was low, the average number of leaves produced per sapling declined during the four years of the study, indicating that combined mammalian and insect herbivory coupled with resource limitation will prevent most saplings from reaching maturity.