Effects of moose Alces alces population density and site productivity on the canopy geometries of birch Betula pubescens and B-pendula and Scots pine Pinus sylvestris

摘要

The instantaneous rate 4 food intake for mammalian herbivores is controlled by the geometry of plant communities which regulates the encounter rate with bites, bite mass and the processing rate of bites. The geometry of plant canopies is fractal because bite density only occupies a fraction of the entire dimension of the tree canopy and scales allometrically with the search resolution of herbivores. We tested the hypothesis that both the population density of moose Alces alces and site productivity alter the fractal geometry of plant canopies as well as bite mass, and therefore the mechanisms regulating herbivore functional response. Sapling birch Betula pubescens and B. pendula and Scots pine Pinus sylvestris in northern coastal Sweden were sampled in five exclosures that spanned the range of site productivities for the region. Within each exclosure and over four years, the effects of four population densities of moose (0, 10, 30, and 50 moose/1,000 ha) were experimentally simulated within treatment plots. The fractal dimension of bite density of Scots pine decreased non-linearly with increasing moose population density regardless of site productivity. In contrast, the fractal dimension of bite density for birch increased strongly from low to intermediate moose densities regardless of site productivity, but decreased with further increases in moose population density. For birch, the decreases in fractal dimension were greater on sites of low productivity. Bite mass decreased linearly with increases in experimental moose density for both species, but the decrease was moderated by increases in site productivity. The different geometric responses of birch and Scots pine saplings to moose population density resulted in an increase in predicted plant biomass for birch, a decrease in plant biomass for Scots pine, and decreases in predicted herbivore intake rate for both species with increased moose population density. Thus, there are strong feedbacks between population density, site productivity and the geometry of plant canopies regulating herbivore functional response.