Effects of natural winter browsing and simulated summer browsing by moose on growth and shoot biomass of birch and its associated invertebrate fauna

摘要

Plant responses to mammalian herbivores can be manifold. Browsing by large mammals such as moose may change plant growth and morphology, which, in turn, will change the food quality and availability for e.g. invertebrate herbivores sharing the same forage plant. Furthermore, the intensity and timing of herbivory may affect plant responses and future herbivore attack. In a field experiment, we tested whether simulated summer browsing and natural winter browsing by moose affects growth and morphology of birch Betula pendula, and whether possible changes had effects on abundance and defoliation by herbivorous invertebrates. The simulated summer browsing treatment was applied in two intensities (50% and 100% defoliation of long shoots) during two different periods of the growth season (mid-June and mid-July). Simulated summer browsing delayed the timing of budburst, reduced height and diameter growth and resulted in reduced defoliation by invertebrate herbivores indicating an induced defence. Winter browsing by moose reduced diameter growth, leaf biomass of short shoots but increased height growth, the abundance of aphids and defoliation by insect herbivores. The effects of herbivory on the timing of budburst, leaf biomass of long and short shoots and the abundance of aphids were more pronounced when the birches were both browsed in winter and artificially stripped. The timing of the artificial leaf stripping treatment was important for nearly all tested variables. Concluding, summer and winter browsing by moose can have opposing or additive effects on plant morphology and growth. Therefore, also indirect effects of mammalian herbivory on invertebrate communities feeding on the same plant may differ, depending on the browsing intensity, the season of browsing and even the timing of browsing within the same season.