Studies of the interactions between vegetation processes and mammalian herbivory have been part of BNZ LTER research for over 20 years. We have found that browsing by moose and snowshoe hares controls vegetation development and nutrient cycling at a variety of scales. Mammalian herbivores control species composition, nutrient cycling, and plant population dynamics at the stand and landscape levels, and these effects are manifested both in early and late succession via herbivore effects on the interaction of biotic and abiotic processes.
Primary succession on the Tanana River floodplain is initiated by plant establishment on newly formed silt bars formed by flooding. Whereas all the major plant species (willows, alder, balsam poplar, and spruce) are present in this early wave of colonization, the successional trajectory is characterized by distinct vegetation stages reflecting the life history traits of these dominant species. Because of their high population densities in interior Alaska, mammalian herbivores may consume over 50% of the current annual growth. Consequently, they have major impacts not only on plant growth but also on nutrient cycling processes since litter from browsed plants tends to have higher nutrient concentrations and faster decomposition rates. Perhaps the largest effect of herbivory on nutrient cycling is the selective browsing by moose and snowshoe hares on willows, which leads indirectly to the dominance of alder, an important nitrogen-fixing species.
During periods of high snowshoe hare density, browsing on seedlings of late-successional species such as white and black spruce can result in effects on forest community composition that persist for decades. Studies of snowshoe hare populations at BNZ have shown that, in addition to the classical decadal population cycle, hare abundance varies nearly as much on an intra-annual basis, underscoring the large oscillations of resource availability in boreal forests.
The effects of mammalian herbivory can also alter the activity and abundance of insects and other arthropods. For example, the longer shoots produced by browsed willows experience higher rates of sawfly infestations than the shorter shoots on mature-growth form plants. Moreover, the generally warmer and drier microclimate on the forest floor, caused by browsing-induced changes in canopy structure, results in altered composition in the guilds of ground-dwelling insects. Thus, mammalian herbivory exerts significant control over biogeochemistry and successional dynamics at the level of the species, community and the ecosystem.