EFFECTS OF LARGE MAMMALIAN BROWSERS ON ARCHITECTURE, BIOMASS, AND NUTRIENTS OF WOODY-PLANTS

Large generalist browsers commonly feed on many species of woody plants, which are used at different intensities and frequencies. Some plant species are used during all seasons, while others are taken only during a short period of time. We have chosen to concentrate our review on a relatively well-studied system, the large generalist herbivore the moose (Alces alces) and its use of and impact on some species of woody plants. By this choice, we can examine a set of mammal-plant interactions within a single environment in different seasons. As a result of our review, we conclude that there is no single coherent response of woody plants to herbivory by moose. Winter browsing and summer leaf-stripping generally lead to the following responses: decreased food resources for moose in terms of plant biomass (in some situations the response may be neutral or even slightly positive); shoot size increases after medium and high levels of winter browsing, especially for deciduous species, but shoot size decreases after summer browsing; branching of annual shoots increases after winter browsing, but decreased by summer browsing; leaf size increases after winter browsing, but the response to summer browsing is variable (no response to increase); winter browsing has a neutral or negative impact on nutrient characteristics of shoots, and fiber concentration increases; winter browsing increases leaf concentrations of K, Ca, and N, while the concentration of protein-precipitating compounds decreases; early-summer browsing decreases nitrogen concentration in leaves of the next summer, while late-summer browsing has the opposite effect, at least for 1 year. Repeated leaf-stripping during late summer, however, tends to decrease nitrogen concentration. Some of these responses (e.g., changes in plant biomass and size of food item) have the potential of being important to the herbivores that feed on the plants. As the food production of a whole tree most often decreases after browsing and some of the chemical properties are affected negatively from the animal's point of view, the most logical response by the herbivores should be to avoid browsed plants. However, some studies have shown that there is a higher probability for a browsed tree than for an unbrowsed tree to be browsed 1 year later.