Effects of reindeer browsing on tundra willow and its associated insect herbivores.
Abstract
Browsing by large mammals may strongly constrain the growth and reproduction of woody plants, and may alter the food quality and resource availability for herbivorous insects. The response of the plants may vary between different growth stages, and the preference of herbivores may be related to plant age. Understanding the effects of reindeer Rangifer tarandus browsing on the growth of woody forage plants is important in formulating guidelines for reindeer grazing management, especially in low productivity subarctic environments. We studied the effects of summer browsing by reindeer on the growth and reproduction of willow Salix phylicifolia and on the abundance of its insect herbivores, by studying plants inside and outside exclosures over a period of 6 years. The experiment was run in northern Finland and included 80 willow genets in an area near the timberline formed by mountain birch Betula pubescens. At the beginning of the experiment, half of the willows were cut at ground level to rejuvenate ramets. Reindeer feeding was more intense on rejuvenated willow compared with old willow, and the effects of browsing were more pronounced on rejuvenated plants. Reindeer browsing reduced the height of willow by c. 50%, shoot length by c. 30% and accelerated dieback of the shoots by c. 50%. Browsed willow produced fewer shoots, with fewer buds and floral catkins, than unbrowsed willow. Browsing also reduced the densities of the most common insect herbivores: leaf beetles of the genus Gonioctena (Coleoptera: Chrysomelidae) and gall-inducing sawflies (Hymenoptera: Tenthredinidae) belonging to the genera Phyllocolpa, Eupontania and Euura. Synthesis and applications. We show that reindeer browsing in summer reduces biomass and diminishes reproductive success of willow; it also lowers the numbers of its associated insect herbivores. Our results suggest that this effect will be most evident in low-productivity tundra heaths where alternative forage plants, such as relatively palatable and productive graminoids, are scarce. We advise that reindeer should be maintained below the present levels of 2-3 reindeer km-2 to sustain the long-term persistence of important forage plants such as willow in these low productivity habitats.