Grazing and browsing by ungulates have marked effects on the abundance of deciduous scrubs and other plants in tundra ecosystems (Henry & Svoboda 1994). Manseau, Huot & Crête (1996) reported higher abundance of Salix planifolia in ungrazed areas than in areas with caribou Rangifer tarandus caribou (reindeer) grazing, noting that this willow was absent from some grazed areas. Long-term experiments on the Fennoscandian tundra have suggested that exclusion of grazers leads to increased abundance of woody plants (Moen & Oksanen 1998), whereas intense summer grazing by reindeer Rangifer t. tarandus virtually eliminates woody plants (Olofsson et al. 2001). There are several studies from other ecosystems showing that wild ungulates and domestic livestock can affect growth, plant structure and reproduction of willow Salix spp. (Maschinski 2001; Peinetti, Menezes & Coughenour 2001; Brookshire et al. 2002).
Ungulate browsing may cause a variety of morphological and physiological changes in plants, which depend on browsing intensity and frequency, the type of herbivory and the time of year when browsing occurs (Danell, Huss-Danell & Bergström 1985; Danell, Bergström & Edenius 1994). For instance, winter browsing by moose Alces alces may stimulate shoot growth in the next growing season (Danell, Bergström & Edenius 1994; Bergman 2002), while leaf-stripping in summer by moose or reindeer generally has the opposite effect (Danell, Bergström & Edenius 1994). Simulated browsing experiments have shown that clipping reduced the growth of willow Salix lanata (Ouellet, Boutin & Heard 1994), demonstrating that effects of browsing can be particularly pronounced in low productivity habitats. However, there are few studies showing how growth of willow is affected when it is exposed to long-term mammal browsing. Responses may also vary between different plant species, growing conditions and growth stage (Edenius, Danell & Bergström 1993). Swihart & Bryant (2001) found that winter herbivory is heavier on mature plants. Unfortunately, there are no experimental studies examining the effects of summer feeding by reindeer Rangifer tarandus L. on willow at different stages of development. Martinsen, Driebe & Whitham (1998) found that resprouting juvenile cottonwoods Populus fremontii × Populus angustifolia not only had a high nitrogen content but also contained high levels of compounds that defend against mammalian herbivores. Similarly, Tahvanainen et al. (1985) found increased levels of defensive compounds in juvenile willow, and hare Lepus timidus preferred mature willow shoots to juvenile ones. We might thus expect that reindeer would not heavily browse resprouting juvenile willow.
It has been proposed that browsing by ungulates affects the reproduction of plants in different ways (Mulder 1999). Loss of foliage leads to restricted carbon and nitrogen gain (Ouellet, Boutin & Heard 1994) and fewer resources are left for flower production. Bergström & Danell (1987) found that moose browsing reduced the production of female catkins in birch Betula pendula and Betula pubescens, and Elmqvist et al. (1987) showed that moderately browsed willow maintained flower production. In addition, reindeer may browse flower buds and developing catkins. There are no detailed studies in northern systems, but we expect reindeer to have a negative effect on flowering and seed production because resource limitation may prevent compensatory responses (Bryant, Chapin & Klein 1983).
The effects of browsing on the insect guild utilizing willows can operate in several ways. Induced-chemical responses of the plant to herbivory may change the nutritional quality of the plant, and therefore the quality of the plant as food (Bryant et al. 1991; Martinsen, Driebe & Whitham 1998). Many of these induced responses are long lasting (Bryant et al. 1991 and references therein). Browsing may reduce foliage available for leaf-feeding insects (leaf-gallers and leaf-beetles) and reduce buds available to bud-galling sawflies. Alternatively, browsing may stimulate development of long shoots, leading to increased resources for these insects (Price 1991). Winter browsing by moose improves the food quality for herbivorous insects, as indicated by higher insect densities on previously browsed trees (Danell & Huss-Danell 1985; Roininen, Price & Bryant 1997). In general, moose browsing leads to an improvement in leaf quality (Danell & Bergström 1989; Bryant et al. 1991) and growth of longer and more vigorous shoots (Bryant et al. 1991). For shoot- and leaf-galling sawflies in the genera Euura and Eupontania, shoot length has been found to correlate positively with the number of ovipositing females and survival of the larvae (Price, Craig & Roininen 1995; Roininen, Price & Bryant 1997), and galls were more common on browsed trees or ramets. Compared with the moose–woody plant–insect system, there is little information on the reindeer–woody plant–insect system. However, Olofsson & Strengbom (2000) found more gall-forming insects on reindeer-browsed willow S. lanata, even though shoot length was reduced by leaf-stripping.
In northern Fennoscandia, Salix phylicifolia L. is one of the common willow species browsed by reindeer (Haukioja & Heino 1974; Nieminen & Heiskari 1989). The objective of our study was to compare the growth and reproductive response of S. phylicifolia to summer browsing by reindeer. We measured several growth characteristics inside and outside exclosures 4 and 5 years after establishment of the experiment. Furthermore, we examined the impact of reindeer feeding on herbivorous insects on willows, in order to test the hypothesis that summer browsing would adversely affect both willow and its associated insect herbivores.