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...and 3.9% browsed snowshoe hares (Lepus americanus). Foraging moose removed, on average, 15.1 mm of current annual growth from willow twigs, which averaged 24.1 mm in length (62.3% removed). Twigs re-growing from 2-year-old stems that were browsed previously had larger diameters at their bud scale scar than those re-growing from stems that were not browsed in the previous year. Browsing history by moose, however, had no effect on nitrogen content, in vitro dry matter digestibility, or tannin content of willow twigs. Willows did not respond to browsing on individual twigs with an inducible defense system that involved tannins. Diameter at point of browsing (bite size) was larger on twigs that had been browsed previously than for twigs re-growing from second-year growth that had not been browsed. Moose did not exhibit an optimal bite size, but took larger-diameter bites from larger compared with smaller leaders of current annual growth. Forage selection by moose for previously browsed twigs likely relates to greater forage biomass on those twigs rather than to forage quality. We caution, however, that foraging behavior by moose cannot be understood fully without considering additional factors, including predation risk in relation to forage availability.
Keywords: Alaskan moose, Alces alces gigas, browsing history, digestibility, feltleaf willow, foraging behavior, nitrogen, Salix alaxensis, tannins
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Moose (Alces alces) are among the largest browsing mammals, with adult males of A. a. gigas (the largest subspecies) attaining > 770 kg and adult females reaching > 570 kg (Schwartz et al. 1987). Moose possess a narrower incisor arcade relative to body mass than do other ruminants, especially grazers--an allometric relationship that ostensibly is an adaptation for selective browsing (Spaeth et al. 2001). Indeed, browse is a critical component in the diet of moose throughout their distribution in North America, especially during winter (Peek 1974, Ludewig and Bowyer 1985). Moreover, in Alaska, USA, willows (Salix spp.) are the mainstay in the diet of moose (Molvar et al. 1993, Bowyer and Bowyer 1997, MacCracken et al. 1997, Weixelman et al. 1998, Bowyer et al. 2001, and many others), and those shrubs are consumed year-round in some areas (Van Ballenberghe et al. 1989; Molvar et al. 1993; Bowyer and Bowyer 1997; Bowyer et al. 1998, 1999a). A more-complete knowledge of interactions between moose and this crucial food supply is essential for understanding their distribution (Teller 1978), population dynamics (Bowyer et al. 1999b), reproductive performance (Schwartz and Hundertmark 1993), life-history characteristics (Keech et al. 2000), and effects on ecosystem structure and function (Pastor and Naiman 1992, Molvar et al. 1993, Bowyer et al. 1997, Berger et al. 2001).
Large herbivores tend to congregate in areas where they have foraged previously (Fryxell 1991) and use of traditional areas is common among some ungulates (Hjeljord 2001). For instance, moose sometimes use the same migratory routes (Andersen 1991) as well as locations for mating (Van Ballenberghe and Miquelle 1993) and giving birth (Bowyer et al. 1999a). Likewise, cervids (Duncan et al. 1998, Moore et al. 2000), including moose (Molvar et al. 1993, Bowyer and Bowyer 1997, Bergquist et al. 2001), preferentially forage on leaders of new growth that have re-grown from twigs that were browsed previously. Such regrowth on previously browsed twigs is characterized by larger twigs and leaves than on unbrowsed leaders (Bergstrom and Danell 1987, Molvar et al. 1993, Bowyer and Bowyer 1997). Whether previous browsing of plants and their subsequent reuse by foraging herbivores influences use of traditional areas by these large mammals is unknown. We contend, however, that understanding why large herbivores re-browse particular plants, or parts thereof, is an essential step in understanding the overall process of diet selection and habitat use.
The browsing history on trees and shrubs is well known to affect subsequent foraging by moose (Molvar et al. 1993, Bowyer and Bowyer 1997, Bergquist et al. 2001). Consequently, we tested whether re-browsing by Alaskan moose on leaders of new growth on feltleaf willow (S. alaxensis) was a result of increased size of twigs, quality of new growth, or both variables. We also tested whether moose would vary bite size in relation to the size of twigs available to browse.
STUDY AREA
We conducted research concerning moose browsing on feltleaf willows in interior Alaska, USA, about 15 km northwest of Fairbanks (64[degrees] 54' N, 147[degrees] 50' W). The study site...
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