Trophic dynamics in a changing Arctic: interactions between ptarmigan and willows in Northern Alaska

Abstract

Shrubs have been expanding in the Arctic over the past century, with important consequences for ecosystem functioning, plant community composition, and wildlife habitat. Herbivores have the capacity to strongly moderate the growth and biomass of shrubs, and therefore need to be considered when attempting to understand and project future changes to Arctic ecosystems. Ptarmigan (Lagopus lagopus, L. muta) are common and widespread in many tundra regions, and feed on shrubs throughout their life cycle. Ptarmigan are likely to be an important herbivore in northern Alaska where shrub expansion is rapidly occurring; however, little is known about their spatial and temporal distribution in the Arctic, or the effect of their browsing on shrubs. This dissertation provides novel information on ptarmigan population ecology and herbivory in northern Alaska. Ptarmigan occupancy in northeastern Alaska increased from March through May, lending support to the idea that they undergo a spring migration from southern wintering grounds to breeding grounds north of the Brooks Range. Ptarmigan distributions were strongly linked to the presence of shrubs; occupancy was greatest in dense patches of riparian willows that grew tall enough to exceed snow depth. The frequency and intensity of ptarmigan browsing in feltleaf willow (Salix alaxensis) stands in northeastern and northwestern Alaska was high, such that ptarmigan browsed 82-89% of willows and removed 30-39% of buds. Browsed willow branches produced fewer catkins than un-browsed branches, but doubled the volume of current annual growth produced the following summer. These longer, larger-diameter shoots bore 40-60% more buds than shoots on unbrowsed branches. The removal of distal buds stimulated dormant buds at the base of the branch to produce shoots, resulting in a "broomed" architecture. Despite their tendency to produce longer shoots when browsed, highly broomed willows with a history of browsing were shorter than un-broomed willows. Broomed willows were more likely to be re-browsed by ptarmigan. Moose browsing was not as prevalent (17-44% of willows browsed) as ptarmigan browsing and resulted in reduced catkin production and increased shoot volume. Simulated ptarmigan browsing of feltleaf willows caused a similar response to that observed in the wild. Browsed willows produced fewer catkins and more buds per shoot, although buds were smaller than on un-browsed willows. Browsing altered the architecture and bud production of willows such that the biomass of easily accessible buds (within 50 cm of snow level) was greater (129 ± 30 mg) on browsed willows than un-browsed willows (113 ± 50 mg). Browsing did not affect nitrogen concentrations, but slightly reduced carbon concentrations and protein precipitation capacity (tannins) in buds produced the following winter. In a feeding preference study, when broomed and un-broomed willow branches were placed in the snow at equal heights, wild ptarmigan showed no preference for either type but obtained more buds from broomed willows. A synthesis of original and published research showed that browsing by vertebrate herbivores in the Arctic is not uniform, and that certain shrubs (such as willows) are more heavily browsed than others (such as evergreen ericoids, resin birches, and Siberian alder (Alnus viridis fruticosa)). These differences in preference translate to variation in the degree to which herbivores regulate Arctic shrub growth and community structure. As shrubs expand in the Arctic, unpalatable, fast-growing species such as alder may have an advantage over more palatable species such as willows. Collectively, this research fills critical gaps in our knowledge of ptarmigan population ecology in Alaska, provides novel insights into how ptarmigan regulate their food source for their own benefit, and enhances our understanding of how herbivores influence shrub expansion in the Arctic.