Graduate Thesis Or Dissertation

Lichen communities and caribou habitat in Denali National Park and Preserve, Alaska, USA

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  • Lichens play many important roles in subarctic terrestrial ecosystems by fixing nitrogen, colonizing rock and gravel, stabilizing otherwise bare soil, adding significantly to vegetation biodiversity and serving as the primary food for caribou in the winter. In these chapters, I analyzed lichen community and ecological trait structure along environmental gradients, map lichens using some of their unique spectral properties to generate lichen maps and study caribou habitat selection patterns in relation to lichens and other habitat variables. Morphological and life history traits of lichens influence their ecological roles through physiological limitations imposed by their form and photobionts, the algal or cyanobacterial partner. In chapter 2, I analyzed the lichen traits in relationship to environmental gradients, other forms of vegetation and time since fire in Denali National Park and Preserve, Alaska. Lichens with different photobionts reached different maxima along environmental gradients, these corresponding to variable water availability or specific biotic factors thought to favor that photobiont. Green algal lichens were most abundant in the alpine whereas cyanolichens peaked where shrub cover increased. Tripartite lichens were most abundant in middle elevation, mossy areas. Lichen growthforms peaked along desiccation and water absorption gradients. Lichens with small vegetative propagules were most abundant in lowland forests. Recent fire favored simple, Cladonia-form lichens with soredia that grow on wood whereas erect branched fruticose lichens, the "reindeer lichens", had only partially recovered 20-100 years after fire. These results imply interacting forces of water regulation, dispersal and optimum conditions for photosynthesis drive lichen trait frequency and abundance. The fungal partner within the lichen symbiosis produces many unique compounds that are often brightly colored. Other studies have attempted to map lichens using their distinctive spectral properties but no study has yet to target specific lichen compounds in order to better model lichen cover. In chapter 3, I focused on one yellow lichen compound, usnic acid, as the target for modeling lichen cover using Landsat 7 ETM+ satellite data. Usnic lichen cover had non-linear relationships with the three best predictors; elevation, blue and near-infrared bandpasses. Using these three predictors, I generated an usnic lichen map for Denali, which I use in chapter 4 for analyzing caribou habitat selection. I also modeled and mapped other vegetation groups corresponding to caribou diet items used later. My results show that some lichens may be directly mapped from space by targeting this specific compound produced by the fungus. Caribou depend on lichens for up to 66% of their winter diet but other factors, such as snow, affect their access to the lichens. In chapter 4, I analyzed caribou habitat selection over 20 years in Denali using vegetation maps from chapter 3, climate data and other environmental variables. Over the two-decade period, caribou selected middle elevation, open areas with high graminoid cover and earlier snow-free dates. As each winter progressed, caribou aggregated where there was higher lichen cover and earlier snow-melt. Caribou selected habitat differently between years, which I collapsed into three different habitat/year groups: (1) years where most animals were in low elevation, flat terrain where there was low lichen and conifer cover but high graminoid and shrub cover with variable snow; (2) years caribou went to middle elevations with deeper snow and rugged terrain and moderate graminoid and lichen cover; and (3) years where caribou were dispersed west in low elevation woodlands with high lichen cover. My results show interacting factors determine caribou habitat selection at multiple spatial scales, specifically the importance of open, tussock tundra and long-term trends in snow melt at long time scales and lichen and snow-melt at shorter time scales. This research improves our understanding of the regional distribution and abundance of lichens in relation to higher plants, fire, and caribou.
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