Implications of a changing climate in coastal Labrador for caribou and their forage

Abstract

The majority of Labrador’s coastal lands are below the 60th latitudinal parallel. Even so, the cold Labrador Sea currents, late-lasting sea ice, and frequent high winds, these coastal ecosystems often resemble Arctic and Subarctic ecosystems that are further north. Historically, these landscapes exhibited stunted shrub growth, but as the climate changes, the shrinking sea ice season and the cooling effects of coastal sea ice on nearby landscapes begin to subside, shrubs are now overtaking tundra vegetation communities. These shrubs threaten ground lichen communities in coastal regions. Understanding changes in vegetation allows us to predict changes in the forage available to caribou, a culturally and ecologically significant species in Labrador and other northern regions. Caribou tend to rely heavily on lichen in the wintertime to meet their dietary needs. In this dissertation, I addressed three main research questions: 1) How much lichen forage do caribou need to satisfy their energetic needs over winter? 2) With the state of ground lichen availability at three Labrador sites, what caribou density can be sustainably achieved at each location? 3) With climate change increasing coastal fog, how will lichen productivity respond to fog as a source of hydration? I used a time-series simulation to estimate caribou energetics over winter. After the simulation results, I assessed the state of ground lichen at three sites in coastal Labrador. The combined results are used to determine a sustainable caribou density at each location based on available winter forage. The caribou energetics simulation results showed that the average caribou must eat 1330 kg of lichen over the winter to avoid weight loss. The ground lichen estimates in open tundra at each of the three sites were 0.66 kg/m2 in Pinware (caribou free site), 0.1 kg/m2 in Cartwright (Mealy Mountain caribou) and 0.04 kg/m2 in Nain (George River caribou herd). With current lichen biomass estimates and an assumed 5% annual growth rate, I was able to derive sustainable caribou densities at each of the three sites (Pinware: 24 caribou/km², Cartwright: three caribou/km², Nain: one caribou/km²). I also studied how lichen productivity may be impacted by increased fog-based precipitation. Although lichens cannot compete vertically with shrubs, they may respond to climate change by becoming more productive when using fog water to increase growth in the growing season. Lichens readily use non-rainfall sources such as fog for their metabolism. Using a simulation model for lichen metabolism, I found that fog can encourage productivity in lichens, with my model showing a carbon uptake of 20 g on a 1 m2 ground lichen mat over four months from only observed fog events. Other promising findings from this study show that fog events happen much more frequently in the morning, hydrating the lichens before peak solar radiation. That fog alone will not block enough sunlight to achieve net photosynthesis. However, some troubling findings are that warmer months result in lower lichen productivity due to fog, as respiration begins to outpace photosynthesis at warmer temperatures. Fog water deposition is likely to increase in these environments, potentially altering lichen productivity in the north. Bottom-up constraints to caribou herds, such as a lack of forage, are essential to identify. Considering other threats caribou face can help herd managers and Indigenous people, who rely on caribou for a food supply, determine when intervention is required in a changing northern environment.