Forage Quality
The type and nutritional quality of vegetation is also changing throughout the Arctic. Woody plants such as shrubs are expanding into more northern regions and becoming more abundant in the Arctic as temperatures rise, but these plants have strong chemical and structural defenses against herbivory and have less available protein than other types of forage that Rangifer typically eat (Mallory & Boyce, 2017).
Increased Arctic sea ice loss promotes local warming over nearby land areas, lengthening the growing season and thawing permafrost. This leads to an increase in terrestrial primary production and plant biomass in pastures that caribou in North America graze during the summer. However, this increase was associated with declining caribou populations, indicating a deterioration in the quality of the forage. Some tall deciduous shrubs that are expanding into Arctic regions have antibrowsing toxins that prevent against caribou grazing. Expanding shrub cover also reduces the availability of lichen, which caribou forage in the winter (Fauchald, et al., 2017).
Fauchald, et al., 2017
In the summer, reindeer graze on grasses, shrubs, herbs, and some deciduous tree species but prefer lichens in the winter. In Fennoscandia (a region of Northern Europe that includes Finland, Norway, Sweden, and part of Russia), the tree line is moving northward, increasing the range of boreal coniferous forests and decreasing tundra and mountain regions that serve as reindeer pastures and summer habitats (Turunen, et al., 2009).
Studies in Sweden have shown that rising temperatures decrease nitrogen concentration in some forage plant species due to carbon accumulation from increased growth. Similar results have been found in Finland, where high carbon dioxide levels reduce nitrogen concentration in the leaves of some tree species. High carbon dioxide concentrations have also been associated with low soluble protein concentrations in some trees. Increases in UV-B radiation have also been shown to increase the concentration of secondary compounds such as phenolics in summer and winter reindeer forage plants. Phenolics can act as antimicrobials and are very resistant to microbial degradation. These changes pose problems because reindeer generally prefer young leaves with high concentrations of nutrients like nitrogen. Additionally, phenolic compounds such as tannins can reduce protein availability, and animals that regularly consume tannins have been observed to develop defensive mechanisms against them. Furthermore, in the long term, increased phenolic concentrations may cause declines in the decomposition rates of organic matter, preventing the release of nutrients into the soil and lowering the nutritional quality of reindeer forage even further (Turunen, et al., 2009).