Caribou and reindeer herbivory may help mitigate climate change impacts in arctic tundra

Background/Question/Methods

Climate change impacts to arctic tundra vegetation are already observable. Increases in deciduous shrub growth over the past 30 years have been widely reported and correlate with increasing mean annual temperatures. Meanwhile, many populations of caribou and reindeer (Rangifer tarandus) have been in decline, and the potential role of this decreased browsing intensity in past shrub dynamics and future trajectories is only beginning to be recognized. As arctic plant growth is most strongly limited by soil nutrient availability, any impact of herbivory on nutrient cycling may either enhance or inhibit shrub growth responses to warming. However, work to date on the role of arctic herbivores in shrub expansion has only considered the direct top-down control on plant production and has largely overlooked the indirect bottom-up effects on nutrient cycling that may be of great importance in the Arctic. Here we investigate the relative importance of natural low-intensity caribou browsing and experimental warming on plant and soil nutrient pools using caribou exclosures and experimental greenhouses in mesic birch hummock tundra in the central Canadian low arctic. We also explore browsing and warming impacts to the biomass, productivity and chemical composition of the seven dominant tundra vascular plants to enable a mechanistic interpretation of the changes to nutrient pools.

Results/Conclusions

We found that caribou browsing decreased shrub growth by 21-42%, decreased shrub nutrient pools, but did not alter soil nutrient pools. As old leaves in the browsed plots had higher total phenolics concentrations and lower total biomass, we identify browsing-induced changes to litter pool quality and quantity as a possible mechanism for reducing nutrient availability in this system. Experimental warming increased shrub growth by 16-41%, increased shrub nutrient pools, but did not consistently alter soil nutrient pools. Further chemical analyses of the greenhouse samples are pending. We conclude that the effective management of caribou and reindeer herds may be an important mechanism for mitigating climate change impacts to arctic tundra.