Does Reindeer Grazing Dictate Understory CO2 Exchange Responses to Snow Depth Changes in Boreal Forests?

Northern boreal forests are globally crucial sinks for atmospheric carbon dioxide (CO₂) but, due to climate change, these sinks are at risk of switching to CO₂ sources. Climate change affects winters for example by altering snow depth, with regional increases or decreases in snowfall. Since snow cover regulates key ecosystem processes in boreal forests, such changes may affect ecosystem functioning with possible consequences for CO₂ exchange year-round. Climate change does not act alone but together with other factors such as herbivory. In boreal forests, reindeer (Rangifer tarandus L.; caribou in North America) is a key herbivore that affects understory vegetation — particularly lichen cover — which may, in turn, affect understory CO₂ exchange. Yet, it remains largely unknown how the changing snow depth together with divergent reindeer grazing conditions affects CO₂ exchange in the boreal forests.

To study these snow-grazer interactions on understory CO₂ exchange, we conducted CO₂ flux measurements during the snow-free season over four years in two Scots pine (Pinus sylvestris L.) forests in northern Fennoscandia. Using a manual chamber method, we measured CO₂ exchange across snow depth manipulation treatments (i.e., ambient, reduced and increased snow depth) in areas of active reindeer grazing and adjacent areas where grazing had been excluded for 25 and 55 years.

We found that reduced and increased snow depths had opposing impacts on understory CO₂ exchange, but only if reindeer had been excluded. Specifically, reduced snow depth increased the understory CO₂ source strength while increased snow depth decreased it when reindeer had been excluded. In contrast, snow depth did not affect CO₂ exchange in the presence of continuous reindeer grazing. These findings suggest that, in northern pine forests, changes in snow depth may unbalance the understory CO₂ exchange with long-term reindeer absence. On the contrary, the presence of continuous reindeer grazing may enhance ecosystem resistance to changes in snow depth. We propose considering these snow-grazer interactions for accurate global C budget estimates in a changing winter climate in boreal forests.