Plant interactions, climate, and the reindeer (Rangifer tarandus) interdependently shape vegetation in northern Finland.

There is limited understanding on how complex interdependencies among large herbivore grazing, competitive and facilitative interactions among plants, and the changes in temperatures and precipitation shape northern boreal and subarctic ecosystems. Here, we assessed changes in dwarf shrub and lichen cover and height using data from 617 field sites monitored a decade apart (2005-2008 and 2016-2018) in semi-dry and dry habitats in northernmost Finland, where reindeer herding constitutes a traditional livelihood with reindeer moving freely across landscapes and grazing on seasonally available forage plants. We hypothesized that several direct and indirect factors related to climate and reindeer herding regimes drive changes in vegetation. We predicted that over the 10 years, shrub cover and height would increase and that lichen cover and height would decrease in response. We also expected that the decline in lichen cover and height would be more pronounced in summer-grazed areas and in areas with higher reindeer densities. We observed that shrub cover and height had increased drastically, and the change in lichen cover was negatively correlated with change in shrub cover. However, the change in lichen height was positively correlated with change in shrub height, which reveals a dual relationship between dwarf shrub and lichen vegetation. The seasonal timing of reindeer grazing was also important: lichen cover decreased less in winter than summer and year-round ranges. The direction of the change in lichen height was even opposite among seasonal ranges with decreased height in summer and year-round ranges and increased height in winter ranges. Lichen cover and height responded negatively to higher reindeer densities in both summer and winter ranges. While shrub cover had increased in summer and year-round ranges, shrub cover was unchanged in winter ranges, and shrub height increased less with increasing reindeer densities. These results indicate that reindeer grazing may partially counteract "shrubification" in areas that are grazed only during winter. Our results demonstrate how differing large herbivore grazing regimes, together with their complex interdependencies between climate warming and associated changes in plant-plant interactions, contribute to spatially variable vegetation trajectories. Due to the direct and the indirect mechanisms by which climate warming affects dwarf shrub and lichen vegetation, for maintaining good lichen grounds for reindeer herding, the benefit of seasonal range rotation will likely even increase in the future.