The Arctic is undergoing dramatic increases in primary productivity. This shift has been largely attributed to proliferation of shrubby vegetation, driven by increased arctic temperatures. An important part of predicting the impacts of this change involves improving our understanding of how tundra shrubs interact with their local environment. Tall shrubs such as green alder (Alnus viridis) have the potential to alter patterns of evapotranspiration and light attenuation, active layer development, and snow distribution. For these reasons we expected conditions (e.g. soil moisture and thaw depth) within alder patches (discrete alder communities) to differ from surrounding alder-free tundra and for the magnitude of these differences to be related to the structure and size of patches. We further hypothesized that these differing conditions may explain variation in understory plant community composition. To examine this, soil moisture, thaw depth, organic matter depth, relative decomposition rates, plant available nutrients, and plant community composition were measured in paired, replicated alder patch and adjacent tundra communities. Transects running from upslope to downslope positions were established through these communities to assess whether slope position played a role in governing environmental conditions. Size and position of mature alder were also recorded and mapped along these transects.
Alder patches were drier and had more shallow active layers than alder-free tundra, which may be related to the increased transpirative demands of alder relative to typical open-tundra vegetation. The depth of organic matter accumulation was also greater in alder patches, which is likely related to the much larger input of leaf litter into the system. We also observed elevated levels of plant available ammonium and potassium in patches. This, in conjunction with greater alder regeneration within patches is particularly important in the context of shrub expansion as it suggests that alder patches themselves may support further increases in productivity.
These results provide insight into the dynamics of alder patches and broaden our understanding of what changing land cover may mean for tundra ecosystem function. It will also aid in extrapolating future habitat availability for arctic plant and wildlife populations, many of which provide an important source of traditional foods for northern communities.