The interacting roles of climate and soil in plant species range shifts in subalpine and alpine meadows

Background/Question/Methods

As one ascends a mountain slope, closed-canopy forests give way to subalpine and then alpine meadows, which give way to bare ground and ice. Climate change is expected to result in the upward movement of these plant species and biomes. However, climate is not the only constraint on plant performance in these habitats, with soil quality being another important factor that also varies dramatically across elevation. Bare soils above the meadows have much lower organic matter content, water holding capacity and concentrations of key nutrients compared to lower elevation meadow soils. And while climate is likely to change rapidly in the coming decades, soil development will likely be relatively slow and lag behind climate change. Thus, seedling establishment above the meadows may be inhibited by soil conditions even if climate becomes suitable. To examine these potentially interacting range determinants, we transplanted seedlings to plots spanning the elevational range of the meadows where we measured microclimate, and within each plot planted half the seedlings in soil collected from the meadows’ lower margin and half in soil from bare ground above the meadows, and then monitored seedling survival and growth.

Results/Conclusions

We found that climate and soil had important and interacting effects on seedling establishment with implications for range shift dynamics. Surprisingly, initial survival declined with earlier snow disappearance, with more severe declines for seedlings in bare soil. These patterns imply that earlier snow disappearance brought by climate change will increase water stress and mortality for young seedlings, especially above the meadows where the low water holding capacity of bare soils exacerbates this stress. In addition, growth increased with earlier snow disappearance for seedlings in meadow soil, while seedlings in bare soil showed no response, suggesting that longer growing seasons will primarily increase seedling growth in meadows, where soil resources are not limiting. Thus, climate change will likely favor seedlings in meadow soils relative to those in bare soils because seedlings in meadows will experience less of the negative effects on survival and more of the positive effects on growth. These establishment patterns suggest that lower elevation tree species establishing in meadows will undergo relatively rapid upward range expansions, at the expense of the shade-intolerant meadow species, while meadow plants will experience relatively slow upward expansions onto bare ground, likely leading to range reductions for the meadow species.