Biogeochemical and plant functional group response to long-term snow manipulation in a subalpine grassland
Holsinger, Jordan Paul.
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Snow represents an important control over plant communities in seasonally snow-covered ecosystems. It constrains the growing season and affects the availability of important resources including water, nitrogen (N) and phosphorus (P). Snow depth, distribution and duration have been affected by global climate change making it increasingly important to understand the effects of changing snow regimes on terrestrial ecosystems. Here we leverage a 43-year snow manipulation experiment to examine the effects of long-term changes in snow depth on plant community structure, resource availability and interactions therein in a common grassland type of the northern Rocky Mountains in western North America. Long-term experimental doubling and quadrupling of snowpack was associated with a significant shift in plant functional group distribution to a more forb rich community. Snow addition has resulted in a two to three-fold increase in forb to grass biomass ratios over time. Forbs consistently had greater N and P contents and lower nutrient use efficiencies compared to grasses. Forbs also displayed higher rates of net photosynthesis relative to grasses and sustained positive carbon (C) fixation rates late into the growing season after grasses had ceased. Though there is evidence that water exerts considerable control over ecosystem processes, increased snow depth did not have affect soil water availability through the growing season. However, snow depth was associated with significant differences in plant-available phosphate across the entire growing season with approximate 15% and 31% increases in pools of available P relative to ambient snowpack depth for doubled and quadrupled snowpacks respectively. Estimates of direct P inputs via dust and the ratio of available P to total P in the soil suggest that internal cycling was largely responsible for the observed differences in pools of available P. However, growing season net mineralization rates do not differ across treatments. This may suggest that winter processes make significant contributions to nutrient cycles. It is possible that the increased availability of P favors the shift to a forb-rich community under deeper snow because of their increased productivity under dry conditions and that the increased litter quality of forbs likewise promotes increased litter decomposition and mineralization, especially of P.