The productivity of biomass by vegetation is a crucial component of the energy, carbon and nutrient fluxes that occur in ecosystems. The spatial distribution, abundance, and productivity of plants in the subalpine forest and alpine tundra are controlled by physical forces, like wind, snow, and topography, as well as biotic factors, like other plants and soil microbial communities. Historic research at NWT has given much attention to the influence of physical forces on plant communities in the tundra; more recently, NWT scientists have begun to investigate the role of biotic factors. Changes in environmental conditions due to climate change (temperature, precipitation) and dust and nitrogen deposition (nutrient availability) are also affecting plant communities in high mountain ecosystems, and are the focus of a number of research experiments at NWT.
Indirect and interactive effects of changes in nutrient availability, temperature, and precipitation (snowpack) patterns on alpine tundra ecosystems are being tested in an on-going global change experiment (Farrer et al. 2014). Over time, enhanced nitrogen (N) availability and winter snowpack were found to increase the abundance of a grass species, Deschampsia caespitosa, and decreasedthe abundance of a forb, Geum rossii. By changing the abundances of these two prominent tundra plant species, N and snow additions had a strong indirect effect on overall plant community diversity, with the strength of this effect increasing over time (Fig. 1). Increased N, summer temperature, and winter snowpack also directly affected ecosystem functions like plant productivity, N mineralization, and winter N availability. However, over time, the indirect effects are stronger than the direct effects of these environmental drivers. For example, most of the negative effect on plant community diversity caused by N addition was due to nitrogen’s effect of increasing Deschampsia abundance. Overall, these results suggest that explicitly accounting for changes in dominant plant species abundances may be necessary for forecasting plant community response to environmental change, but predicting ecosystem function without knowledge of plant responses to global change may also be possible.
Investigators: Katie Suding, Emily Farrer
The expansion of woody plant cover in historically herbaceous-dominated plant communities has been observed in a range of ecosystems around the world, including arctic and alpine tundra. These changes are being driven by various factors including climate and other global factors, like N deposition, as well as land management strategies. Depending on the rate and extent of woody encroachment in these systems, such shifts in vegetation cover could have major implications for important ecosystem processes like nutrient and carbon cycling and storage.
Formica et al. (2014) quantified the rate of shrub expansion from 1946 to 2008 in an 18 ha area of alpine tundra on Niwot Ridge by analyzing aerial photographs and used a global change manipulation experiment to assess the effects of three important global change factors – N deposition, temperature, and precipitation (snowpack) – on willow shrub seedling survival and growth. They found that over the 62-year period willow cover increased by 441% and at an exponential rate (Fig. 2), translating to a 137 kg ha-1 increase in carbon storage. Increased snowpack was shown to increase willow seedling survival and increased N deposition and warmer summer temperatures to facilitate willow growth (Fig. 3), supporting the conclusion that, in addition to release from grazing pressure, global change factors may be driving shrub expansion in the alpine tundra.
Investigators: Katie Suding, Emily Farrer
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This material is based upon work supported by the National Science Foundation under Cooperative Agreement #DEB-1027341. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necesarily reflect the views of the National Science Foundation.
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