Ecosystem function in complex mountain terrain: Combining models and long-term observations to advance process-based understanding

Wieder, W. R., Knowles, J. F., Blanken, P. D., Swenson, S. C., & Suding, K. N. (2017). Ecosystem function in complex mountain terrain: Combining models and long‐term observations to advance process‐based understanding. Journal of Geophysical Research: Biogeosciences, 122(4), 825-845.

Abstract

Abiotic factors structure plant community composition and ecosystem function across many different spatial scales. Often, such variation is considered at regional or global scales, but here we ask whether ecosystem‐scale simulations can be used to better understand landscape‐level variation that might be particularly important in complex terrain, such as high‐elevation mountains. We performed ecosystem‐scale simulations by using the Community Land Model (CLM) version 4.5 to better understand how the increased length of growing seasons may impact carbon, water, and energy fluxes in an alpine tundra landscape. The model was forced with meteorological data and validated with observations from the Niwot Ridge Long Term Ecological Research Program site. Our results demonstrate that CLM is capable of reproducing the observed carbon, water, and energy fluxes for discrete vegetation patches across this heterogeneous ecosystem. We subsequently accelerated snowmelt and increased spring and summer air temperatures in order to simulate potential effects of climate change in this region. We found that vegetation communities that were characterized by different snow accumulation dynamics showed divergent biogeochemical responses to a longer growing season. Contrary to expectations, wet meadow ecosystems showed the strongest decreases in plant productivity under extended summer scenarios because of disruptions in hydrologic connectivity. These findings illustrate how Earth system models such as CLM can be used to generate testable hypotheses about the shifting nature of energy, water, and nutrient limitations across space and through time in heterogeneous landscapes; these hypotheses may ultimately guide further experimental work and model development.

Plain Language Summary

Projecting ecosystem response to environmental change presents enormous challenges that are critical to understand for multiple stakeholders. These projections are complicated by complex interactions between physical drivers, like temperature and precipitation, and biotic agents like plants, animals, and soil microbes. Using long‐term observations from a heterogeneous alpine ecosystem and a state‐of‐the‐art land model, we explore how the physical environment shapes ecosystem function and how the function of this ecosystem may respond to climate change. We found that the land model was able to capture observed water, energy, and carbon fluxes from this well‐studied alpine ecosystem, lending credibility to our results. Our simulations also indicated that earlier snowmelt and warmer summertime temperatures might drive divergent plant responses across the landscape. Notably, climate change may decouple the timing of snowmelt that delivers critical water resources from periods when plants experience the greatest water demand, thus altering plant productivity. Additionally, this work raises ecological questions that can be addressed with additional experimentation and/or model development.