The SPRUCE experiment: Learning from the past to better predict the future

Background/Question/Methods

Northern peatland ecosystems are expected to be especially vulnerable to climatic warming because of the vast stocks of carbon they hold and the expectation that warming will lead to rapid release of carbon to the atmosphere as CO₂ or CH₄. To meet the mandate to evaluate the vulnerability of critical ecosystems to atmospheric and climatic change and their feedbacks to the climate system, the Spruce and Peatland Responses Under Climatic and Environmental Change (SPRUCE) experiment has been established in an ombrotrophic bog in northern Minnesota. This ecosystem is characterized by a Picea mariana forest over a ground cover of Sphagnum, other mosses, and ericaceous shrubs; several meters of peat are perched above the regional groundwater table. Previous research on forest responses to warming necessarily involved compromises resulting from the limitations imposed by the stature of forests and the limitations of available warming technology. Recognizing the importance of having a whole-ecosystem perspective, the SPRUCE experiment will employ a novel warming technology that permits exposure of thethe intact ecosystem—including trees, shrubs, moss, and peat—to a range of temperatures up to 9 ◦C greater than ambient. Threshold responses to elevated temperature are especially important; hence, the experimental design takes a regression approach, as long advocated by modelers; we will also investigate the effect of elevated atmospheric CO₂ on the temperature response surface. Pretreatment characterization has encompassed the full range of responses that will be important throughout the 10-year experiment: tree growth and physiology, plant community composition and production of the shrub and moss layers, root growth and distribution, biogeochemical cycling, hydrology and water chemistry, soil microbial composition and responses, and water and trace gas (CO₂ and CH₄) fluxes. These components of ecosystem response are all interrelated, requiring a strong model framework to guide the measurements and evaluation of results.

Results/Conclusions

Preliminary measurements have demonstrated the importance of small-scale heterogeneity in this ecosystem. Fine roots of spruce and larch trees are exclusively in hummocks, whereas shrub roots also occur beneath hollows. The moss community differentiates across the hummock-hollow gradient; the different morphological adaptations of the Sphagnum and Polytrichummosses will likely lead to changes in moss community composition in response to warming. Carbon budget analysis has indicated that net flux is small relative to influx and efflux and can be either positive or negative; the pretreatment analysis has helped to define critical ecosystem measurements that must be made when the treatments commence.