February 02, 2021

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Boreal Trees and Shrubs Exhibit Differential Water Stress When Faced with Whole Ecosystem Warming

Species with more water stress do not benefit from elevated CO2

A 12.8-m diameter open-top whole ecosystem warming enclosure installed around intact vegetation in a southern boreal bog ecosystem. The SPRUCE experiment consists of five warming treatments (+0 to +9°C) with or without the addition of elevated CO2 (+500 ppm).

[Courtesy Oak Ridge National Laboratory.]

The Science

Researchers increased the soil and air temperature in a boreal wetland forest and measured water stress of the two main shrub species and two main tree species. The higher temperatures increased water use by tamarack trees but reduced water use by spruce trees. As a result, the tamarack trees displayed more water stress than spruce trees. Water stress was also greater for the leatherleaf shrubs as compared with the Labrador tea shrubs. The addition of higher carbon dioxide (CO2) in the air reduced water stress in spruce and Labrador tea but not for tamarack or leatherleaf plants.

The Impact

The trees and shrubs that show greater water stress with warming may be damaged and die back during extreme drought or heat in the future. This could change the productivity of those species and their competitive ability. The result could be a change in species composition and subsequently whole-ecosystem productivity. Since boreal wetlands store a lot of carbon in the soil and plants, a loss in productivity by some species would contribute negatively to climate change.


Boreal peatland forests have relatively low species diversity and thus impacts of climate change on one or more dominant species could change how the ecosystem functions. Despite abundant soil water availability, shallowly rooted plants within peatlands may not be able to meet canopy demand for water under drought or heat events. As rates of leaf transpiration increase, there must be greater root uptake and transport of water to the leaves. Under such conditions, some plants will limit transpiration by closing the stomatal pores in the leaves, while others maintain water use, which can lead to water stress and even plant mortality. Elevated atmospheric CO2 can lead to partial stomatal closure since the higher concentrations exceed that needed for photosynthesis within the leaf, which can buffer water stress. In this study, the tamarack and leatherleaf kept their stomata open under warming treatments, which may maintain rates of photosynthesis, but they had increased water stress. Alternately, the black spruce and Labrador tea closed stomata and maintained greater hydraulic safety. These latter species also responded to elevated CO2, which further reduced water stress. The species-specific responses of peatland plant communities to drier or hotter conditions will shape boreal peatland composition and function in the future.

Principal Investigator

Jeffery M. Warren
Oak Ridge National Laboratory

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science


This research was supported by the Office of Biological and Environmental Research (BER) within the U.S. Department of Energy’s (DOE) Office of Science. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for DOE.


Warren, J.M., et al. "Divergent Species-Specific Impacts of Whole Ecosystem Warming and Elevated CO2 on Vegetation Water Relations in an Ombrotrophic Peatland." Global Change Biology 27 (9), 1820–1835  (2020). https://doi.org/10.1111/gcb.15543.