Whole ecosystem warming induces hydraulic dysfunction in boreal bog trees

Jennifer M. R. Peters, Environmental Science Division and Climate Change Science Institute, Oak Ridge National Laboratory, Oak Ridge, TN, Jeffrey M. Warren, Environmental Sciences Division and Climate Change Science Institute, Oak Ridge National Laboratory, Oak Ridge, TN and Paul J. Hanson, Climate Change Science Institute and Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN

Background/Question/Methods
As the atmospheric CO2 concentration increases, the resulting changes in climate are on track to increase global temperature by > 2 °C by 2100. Temperature increases are expected to be substantially greater in the boreal and arctic biomes, which could have significant impacts on net ecosystem carbon uptake and atmospheric feedbacks. At the southern edge of the boreal forest large, open-topped enclosures are exposing a natural peatland to whole-ecosystem warming × CO2 enrichment (https://mnspruce.ornl.gov/). After only a few years of warming (+0, +2.25,+4.5, +6.75, +9 °C) and CO2 (+0, +500ppm) treatments, the plant community in this bog ecosystem is showing signs of stress with crown damage, branch tip dieback and even mortality in the conifer species: the deciduous Larix laricina and evergreen Picea mariana. We examined the mechanisms of this damage, hypothesizing that, even in this ecosystem characterized by saturated soils, evaporative demand created by elevated temperatures could exert critical tension on the hydraulic system, leading to dysfunction in the xylem.

Results/Conclusions
We found increased vapor pressure deficient in warmed enclosures, which translated into greater plant water stress. Interestingly, diurnal water potential patterns revealed contrasting hydraulic strategies in the two tree species. Across the treatments P. mariana water potentials significantly declined by 0.046MPa per degree warming at predawn and declined more dramatically by 0.098MPa per degree at midday. L. laricina exhibited the opposite pattern, with similarly low midday water potentials across treatments, but a decline of 0.10MPa per degree warming at predawn with predawn measurements in the warmest treatments approaching midday values in control treatments, -1.54MPa and -1.84MPa respectively.
Despite the divergence in water stress patterns, both species experienced a significant increase in hydraulic dysfunction, measured as native embolism, with air temperature, from 9.0% and 8.1% loss of hydraulic conductivity in control enclosures to 18.7% and 21.6% in +9°C enclosures for P. mariana and L. laricina respectively. Together out results show that elevated evaporative demand can strain plants hydraulic system even in regions not at risk of soil water limitation.