Sap flow and water relations in an ombrotrophic Picea mariana – Sphagnum bog
The Spruce and Peatland Responses under Climatic and Environmental Change (SPRUCE) climate change experiment (http://mnspruce.ornl.gov/) in Northern Minnesota, USA, will expose 13 m diameter plots of an ombrotrophic Picea mariana – Ericaceous shrub – Sphagnum bog ecosystem to long-term temperature × CO2 treatments. Whole-ecosystem treatments scheduled to begin in 2015 are expected to change soil water availability, vapor pressure deficit, evapotranspiration (ET) and relative species composition. Here, we examine plant water relations of woody species at the site to assess pretreatment physiological and environmental controls on patterns of ET. Plant water use as sap flow was investigated in ca. 15-40 year-old black spruce and larch trees using commercially available Granier-style thermal dissipation probes (TDP). To improve the accuracy of sap flow estimates for use in scaling across the site, species-specific calibrations were conducted in situ by cutting instrumented trees and measuring their actual water uptake. Spruce hydraulic conductivity under drying conditions was measured in excised roots, branches and foliage using vulnerability curves and pressure-volume curves. Shrub and sphagnum evapotranspiration was measured monthly during the growing season using a 1-m diameter chamber. Predawn and diel patterns of plant water potential (ψ) were also measured seasonally.
In the trees, sap flow began by late May and was fairly constant over the season until declining in mid-September and ceasing as temperatures dropped below zero. Midday mean summer ψ was -1.0 MPa (shrubs), -1.5 MPa (spruce) and -1.9 MPa (larch). Laboratory measurements indicated specific leaf conductivity of spruce declined as drought stress increased beyond -1.2 MPa, with the average turgor loss point (TLP) reached by -2.5 MPa, slightly lower than the turgor loss point of larch (-1.95 MPa). While summer spruce ψ remained higher that the TLP, larch often reached the TLP indicating substantial loss of hydraulic conductivity on a daily basis. Warming and CO2 treatments will likely force a change of community composition within this ecosystem due to treatment effects on water availability, and resultant differential water stress among the species. We anticipate being able to capture this response through targeted physiological measurements and more integrated approaches such as chamber-based estimates of system transpiration, changes in soil water content, and through analysis of hydrologic budgets. Insights generated through these studies will be used to improve our understanding of peatland response and resiliency to a changing climate and models of those systems at local to global scales.