Divergent species‐specific impacts of whole ecosystem warming and elevated CO2 on vegetation water relations in an ombrotrophic peatland

• Published in: Global change biology 2021-05, Vol.27 (9), p.1820-1835
• Main Authors: Warren, Jeffrey M., Jensen, Anna M., Ward, Eric J., Guha, Anirban, Childs, Joanne, Wullschleger, Stan D., Hanson, Paul J.
• Format: Article
• Language: English
• Subjects: Availability; BASIC BIOLOGICAL SCIENCES; black spruce; Bogs; boreal forest; Boreal forests; Carbon dioxide; Chamaedaphne calyculata; Climate change; Divergence; Dominant species; Drought; Ecological function; Ecosystems; Environmental impact; Extreme drought; High temperature; hydraulic stress; Hydraulics; Larix laricina; Moisture content; Peatlands; Plants; Pretreatment; sap flow; Soil water; Species diversity; Surface water; Surface water availability; Temperature; Turgor; Uptake; Vapor pressure; Vapour pressure; Water availability; Water demand; Water potential; Water relations
• ISSN: 1354-1013; 1365-2486
• DOI: 10.1111/gcb.15543

Boreal peatland forests have relatively low species diversity and thus impacts of climate change on one or more dominant species could shift ecosystem function. Despite abundant soil water availability, shallowly rooted vascular plants within peatlands may not be able to meet foliar demand for water under drought or heat events that increase vapor pressure deficits while reducing near surface water availability, although concurrent increases in atmospheric CO2 could buffer resultant hydraulic stress. We assessed plant water relations of co‐occurring shrub (primarily Rhododendron groenlandicum and Chamaedaphne calyculata) and tree (Picea mariana and Larix laricina) species prior to, and in response to whole ecosystem warming (0 to +9°C) and elevated CO2 using 12.8‐m diameter open‐top enclosures installed within an ombrotrophic bog. Water relations (water potential [Ψ], turgor loss point, foliar and root hydraulic conductivity) were assessed prior to treatment initiation, then Ψ and peak sap flow (trees only) assessed after 1 or 2 years of treatments. Under the higher temperature treatments, L. laricina Ψ exceeded its turgor loss point, increased its peak sap flow, and was not able to recover Ψ overnight. In contrast, P. mariana operated below its turgor loss point and maintained constant Ψ and sap flow across warming treatments. Similarly, C. calyculata Ψ stress increased with temperature while R. groenlandicum Ψ remained at pretreatment levels. The more anisohydric behavior of L. laricina and C. calyculata may provide greater net C uptake with warming, while the more conservative P. mariana and R. groenlandicum maintained greater hydraulic safety. These latter species also responded to elevated CO2 by reduced Ψ stress, which may also help limit hydraulic failure during periods of extreme drought or heat in the future. Along with Sphagnum moss, the species‐specific responses of peatland vascular communities to drier or hotter conditions will shape boreal peatland composition and function in the future. Plant water relations of co‐occurring shrub and tree species growing in a boreal peatland were assessed after exposure to whole ecosystem warming (0 to +9°C) and elevated CO2. Under higher temperatures, larch water use increased (as sap flux density [Fd]), and leaf water stress was apparent in larch trees and leatherleaf shrubs, but not in black spruce trees or Labrador tea shrubs. With the addition of elevated CO2, spruce and Labrador tea showed further redu