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

Abstract

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.

Warming induces divergent stomatal dynamics in co‐occurring boreal trees

“…Taken together, these data show that tamarack exhibited more anisohydric behavior, with weak stomatal regulation in response to warming, increasing VPD and changes in the water table depth generating more negative water potentials, while black spruce showed more isohydric behavior (Sullivan et al, 2017;Warren et al, 2021). The branch-level data in Warren et al (2021) are also consistent with sap-flux results from SPRUCE, which show greater stimulations of tree-level water use with warming in tamarack than in spruce (Figure 5 in Warren et al, 2021), which imply that the leaf level responses in stomatal conductance are consistent with tree level water use patterns. While our findings contrast with earlier sap-flux based attributions of the isohydricity of these two species (Pappas et al, 2018), our data imply that black spruce regulates its stomata to prioritize hydraulic safety under warmer and high VPD air conditions over the ability to fix extra CO 2 .…”

“…In 2016, these differences in stomatal responses were also reflected in differences in branch water potential, where spruce maintained a relatively constant branch water potential throughout the growing season, while branch water potentials declined in tamarack in August when the water table depth increased (Table S3 in Warren et al, 2021). Taken together, these data show that tamarack exhibited more anisohydric behavior, with weak stomatal regulation in response to warming, increasing VPD and changes in the water table depth generating more negative water potentials, while black spruce showed more isohydric behavior (Sullivan et al, 2017;Warren et al, 2021).…”

“…In 2016, these differences in stomatal responses were also reflected in differences in branch water potential, where spruce maintained a relatively constant branch water potential throughout the growing season, while branch water potentials declined in tamarack in August when the water table depth increased (Table S3 in Warren et al, 2021). Taken together, these data show that tamarack exhibited more anisohydric behavior, with weak stomatal regulation in response to warming, increasing VPD and changes in the water table depth generating more negative water potentials, while black spruce showed more isohydric behavior (Sullivan et al, 2017;Warren et al, 2021). The branch-level data in Warren et al (2021) are also consistent with sap-flux results from SPRUCE, which show greater stimulations of tree-level water use with warming in tamarack than in spruce (Figure 5 in Warren et al, 2021), which imply that the leaf level responses in stomatal conductance are consistent with tree level water use patterns.…”

“…Tamarack also showed greater thermal acclimation of respiration than did spruce, which, taken with the photosynthetic results, implies that tamarack will be better able to maintain favorable carbon balance and growth under warmer climates, provided soil water is available. However, the higher stomatal conductance (and accompanying lower branch water potentials and greater sap flux rates reported in Warren et al, 2021) may mean that tamarack will be more sensitive to drought than spruce in coming decades. These data suggest that species-specific responses to future climate change may dictate how forest carbon and water fluxes change over the next few decades in boreal forests.…”

To what extent can rising [CO2] ameliorate plant drought stress?

“…Experimental data on eCO 2 impacts during more severe, long‐lasting droughts, or droughts combined with heatwaves – conditions that may generate mortality in the field – remain rare (but see Warren et al ., 2011b, 2021). Most eCO 2 field studies have addressed the potential for system‐wide water savings under mild or moderate drought conditions.…”