Peatland Woody Vegetation Growth Response to Warming and Implications for Ecosystem Carbon Gain or Loss

The Spruce and Peatland Responses Under Changing Environments (SPRUCE) experiment is being conducted to identify response functions for organisms and ecosystems to rapidly changing environments. The in situ whole ecosystem warming facilities combine air warming with active deep peat heating in a system designed for a decade of continuous operation (2016 through 2025) at the following target warming treatments: +0, +2.25, +4.5, +6.75 and +9 °C. All designated permanent plots were sampled annually for shrub-layer production in the middle of August. In each plot, two 0.25 m² clipped plots were sampled. All vegetation above the peatland moss layer were clipped, collected, and sorted by species and tissues were separated into current-year vs. older tissues. The cumulative mass of current-year growth for leaves and stem tissues were interpreted as the shrub-layer net primary production (NPP) for the designated plot and expressed in C units. Tree (Picea and Larix) growth measures were initiated in 2011 have been repeated on an annual basis in winter and include measurements of circumference at 1.3 m (diameter at breast height assessments; DBH) above the nominal bog hollow surface. Circumference measurements to the nearest mm are converted to DBH and basal area at DBH (cm²). Allometric data are used to convert the dimensional data for trees to mass in C units. Ecosystem scale implications for growth changes in the context of the C cycle are calculated from a combined analysis with CO₂ and CH₄ efflux data.

In the first two years of warming (2016 and 2017), the shrub layer was dominated by two ericaceous shrubs and the sedge/forb community. The combined shrub layer showed significant increases in NPP with warming, however, the response varied by species with some increasing and some declining with warming. Tree growth exhibited a consistent reduction in annual growth rates with warming treatments that are primarily driven by changes in P. mariana rather than L. laricina trees. When these growth data are combined with assessments of C loss from peatland heterotrophic organisms, a consistent negative relationship with temperature exists, suggesting that warming is leading to a loss of C uptake potential driven by reduced NPP and some enhancement of C losses from CO₂ and CH₄ emissions.