Peatland Plant Community Changes in Annual Production and Composition Through 8 Years of Warming Manipulations Under Ambient and Elevated CO₂ Atmospheres

Abstract The Spruce and Peatland Responses Under Changing Environments (SPRUCE) experiment has operated five whole‐ecosystem warming manipulations (+0, +2.25, +4.5, +6.75, and +9°C) with paired ambient and elevated CO 2 atmospheres (eCO 2 , +500 ppm) for 8 full calendar years (since August 2015). We tracked shrub‐layer vegetation responses to the treatments using annual destructive plot sampling. Tree ( Picea and Larix ) responses were assessed annually using nondestructive dimensional analyses and allometric conversions. Shrub community changes were assessed for key ericaceous shrubs ( Rhododendron, Chamaedaphne, and Kalmia ), two Vaccinium species ( V. angustifolium, V. oxycoccos ), graminoid species (mostly Eriophorum ), and one common forb ( Maianthemum trifolium ), plus minor understory species. We tracked annual aboveground net primary production (ANPP) for vascular plant species in gC m −2 y −1 and overall stand contribution in dry mass. We observed a linear increase in shrub‐layer aboveground biomass accumulation with warming over time due primarily to an increase in ericaceous shrub abundance. Cumulative biomass increases across the shrub community showed overall positive responses to eCO 2 after 8 years. Community composition also changed with warming, with increases in woody shrub density, and the reduction or loss of forbs. The tree community showed minimal initial responses to warming early in the treatments, but since 2020, has shown significant increases in ANPP and individual tree growth with warming. The main driver of change in the vascular plant community was temperature, with less pronounced effects of eCO 2 evident. These results indicate an overall increase in ANPP with warming from both the tree and shrub layers of peatland vegetation.