The SPRUCE climate change experiment (http://mnspruce.ornl.gov/) has been exposing a southern boreal forested bog to whole ecosystem warming and elevated CO2 (eCO2) since 2016. This long-term project was designed to test mechanistic responses of a vulnerable ecosystem to changing environment conditions in order to inform and improve prognostic terrestrial biosphere models. Using large open-topped chambers the ecosystem was subjected to increases in temperature (Tair) up to +9°C that has accelerated spring phenology in the warmest plots by up to 6 weeks. Together with T-driven increases in nutrient availability and vapor pressure deficit, we hypothesized that the treatments would induce changes in morphological structure and anatomical function of dominant woody plants. In June and August 2017 we collected 2-year-old terminal shoots that fully developed under treatments from the primary woody species: black spruce (Picea mariana), leatherleaf (Chamaedaphne calyculata), tamarack (Larix laricina), and bog Labrador tea (Rhododendron groenlandicum). Plant traits measured included stem length, diameter, leaf mass per area (LMA), leaf size, projected branch silhouette to leaf area ratio (SPAR), C:N, apparent chlorophyll content (SPAD) and xylem hydraulic anatomy (microscopy).
Results/Conclusions
In June, LMA and chlorophyll content in C. calyculata increased with temperature, but LMA decreased for L. laricina and P. mariana. Changes were largely due to T-driven shifts in timing of budbreak, so samples from different plots were at different phenological stages. By August, LMA declined with increasing T in L. laricina exposed to eCO2 but increased for aCO2. In contrast, LMA of current and prior year cohorts declined with T in P. mariana under both CO2 treatments (R2=0.63). In both June and August samples, whole shoot SPAR values declined with T for L. laricina by up to 14%, but SPAR increased with T for P. mariana by 8% under +9°C. In contrast, the R. groenlandicum shrub SPAR values exhibited a threshold response, declining with T up to +6.75°C, but then increasing dramatically for +9°C treatments, regardless of CO2 treatment. We expect there are species-specific limits to foliar plasticity to T that may result in ecological advantages those species with broader acclimation capacity. The divergent responses of the trees may reflect their different anisohydric (Larix) or isohydric (Picea) strategies. Analysis, including microscopy stem hydraulic traits is ongoing and additional results will be presented at the meeting. Results will be used to improve mechanistic understanding of plant responses to future environmental conditions, including net primary productivity, water use and biogeochemical cycling.