Impacts of environmental and atmospheric changes on carbon storage and exchange in upland deciduous forests: Current patterns and future possibilities

Background/Question/Methods

Observed net ecosystem carbon exchange responses of upland-oak vegetation of an eastern deciduous hardwood forest were derived for changing CO₂, temperature, precipitation and tropospheric ozone (O₃) from field studies.  The measured field responses were interpreted with a stand-level model (INTRASTAND) for an 11-year range of environmental conditions.  Analyses were extended to a number of plausible environmental futures to clarify the relative importance of various combined environmental drivers for upland forest ecosystem carbon exchange.  Scenarios for the next century included elevated [CO₂] and [O₃] (+385 ppm and +20 ppb, respectively), warming (+4 °C), and increased winter precipitation (+20% November through March). Simulations were run with and without the inclusion of acclimation processes or biomass changes to reveal the magnitude and importance of including lessons-learned from manipulative experiments in simulations.  Forest biomass and nitrogen dynamics over multiple decades were further evaluated with the succession model - LINKAGES v2.2. 

Results/Conclusions

Initial simplistic model runs for single-factor changes in CO₂ and temperature predicted substantial increases (+191% or 508 gC m^-2 y^-1) or decreases (-206% or -549 gC m^-2 y^-1), respectively, in mean annual net ecosystem carbon exchange (NEEa ≈ 266±23 gC m^-2 y^-1 from 1993 to 2003).  Conversely, single-factor changes in precipitation or O₃ had comparatively small effects on NEEa (0 and –35%, respectively).  The combined influence of all environmental changes yielded a 29% reduction in mean annual NEEa. The results suggest that future CO₂-induced enhancements of gross photosynthesis would be largely offset by temperature-induced increases in respiration, exacerbation of water deficits, and O₃-induced reductions in photosynthesis. When experimentally observed physiological adjustments were included in the simulations (e.g., acclimation of leaf respiration to warming), the combined influence of the year-2100 scenario resulted in a 20% increase in NEEa not a decrease. Simulations with a forest succession model run for gradually changing conditions from 2000 to 2100 indicated an 11% increase in stand wood biomass in future compared to current conditions. Satisfactory explanations for multi-year changes in soil C and N stocks of the target upland forests have not been captured by integrated simulations and represent a key area of emphasis in future experimental manipulations and process studies.