Modeling spatial, temporal, and taxonomic heterogeneity: Landscape-scale estimates of forest C dynamics

Background/Question/Methods

Landscape-scale carbon dynamics are a consequence of NPP, heterotrophic respiration, and disturbances.  Each is sensitive to changing community composition and each is variable across many time scales:  annual, decadal, and longer.  In addition, spatial heterogeneity and spatial interactions across landscapes add further complexity.  Natural disturbances – notably wildfire and insect outbreaks – are highly spatial processes that may strongly affect carbon balance and may be amplified by future climate changes.  We estimated carbon balances, including forest soil carbon, as a consequence of wildfires, insect outbreaks, and climate change in the New Jersey pine barrens (NJPB) over the next 100 years.  The NJPB are edaphically complex with diverse tree species and many potential successional trajectories.  We modeled carbon balance using the LANDIS-II succession and disturbance model combined with the CENTURY soil model. The model was calibrated and validated using data from three eddy flux towers.  We also estimated how N limitations influence forest responses to climate change and disturbance. 

Results/Conclusions

Our results suggest that climate change will not appreciably increase fire sizes and intensity.  Insect outbreaks, however, may become a major determinant both of forest C and N cycling.  The recovery of C stocks following substantial disturbances at the turn of the 20^th century will continue to play a significant role in this system.  Our projections further indicate that climate change will increase growth, live biomass and decomposition over the next 100 years in upland and wetland forests but soil sequestration stabilizes.  In the driest parts of the NJPB, low soil moisture leads to losses of C and these forests switch from carbon sinks to carbon sources towards the latter part of the 21^st century.  In all areas, the regeneration of many key tree species declines with climate change.  Gypsy moth (and other forest pests) may exacerbate climate change effects.  In summary, the system is initially C conservative but by the end of the 21^st century, there is increasing risk of de-stabilization due to disturbances and declining regeneration.

In conclusion, regional or landscape-scale models of carbon balance – including respiration –must incorporate spatial, temporal, and taxonomic (changing species and life forms) heterogeneity.  Disturbances are key to understanding carbon balances at all spatial and temporal scales.  Further research is required to reduce uncertainty of dynamics of soil organic matter and DOC leaching, in particular.