Defining plant functional types for xeric habitats with disturbance regimes

Background/Question/Methods

The development of realistic projections for the terrestrial carbon budget will require that terrestrial biosphere models faithfully capture ecosystem responses to environmental change. Important environmental changes may include changes in the physical climate as well as disturbances such as insect outbreaks and fires. Here, the impact of insect disturbance at a xeric site with the Ecosystem Demography model 2, originally calibrated for mesic sites, is investigated. Eddy-covariance measurements of carbon dioxide fluxes and forest inventory measurements of tree growth and mortality are used to calibrate two new plant functional types to adequately represent this xeric site.  The model’s ability to simulate an extensive gypsy moth defoliation event that occurred in 2007 is also evaluated.   

Results/Conclusions

It is found that although the original model performs well for a range of mesic sites, it predicts excessive mortality and insufficient summertime carbon uptake at the xeric site.  With the new plant functional tree types, the model is able to realistically simulate the dramatic changes in ecosystem structure and functioning that occur following the 2007 gypsy moth disturbance, including the mortality of ~1/3 of the basal area.  We conclude that the dynamics of oak-pine forests can be highly sensitive to species composition and to disturbances.   Neglect of these heterogeneities by highly-aggregated terrestrial biosphere models will result in biased predictions of future ecosystem structure and functioning.