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Harvard Forest Symposium Abstract 2005

  • Title: Modeling HWA impact at the regional scale with the Ecosystem Demography model
  • Primary Author: Marco Albani (Harvard University)
  • Additional Authors: Aaron Ellison (Harvard University); David Medvigy (Harvard University); Paul Moorcroft (Harvard University); David Orwig (Harvard Forest)
  • Abstract:

    The Ecosystem Demography (ED) model is a mechanistic individual-based terrestrial ecosystem model that is capable of explicitly incorporating ecosystem disturbance and recovery in regional and continental scale simulations. The ED model is a momentum approximation of a traditional `gap model', a stochastic individual model of forest dynamics. As such, ED models both the age and the size structures of the terrestrial ecosystems it simulates. The age structure directly accounts for the distribution of ages since last disturbance in each simulation grid cell, and captures the disturbance impact on the ecosystem's carbon stocks. The size structure is used to model the impact of different kinds of disturbance on ecosystem dynamics and the recovery processes following disturbance.
    We present a set of ED simulations of the carbon dynamics of New England for the period 1700-2100 that account for both natural and anthropogenic ecosystem disturbance and the impact of the Hemlock Wooly Adelgid (HWA), an introduced pathogen of eastern hemlock. While all these disturbances involve tree mortality, their simulated impact on the carbon dynamics is substantially different depending on the timing, frequency, and selectivity of the disturbance.
    We modeled the spread of HWA at the regional scale as a stochastic process where the probability of infestation in any given year is a logistic function of the distance from the closest infested area (Fig 1). The model is fit on the historical expansion of the HWA as recorded by the USDA Forest Service at the county level, and for ED simulation purposes we built a mean spread scenario as the average of 100 runs of the stochastic model from the current conditions (Fig 2).
    We superimposed the impact of HWA over the regional dynamics of forest harvesting and recovery from past land-use, simulated for ~100 soil polygons covering New England (Fig 3), using county-level reconstruction of land-use dynamics and regional reconstructions and predictions of forest harvesting rates. The model predicts a substantial acceleration in the regional decline of the apparent air-to ground flux of carbon. In our simulation the impact of HWA culminates in 2025, when the flux is 6.5 Tg C per year lower and 28% smaller than it would have been in the absence of the infestation (Fig 4).

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