Moderate Forest Disturbance as a Stringent Test for Gap and Big-Leaf Models
Disturbance-induced tree mortality is a key factor regulating the carbon balance of a forest, but tree mortality and its subsequent effects are poorly represented processes in terrestrial ecosystem models. Thus unclear is whether models can robustly simulate moderate (noncatastrophic) disturbances, which tend to increase biological and structural complexity and are increasingly common in aging U.S. forests. Researchers recently tested whether three forest ecosystem models—Biome-BGC (BioGeochemical Cycles), a classic big-leaf model, and the ZELIG and ED (Ecosystem Demography) gap-oriented models—could reproduce the resilience to moderate disturbance observed in an experimentally manipulated forest (Forest Accelerated Succession Experiment in northern Michigan, where 38% of canopy dominants were stem girdled and compared to control plots). Each model was parameterized, spun up, and disturbed following similar protocols and run for 5 years post-disturbance. The models replicated observed declines in aboveground biomass well. Biome-BGC captured the timing and rebound of observed leaf area index (LAI), while ZELIG and ED correctly estimated the magnitude of LAI decline. None of the models fully captured the observed post-disturbance carbon fluxes, in particular gross primary production or net primary production (NPP). Biome-BGC NPP was correctly resilient but for the wrong reasons, and could not match the absolute observational values. ZELIG and ED, in contrast, exhibited large, unobserved drops in NPP and net ecosystem production. The biological mechanisms proposed to explain the observed rapid resilience of the carbon cycle typically are not incorporated by these or other models. Thus, an open question is whether most ecosystem models will simulate correctly the gradual and less extensive tree mortality characteristic of moderate disturbances.
Bond-Lamberty, B., J. P. Fisk, J. A. Holm, V. Bailey, G. Bohrer, and C. M. Gough. 2015. “Moderate Forest Disturbance as a Stringent Test for Gap and Big-Leaf Models,” Biogeosciences 12(2), 513–26. DOI: 10.5194/bg-12-513-2015.