Moderate forest disturbance as a stringent test for gap and big-leaf models

[摘要] Disturbance-induced tree mortality is a key factor regulating the carbonbalance of a forest, but tree mortality and its subsequent effects arepoorly represented processes in terrestrial ecosystem models. It is thusunclear whether models can robustly simulate moderate (non-catastrophic)disturbances, which tend to increase biological and structural complexityand are increasingly common in aging US forests. We tested whether threeforest ecosystem models – Biome-BGC (BioGeochemical Cycles), a classic big-leaf model, and theZELIG and ED (Ecosystem Demography) gap-oriented models – could reproduce the resilience to moderatedisturbance observed in an experimentally manipulated forest (the ForestAccelerated Succession Experiment in northern Michigan, USA, in which 38%of canopy dominants were stem girdled and compared to control plots). Eachmodel was parameterized, spun up, and disturbed following similar protocolsand run for 5 years post-disturbance. The models replicated observeddeclines in aboveground biomass well. Biome-BGC captured the timing andrebound of observed leaf area index (LAI), while ZELIG and ED correctlyestimated the magnitude of LAI decline. None of the models fully capturedthe observed post-disturbance C fluxes, in particular gross primaryproduction or net primary production (NPP). Biome-BGC NPP was correctlyresilient but for the wrong reasons, and could not match the absoluteobservational values. ZELIG and ED, in contrast, exhibited large, unobserveddrops in NPP and net ecosystem production. The biological mechanismsproposed to explain the observed rapid resilience of the C cycle aretypically not incorporated by these or other models. It is thus an openquestion whether most ecosystem models will simulate correctly the gradualand less extensive tree mortality characteristic of moderate disturbances.

[学科分类] 地球化学与岩石