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

Abstract

Abstract. 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. It is thus unclear whether models can robustly simulate moderate (non-catastrophic) disturbances, which tend to increase biological and structural complexity and are increasingly common in aging US forests. We 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 (the Forest Accelerated Succession Experiment in northern Michigan, USA, in which 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 C 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 C cycle are typically not incorporated by these or other models. It is thus an open question whether most ecosystem models will simulate correctly the gradual and less extensive tree mortality characteristic of moderate disturbances.

Net primary production of a temperate deciduous forest exhibits a threshold response to increasing disturbance severity

“…A nonlinearly relationship between production and disturbance can be inferred from separate studies showing that forest growth changes very little or not at all following a single level of low‐intensity disturbance (Granier et al 2008, , , ) and, conversely, declines significantly at high disturbance severities (Campbell et al 2004, ). A high disturbance threshold, however, is not supported by terrestrial ecosystem models that simulate declines in production (Bond‐Lamberty et al 2014), and aquatic studies showing high functional sensitivity to the loss of a subset of primary producers (Cardinale et al 2011). A nonlinear response to increasing disturbance severity by some ecosystems has important implications, indicating that some ecosystems undergo substantial structural shifts without a corresponding functional change.…”

Redefining temperate forest responses to climate and disturbance in the eastern United States: New insights at the mesoscale

“…We believe that even greater insight is possible from combining multi‐century, site‐based studies across broad geographical regions with modelling to span spatial and temporal scales further. Relatively untapped sources include historical documentary data (Mock, Mojzisek, McWaters, Chenoweth, & Stahle, ), historical structures (de Graauw, ; Trouet, Domínguez‐Delmás, Pearson, Pederson, & Rubino, ), archaeological data (Scharf, ; Trouet et al, ), fine‐scale pollen analysis (Fuller, Foster, McLachlan, & Drake, ) and the continued development of tree‐ring networks (Babst, Poulter, Bodesheim, Mahecha, & Frank, ; Pederson, Young, Stan, Ariya, & Martin‐Benito, ) along with simulation modelling (Bond‐Lamberty et al, ).…”

Preface: Impacts of extreme climate events and disturbances on carbon dynamics

“…Disturbance-induced tree mortality regulates the forest carbon balance, but tree mortality and its carbon consequences are not well represented in ecosystem models (Bond-Lamberty et al, 2015). Bond-Lamberty et al (2015) tested whether three ecosystem models -the classic bigleaf model Biome-BGC (BioGeochemical Cycles) and the gap-oriented models ZELIG, a gap model, and ED (ecosystem demography) -could reproduce the resilience of forest ecosystems to moderate disturbances.…”

“…Bond-Lamberty et al (2015) tested whether three ecosystem models -the classic bigleaf model Biome-BGC (BioGeochemical Cycles) and the gap-oriented models ZELIG, a gap model, and ED (ecosystem demography) -could reproduce the resilience of forest ecosystems to moderate disturbances. The models replicated observed declines in aboveground biomass well but could not fully capture observed post-disturbance carbon fluxes.…”