Net Primary Production of Temperate Deciduous Forest Exhibits Threshold Response to Increasing Disturbance Severity
The global carbon balance is vulnerable to disturbances that alter terrestrial carbon storage. Disturbances to forests occur along a continuum of severity, from low-intensity disturbance causing the mortality or defoliation of only a subset of trees to severe stand-replacing disturbance that kills all trees; yet, considerable uncertainty remains in how forest production changes across gradients of disturbance intensity. In a recent study, researchers used a gradient of tree mortality in an upper Great Lakes forest ecosystem to: (1) quantify how aboveground wood net primary production (ANPPw) responds to a range of disturbance severities and 2) identify mechanisms supporting ANPPw resistance or resilience following moderate disturbance. They found that ANPPw declined nonlinearly with rising disturbance severity, remaining stable until > 60 % of the total tree basal area senesced. As upper canopy openness increased from disturbance, greater light availability to the subcanopy enhanced the leaf-level photosynthesis and growth of this formerly light-limited canopy stratum, compensating for upper canopy production losses and a reduction in total leaf area index (LAI). As a result, whole-ecosystem production efficiency (ANPPw/LAI) increased with rising disturbance severity, except in plots beyond the disturbance threshold. These findings provide a mechanistic explanation for a nonlinear relationship between ANPPw and disturbance severity, in which the physiological and growth enhancement of undisturbed vegetation is proportional to the level of disturbance until a threshold is exceeded. These results have important ecological and management implications, demonstrating that in some ecosystems moderate disturbance levels minimally alter forest production.
Stuart-Haëntjens, E., P. S. Curtis, R. T. Fahey, C. S. Vogel, and C. M. Gough. 2015. “Net Primary Production of a Temperate Deciduous Forest Exhibits a Threshold Response to Increasing Disturbance Severity,” Ecology 96, 2478–87. DOI: 10.1890/14-1810.1