title: Net primary production of a temperate deciduous forest exhibits a threshold response to increasing disturbance severity.
abstract: The global carbon (C) balance is vulnerable to disturbances that alter terrestrial C 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. We used a gradient of tree mortality in an upper Great Lakes forest ecosystem to: (1) quantify how aboveground wood net primary production (ANPP,) responds to a range of disturbance severities; and (2) identify mechanisms supporting ANPPw resistance or resilience following moderate disturbance. We 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. Our results have important ecological and management implications, demonstrating that in some ecosystems moderate levels of disturbance minimally alter forest production.
authors.forename: Ellen J
authors.initials: EJ
authors.lastname: Stuart-Haëntjens
authors.forename: Peter S
authors.initials: PS
authors.lastname: Curtis
authors.forename: Robert T
authors.initials: RT
authors.lastname: Fahey
authors.forename: Christoph S
authors.initials: CS
authors.lastname: Vogel
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Gough
mesh_terms: D003247:Conservation of Natural Resources; D004784:Environmental Monitoring; D065928:Forests; D006114:Great Lakes Region; D006802:Human Activities; D006801:Humans; D008824:Michigan; D008954:Models, Biological; D009584:Nitrogen; D010944:Plants / Q000145:classification*; D014197:Trees / Q000502:physiology*
chemical_list: D009584:Nitrogen