Forest Carbon Cycling Across Gradients of Disturbance Severity: Patterns and Underlying Mechanisms

Christopher Michael Gough, Virginia Commonwealth University, Biology, Richmond, VA, United States, Gil Bohrer, The Ohio State University, Department of Civil, Environmental and Geodetic Engineering, Columbus, United States, Lucas E Nave, University of Michigan, Ann Arbor, MI, United States, Knute John Nadelhoffer, Univ of Mich- Eco & Evol Bio, Ann Arbor, MI, United States, Christoph S Vogel, University of Michigan Biological Station, Ann Arbor, United States, Ben P Bond-Lamberty, Pacific Northwest National Laboratory, Richland, United States, Ellen JoAnne Stuart-Haëntjens, U.S. Geological Survey, California Water Science Center, Sacramento, CA, United States and Peter Curtis, Ohio State University Main Campus, Columbus, OH, United States
Abstract:
Ecological disturbances alter biogeochemical processes central to forest carbon (C) storage. Disturbances to forests occur along a continuum of severity, from low intensity disturbance causing the mortality of only a subset of trees to severe stand-replacing disturbance that kills all trees; yet, considerable uncertainty exists in how and why the forest C cycle changes across gradients of disturbance intensity, and whether ecosystem models robustly simulate these responses.

At the University of Michigan Biological Station, we are using multiple ecosystem-scale experiments to examine how disturbance intensity affects C cycling and to identify the underlying mechanisms that support recovery of the C cycle. The Forest Accelerated Succession Experiment (FASET), in which a third of all canopy trees were stem girdled within a 39 ha area, employs C cycling measurements within paired treatment and control meteorological flux tower footprints. A separate study examines forest C cycling following stand-replacing clear-cut harvest and fire.

We found that net ecosystem production (NEP) was highly resilient following moderate disturbance, but experienced long-term reductions following stand-replacing disturbance. Using a gradient of disturbance severity within the FASET treatment, we found that forest production declined non-linearly with rising disturbance intensity, remaining stable until a threshold of ~60 % tree mortality was exceeded. NEP was sustained following moderate disturbance because of improved canopy light-use efficiency, which compensated for a temporary reduction in leaf area index. Contrastingly, NEP was reduced for several decades at the highest levels of disturbance severity. A model assessment revealed that neither big-leaf nor gap models captured the observed high resilience in NEP following low intensity disturbance, suggesting inadequate representation of the mechanisms supporting C cycling resilience.

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