Disturbance severity and net primary production resilience of a Great Lakes forest ecosystem
Abstract
As many Eastern deciduous forests of North America transition from early to mid-succession, the future of regional terrestrial carbon (C) storage is uncertain. The gradual, patchy senescence of early-successional trees accompanying this transition is comparable in severity to moderate disturbances such as silvicultural thinnings or insect outbreaks. While stand-replacing disturbance causes forests to temporarily become C sources, more moderate disturbances may inflict little to no decline in C sequestration. Identifying the disturbance severity at which net primary production (NPP) declines and the underlying mechanisms that drive forest C storage resistance to disturbance is increasingly important as moderate disturbances increase in frequency and extent across the region. The Forest Accelerated Succession ExperimenT (FASET) at the University of Michigan Biological Station subjected 39 ha of forest to moderate disturbance in 2008 by advancing age-related tree mortality through the stem girdling of early successional aspen and birch. Stand-scale disturbance severity, expressed as relative basal area of girdled aspen and birch, was 39% but plot-scale severity varied substantially within the experimental area (9 to 66% in 0.1 ha plots) because of the heterogeneous distribution of aspen and birch. We used this disturbance severity gradient to examine: 1) the relationship between NPP resilience and disturbance severity; 2) the disturbance severity at which NPP resilience prompts a shift in dominance from canopy to subcanopy vegetation; 3) how NPP resilience relates to disturbance-driven changes in resource-use efficiency, and 4) how disturbance severity shapes emerging forest communities We found that NPP is highly resilient to low to moderate levels of disturbance, but that production declines once a higher disturbance threshold is exceeded. Several complementary mechanisms, including canopy structural reorganization and the reallocation of growth-limiting light and nitrogen resources, appear to maintain NPP up to the disturbance severity threshold. Our results suggest that both canopy and subcanopy trees reacted rapidly to compensate for canopy tree mortality, but at higher disturbance severities, subcanopy trees provided an important buffer in support of NPP resilience. Our data also suggests a larger increase in the growth rate of red maples (Acer rubrum) following disturbance than subcanopy red oak (Quercus rubra) and white pine (Pinus strobus), as well as a greater contribution to overall plot-level production in more severely disturbed plots. These findings demonstrate that some forests can tolerate substantial disturbance without a reduction in NPP, suggesting that the relationship between disturbance severity and declining production may be non-linear. This result has important implications for the region's C cycle, suggesting that moderate disturbances may not cause a decline in forest C sequestration but may actually stimulate new growth to maintain NPP.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.B13D0542G
- Keywords:
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- 0439 BIOGEOSCIENCES Ecosystems;
- structure and dynamics;
- 0428 BIOGEOSCIENCES Carbon cycling