Mechanisms sustaining high rates of carbon storage in mature forests of northern Michigan.

Abstract

As atmospheric carbon dioxide levels rise, it becomes increasingly important to understand the role of forests as carbon sinks. Disturbance events that alter the structure of forest ecosystems play a large role in determining the strength of a forest as either a source or a sink for carbon dioxide (Pan et al. 2011). Moderate disturbances that only partially defoliate the canopy have very different effects on forest carbon storage than severe stand-replacing disturbances (Nave et al. 2011). For example, the mixed deciduous forests of northern Michigan are currently undergoing moderate disturbance due to a decline in bigtooth aspen (Populus grandidentata) and white birch (Betula papyrifera) stands (Gough et al. 2010). Following clear-cut logging and wildfires in the early 20th century, aspen and birch became the dominant species. However, due to their relatively short life span, these species are now senescing. The Forest Accelerated Succession Experiment (FASET) at the University of Michigan Biological Station simulated this age-related senescence through the girdling of >6,700 aspen and birch in 2008. Prior research in FASET found that despite a 44% decrease in leaf area by 2010, net ecosystem production, a primary component of total ecosystem C storage, has been resistant (Gough et al., in preparation). Though carbon storage resistance to moderate disturbance has been documented, the ecological mechanisms supporting this resistance are largely unknown. Though NPP generally increases with a decrease in tree density, dominant woody plants are usually better at utilizing resources than subdominant plants. This indicates that there is a threshold of subdominant canopy contribution to dominant canopy removal that sustains NPP during disturbance (Sabo et al. 2008). Gaining a better understanding of these community and ecosystem processes that support carbon storage resistance to moderate disturbance may help improve ecosystem models, and guide forest management practices. Building on pre-disturbance (2006) and peak disturbance (2010) data collected from the FASET site, the objective of this study was to link canopy disturbance severity with changes in community composition and ecosystem functions, such as carbon and nitrogen cycling. Over a gradient of moderate disturbance severities, I determined the relative contributions of dominant and subdominant species to wood net primary production (NPP) resistance to partial canopy defoliation. Additionally, I examined how resource reallocation of nitrogen mediates the relative contributions of subdominant and subdominant species to wood NPP. I hypothesized that in areas of greater disturbance severity, the ratio of dominant to subdominant NPP contributions would be lower. This is because larger canopy gaps may allow for greater subdominant response as light and nitrogen are more readily reallocated to the understory.