Natural disturbances are major factors affecting the carbon balance of ecosystems because they disrupt carbon stocks and increase atmospheric emissions. Disturbances of forests in western North America are expected to increase in frequency, extent, intensity and variety. However, fundamental questions remain about the effects of disturbance on carbon sequestration. The goal of this project is to improve understanding of the consequences of multiple disturbances on carbon sequestration in forests. Measured tree regeneration and carbon contents, geographic information systems, and modeling will be used to examine the outcomes of multiple interacting disturbances for carbon sequestration. A forest growth model will be used to estimate biomass accumulation and evaluate carbon sequestration across the landscape and under a variety of management and disturbance scenarios.

Cascading large-scale disturbances such as drought, insect infestations, and fire will interact to shape future forest landscapes and consequently carbon balances. Forest management activities will also influence and be influenced by disturbances. This research builds on a database of soils and regeneration in northern Colorado that shows evidence of thresholds in forest recovery resulting from disturbance interactions. The proposed project will lend valuable insights into management options. A significant component of this work will be devoted to the support of two Doctoral students and the participation of several undergraduates. Every effort will be made to recruit graduate and undergraduate students from underrepresented minority groups through the Colorado Diversity Initiative in Science, Math, and Engineering. Research activities will be integrated into ecological education using case studies as experiential learning tools. This work will also be used in University of Colorado Landscape Ecology courses for students in interdisciplinary decision making curricula. Dialog with U.S. Forest Service offices in the Rocky Mountain Region will be maintained throughout this project, with a workshop conducted for resource managers in the final year of the project.

Project Report

Forest disturbances in the Western US are expected to increase in frequency, extent, intensity and variety with climate warming. Historically, natural disturbances have had a strong influence on regional carbon balance because they disrupt carbon storage and increase emissions of carbon to the atmosphere. With increasing frequency and extent of disturbances, opportunities for disturbances to overlap and interact will increase substantially, and in ways that are unprecedented and, very likely, unpredictable. These interactions will have significant impacts on short- and long-term carbon storage, but it is uncertain what the magnitude of those impacts will be. This project investigated the potential vulnerabilities of forest ecosystems and their carbon stocks to multiple disturbances and their interactions. Our research focused on a region of subalpine coniferous forest in the Colorado Rocky Mountains that has experienced several disturbances over a short period of time, far shorter than the recovery time of the forest. We investigated the recovery trajectories of forests disturbed by combinations of windthrow, logging, fire, and beetlekill, and how those trajectories will influence carbon stocks 100 years into the future. Current carbon stocks were surveyed in the field and future carbon stock recovery was simulated using a forest growth model developed by the USDA Forest Service. Single catastrophic disturbances reduced carbon stocks substantially, as was expected. However, each additional disturbance not only lowered carbon stocks further, but also impaired regeneration mechanisms such as seed availability enough to change the types of species that could establish in some areas (and thus influence carbon recovery). We had expected to see an increased amount of carbon in the form of charcoal (black carbon) in the multiple disturbances that included fire. It appears that subalpine fire regimes, which have a fire-return interval of ~200-400 years, maintain a relatively constant level of black carbon over time; in other words, the amount of black carbon is not significantly different between the green forest and the areas that burned. Our results suggest that primary black carbon loss mechanisms in these subalpine ecosystems are either re-combustion of black carbon during fires or erosion within the first 10 years after fire. We did find evidence that multiple high-intensity disturbances could result in a net loss over time. Based on our surveys of seedlings in disturbed areas, we used the model to grow forests over 100 years under different climate and management scenarios. Carbon stocks were initially more resilient than the coniferous forest; areas with little conifer regeneration recovered carbon at a similar pace due to the influx of deciduous seedlings. In the near term, aspen establishment more than compensated for any loss of coniferous species, to the point where total carbon stocks were similar between plots with zero conifer seedlings and those with ample conifer regeneration. The heterogeneity of recovery we saw across the landscape (domination of conifer seedlings in some areas, deciduous or grasses in others) persisted through the coming century. However, under a changing climate (using IPCC climate change scenarios), these landscapes transitioned to non-forests. In other words, 100% of the trees died with warming temperatures and failed regeneration could not sustain the forest. Complete mortality occurred even when forest recovery was assisted with a management strategy that planted local species. Any seedlings of the native conifer species, naturally occurring or planted, could not successfully establish in a warmer climate. However, when actions were taken to plant species more suited to the changed climate (adaptation-oriented management), the forest structure and carbon stocks were maintained, albeit at lower densities. In other words, assisted colonization of new tree species preserved the presence of a forest and some ecosystem services (e.g. carbon stocks, forest-type habitat) in the face of subalpine forest failure under climate warming. An important outcome of these modeling studies is the suggestion that disturbances coupled with climate warming are likely to inhibit regeneration of subalpine forest species and may lead to ecosystem change. Our results suggest that disturbance interactions impact forest ecosystems in two primary ways: (i) the combination of disturbances can, together, have greater power to damage the ecosystem than a single disturbance, and (ii) the disturbance interactions can affect the ability of individual species to recover, potentially leading to the establishment of different plant communities, and impacting future carbon stocks. An increase in extent and severity of forest disturbances such as drought, windthrow, insect infestations, and fire will increase the potential for disturbance interactions. The resilience of forest ecosystems experiencing multiple catastrophic disturbances is uncertain, and carbon stocks are likely to change. Disturbance events in a warming climate may create conditions that inhibit regeneration and, as a consequence, inhibit subalpine forest recovery. Future vegetation composition and structure may differ notably from historic subalpine patterns. The role and impact of forest management will, no doubt, influence and be influenced by these disturbances.

National Science Foundation (NSF)
Division of Environmental Biology (DEB)
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Henry L. Gholz
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University of Colorado at Boulder
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