Insect outbreaks are a significant factor in models of Southeastern vegetation dynamics under climate change

Background/Question/Methods

Bark beetles (Scolytinae) are one of the major forces in the life cycle of temperate and boreal forests worldwide. Throughout our focal area of the South-eastern United States, the most important forest pest is the Southern Pine Beetle (SPB), Dendroctonus frontalis. The effect of the beetles on the forest dynamics occurs in pulses, and as such is difficult to appreciate with a limited time-window. However, throughout the complete forest development cycle (50-100 years), the effect of bark beetle may be as large as that of fire or timber industry. Therefore, a model that jointly simulates forest dynamics, forest management, and SPB infestations over several decades for a large region is necessary to inform regional forestry or land management policies.

To model the baseline dynamics of loblolly-pine dominated landscape, we used the VDDT framework  (Vegetation Dynamics Development Tool), parameterized with data on loblolly-dominated forests. To create a realistic spatially-explicit model, we implemented this dynamics into a virtual landscape derived from satellite imagery and translated into the TELSA framework (Tool for Exploratory Landscape Scenario Analyses). To model the dynamic interactions between the SPB infestation and forest development, we emulated outbreak probabilities from previously documented scenarios (ranging from background SPB presence to large-scale outbreaks, Forest Service Database) in the focal region, and created dynamic probabilistic links between them. With regards to the potential effect of global warming on the forest-insect interactions, we parameterized the model by values from two historical scenarios: low (background) outbreak probability (data from the focal area during 1990’s) and high outbreak probability, following a favorable climatic conditions (data from East Texas, 1960’s).

Results/Conclusions

Preliminary assessment of the model output suggests that occasional clusters of infested stands occur even in the low-infestation scenario. However, large-scale outbreaks are unlikely to arise spontaneously, unless triggered by favorable climate. Under the “low beetle pressure” scenario, current high levels of management result in relatively low SPB activity over time, and decreased management leads to only slightly increased beetle activity. The influence of management is more important under the high outbreak probability scenario. High probability of SPB contagion dramatically influences the landscape-level forest  structure, unless high intensity of forest management is implemented. That itself removes large portions of natural stands. Thus, natural stands of loblolly appear unstable over a long periods of time of climatic conditions conducive to bark beetle attacks.