Modeling biological disturbances in LANDIS: a module description and demonstration using spruce budworm

Insects and diseases are common disturbance agents in forested ecosystems. Severe outbreaks can cause significant changes in tree species composition, age structure, and fuel conditions over broad areas. To investigate the role of biological disturbances in shaping forest landscapes over time, we constructed a new “biological disturbance agent” (BDA) module for a landscape-level forest succession and disturbance simulator, LANDIS. The BDA module is designed to simulate tree mortality following major outbreaks of insects and/or disease. Major outbreaks are defined as those significant enough to influence forest succession, fire disturbance, or harvest disturbance at landscape scales. Module design is flexible to accommodate a diversity of life history traits characterizing destructive insects and diseases, and more than one BDA can be simulated to examine their interactions. Five main elements control the probability of biological disturbance within the module: (1) local host dominance on a given site; (2) host value modifiers that reflect environmental conditions and recent disturbance history; (3) host dominance within a user-specified neighborhood; (4) the temporal outbreak pattern characteristic of the BDA, and (5) BDA dispersal in cases where the annual dispersal range of the BDA is small relative to the study area. In this paper, we describe the first four elements of the BDA module, and present the initial testing of the module on a variety of neutral landscape patterns, using Eastern spruce budworm as a test case. Our results are consistent with published successional patterns of spruce-fir and mixed forests affected by spruce budworm, but also highlight areas of uncertainty in the spatio-temporal patterns of budworm-caused tree mortality and biological disturbances in general. We suggest that the behavior of the module is consistent with the design and intended purpose of LANDIS as a probabilistic landscape-level simulator of forest disturbance and succession.