Directions for Future Research

Recently, LANDIS II has been released. In a major change from LANDIS, the life history parameters have been updated to include both minimum and maximum age of resprouting as well as a postfire resprout function, which allows for serotiny or resprout. LANDIS-II also allows for the calculation of aboveground live and dead biomass (as kg/ha) and tracks woody and leaf litter dead biomass. Biomass can also be used as an alternative to the original succession function using species age. Disturbances that can be modeled follow those of LANDIS. In LANDIS-II, each ecological process operates on its own individual time step (units: year). For example, fire may operate at a 5- year time step, whereas SPB occurs at a 7-year time step and HWA at a 1-year time step. Also, while LANDIS was limited to 30 species, LANDIS II can have an unlimited number of plant species. Changing to LANDIS-II will undoubtedly aid in sorting out some of the problems in the HWA case, as we will be able to incorporate species such as rhododendron and invasive species, as well as have the ability to model HWA annually rather than at 10-year increments.

The Harvesting module in LANDIS II allows for both tree removal and planting within user-defined management areas. There are several functions for species removal. Clearcut removes all species within a stand. Individual species can be removed either as all, a percentage of the species, the oldest cohort within the species, all cohorts except the oldest, the youngest cohort, and all cohorts except the youngest cohort. There are several other differences between the two-model versions. First, climate change scenarios are now possible as land type parameters can be altered according to temporal grouping. Second, the order of disturbances can be randomized so that within a series of runs you might, for example, have fire run either before or after the BDA is run. Finally, LANDIS-II is modular. This modularity allows for the relatively easy incorporation of new modules (such as ice storm disturbance) as well as the alteration of existing modules to meet research needs.

Our goal for future research is to test the capacity of LANDIS II using a landscape modeling environment to evaluate changes in composition and structure of Eastern United States forests undergoing multiple interacting environmental threats. Specifically, we are adapting LANDIS II to model the combined effects of key invasive biological disturbance agents and non-native invasive plant species on the composition and structure of Southern Appalachian forests. This will allow us to determine the effects of changes in forest structure and composition on fire regimes, biodiversity and wildlife habitat, to investigate strategies for restoring key ecosystems that may be significantly impacted by multiple-threat interactions, and to test contemporary ecological theory, such as the relationship between biodiversity and invasibility. Also, by incorporating Gap models, we will be able to address additional questions beyond those for which LANDIS II is suitable.

Gap models simulate the establishment, growth, and death of individual trees on small plots (Perry and Enright 2006). Unlike LANDIS, they do not consider the influence of landscape structure on disturbance and succession. Their value lies in their ability to simulate interactions among individual plants in a detailed, mechanistic way. Such local-scale interactions between individual plants that vary in size, growth rate, shade-tolerance, moisture/nutrient requirements, and other attributes are thought to govern successional processes, including exotic species invasions (Huston 2004, Shea and Chesson 2002). Gap models also are capable of representing the interactions between different plant functional types, e.g., trees and shrubs. Although most commonly applied to problems of forest succession, gap models have been used to investigate the dynamics of herbaceous vegetation as well, (e.g., Peters 2002). By employing a combined approach of gap and landscape modeling, we will be able to rectify the problems encountered in the HWA study and provide more detailed succession projections.