Bill Hoffmann

Understanding the factors that determine the current distribution of biomes is fundamental for projecting how vegetation will respond to future climates and disturbance regimes. Tropical savanna-forest boundaries mark the transition between the two most extensive tropical biomes, yet the main factors determining the location, structure, and dynamics of savanna-forest boundaries are poorly understood. Our understanding has been limited by strong positive feedbacks and other non-linear processes that result in complex dynamics and hysteresis (the dependence of a system not only on its current environment but also on its past environment). Hysteresis is particularly problematic because it can introduce large errors in model behavior if key processes are not represented realistically. We will combine field data and modeling to test for and quantify sources of hysteresis in savanna-forest dynamics at multiple scales. The primary objectives of this research are: (1) quantify processes that underlie switches in biome states between savanna and forest; (2) use this information to refine and parameterize the CLM(ED-SPITFIRE) model for simulating savanna-forest dynamics; and (3) perform simulations to understand environmental controls on the distribution of tropical savanna and forest ecosystems, with particular attention to understanding causes of hysteresis. Within the first objective, we have three specific aims designed to fill gaps in our understanding of vegetation-fire feedbacks that regulate the dynamics of savanna. These aims are: (1) understand the unique roles of savanna and forest tree species for mediating ecosystem dynamics during biome switches from savanna to forest; (2) understand the roles of savanna and forest tree species under the reverse process of forest retreat; (3) quantify flammability thresholds across savanna-forest gradients to understand controls over vegetation-fire feedbacks during biome switches.

Abstract 1. Objective: Management of military lands aims to ensure long-term persistence of threatened and endangered species while sustaining the military mission (DoD Instruction 4715.03). Evaluating local extinction risk of plants is complicated by the long lifespan and slow rate of dynamics of most species. To this end, we will combine new data with a rare-plant monitoring study that is putatively the largest, longest, and most comprehensive undertaken on military lands with the aim to 1) quantify the factors that influence local extinction, including population structure, management activities, and physical environment, 2) generalize these results across 3) provide resource managers with guidelines for assessing risk of local extinction and for remediating this risk. In addition to meeting these aims, we will re-evaluate a set of hypotheses that were originally conceived for this study and that were previously tested by Gray et al (2003) when only relatively short data series was available. 2. Technical Approach: For over 20 years, the Endangered Species Branch of Ft. Bragg has monitored 1396 populations of 45 rare plant species, resulting in the Monitoring and Assessment of Rare Species (MARS) database. The MARS database has over 32,000 population-years of data, thereby offering a rich resource for understanding the drivers of local extinction. We will combine this database with information on plant species traits, local site characteristics, management history to identify generalizable risk factors for local extirpation of rare plant populations. Identifying these risk factors will require detailed information regarding each population and species in the MARS database. Information on population size and distance to nearest conspecific population extracted from the MARS database. Information on management history and local conditions will be extracted by overlaying the geographic coordinates of each population on existing GIS layers. The influence of these factors on population persistence will be evaluated by using logistic regression and GLM methods to test for their influence on the probability that an existing population disappears with in the interval between successive censuses. 3. Benefits: The findings will provide DoD resource managers on multiple southeastern installations with guidance related to species management. The expected benefits of the proposed work are: ��������������� Deeper understanding of the metapopulation dynamics of the 45 rare plant species on Fort Bragg ��������������� Extension of the initial inferences to other species and installations in the southeastern United States through incorporation of functional traits ��������������� Provide a model framework for other ecosystems

The proposed research will examine the effects of fire on population dynamics and physiological ecology of common and rare species in inclusional wetlands

Our objective is to quantify the effect of warming on assimilation, transpiration, and growth of saplings of four dominant tree species within an existing warming experiment. We will combine field measurements and process modeling to deconvolve the temperature and moisture effects of warming on assimilation and transpiration. Finally, we will generate quantitative estimates of the uncertainty introduced by our incomplete knowledge of future changes in soil and atmospheric moisture under warming scenarios.

Habitat loss, fragmentation, and altered disturbance regimes are among the greatest threats to biodiversity and species conservation. These situations can be ameliorated with appropriate management, but only when there is sufficient information on which to base this management. Unfortunately, it is often necessary to implement management strategies with an incomplete understanding of the demography and natural history of the target species. We propose population and community studies to provide this information for five species (Amorpha georgiana var. georgiana, Astragalus michauxii, Lilium pyrophilum, Pyxidanthera brevifolia, Stylisma pickeringii) selected for intensive study at Ft. Bragg. For L. pyrophilum, we also propose a population genetics study to quantify genetic structure, outcrossing rates, and level of genetic differentiation in relation to its sister species.