Disturbance by Fire and Its Role in the Population Biology and Distribution of a Rare Temperate Forest Herb
Bourg, Norman Alan
Gill, Douglas E.
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This research comprises a comprehensive investigation of the role and effects of disturbance by fire on the population biology of turkeybeard (<em> Xerophyllum asphodeloides </em>: Melanthiaceae) a rare forest herb of the Appalachian Mountains. I first employ a long term monitoring dataset from a primary study population to examine demography and flowering patterns. The effects of forest disturbance on survival, fruit and seed production in this population are next investigated by evaluating the outcome of a controlled, fire and canopy alteration 'pulse' experiment. The pollination biology of <em> X. asphodeloides </em> in the context of the above habitat manipulation is then determined from hand pollination experiments over three flowering seasons. Lastly, the role of fire and other environmental variables in the distribution of <em> X. asphodeloides </em> populations at the landscape scale is assessed via the construction, cross-validation, and ground-truthing of a classification tree and geographic information system (GIS)-based predictive habitat model for the mountains of northwestern Virginia, U.S.A. The major results demonstrate that <em> X. asphodeloides </em> is one of the few definitively fire-adapted forest understory herbs in the eastern United States. This is due to a number of factors, including high survival and rapid resprouting after burning, fire-induced mass flowering, significantly enhanced fruit and seed production in burnt and canopy-altered habitat, and greatly increased seed production in plants with larger floral displays. Additionally, support is found for characterizing <em> X. asphodeloides </em> as a primarily outcrossing species with what is highly likely to be a "leaky" self-incompatibility system. Populations typically exhibit low flowering levels in undisturbed forest which in combination with their self-incompatibility subjects them to Allee effects due to pollinator limitation in most years. Disturbance by fire results in release from these limiting factors by inducing mass flowering, altering the forest habitat and increasing pollinator activity, thus facilitating outcrossing and seed set. Finally, fire frequency, elevation, slope and forest type are indicated as the main explanantory variables for predicting suitable habitat in the classification tree/GIS model. This model correctly classifies 74% of known turkeybeard presence areas and 90% of known absence areas, and results in the discovery of eight new occupied habitat patches during ground-truthing exercises. Results of this research project are valuable not only for the conservation and management of <em> X. asphodeloides </em>, but also make a major contribution to the understanding of disturbance regimes in Appalachian forests and have important implications for improving ecologically based management efforts of these lands.