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{"Abstract":["The encroachment of woody plants into grasslands is a global\n phenomenon with implications for biodiversity and ecosystem\n function. Understanding and predicting the pace of expansion and the\n underlying processes that control it are key challenges in the study\n and management of woody encroachment. Theory from spatial population\n biology predicts that the occurrence and speed of population\n expansion should depend sensitively on the nature of conspecific\n density dependence. If fitness is maximized at the low-density\n encroachment edge then shrub expansion should be "pulled"\n forward. However, encroaching shrubs have been shown to exhibit\n positive feedbacks, whereby shrub establishment modifies the\n environment in ways that facilitate further shrub recruitment and\n survival. In this case there may be a fitness cost to shrubs at low\n density causing expansion to be "pushed" from behind the\n leading edge. We studied the spatial dynamics of creosotebush\n (Larrea tridentata), which has a history of\n encroachment into Chihuahuan Desert grasslands over the past\n century. We used demographic data from observational censuses and\n seedling transplant experiments to test the strength and direction\n of density dependence in shrub fitness along a gradient of shrub\n density at the grass-shrub ecotone. We also used seed-drop\n experiments and wind data to construct a mechanistic seed dispersal\n kernel, then connected demography and dispersal data within a\n spatial integral projection model (SIPM) to predict the dynamics of\n shrub expansion. The SIPM predicted that, contrary to expectations\n based on potential for positive feedbacks, the shrub encroachment\n wave is "pulled" by maximum fitness at the low-density\n front. However, the predicted pace of expansion was strikingly slow\n (ca. 8 cm/yr), and this prediction was supported by independent\n re-surveys of the ecotone showing little to no change in spatial\n extent of shrub cover over 12 years. Encroachment speed was acutely\n sensitive to seedling recruitment, suggesting that this population\n may be primed for pulses of expansion under conditions that are\n favorable for recruitment. Our integration of observations,\n experiments, and modeling reveals not only that this ecotone is\n effectively stalled under current conditions, but also\n why that is so and how that may change as the\n environment changes."]}