Density-dependent demography of creosote bush (Larrea tridentata) along grass-shrub ecotones

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

木本植物入侵草原是一种全球性现象，对生物多样性与生态系统功能具有深远影响。理解并预测其扩张速率与背后的调控过程，是木本入侵研究与管理领域的核心挑战。空间种群生物学理论预测，种群扩张的发生与速度应敏感地取决于同种密度依赖的性质。若在低密度入侵前缘处适合度达到最大化，则灌木扩张应是"拉动型（pulled）"的。然而已有研究证实，入侵灌木存在正反馈效应：灌木定植会改变局部生境，进而促进后续灌木的定植与存活。在此情形下，低密度区域的灌木可能面临适合度成本，导致扩张由前缘后方"推动型（pushed）"驱动。我们以过去一个世纪以来持续入侵奇瓦瓦沙漠草原的石炭酸灌木（creosotebush, *Larrea tridentata*）为研究对象，开展了空间动态研究。我们通过野外样方普查与幼苗移植实验获取的种群统计数据，检验了灌丛-草原交错带（grass-shrub ecotone）沿灌丛密度梯度的灌木适合度所对应的密度依赖强度与方向。我们还通过种子下落实验与风速数据构建了机械性种子扩散核（seed dispersal kernel），随后将种群统计与扩散数据整合进空间积分投影模型（spatial integral projection model, SIPM）中，以预测灌木扩张的动态。该空间积分投影模型的预测结果与正反馈效应的预期相悖，显示灌木入侵波是由低密度前缘的最大适合度所"拉动"的。不过预测的扩张速率异常缓慢（约8厘米/年），这一结果得到了独立的交错带重测数据的支持：12年间灌丛覆盖的空间范围几乎未发生变化。入侵速率对幼苗定植极为敏感，表明该种群在利于定植的环境条件下，可能会出现爆发式扩张。我们整合观测数据、实验结果与模型模拟的研究不仅揭示了该灌丛-草原交错带在当前环境下实际处于停滞状态，还阐明了其停滞的原因，以及环境变化可能如何改变这一现状。