Table_1_Simulating Groundcover Community Assembly in a Frequently Burned Ecosystem Using a Simple Neutral Model.docx

Fire is a keystone process that drives patterns of biodiversity globally. In frequently burned fire-dependent ecosystems, surface fire regimes allow for the coexistence of high plant diversity at fine scales even where soils are uniform. The mechanisms on how fire impacts groundcover community dynamics are, however, poorly understood. Because fire can act as a stochastic agent of mortality, we hypothesized that a neutral mechanism might be responsible for maintaining plant diversity. We used the demographic parameters of the unified neutral theory of biodiversity (UNTB) as a foundation to model groundcover species richness, using a southeastern US pine woodland as an example. We followed the fate of over 7,000 individuals of 123 plant species for 4 years and two prescribed burns in frequently burned Pinus palustris sites in northwest FL, USA. Using these empirical data and UNTB-based assumptions, we developed two parsimonious autonomous agent models, which were distinct by spatially explicit and implicit local recruitment processes. Using a parameter sensitivity test, we examined how empirical estimates, input species frequency distributions, and community size affected output species richness. We found that dispersal limitation was the most influential parameter, followed by mortality and birth, and that these parameters varied based on scale of the frequency distributions. Overall, these nominal parameters were useful for simulating fine-scale groundcover communities, although further empirical analysis of richness patterns, particularly related to fine-scale burn severity, is needed. This modeling framework can be utilized to examine our premise that localized groundcover assemblages are neutral communities at high fire frequencies, as well as to examine the extent to which niche-based dynamics determine community dynamics when fire frequency is altered.

火是驱动全球生物多样性格局的关键生态过程。在频繁受火的火依赖型生态系统中，地表火制度可在细尺度上维持高植物多样性共存，即便土壤条件均质。然而，火如何影响地被植物群落动态的机制仍未得到充分解析。鉴于火可作为随机性死亡因子发挥作用，我们提出假说：中性机制可能是维持植物多样性的核心驱动因素。我们以生物多样性统一中性理论（Unified Neutral Theory of Biodiversity, UNTB）的种群统计参数为基础，以美国东南部松林林地为研究案例，构建地被植物物种丰富度模型。我们在美国佛罗里达（FL）西北部的长叶松（Pinus palustris）高频火烧样地中，对123种植物的7000余株个体开展了为期4年、历经两次计划性火烧（prescribed burns）的动态跟踪监测。基于上述实测数据与基于UNTB的假设，我们构建了两类简约自治智能体模型，二者的核心差异在于局域补充过程是否具备空间显性特征。通过参数敏感性试验，我们分析了实测估计值、输入物种频率分布以及群落规模对输出物种丰富度的影响。研究发现，扩散限制是影响力最强的参数，其次为死亡率与出生率，且这些参数的效应随频率分布的尺度发生变化。总体而言，尽管后续仍需针对物种丰富度格局开展进一步实测分析（尤其需聚焦细尺度火烧烈度相关的研究），但本研究采用的参数集可有效用于模拟细尺度地被植物群落。本建模框架可用于验证我们的核心前提：在高频火烧条件下，局地地被植物组合属于中性群落；同时也可用于解析当火烧频率发生改变时，基于生态位的群落动态对群落结构的调控程度。