Data from: Deterministic and stochastic processes lead to divergence in plant communities 25 years after the 1988 Yellowstone fires

Young, recently burned forests are increasingly widespread throughout western North America, but forest development after large wildfires is not fully understood, especially regarding effects of variable burn severity, environmental heterogeneity, and changes in drivers over time. We followed development of subalpine forests after the 1988 Yellowstone fires by periodically re-sampling permanent plots established soon after the fires. We asked two questions about patterns and processes over the past 25 years: (1) Are plant species richness and community composition converging or diverging across variation in elevation, soils, burn severity, and post-fire lodgepole pine (Pinus contorta var. latifolia) density? (2) What are the major controls on post-fire species composition, and has the relative importance of controls changed over time? For question 1, we sampled 10-m2 plots (n=552) distributed among three geographic areas that differ in elevation and substrate; plots spanned the spectrum of fire severities and were re-sampled periodically from 1991-2013. For question 2, we sampled 0.25-ha plots (n=72), broadly distributed across areas that burned as stand-replacing fire, in 1999 and 2012. Richness and species composition diverged early on between infertile low-elevation areas (lower richness) and more fertile high-elevation areas (greater richness). Richness increased rapidly for the first five years post-fire, then leveled off or increased only slowly thereafter. Only 6% of 227 recorded species were non-native. Some annuals and species with heat-stimulated soil seed banks were associated with severely burned sites. However, most post-fire species had been present before the fire; many survived as roots or rhizomes and regenerated rapidly by sprouting. Among the 72 plots, substrate, temperature, and precipitation (the abiotic template) were consistently important drivers of community composition in 1999 and 2012. Post-fire lodgepole pine abundance was not significant in 1999 but was the most important driving variable by 2012, with a negative effect on presence of most understory species, especially annuals and shade-intolerant herbs. Burn severity was significant in 1999 but not in 2012, and distance to unburned forest had no influence in either year. The 1988 fires did not fundamentally alter subalpine forest community assemblages in Yellowstone, and ecological memory conferred resilience to high-severity fire.近期过火的幼龄林在北美西部的分布日益广泛，但大型野火后的森林群落发育过程尚未被完全阐明，尤其是在不同火烧烈度、环境异质性以及驱动因子随时间变化这几个方面。本研究以1988年黄石公园野火后的亚高山森林为研究对象，通过定期重采样火灾后不久设立的永久样地，对过去25年间的群落格局与过程展开了两项探究：其一，在海拔、土壤、火烧烈度以及过火后黑松（Pinus contorta var. latifolia）密度的梯度变化下，植物物种丰富度与群落组成是趋于聚合还是分化？其二，调控过火后物种组成的主要因子是什么？且这些调控因子的相对重要性是否随时间发生了改变？针对第一项问题，我们在海拔与基质类型存在差异的三个地理区域内，布设了总计552个面积为10平方米的样地，这些样地覆盖了全部火烧烈度梯度，并于1991年至2013年间定期进行重采样。针对第二项问题，我们于1999年和2012年对72个面积为0.25公顷的样地进行了采样，这些样地广泛分布于经历全林分更替火烧的区域内。研究结果显示，物种丰富度与群落组成在早期便出现分化：贫瘠的低海拔区域物种丰富度较低，而肥力较高的高海拔区域物种丰富度更高。过火后前五年，物种丰富度快速增长，随后便趋于平稳或仅缓慢提升。在记录到的227个物种中，仅有6%为外来物种。部分一年生植物以及土壤种子库受热量激活的物种，与重度火烧样地存在显著关联。但绝大多数过火后出现的物种在火灾前便已存在于该区域；其中多数通过根系或根茎存活，并通过萌蘖快速完成更新。在这72个样地中，基质、温度与降水（即非生物环境模板）始终是1999年和2012年群落组成的重要调控因子。过火后黑松的多度在1999年时未表现出显著影响，但到2012年已成为最关键的调控变量，对多数林下物种的存在产生了负面影响，尤其是一年生植物与耐阴性弱的草本植物。火烧烈度在1999年对群落组成存在显著影响，但在2012年则无此效应；而与未火烧森林的距离在两个年份均未对群落组成产生影响。1988年的野火并未从根本上改变黄石公园的亚高山森林群落组成，生态记忆赋予了该生态系统应对高强度火烧的恢复力。