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.

北美西部新近过火的幼龄森林分布范围日益扩大，但大型野火后的森林发育过程仍未被完全阐明，尤其是在火烧烈度（burn severity）的空间变异、环境异质性以及驱动因子随时间变化的影响方面。本研究通过定期重采样1988年黄石公园大火后不久设立的永久样地，追踪了该区域亚高山森林的演替发育历程。针对过去25年间的群落格局与生态学过程，我们提出两个核心研究问题：（1）在海拔、土壤、火烧烈度以及灾后扭叶松（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趋同还是趋异？（2）调控灾后物种组成的主要驱动因子是什么？且这些调控因子的相对重要性是否随时间发生改变？

针对问题1，我们在3个海拔与基质存在差异的地理区域内布设了552个10平方米样地（n=552），样地覆盖了完整的火烧烈度梯度，并于1991年至2013年间定期开展重采样。针对问题2，我们于1999年与2012年对72个0.25公顷样地（n=72）进行了采样，这些样地广泛分布于经历全林分火烧（stand-replacing fire）的区域。

研究结果显示，物种丰富度与群落组成在火灾早期便出现了显著分异：贫瘠低海拔区域的物种丰富度较低，而较为肥沃的高海拔区域物种丰富度更高。火灾发生后的前5年，物种丰富度快速上升，此后则趋于平稳或仅缓慢增长。本次调查共记录到227个植物物种，其中仅6%为外来种。部分一年生植物以及具有热激活型土壤种子库的物种与重度火烧样地存在显著关联。然而，绝大多数灾后出现的物种在火灾前便已存在于该区域：多数个体通过根系或根茎存活，并通过萌蘖快速完成更新。

在72个样地中，基质、温度与降水（即非生物生境模板）在1999年与2012年均为影响群落组成的关键驱动因子。灾后扭叶松的丰度在1999年未表现出显著影响，但到2012年已成为最重要的驱动变量，其对多数林下物种的存在产生了负面影响，尤其对一年生植物与不耐阴草本植物的抑制作用更为明显。火烧烈度在1999年对群落组成存在显著影响，但在2012年则无显著效应；而与未火烧森林的距离在两个调查年份均未表现出任何影响。

1988年的黄石公园大火并未从根本上改变该区域的亚高山森林群落组成，生态记忆赋予了群落应对高强度火烧的恢复力。