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林分布范围日益扩大，但大型野火后的森林发育过程尚未得到充分阐释，尤其是在可变火烧烈度（burn severity）、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年黄石公园火灾后不久设立的永久样地（permanent plots），追踪了该区域亚高山森林（subalpine forests）的发育进程。我们针对过去25年间的生态格局与过程提出两个研究问题：（1）在海拔、土壤、火烧烈度以及火烧后黑松（Pinus contorta var. latifolia）密度的变异梯度下，植物物种丰富度（plant species richness）与群落组成（community composition）正趋于趋同还是趋异？（2）火烧后物种组成的主要调控因子是什么？这些调控因子的相对重要性是否随时间发生变化？针对问题1，我们在海拔与基质存在差异的三个地理区域内布设了共计552个10平方米样地（10-m2 plots），样地覆盖了全梯度的火烧烈度，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年在均发生过林分替换型火烧（stand-replacing fire）的区域内，广泛布设了72个0.25公顷样地（0.25-ha plots）并完成采样。研究发现，贫瘠低海拔区域（物种丰富度较低）与高肥力高海拔区域（物种丰富度较高）之间，物种丰富度与群落组成在早期便出现了显著分化。火烧后前五年，物种丰富度快速提升，随后趋于平稳或仅缓慢增长。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具有热激发土壤种子库（soil seed banks）的物种与重度火烧样地存在显著关联。然而，绝大多数火烧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（rhizomes）存活，并通过萌芽实现快速更新。在72个样地中，基质、温度与降水（即非生物环境模板，abiotic template）在1999年与2012年始终是群落组成的重要驱动因子。火烧后黑松的丰度在1999年未表现出显著影响，但到2012年已成为最关键的驱动变量，对多数林下物种（understory species）的存在产生了负面影响，尤其是一年生植物与不耐阴草本植物（shade-intolerant herbs）。火烧烈度在1999年对群落组成具有显著影响，但在2012年不再显著；而与未烧林地的距离在两个年份均未表现出影响。1988年的黄石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（ecological memory）为该生态系统应对高强度火烧提供了恢复力。