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.

新近过火的幼龄林在北美西部愈发普遍，但目前学界对大型野火后的森林发育过程仍缺乏充分认知，尤其是在火烧烈度差异（variable burn severity）、环境异质性（environmental heterogeneity）以及驱动因子随时间变化的影响方面。本研究以1988年黄石公园野火后的亚高山森林（subalpine forests）为研究对象，通过定期对火灾后不久设立的永久样地（permanent plots）进行重复采样，追踪了过去25年间的森林发育动态。本研究围绕两个模式与过程问题展开探讨：（1）在海拔、土壤、火烧烈度以及火烧后扭叶松（lodgepole pine, Pinus contorta var. latifolia）密度的梯度变化下，植物物种丰富度（plant species richness）与群落组成（community composition）正趋于聚合还是分化？（2）调控火烧后群落组成的主要因子是什么？这些因子的相对重要性是否随时间发生改变？针对问题1，我们在海拔与土壤基质（substrate）存在差异的三个地理区域内布设了552个10平方米的样地，样地覆盖了全部火烧烈度梯度，并于1991年至2013年间进行了周期性重采样。针对问题2，我们于1999年与2012年在以皆伐式野火（stand-replacing fire，即过火后林分完全被摧毁的野火）过火的区域内广泛布设了72个0.25公顷的样地进行采样。研究结果显示，物种丰富度与群落组成在早期便出现分化：低海拔贫瘠区域的物种丰富度较低，而高海拔肥沃区域的物种丰富度更高。火烧后前5年物种丰富度快速增长，随后趋于平稳或仅缓慢提升。在记录到的227个物种中，仅6%为外来物种。部分一年生植物以及土壤种子库受热刺激萌发的物种与重度火烧样地存在关联。但绝大多数火烧后出现的物种在火灾前便已存在，其中多数通过根系或根状茎存活，并通过萌蘖快速完成更新。在72个样地中，土壤基质、温度与降水（即非生物环境模板，abiotic template）在1999年与2012年均为群落组成的关键驱动因子。火烧后扭叶松的丰度在1999年尚未表现出显著影响，但到2012年已成为最关键的驱动变量，对多数林下物种（尤其是一年生植物与不耐阴草本）的存在产生负面影响。火烧烈度在1999年对群落组成具有显著影响，但在2012年则无此效应；而与未过火森林的距离在两个年份均未表现出影响。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合，生态记忆（ecological memory）赋予了森林应对高强度火烧的恢复力。