Data from: Twenty-four years after the Yellowstone fires: are postfire lodgepole pine stands converging in structure and function?

Disturbance and succession have long been of interest in ecology, but how landscape patterns of ecosystem structure and function evolve following large disturbances is poorly understood. After nearly 25 years, lodgepole pine (Pinus contorta var. latifolia) forests that regenerated after the 1988 Yellowstone Fires (Wyoming, USA) offer a prime opportunity to track the fate of disturbance-created heterogeneity in stand structure and function in a wilderness setting. In 2012, we resampled 72 permanent plots to ask (1) How have postfire stand structure and function changed between 11 and 24 yr postfire, and what variables explain these patterns and changes? (2) How has landscape-level (among-stand) variability in postfire stand structure and function changed between 11 and 24 yr postfire? We expected to see evidence of convergence beginning to emerge, but also that initial postfire stem density would still determine trajectories of biomass accumulation. After 24 yr, postfire lodgepole pine density remained very high (mean = 21,738 stems ha−1, range = 0 to 344,067 stems ha−1). Stem density increased in most plots between 11 and 24 yr postfire, but declined sharply where 11-yr-postfire stem density was > 72,000 stems ha−1. Stems were small in high-density stands, but stand-level lodgepole pine leaf area, foliage biomass, and live aboveground biomass increased over time and with increasing stem density. After 24 yr, mean annual lodgepole pine aboveground net primary production (ANPP) was high (mean = 5 Mg ha−1 yr−1, range = 0 to 16.5 Mg ha−1 yr−1). Among stands, lodgepole pine ANPP increased with stem density, which explained 69% of the variation; another 8% of the variation was explained by environmental covariates. Early patterns of postfire lodgepole pine regeneration, which were contingent on prefire serotiny and fire severity, remained the dominant driver of stand structure and function. We observed mechanisms that would lead to convergence in stem density (structure) over time, but it was landscape variation in functional variables that declined substantially. Stand structure and function have not converged across the burned landscape, but our evidence suggests function will converge sooner than structure.

干扰与演替长期以来都是生态学领域的核心研究议题之一，但大型扰动发生后，生态系统结构与功能的景观格局将如何演化，学界对此的认知仍较为有限。时隔近25年后，1988年黄石大火（美国怀俄明州）后自然更新的扭叶松（lodgepole pine, Pinus contorta var. latifolia）林，为在荒野背景下追踪扰动驱动的林分结构与功能异质性的动态变化提供了绝佳的研究契机。2012年，我们对72块永久样地进行了重采样，旨在解答两个科学问题：（1）在火后11年至24年期间，火烧迹地的林分结构与功能发生了哪些变化？哪些变量能够解释这些格局与演变规律？（2）在火后11年至24年期间，景观尺度（林分间）的火烧迹地林分结构与功能的空间异质性发生了何种变化？我们曾推测，林分结构的趋同现象会开始显现，但火后初期的林木株密度仍将决定生物量积累的动态轨迹。至火后24年时，火烧迹地扭叶松的株密度仍维持在较高水平（平均值为21738株·公顷⁻¹，区间为0至344067株·公顷⁻¹）。多数样地的株密度在火后11年至24年期间有所上升，但在火后11年株密度超过72000株·公顷⁻¹的样地中，株密度出现了显著下降。高密度林分中的林木个体偏小，但林分水平的扭叶松叶面积、枝叶生物量以及地上活体生物量均随时间推移以及株密度的增加而上升。至火后24年时，扭叶松年平均地上净初级生产力（aboveground net primary production, ANPP）处于较高水平（平均值为5 Mg·公顷⁻¹·年⁻¹，区间为0至16.5 Mg·公顷⁻¹·年⁻¹）。在不同林分之间，扭叶松的ANPP随株密度的升高而增加，这一因素解释了69%的变异；另有8%的变异可由环境协变量解释。火后初期扭叶松的更新格局受制于火前的种子闭鳞特性与火烧烈度，仍为林分结构与功能的主导驱动因子。我们观测到了随时间推移会导致株密度（林分结构）趋同的机制，但功能变量的景观异质性出现了大幅下降。整个火烧迹地的林分结构与功能并未出现整体趋同，但我们的研究结果表明，林分功能的趋同进程将早于林分结构。