Looking beyond the mean: Drivers of variability in postfire stand development of conifers in Greater Yellowstone

High-severity, infrequent fires in forests shape landscape mosaics of
stand age and structure for decades to centuries, and forest structure
can vary substantially even among same-aged stands. This variability
among stand structures can affect landscape-scale carbon and nitrogen
cycling, wildlife habitat availability, and vulnerability to
subsequent disturbances. We used an individual-based forest process
model (iLand) to ask: Over 300 years of postfire stand development,
how does variation in early regeneration densities versus abiotic
conditions influence among-stand structural variability for four
conifer species widespread in western North America? We parameterized
iLand for lodgepole pine (Pinus contorta var. latifolia), Douglas-fir
(Pseudotsuga menziesii var. glauca), Engelmann spruce (Picea
engelmannii), and subalpine fir (Abies lasiocarpa) in Greater
Yellowstone (USA). Simulations were initialized with field data on
regeneration following stand-replacing fires, and stand development
was simulated under historical climatic conditions without further
disturbance. Stand structure was characterized by stand density and
basal area. Stands became more similar in structure as time since fire
increased. Basal area converged more rapidly among stands than tree
density for Douglas-fir and lodgepole pine, but not for subalpine fir
and Engelmann spruce. For all species, regeneration-driven variation
in stand density persisted for at least 105 years postfire, and for
lodgepole pine, early regeneration densities dictated among-stand
variation for 203 years. Over time, stands shifted from
competition-driven convergence to environment-driven divergence, in
which variability among stands was maintained or increased. The
relative importance of drivers of stand structural variability
differed between density and basal area and among species due to
differential species traits, growth rates, and sensitivity to
intraspecific competition versus abiotic conditions. Understanding
dynamics of postfire stand development is increasingly important for
anticipating future landscape patterns as fire activity increases. This dataset was used in the publication Braziunas et al. (2018),
Forest Ecology and Management
https://doi.org/10.1016/j.foreco.2018.08.034

森林中高强度、低发生频率的火灾会在数十年至数百年间塑造林分年龄与结构的景观镶嵌格局；即便同龄林分之间，其森林结构也会存在显著差异。这种林分结构间的变异会影响景观尺度的碳氮循环、野生动物栖息地可获得性，以及林分对后续干扰的脆弱性。我们采用基于个体的森林过程模型（iLand）开展研究，旨在解答：在火灾后林分长达300年的发育周期中，早期更新密度与非生物条件的差异，会如何影响北美西部广泛分布的4种针叶树种的林分结构变异？我们针对美国大黄石地区的4个树种对iLand模型进行了参数化：扭叶松（Pinus contorta var. latifolia）、花旗松（Pseudotsuga menziesii var. glauca）、恩氏云杉（Picea engelmannii）以及亚高山冷杉（Abies lasiocarpa）。模拟以全林分火灾后的野外更新实地数据作为初始条件，并在无后续干扰的历史气候情景下开展林分发育模拟。本研究以林分密度与断面积作为林分结构的表征指标。随着火灾后时间推移，各林分的结构相似度逐渐提升。对于花旗松与扭叶松而言，林分间的断面积收敛速度快于树木密度；但亚高山冷杉与恩氏云杉并无此特征。对于所有受试树种而言，由更新驱动的林分密度差异在火灾后至少持续105年；其中扭叶松的早期更新密度对林分间差异的调控作用可长达203年。随着时间推移，林分结构的变化模式从竞争驱动的收敛转向环境驱动的分化，此时林分间的结构差异得以维持甚至进一步增大。由于不同树种的功能性状、生长速率以及对种内竞争与非生物条件的敏感性存在差异，林分结构变异驱动因子的相对重要性在林分密度与断面积之间、以及不同树种之间均有所不同。随着火灾活动愈发频繁，明晰火灾后林分的发育动态，对预测未来景观格局而言愈发重要。本数据集已应用于Braziunas等人（2018）发表于《森林生态学与管理（Forest Ecology and Management）》的研究论文，DOI链接：https://doi.org/10.1016/j.foreco.2018.08.034