Data from: Demographic stimulation of the obligate understorey herb, Panax quinquefolius L., in response to natural forest canopy disturbances

1.Natural and anthropogenic forest canopy disturbances significantly alter forest dynamics and lead to multi-dimensional shifts in the forest understorey. An understorey plant's ability to exploit alterations to the light environment caused by canopy disturbance leads to changes in population dynamics. The purpose of this work was to determine if population growth of a species adapted to low light increases in response to additional light inputs caused by canopy disturbance, or alternatively, declines due to long-term selection under low light conditions. 2.To address this question, we quantified the demographic response of an understorey herb to three contrasting forest canopy disturbances (ice storms, tent caterpillar defoliation and lightning strikes) that encompass a broad range of disturbance severity. We used a model shade-adapted understorey species, Panax quinquefolius, to parameterize stage-based matrix models. Asymptotic growth rates, stochastic growth rates and simulations of transient dynamics were used to quantify the population-level response to canopy disturbance. Life table response experiments were used to partition the underlying controls over differences in population growth rates. 3.Population growth rates at all three disturbed sites increased in the transition period immediately after the canopy disturbance relative to the transition period prior to disturbance. Stochastic population models revealed that growth rates increased significantly in simulations that included disturbance matrices relative to those simulations that excluded disturbance. Additionally, transient models indicated that population size (n) was larger for all three populations when the respective disturbance matrix was included in the model. 4.Synthesis Obligate shade species are most likely to be pre-adapted to take advantage of canopy gaps and light influx to a degree, and this pre-adaptation may be due to long-term selection under dynamic old growth forest canopies. We propose a model whereby population performance is represented by a parabolic curve where performance is maximized under intermediate levels of canopy disturbance. This study provides new evidence to aid our understanding of the population-level response of understorey herbs to disturbances whose frequency and intensity are predicted to increase as global climates continue to shift.1. 自然与人为林冠干扰（forest canopy disturbance）可显著改变森林动态（forest dynamics），并引发森林林下植被（forest understorey）的多维度变化。林下植物利用林冠干扰所引发的光环境改变的能力，会驱动其种群动态（population dynamics）发生改变。本研究旨在探究：适应低光（low light）环境的物种种群增长，是否会因林冠干扰带来的额外光照输入而提升，抑或是会因长期低光环境下的选择压力而出现下降。 2. 为解答上述问题，我们量化了林下草本植物（understorey herb）对三类典型林冠干扰——冰风暴、天幕毛虫落叶危害与雷击——的种群统计响应（demographic response），这三类干扰涵盖了大范围的干扰强度梯度。本研究以适应遮阴环境的林下模式物种西洋参（Panax quinquefolius）为研究对象，对基于阶段的矩阵模型（stage-based matrix model）进行参数化。我们通过渐近增长率（asymptotic growth rate）、随机增长率（stochastic growth rate）以及瞬时动态（transient dynamics）模拟，量化了种群对林冠干扰的响应水平；并借助生命表响应实验（life table response experiment），解析了种群增长率差异背后的调控机制。 3. 相较于干扰前的过渡期，三处受干扰样地的种群增长率均在林冠干扰后的即刻过渡期内有所提升。随机种群模型结果显示，相较于未纳入干扰矩阵的模拟，纳入干扰矩阵的模拟中种群增长率显著升高。此外，瞬时模型结果表明，当模型中纳入对应干扰矩阵时，三处种群的个体总数（n）均更高。 4. 研究总结：专性阴生植物在一定程度上大概率已预先适应利用林冠间隙（canopy gaps）与光照输入，这种预先适应性可能源于动态老龄原始林（old growth forest）林冠下的长期选择压力。我们提出了一个种群表现模型：种群表现以抛物线形式呈现，在中等强度的林冠干扰下达到峰值。本研究为我们理解林下草本植物对干扰的种群响应提供了新证据——随着全球气候变化持续演进，这类干扰的发生频率与强度预计将不断提升。