Data from: Canopy closure exerts weak controls on understory dynamics: a 30-year study of overstory-understory interactions

Stem exclusion and understory re-initiation are commonly described, but poorly understood stages of forest development. It is assumed that overstory trees exert strong controls on understory herbs and shrubs during the transition from open to closed-canopy forests, but long-term observations of this process are rare. We use long-term data from 188 plots to explore patterns and correlates of variation in understory richness and abundance 15-45 yr after clear-cut logging and burning of two experimental watersheds in western Oregon, USA. We test whether variation in the temporal dynamics of plots can be explained by topoedaphic factors that influence resource availability (insolation and soil moisture), variation in the pace and intensity of overstory development, or characteristics of the vegetation prior to canopy closure. Changes in forest structure were substantial over the study period; canopy cover increased four-fold, stem density by 75%, and bole biomass by two orders of magnitude, although trends were highly variable among individual plots. In contrast, understory richness, foliar cover, and biomass declined only 30-40%, driven by loss of early-seral colonists, not residual forest species. Canopy closure occurred earlier on north aspects but declines in understory biomass, reflecting loss of colonizing shrubs (without concomitant increases in forest shrubs), were limited to south aspects. In contrast, variation in effective soil moisture had little influence on the pace of decline. Temporal trends were highly asynchronous among plots nearly 50% of plots experienced some form of decline, but >35% showed no discernible trend. Declines were more likely in plots with greater tree influence before or at peak overstory development, but also in plots with greater understory development prior to canopy closure. Quantile regression models indicated weak relationships between understory biomass and overstory structure at most points in time. Our long-term data support a model of understory dynamics in which characteristics of the pre-closure vegetation are as important as overstory structure in determining the timing and nature of decline. Long-term studies are critical for elucidating patterns and processes that cannot be inferred from short-term experiments or space-for-time substitutions.

干材稀疏（stem exclusion）与林下重新启动（understory re-initiation）是森林发育过程中被广泛记述却鲜有深入阐明的典型阶段。学界普遍认为，在从开阔林分向郁闭林冠过渡的过程中，林冠层树木会对林下草本与灌木产生强烈调控作用，但针对这一过程的长期观测研究却极为匮乏。本研究依托美国俄勒冈州西部两处实验集水区皆伐火烧后15至45年的188块样地长期监测数据，探究林下植被物种丰富度与多度的变化模式及其关联因素。我们旨在检验样地时间动态的变异是否可由影响资源可获得性（日照辐射与土壤湿度）的地形土壤因子、林冠层发育的速率与强度，抑或是林冠郁闭前的植被特征加以解释。研究期内森林结构发生了显著变化：林冠覆盖度增长4倍，茎干密度提升75%，树干生物量增至原本的两个数量级，不过各单样地的变化趋势存在极高异质性。与之形成对比的是，林下植被的物种丰富度、叶面积盖度与生物量仅下降了30%至40%，这一变化由演替早期拓殖物种的消失所驱动，而非残留林分物种的减少。北坡样地的林冠郁闭发生更早，但林下生物量的下降（反映为拓殖灌木的消失，且未伴随林内灌木的相应增长）仅出现在南坡样地。与之相反，有效土壤湿度的变异对林下生物量下降的速率几乎无影响。各样地的时间动态呈现高度异步性：近50%的样地出现了不同程度的下降趋势，但超过35%的样地未显现出可辨识的变化趋势。在林冠层发育前期或峰值期受林木影响更强的样地，以及林冠郁闭前林下植被发育更为旺盛的样地，其林下植被下降的概率更高。分位数回归模型（quantile regression models）显示，在多数时间节点上，林下生物量与林冠层结构之间仅存在微弱的关联。本研究的长期数据支持如下林下动态模型：林冠郁闭前的植被特征与林冠层结构在决定林下植被下降的时间与特征方面具有同等重要的作用。长期监测研究对于阐明无法通过短期实验或空间替代时间的方法推断的格局与过程至关重要。