Canopy closure exerts weak controls on understory dynamics: a 30-year study of overstory-understory interactions
Canopy closure exerts weak controls on understory dynamics: a 30-year study of overstory-understory interactions
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DOI：
10.1890/12-1696.1
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发表时间：
2013-05-01
影响因子：
6.1
通讯作者：
Lutz, James A.
中科院分区：
文献类型：
--
作者：
Halpern, Charles B.;Lutz, James A.
树干排除和林下植被再发生是森林发展的两个阶段，但人们对此知之甚少。据推测，上层树木施加强大的控制林下草本植物和灌木的过渡过程中，从开放到封闭的冠层森林，但这一过程的长期观察是罕见的。我们使用188个地块的长期数据，探索模式和相关的林下植物丰富度和丰富度的变化后15-45年的两个实验流域在美国西部俄勒冈州，皆伐和燃烧。我们测试的时间动态的地块的变化是否可以解释的地形因子，影响资源的可用性（日照和土壤水分），变化的速度和强度的上层发展，或植被的特点之前，树冠关闭。森林结构的变化是巨大的，在研究期间，树冠覆盖增加了四倍，干密度的75%，树干生物量的两个数量级，虽然趋势是高度可变的个别地块。相比之下，林下植物的丰富度，叶覆盖，生物量下降只有30- 40%，由早期seral殖民者的损失，而不是残留的森林物种。冠层关闭发生较早的北部方面，但林下生物量的下降，反映了殖民灌木的损失（没有随之而来的森林灌木的增加），仅限于南部方面。相反，有效土壤水分的变化对下降的速度影响不大。各地块的时间趋势高度不同步：近50%的地块经历了某种形式的下降，但> 35%的地块没有明显的趋势。下降更有可能在地块更大的树的影响之前或在高峰上层发展，但也在地块更大的林下发展之前，树冠关闭。分位数回归模型表明，在大多数时间点林下生物量和上层结构之间的关系较弱。我们的长期数据支持林下动态的模型，其中关闭前植被的特征是一样重要的上层结构，在确定的时间和性质的下降。长期研究对于阐明不能从短期实验或空间换时间的替代中推断出的模式和过程至关重要。
Stem exclusion and understory reinitiation 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 years 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 fourfold, 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.