Quevedo Rojas Ana Mercedes, Schwarzkopf Teresa, García Carlos, Jerez Rico Mauricio
Rev Biol Trop. 2016 Dec;64(4):1699-1707.
The light environment in the understory of cloud forests is highly heterogeneous and determined by species composition, canopy structure, site conditions, and seasonality. This study was carried out at San Eusebio cloud forest, Venezuela (2 300 - 2 500 masl). The impact of canopy structure variations on understory light availability was estimated in the dry (December-February) and rainy (March-November) seasons, in sites under continuous canopy cover and gaps of various sizes. Hemispherical photographs were taken to estimate the percentage of canopy openness, leaf area index, percentages of transmitted direct and diffuse light, and duration and frequency of sunflecks. A light index was calculated from the relative proportions of direct and diffuse light transmitted to the understory. For most variables, there were significant differences between seasons, as well as among different gap sizes and under closed canopy. The light index was low (0.25 to 26 of a maximum = 100), even for the largest gaps, indicating a highly shaded light environment, especially beneath closed canopy in the rainy season. Patterns and interactions among factors were analyzed (gaps vs. continuous canopy, gap sizes, location within the gaps, and seasonality) with a mixed effects repeated measures Anova design. Results showed that the amount of light reaching the understory is low in both gaps and closed canopy. However, small but significant differences in light availability existed for both seasonality and magnitude of the perturbations. These differences could contribute to explain the dynamics of tree species regeneration in this forest. The knowledge of the factors conditioning light availability in the understory where tree regeneration begins is crucial in cloud forests because of energetic limitations in this ecosystem, and might be essential for future restoration and conservation plans concerning the preservation of the diversity and integrity of these forests.
云雾森林林下的光照环境高度异质，由物种组成、树冠结构、立地条件和季节性决定。本研究在委内瑞拉的圣欧塞维奥云雾森林（海拔2300 - 2500米）进行。在旱季（12月至2月）和雨季（3月至11月），对连续树冠覆盖区域和不同大小林窗下的林下光照可利用性受树冠结构变化的影响进行了估算。通过拍摄半球形照片来估算树冠开阔度百分比、叶面积指数、直射和散射光透射百分比以及光斑持续时间和频率。根据透射至林下的直射光和散射光的相对比例计算出一个光照指数。对于大多数变量，季节之间、不同林窗大小之间以及郁闭树冠下均存在显著差异。即使是最大的林窗，光照指数也较低（最大值为100时为0.25至26），这表明光照环境高度荫蔽，尤其是在雨季的郁闭树冠下。采用混合效应重复测量方差分析设计对各因素之间的模式和相互作用（林窗与连续树冠、林窗大小、林窗内位置以及季节性）进行了分析。结果表明，到达林下的光照量在林窗和郁闭树冠下均较低。然而，在季节性和干扰强度方面，光照可利用性存在微小但显著的差异。这些差异有助于解释该森林中树木物种更新的动态。由于该生态系统存在能量限制，了解树木更新起始的林下光照可利用性的影响因素在云雾森林中至关重要，并且可能对未来有关这些森林多样性和完整性保护的恢复和保护计划至关重要。
