Vaughn Nicholas R, Asner Gregory P, Giardina Christian P
Ecol Appl. 2014;24(7):1638-50. doi: 10.1890/13-1568.1.
Fragmentation poses one of the greatest threats to tropical forests with short-term changes to the structure of forest canopies affecting microclimate, tree mortality, and growth. Yet the long-term effects of fragmentation are poorly understood because (1) most effects require many decades to materialize, but long-term studies are very rare, (2) the effects of edges on forest canopy structure as a function of fragment size are unknown, and (3) edge effects are often confounded by fragment shape. We quantified the long-term (centennial) effects of fragmentation on forest canopy structure using airborne light detection and ranging (LiDAR) of 1060 Hawaiian rain forest fragments ranging in size from 0.02 to 1000 ha, created more than 130 years ago by flowing lava. Along with distance from edge, we developed a metric, minimum span, to gain additional insight into edge effects on three measures of canopy structure: canopy height, height variation, and gap fraction. Fragment size was a strong determinant of the three structural variables. Larger fragments had greater average height, larger variation in height, and smaller gap fraction. Minimum span had a large effect on the depth and magnitude of edge effects for the three structural variables. Locations associated with high span values (those surrounded by more forest habitat) showed little effect of distance to fragment edge. In contrast, locations with low span values (those more exposed to edges) were severely limited in canopy height, showed lower height variation, and were associated with greater gap fraction values. The minimum span attribute allows for a more accurate characterization of edge as well as fragment-level effects, and when combined with high resolution imagery, can improve planning of protected areas for long-term ecological sustainability and biodiversity protection.
森林碎片化对热带森林构成了最严重的威胁之一，森林冠层结构的短期变化会影响小气候、树木死亡率和生长。然而，碎片化的长期影响却鲜为人知，原因如下：（1）大多数影响需要数十年才能显现，但长期研究非常罕见；（2）边缘对森林冠层结构的影响作为碎片大小的函数尚不清楚；（3）边缘效应常常与碎片形状相互混淆。我们利用机载激光雷达（LiDAR）对1060个夏威夷雨林碎片进行了长期（百年）研究，这些碎片大小从0.02公顷到1000公顷不等，是130多年前由流动的熔岩形成的，以此来量化碎片化对森林冠层结构的影响。除了与边缘的距离外，我们还开发了一个指标——最小跨度，以进一步了解边缘对冠层结构的三个指标的影响：冠层高度、高度变化和间隙率。碎片大小是这三个结构变量的一个重要决定因素。较大的碎片平均高度更高，高度变化更大，间隙率更小。最小跨度对这三个结构变量的边缘效应的深度和幅度有很大影响。与高跨度值相关的位置（那些被更多森林栖息地包围的位置）显示出距离碎片边缘的影响很小。相比之下，低跨度值的位置（那些更靠近边缘的位置）冠层高度受到严重限制，高度变化较小，且间隙率值更大。最小跨度属性能够更准确地描述边缘以及碎片级别的效应，并且与高分辨率图像相结合时，可以改善保护区的规划，以实现长期生态可持续性和生物多样性保护。
