Department of Geography, University of British Columbia, 1984 West Mall, Vancouver, British Columbia, V6T 1Z2, Canada.
Department of Ecology, Evolution, and Behavior, University of Minnesota, 1479 Gortner Avenue, St. Paul, Minnesota, 55108, USA.
Ecology. 2019 Jun;100(6):e02677. doi: 10.1002/ecy.2677. Epub 2019 Apr 29.
Climate models predict increases in drought conditions in many parts of the tropics. Yet the response of tropical forests to drought remains highly uncertain, especially with regards to the factors that generate spatial heterogeneity in drought response across landscapes. In this study, we used Landsat imagery to assess the impacts of a severe drought in 2015 across an ~80,000-ha landscape in Puerto Rico. Specifically, we asked whether drought effects varied systematically with topography and with forest age, height, and fragmentation. We quantified drought impacts using anomalies of two vegetation indices, the enhanced vegetation index (EVI) and normalized difference water index (NDWI), and fit random forest models of these metrics including slope, aspect, forest age, canopy height, and two indices of fragmentation as predictors. Drought effects were more severe on drier topographic positions, that is, steeper slopes and southwest-facing aspects, and in second-growth forests. Shorter and more fragmented forests were also more strongly affected by drought. We also assessed which factors were associated with stronger recovery from drought. Factors associated with more negative drought anomalies were also associated with more positive postdrought anomalies, suggesting that increased light availability as a result of drought led to high rates of recovery in forests more severely affected by drought. In general, recovery from drought was rapid across the landscape, with postdrought anomalies at or above average across the study area. This suggests that forests in Puerto Rico might be resilient to a single-year drought, though vulnerability to drought varies depending on forest characteristics and landscape position.
气候模型预测，热带地区的许多地方干旱条件将会增加。然而，热带森林对干旱的反应仍然高度不确定，尤其是在产生景观中干旱反应空间异质性的因素方面。在这项研究中，我们使用 Landsat 图像评估了 2015 年波多黎各一个约 8 万公顷景观中严重干旱的影响。具体来说，我们询问了干旱效应是否与地形以及森林年龄、高度和破碎化系统地变化。我们使用两个植被指数（增强植被指数 EVI 和归一化差异水指数 NDWI）的异常值来量化干旱影响，并拟合了这些指标的随机森林模型，包括坡度、方位、森林年龄、冠层高度和两个破碎化指数作为预测因子。在较干燥的地形位置（即较陡的坡度和西南朝向的方位）和次生林，干旱影响更为严重。较短和较破碎的森林也受到干旱的强烈影响。我们还评估了哪些因素与干旱后的更强恢复有关。与更负的干旱异常相关的因素也与更积极的旱后异常相关，这表明干旱导致的光照可用性增加导致受干旱影响更严重的森林恢复速度更快。总体而言，干旱后的恢复在整个景观中非常迅速，受旱后异常在研究区域内处于或高于平均值。这表明波多黎各的森林可能对单一年份的干旱具有弹性，尽管对干旱的脆弱性取决于森林特征和景观位置。
