Department of Forestry, Michigan State University, East Lansing, MI, 48824, USA.
Ecology & Evolutionary Biology, University of Arizona, Tucson, AZ, 85721, USA.
New Phytol. 2019 May;222(3):1284-1297. doi: 10.1111/nph.15726. Epub 2019 Mar 9.
Seasonal dynamics in the vertical distribution of leaf area index (LAI) may impact the seasonality of forest productivity in Amazonian forests. However, until recently, fine-scale observations critical to revealing ecological mechanisms underlying these changes have been lacking. To investigate fine-scale variation in leaf area with seasonality and drought we conducted monthly ground-based LiDAR surveys over 4 yr at an Amazon forest site. We analysed temporal changes in vertically structured LAI along axes of both canopy height and light environments. Upper canopy LAI increased during the dry season, whereas lower canopy LAI decreased. The low canopy decrease was driven by highly illuminated leaves of smaller trees in gaps. By contrast, understory LAI increased concurrently with the upper canopy. Hence, tree phenological strategies were stratified by height and light environments. Trends were amplified during a 2015-2016 severe El Niño drought. Leaf area low in the canopy exhibited behaviour consistent with water limitation. Leaf loss from short trees in high light during drought may be associated with strategies to tolerate limited access to deep soil water and stressful leaf environments. Vertically and environmentally structured phenological processes suggest a critical role of canopy structural heterogeneity in seasonal changes in Amazon ecosystem function.
季节变化会影响亚马逊森林的生产力。然而，直到最近，揭示这些变化背后生态机制的关键精细尺度观测还很缺乏。为了研究叶片随季节和干旱的精细尺度变化，我们在亚马逊森林的一个地点进行了为期 4 年的每月地面激光雷达调查。我们分析了沿着冠层高度和光照环境轴的垂直结构 LAI 的时间变化。上层冠层 LAI 在旱季增加，而下层冠层 LAI 减少。低冠层的减少是由空隙中较小树木的高光照叶片驱动的。相比之下，林下 LAI 与上层冠层同时增加。因此，树木物候策略按高度和光照环境分层。在 2015-2016 年严重厄尔尼诺干旱期间，趋势被放大。冠层下部叶片的面积减少与水分限制一致。在干旱期间，高光下的小树叶片损失可能与耐受有限的深层土壤水和有压力的叶片环境的策略有关。垂直和环境结构的物候过程表明，树冠结构异质性在亚马逊生态系统功能的季节性变化中起着关键作用。
