Institute of Carbon Neutrality, Sino-French Institute for Earth System Science, College of Urban and Environmental Sciences, Peking University, Beijing, China.
State Key Laboratory of Tibetan Plateau Earth System, Resources and Environment, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, China.
Nat Ecol Evol. 2024 May;8(5):912-923. doi: 10.1038/s41559-024-02372-1. Epub 2024 Mar 11.
Vegetation greening has been suggested to be a dominant trend over recent decades, but severe pulses of tree mortality in forests after droughts and heatwaves have also been extensively reported. These observations raise the question of to what extent the observed severe pulses of tree mortality induced by climate could affect overall vegetation greenness across spatial grains and temporal extents. To address this issue, here we analyse three satellite-based datasets of detrended growing-season normalized difference vegetation index (NDVI) with spatial resolutions ranging from 30 m to 8 km for 1,303 field-documented sites experiencing severe drought- or heat-induced tree-mortality events around the globe. We find that severe tree-mortality events have distinctive but localized imprints on vegetation greenness over annual timescales, which are obscured by broad-scale and long-term greening. Specifically, although anomalies in NDVI (ΔNDVI) are negative during tree-mortality years, this reduction diminishes at coarser spatial resolutions (that is, 250 m and 8 km). Notably, tree-mortality-induced reductions in NDVI (|ΔNDVI|) at 30-m resolution are negatively related to native plant species richness and forest height, whereas topographic heterogeneity is the major factor affecting ΔNDVI differences across various spatial grain sizes. Over time periods of a decade or longer, greening consistently dominates all spatial resolutions. The findings underscore the fundamental importance of spatio-temporal scales for cohesively understanding the effects of climate change on forest productivity and tree mortality under both gradual and abrupt changes.
植被变绿被认为是近几十年来的主导趋势，但在干旱和热浪之后，森林中树木大量死亡的严重情况也时有发生。这些观察结果提出了一个问题，即在多大程度上，气候引起的严重树木死亡脉冲可能会影响整个植被的绿色度，无论是在空间粒度上还是在时间尺度上。为了解决这个问题，我们在这里分析了三个基于卫星的去趋势生长季节归一化差异植被指数（NDVI）数据集，这些数据集的空间分辨率从 30 米到 8 公里不等，涵盖了全球 1303 个实地记录的严重干旱或热诱发树木死亡事件的地点。我们发现，严重的树木死亡事件在年度时间尺度上对植被绿色度有独特但局部的影响，但这些影响被广泛的和长期的绿化所掩盖。具体来说，尽管树木死亡年份的 NDVI 异常（ΔNDVI）为负值，但这种减少在较粗的空间分辨率（即 250 米和 8 公里）下会减弱。值得注意的是，30 米分辨率下 NDVI（|ΔNDVI|）的树木死亡引起的减少与本地植物物种丰富度和森林高度呈负相关，而地形异质性是影响各种空间粒度下 ΔNDVI 差异的主要因素。在十年或更长的时间内，绿化始终主导着所有的空间分辨率。这些发现强调了时空尺度对于协调理解气候变化对森林生产力和树木死亡的影响的重要性，无论是在逐渐变化还是突然变化的情况下。
