Seasonal and interannual variability of climate and vegetation indices across the Amazon
Seasonal and interannual variability of climate and vegetation indices across the Amazon
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DOI：
10.1073/pnas.0908741107
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发表时间：
2010-08-17
影响因子：
11.1
通讯作者：
Christman, Mary C.
中科院分区：
文献类型：
--
作者：
Brando, Paulo M.;Goetz, Scott J.;Christman, Mary C.
干旱对热带森林的新陈代谢、碳储量以及最终进入大气的碳通量都有很大的影响。基于卫星的研究表明，由于日照增加，亚马逊森林在干旱期间变绿，而实地研究报告称，在严重干旱期间，树木死亡率增加。为了协调这些明显相互矛盾的发现，我们对气候数据、实地测量和改进的基于卫星的森林光合作用活动测量进行了分析。从1996年到2005年，亚马孙盆地的雨季降雨量和植物有效水(PAW)减少，而从2002年到2005年，光合作用有效辐射(PAR)和空气干燥度(以水汽压差表示)增加。利用改进的增强植被指数(EVI)测量(2000-2008年)，我们发现，在研究的每一年，稀疏树冠覆盖地区的总初级生产力(以EVI表示)随着VPD和PAW的下降而下降。在森林茂密的地区，没有任何气候变量能够充分解释整个流域EVI的年际变化。基于特定地点的研究，我们发现月度EVI对叶面积指数(LAI)相对不敏感，但与田间测量的叶片冲刷和PAR呈正相关。这些发现表明，新叶的产生，即使没有伴随LAI的相关变化，也可能在整个盆地范围内的EVI年际变化中发挥重要作用。由于EVI在下爪区域的变异性最大，我们假设干旱可能通过影响叶芽发育来同步叶片的冲洗，从而增加EVI。
Drought exerts a strong influence on tropical forest metabolism, carbon stocks, and ultimately the flux of carbon to the atmosphere. Satellite-based studies have suggested that Amazon forests green up during droughts because of increased sunlight, whereas field studies have reported increased tree mortality during severe droughts. In an effort to reconcile these apparently conflicting findings, we conducted an analysis of climate data, field measurements, and improved satellite-based measures of forest photosynthetic activity. Wet-season precipitation and plant-available water (PAW) decreased over the Amazon Basin from 1996-2005, and photosynthetically active radiation (PAR) and air dryness (expressed as vapor pressure deficit, VPD) increased from 2002-2005. Using improved enhanced vegetation index (EVI) measurements (2000-2008), we show that gross primary productivity (expressed as EVI) declined with VPD and PAW in regions of sparse canopy cover across a wide range of environments for each year of the study. In densely forested areas, no climatic variable adequately explained the Basin-wide interannual variability of EVI. Based on a site-specific study, we show that monthly EVI was relatively insensitive to leaf area index (LAI) but correlated positively with leaf flushing and PAR measured in the field. These findings suggest that production of new leaves, even when unaccompanied by associated changes in LAI, could play an important role in Basin-wide interannual EVI variability. Because EVI variability was greatest in regions of lower PAW, we hypothesize that drought could increase EVI by synchronizing leaf flushing via its effects on leaf bud development.