Nature2014-06-26 2:22 PM

亚马逊森林旱季“变绿”实为光造成的假象 Amazon forests maintain consistent canopy structure and greenness during the dry season

论文摘要 

最近来自亚马逊的遥感数据显示,旱季植被会变绿,这说明森林生产力的主要限制因素是光而不是水。现在,Douglas Morton及同事对该证据重新进行了分析,发现“变绿”事实上是观察方法在光学上产生的一个假象,是由卫星观察的相对方位角在六月份的夏至和九月份的秋分之间的变化造成的。纠正这一偏差会将“变绿”现象消除,从而为证明亚马逊森林植物生产力的主要驱动因素是水分供应而非光的其他研究进一步提供了支持。

Abstract 

The seasonality of sunlight and rainfall regulates net primary production in tropical forests. Previous studies have suggested that light is more limiting than water for tropical forest productivity, consistent with greening of Amazon forests during the dry season in satellite data. We evaluated four potential mechanisms for the seasonal green-up phenomenon, including increases in leaf area or leaf reflectance, using a sophisticated radiative transfer model and independent satellite observations from lidar and optical sensors. Here we show that the apparent green up of Amazon forests in optical remote sensing data resulted from seasonal changes in near-infrared reflectance, an artefact of variations in sun-sensor geometry. Correcting this bidirectional reflectance effect eliminated seasonal changes in surface reflectance, consistent with independent lidar observations and model simulations with unchanging canopy properties. The stability of Amazon forest structure and reflectance over seasonal timescales challenges the paradigm of light-limited net primary production in Amazon forests and enhanced forest growth during drought conditions. Correcting optical remote sensing data for artefacts of sun-sensor geometry is essential to isolate the response of global vegetation to seasonal and interannual climate variability.

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