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Does ASCAT observe the spring reactivation in temperate deciduous broadleaf forests?
Remote Sensing of Environment ( IF 13.5 ) Pub Date : 2020-12-01 , DOI: 10.1016/j.rse.2020.112042
Isabella Pfeil , Wolfgang Wagner , Matthias Forkel , Wouter Dorigo , Mariette Vreugdenhil

Abstract Scatterometer observations over land are sensitive to the water content in soil and vegetation, but have been rarely used to study seasonal changes in the plant water status and seasonal development of deciduous trees. Here we use Advanced Scatterometer (ASCAT) observations to investigate the sensitivity of C-band backscatter to spring phenology of temperate deciduous broadleaf forests in Austria. ASCAT's multi-angle looking capability enables the observation of backscatter over a large range of incidence angles. The vegetation status affects the slope of the backscatter-incidence angle relationship. We discovered a maximum in the slope around the month April, hereafter referred to as spring peak, predominantly in regions covered by deciduous broadleaf forest. We hypothesized that the spring peak indicates the average timing of leaf emergence in the deciduous trees in the sensor footprint. The hypothesis was tested by comparing the timing of the spring peak to leaf unfolding observations from the PEP725 phenology database, to the increase of leaf area index (LAI) during spring, and to temperature. Our results demonstrate a good agreement between the ASCAT spring peaks, phenology observations and temperature conditions. The steepest increase in LAI however lags behind the ASCAT peak by several days to a few weeks, suggesting that the spring peak in fact marks the timing of maximum woody water content, which occurs right before leaf emergence. Based on these observations, we conclude that the ASCAT signal has a high sensitivity to spring reactivation and in particular water uptake of bare deciduous broadleaf trees. Our findings might provide the basis for novel developments to estimate eco-physiological changes of forests during spring at large scales.

中文翻译:

ASCAT 是否观察到温带落叶阔叶林的春季恢复活动?

摘要 陆地上的散射计观测对土壤和植被中的水分含量很敏感,但很少用于研究植物水分状况的季节性变化和落叶乔木的季节性发展。在这里,我们使用高级散射仪 (ASCAT) 观测来研究 C 波段反向散射对奥地利温带落叶阔叶林春季物候的敏感性。ASCAT 的多角度观察能力可以在大范围的入射角上观察反向散射。植被状况影响反向散射-入射角关系的斜率。我们在 4 月左右的斜坡上发现了一个最大值,以下称为春季峰值,主要位于落叶阔叶林覆盖的地区。我们假设春季峰值表示传感器足迹中落叶树中叶子出现的平均时间。通过比较春季峰值的时间与来自 PEP725 物候数据库的叶片展开观察、春季叶面积指数 (LAI) 的增加以及温度的比较来检验该假设。我们的结果表明 ASCAT 弹簧峰值、物候观察和温度条件之间具有良好的一致性。然而,LAI 的最陡峭增加滞后于 ASCAT 峰值几天到几周,这表明春季峰值实际上标志着最大木质水分含量的时间,它发生在叶子出现之前。基于这些观察,我们得出结论,ASCAT 信号对春季再激活,特别是裸落叶阔叶树的吸水具有高度敏感性。我们的研究结果可能为大规模估计春季森林生态生理变化的新发展提供基础。
更新日期:2020-12-01
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