In-situ spectroscopy and shortwave radiometry reveals spatial and temporal variation in the crown-level radiative performance of urban trees
In-situ spectroscopy and shortwave radiometry reveals spatial and temporal variation in the crown-level radiative performance of urban trees
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DOI：
10.1016/j.rse.2020.112231
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发表时间：
2021-02
影响因子：
13.5
通讯作者：
Jie Deng;B. J. Pickles;Li Shao
中科院分区：
文献类型：
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作者：
Jie Deng;B. J. Pickles;Li Shao
在传统的小气候环境建模和城市热缓解的树木规划策略的发展中，任何给定物种的树冠表面覆盖率被假定为常数。然而，我们最近对城市树木在树冠水平上的辐射性能的研究表明，树冠表面辐射确实随时间而变化。基于先前建立的测量树冠透反射率的原位光谱学协议，结合光谱学和太阳短波辐射测量，探索树冠表面透反射率特性的变化。三种常见的英国本土树种，鹅耳枥，槭campestre，和红豆杉，取样。在固定的太阳高度测量的树冠透反射率的空间分布轮廓进行了归一化的原位分光辐射计。在近红外（NIR）区域的树冠透反射率被发现与树冠表面反射率成比例地联系在一起。在每个物种中，在800-900 nm的近红外区域的平均树冠透反射率约为树冠表面反射率的2.5倍。进一步发现，红外辐射（700-2500 nm）占总透过反射的短波辐射从树冠90%以上。结果表明，树冠表面最大高度与瞬时太阳高度呈线性关系，夏季晴天正午最大高度对应于最大树冠表面最大高度。如果考虑到明显密集的树冠叶，树冠表面跨物种的重叠往往强烈依赖于叶的大小。我们的研究结果提供了重要的见解树木的辐射遮荫效果所造成的树冠表面的时间变化，城市小气候建模和城市热缓解策略的发展的后果。
In conventional microclimate environment modelling, and the development of tree planning strategies for urban heat mitigation, tree crown surface albedo for any given species is assumed to be a constant. However, our recent research into urban tree radiative performance at the crown level implied that tree crown surface albedo changes over time. Based on the in-situ spectroscopy protocols established previously to measure tree crown transflectance, variation in the characteristics of tree crown surface albedo was explored combining spectroscopy and solar shortwave radiometry. Three commonly planted native UK tree species,Carpinus betulus,Acer campestre, andTaxus baccata, were sampled. Spatial distribution profiles of tree crown transflectance measured at fixed solar altitudes were normalised by in-situ spectroradiometry. Tree crown transflectance in the near infrared (NIR) region was found to be proportionally linked to tree crown surface albedo. Within each species, mean tree crown transflectance in the NIR region of 800–900 nm was approximately 2.5 times tree crown surface albedo. It was further found that infrared radiation (700–2500 nm) accounted for more than 90% of the total transflected shortwave radiation from tree crowns. The results demonstrate that tree crown surface albedo linearly increases with momentary solar altitude and the maximum tree crown surface albedo corresponds to maximum solar altitude at solar noon on sunny days in summer. Tree crown surface albedo across species tends to be strongly dependent on leaf size if considering visibly dense crown foliage. Our findings provide important insights into tree radiative shading effects resulting from temporal variation in tree crown surface albedo, with consequences for urban microclimate modelling and the development of urban heat mitigation strategies.