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首页> 外文期刊>Agricultural and Forest Meteorology >Spatial modelling of the fraction of photosynthetically active radiation absorbed by a boreal mixedwood forest using a lidar-hyperspectral approach.
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Spatial modelling of the fraction of photosynthetically active radiation absorbed by a boreal mixedwood forest using a lidar-hyperspectral approach.

机译：使用激光雷达-高光谱方法对北方混合木森林吸收的光合有效辐射部分进行空间建模。

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摘要

The spatial variability of the fraction of photosynthetically active radiation absorbed by the canopy (fPAR) was characterized for a heterogeneous boreal mixedwood forest site located in northern Ontario, Canada, based on relationships found between fPAR and light detection and ranging (lidar) data over different canopy architectures. Estimates of fPAR were derived from radiation measurements made above the canopy at a flux tower and below-canopy radiation was measured across a range of species compositions and canopy architectures. Airborne lidar data were used to characterize spatial variability of canopy structure around the flux tower and a map of mean canopy chlorophyll concentration was derived from airborne hyperspectral imagery. Different volumes of lidar points for the locations directly above each photosynthetically active radiation (PAR) sensor were examined to determine if there is an optimal method of relating lidar returns to estimated fPAR values. The strongest correlations between mean lidar height and fPAR occurred when using points that fell within a theoretical cone which originated at the PAR sensor having a solid angle alpha =55 degrees . For diffuse conditions, the correlation (r) between mean lidar height versus fPAR x chlorophyll was stronger than between mean lidar height versus fPAR by 8% for mean daily fPAR and from 10 to 20% for diurnal fPAR, depending on solar zenith angle. For direct light conditions, the relationship was improved by 12% for mean daily fPAR and 12-41% for diurnal relationships. Linear regression models of mean daily fPAR x chlorophyll versus mean lidar height were used in conjunction with gridded lidar data and the canopy chlorophyll map to generate maps of mean daily fPAR for direct and diffuse sunlight conditions. Site average fPAR calculated from these maps was 0.79 for direct light conditions and 0.78 for diffuse conditions. When compared to point estimates of mean daily fPAR calculated on the tower, the average fPAR was significantly lower than the point estimate. Subtracting the direct sunlight fPAR map from the diffuse sunlight fPAR map revealed a distinct spatial pattern showing that areas with open canopies and relatively low chlorophyll (e.g., black spruce patches) have a higher fPAR under direct sunlight conditions, while closed canopies with higher chlorophyll (e.g., deciduous species) absorb more PAR under diffuse conditions. These findings have implications for scaling from point measurements at flux towers to larger resolution satellite imagery and addressing local scale heterogeneity in flux tower footprints..
机译：基于fPAR与光检测之间的关系以及不同范围的测距（激光）数据之间的关系，对位于加拿大安大略省北部的异质混交林林地而言，通过冠层（fPAR）吸收的光合有效辐射部分的空间变异性得到了表征。冠层架构。 fPAR的估算值是从通量塔顶盖上方进行的辐射测量得出的，而顶盖下方的辐射是通过一系列物种组成和顶盖结构进行测量的。机载激光雷达数据用于表征通量塔周围冠层结构的空间变异性，并从机载高光谱图像中得出平均冠层叶绿素浓度的地图。检查了每个光合有效辐射（PAR）传感器正上方位置的激光雷达点的不同体积，以确定是否存在将激光雷达返回值与估计的fPAR值相关联的最佳方法。当使用落在理论锥内的点产生平均激光雷达高度与fPAR之间的最强相关性，该点起源于立体角为alpha = 55度的PAR传感器。对于漫射条件，平均日平均fPAR的平均激光雷达高度与fPAR x叶绿素之间的相关性（r）比平均激光雷达高度与fPAR之间的相关性强（8％），而对于日间fPAR则为10％至20％，具体取决于太阳天顶角。对于直接光照条件，平均每日fPAR的关系改善了12％，而昼间关系的关系改善了12-41％。日平均fPAR x叶绿素对平均激光雷达高度的线性回归模型与栅格化的激光雷达数据和冠层叶绿素图结合使用，以生成直射和漫射阳光条件下的日平均fPAR图。从这些图计算得出的站点平均fPAR在直射光条件下为0.79，在扩散条件下为0.78。与塔上计算的每日平均fPAR的点估计值相比，平均fPAR显着低于点估计值。从漫射日光fPAR图中减去直接日光fPAR图显示了独特的空间格局，表明在阳光直射条件下，开放冠层和叶绿素相对较低的区域（例如黑云杉斑块）具有较高的fPAR，而封闭冠层具有较高的叶绿素（例如，落叶物种）在扩散条件下吸收更多的PAR。这些发现对于从磁通量塔的点测量到更高分辨率的卫星图像的缩放以及解决磁通量塔足迹中局部尺度的异质性具有意义。

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