Department of Geosciences and Natural Resource Management, University of Copenhagen, Copenhagen, Denmark.
School of GeoSciences, University of Edinburgh, Edinburgh, EH9 3FF, United Kingdom.
Sci Rep. 2017 Jun 14;7(1):3505. doi: 10.1038/s41598-017-03469-3.
There is an urgent need to quantify anthropogenic influence on forest carbon stocks. Using satellite-based radar imagery for such purposes has been challenged by the apparent loss of signal sensitivity to changes in forest aboveground volume (AGV) above a certain 'saturation' point. The causes of saturation are debated and often inadequately addressed, posing a major limitation to mapping AGV with the latest radar satellites. Using ground- and lidar-measurements across La Rioja province (Spain) and Denmark, we investigate how various properties of forest structure (average stem height, size and number density; proportion of canopy and understory cover) simultaneously influence radar backscatter. It is found that increases in backscatter due to changes in some properties (e.g. increasing stem sizes) are often compensated by equal magnitude decreases caused by other properties (e.g. decreasing stem numbers and increasing heights), contributing to the apparent saturation of the AGV-backscatter trend. Thus, knowledge of the impact of management practices and disturbances on forest structure may allow the use of radar imagery for forest biomass estimates beyond commonly reported saturation points.
量化人为因素对森林碳储量的影响已迫在眉睫。由于在某种“饱和”点之上，森林地上生物量（AGV）的变化对卫星雷达图像信号的敏感性明显下降，因此利用基于卫星的雷达图像来实现这一目标具有一定的挑战性。关于饱和的原因存在争议，且往往没有得到充分解决，这对利用最新的雷达卫星来绘制 AGV 图造成了重大限制。本研究利用西班牙拉里奥哈省和丹麦的地面和激光雷达测量数据，调查了森林结构的各种特性（平均茎干高度、大小和数量密度；冠层和林下覆盖物的比例）如何同时影响雷达后向散射。研究结果表明，由于某些特性（例如，茎干尺寸增大）导致的后向散射增加，往往会被其他特性（例如，茎干数量减少和高度增加）导致的同等幅度的减少所抵消，这导致了 AGV-后向散射趋势的明显饱和。因此，了解管理实践和干扰对森林结构的影响可能会允许在通常报告的饱和点之外，利用雷达图像来估算森林生物量。
