落叶阔叶林冠层光合有效辐射分量的遥感模拟与分析

冠层绿色叶片(光合组分)的光合有效辐射分量(绿色FPAR)真实地反映了植被与外界进行物质和能量交换的能力,获取冠层光合组分吸收的太阳光合有效辐射,对生态系统生产力的遥感估算精度的提高具有重要的意义。研究以落叶阔叶林为例,基于SAIL模型模拟森林冠层光合组分和非光合组分吸收的光合有效辐射,研究冠层FPAR变化规律以及与植被指数的相关关系。结果表明,冠层结构的改变会影响冠层对PAR的吸收能力,冠层绿色FPAR的大小与植被面积指数及光合组分面积比相关;在高覆盖度植被区,冠层绿色FPAR占冠层总FPAR的80%以上,非光合组分的贡献较小,但在低植被覆盖区,当光合组分和非光合组分面积相同时,绿色FPAR不及冠层总FPAR的50%;相比于NDVI,北方落叶阔叶林冠层EVI与绿色FPAR存在更为显著的线性相关关系(R~20.99)。

Simulation and analysis on green fraction of absorbed photosynthetically active radiation of deciduous broadleaved forests canopy through remote sensing model

The fraction of absorbed photosynthetically active radiation (FPAR) is an important biophysical variable, widely used in satellite-based production efficiency models to estimate global primary production(GPP) or net primary production. To the forest, the vegetation canopy is composed primarily of photosynthetically active vegetation (PAV; green leaf) and non-photosynthetic vegetation (NPV; e.g., branches and stems). They absorb photosynthetically active radiation(PAR) from the sun, but only the PAR absorbed by PAV is used for photosynthesis. Remote sensing-driven biogeochemical models that use green FPAR(FPAR_PAV) in estimating GPP are more likely to be consistent with plant photosynthesis processes. However, there are no field and laboratory experiments to measure green FPAR at the canopy level. In this study, a method based on the radiative transfer model was used, and the scattering by arbitrary inclined leaves (SAIL) model was modified to classify forest canopy, and to model the canopy spectral reflectance and the PAR absorbed by PAV and NPV of three deciduous broadleaved forests in different scenarios (varied plant area index and leaf area index). Green FPAR of the canopy was calculated based on the PAR absorbed by PAV and total PAR, and the characteristics of the green FPAR and the relation between green FPAR and two vegetation indices (normalize difference vegetation index NDVI] and enhanced vegetation index EVI]) were analyzed. Our results showed that the variation in canopy structure influenced the canopy absorption of PAR, and that green FPAR was related to the plant area index and the ratio of leaf area index to plant area index. In high-coverage forests, the green FPAR was close to the total FPAR of the vegetation canopy(> 80%) and the contribution of NPV to the total FPAR is very low. In contrast, in low-coverage forests or open forests, the difference between green FPAR and the total FPAR was large. Especially when the proportion of the leaf area to plant area low(0.5), the ratio of the green FPAR to the total FPAR was 45.86%. The significant relationships were found between two vegetation index (NDVI and EVI) and green FPAR, but compared with NDVI, EVI was more suitable for describing the variation in the green FPAR.