Canopy Light Absorption and Application of the Light-Use Efficiency Model of Photosynthesis in a Northern Great Plains Grassland

Abstract:

The fraction of absorbed photosynthetically active radiation (f_APAR) is fundamentally important for model calculations of ecosystem productivity across large areas. The main objective of this study was to better understand factors influencing f_APAR, its relationship with seasonal variation in canopy greenness (Normalized Difference Vegetation Index (NDVI)), and the consequences of potential seasonal changes in the NDVI- f_APAR relationship for light-use efficiency model calculations of ecosystem photosynthesis in a semi-arid grassland. We used two approaches to determine f_APAR, (i) incoming and outgoing radiance measurements above and below the canopy, and (ii) an inversion approach based on incident photosynthetically active radiation and the light response curve of net ecosystem productivity (NEP) measured by eddy covariance at low light levels. The two approaches resulted in f_APAR values that were very strongly correlated during the initial development of the canopy until peak leaf area index (LAI) was reached. A strong linear relationship also occurred between f_APAR and NDVI, based on reflectance measurements made along a tram system above the grassland canopy during initial LAI development. After peak LAI, there was hysteresis in the NDVI- f_APAR relationship. Light-use efficiency model calculations of ecosystem photosynthesis made using f_APAR values were strongly correlated with chamber CO₂ exchange measurements during the initial development of the canopy leaf area. After peak LAI, a relative stress function, based on either soil moisture or vapour pressure deficit (VPD) measurements, was necessary to reduce quantum yield and model calculations of ecosystem photosynthesis during periods of relatively low soil moisture and higher VPD later in the growing season. Both stress functions were similarly effective in improving the correlation between modeled and measured ecosystem photosynthesis values.