The estimation of light transmittance through grassland canopy: Searching for a suitable indirect indicator

In grasslands, the transmittance of photosynthetically active radiation through the canopy (RLIg) is a key factor in terms of ecosystem productivity and biodiversity. However, direct RLIg measurement is time-consuming, and the estimation is difficult because the relationship between RLIg and other canopy characteristics has not been sufficiently explored. We aimed to determine the relationships between RLIg for vegetation with a different cover of forbs, and easily measurable, potential proxy indicators: average canopy height (h), fresh aboveground biomass (FM), and several indices obtained from Sentinel-2 satellites: leaf area index (LAI), normalized difference vegetation index (NDVI), enhanced vegetation index (EVI) and structure insensitive pigment index (SIPI). Empirical data was collected from 2020 to 2023 in 22 mesic grasslands located in Poland. A clear pattern of relationship with RLIg was observed for h, FM, and LAI: the increase in these parameters coincided with the decrease in RLIg. This decrease was strong when h, FM and LAI were still low and became moderate when h, FM, and LAI increased to a certain threshold. This shift in the RLIg trend line was more abrupt and occurred earlier in canopies with high herb cover. This relationship can be accurately modeled using a broken line regression. Photosynthetically active radiation (PAR) is almost unavailable near the ground in swards taller than 15 cm, with LAI greater than 4.5 and aboveground biomass greater than 1.0 kg m−2. Due to the complexity of the models obtained, practical estimation of RLIg based on h, FM, or LAI may be difficult.