Estimation of photosynthetic dynamics in forests from daily measured fluorescence and PRI data with adjustment for canopy shadow fraction

Daniel Kováč; Jan Novotný; Ladislav Šigut; Alexander Ač; Josep Peñuelas; John Grace; Otmar Urban

doi:10.1016/j.scitotenv.2023.166386

Estimation of photosynthetic dynamics in forests from daily measured fluorescence and PRI data with adjustment for canopy shadow fraction

Sci Total Environ. 2023 Nov 10:898:166386. doi: 10.1016/j.scitotenv.2023.166386. Epub 2023 Aug 18.

Authors

Daniel Kováč¹, Jan Novotný², Ladislav Šigut², Alexander Ač², Josep Peñuelas³, John Grace⁴, Otmar Urban²

Affiliations

¹ Global Change Research Institute of the Czech Academy of Sciences, Bělidla 986/4a, 603 00 Brno, Czech Republic. Electronic address: kovac.d@czechglobe.cz.
² Global Change Research Institute of the Czech Academy of Sciences, Bělidla 986/4a, 603 00 Brno, Czech Republic.
³ Global Change Research Institute of the Czech Academy of Sciences, Bělidla 986/4a, 603 00 Brno, Czech Republic; CSIC, Global Ecology Unit CREAF-CSIC-UAB, E-08193 Bellaterra, Catalonia, Spain; CREAF, E-08193 Cerdanyola del Vallès, Catalonia, Spain.
⁴ Global Change Research Institute of the Czech Academy of Sciences, Bělidla 986/4a, 603 00 Brno, Czech Republic; School of GeoSciences, University of Edinburgh, Crew Bldg, Kings Bldgs, Alexander Crum Brown Rd, Edinburgh EH9 3FF, UK.

PMID: 37597564
DOI: 10.1016/j.scitotenv.2023.166386

Abstract

We conducted year-long measurements of the photochemical reflectance index (PRI) and solar-induced fluorescence in the O₂A oxygen band (SIFA) at a Norway spruce forest and a European beech forest to study relationships of these remote sensing variables to photosynthesis by trees in grown forest stands. Measured PRI and SIFA values were linked to changes in forest gross primary productivity (GPP) and light-use efficiency (LUE). Changes in the shadow fraction (α_S) within tree crowns influenced PRI and fluorescence signals. In the spruce forest, the quantum yield of SIFA (FYSIFA) decreased around midday together with photosynthesis and GPP. Such decreases in FYSIFA were accompanied by an increase in the α_S. In the beech forest, we detected an increase in FYSIFA together with a decrease in α_S in the afternoon hours. The overall sensitivity of PRI to LUE was variable according to the season, presumably influenced by complex changes in photosynthetic pigments. PRI and FYSIFA showed weak correlations with canopy LUE; however, when considered together, the correlation was strengthened (R² was 0.63 and 0.34 in spruce and beech forest, respectively). Our model predicting LUE dynamics includes a diurnal minimum of PRI and canopy α_S to make allowances for seasonal changes in photosynthetic pigments and for diurnal variability of the shadow fraction in forests. The incorporation of these correcting factors allowed us to estimate LUE at R² = 0.68 (spruce) and 0.53 (beech). The modeling equations appeared sensitive to the absorbed photosynthetically active radiation (APAR), but less sensitive to the GPP of these forests. Substituting pigments correction with introducing differential PRI (ΔPRI) into the model did not significantly improve the LUE estimation across the season. Our results show that the joint use of PRI and fluorescence improves LUE and GPP estimation accuracy in both daily and seasonal observations.

Keywords: 3-FLD; Daily dynamics; Fluorescence; LiDAR; Photochemical reflectance index; Shadow fraction.

MeSH terms

Fagus*
Forests
Photosynthesis
Picea*
Seasons
Sunlight
Trees