Disentangling Structural Controls on Airborne SIF: Unexpected Pasture–Forest Contrasts at Alice Holt Forest

Solar-induced chlorophyll fluorescence (SIF) provides a direct optical signature of photosynthetic light reactions and is used to assess vegetation physiological function across spatial scales. However, top-of-canopy SIF measured from airborne platforms can be strongly modulated by canopy structure, and sun–shade geometry. As part of a larger effort to diagnose structural versus physiological controls on SIF at Alice Holt Forest, we report preliminary findings from high-resolution IBIS airborne SIF and Fenix hyperspectral data acquired over mixed oak-dominated woodland and adjacent pasture in southern England.

Initial analysis confirmed that pasture showed higher mean SIF than the neighbouring forest, despite the forest having higher NDVI, EVI and NIRv. Given the dense, vertically complex canopy at Alice Holt, we hypothesize that shading and within-canopy radiative transfer reduce the apparent SIF emitted from the forest canopy, whereas the pasture—being uniformly sunlit—preserves a higher top-of-canopy signal.

To test this hypothesis, we combine multi-method FLD analyses with targeted radiative-transfer experiments (DART) to quantify how much of the observed pasture–forest SIF contrast arises from canopy geometry rather than physiology.

This study provides high-resolution evaluations of structural biases in airborne SIF over temperate woodland and highlights the need to account for canopy shading when interpreting SIF–photosynthesis relationships.