Impacts of forest structures on canopy SIF modelled for FLEX Cal/Val purposes with Discrete Anisotropic Radiative Transfer

0570664 - ÚVGZ 2024 eng A3 - Přednáška/prezentace nepublikovaná
Malenovský, Z. - Janoutová, Růžena - Homolová, Lucie - Regaieg, O. - Wang, Y. - Lauret, N. - Guilleux, J. - Chavanon, E. - Gastellu-Etchegorry, J. P.
Impacts of forest structures on canopy SIF modelled for FLEX Cal/Val purposes with Discrete Anisotropic Radiative Transfer.
[2022 LIVING PLANET SYMPOSIUM. Bonn, 23.05.2022-27.05.2022]
Způsob prezentace: Poster
Pořadatel akce: ESA
URL akce: https://lps22.esa.int/living-planet-22-archive-website/ 
Grant CEP: GA MŠMT(CZ) LTC20055
Institucionální podpora: RVO:86652079
Klíčová slova: forest structure * solar-induced chlorophyll fluorescence * 3d tree reconstruction * radiative transfer modelling
Obor OECD: Remote sensing

Uncertainties related to the FLEX Earth Explorer space observations, which will measure the canopy solar-induced chlorophyll fluorescence (SIF) of various vegetation types, can be assessed not only through field and airborne validation activities but also through a dedicated computer modelling using modern, physically based radiative transfer models (RTMs). RTMs are highly efficient in evaluating the SIF confounding factors that cannot be directly measured in field (e.g., impacts of forest woody components) and in revealing their importance in spatial three-dimensional (3D) as well as temporal (diurnal to seasonal) contexts. In this work, we used the 3D Discrete Anisotropic Radiative Transfer (DART) model, to analyse canopy structural impacts of three morphologically contrasting forest types, specifically European beech (Fagus sylvatica), white peppermint (Eucalyptus pulchella) and Norway spruce (Picea abies) stands, on their top-of-canopy (TOC) SIF emissions. While the beech canopy was tall (height of c. 25 m), broadleaf and characterized by the planophile leaf angle distribution (LAD), the peppermint and spruce canopies were middle-sized (height of c. 15 m), narrow-/needle-leaf, with the erectophile and spherical LADs, respectively. 3D DART representations of the stands were created from terrestrial laser scans (TLS) of individual trees of respective species. Each stand had canopy cover 80% and was simulated with three leaf area index (LAI) values: low (4-5), medium (7-8), and high (10-11). To ensure full comparability of the modelled results, all forest scenarios shared the same field-measured bark/wood and ground optical properties, the same solar zenith and azimuth angles (a local noon), and the same atmospheric composition. Leaf optical properties (including SIF emissions) were simulated with the Fluspect-Cx RTM for the constant fluorescence quantum efficiently (fqe) of 0.02305. DART was set to produce the TOC red (686 nm) and far-red (740 nm) SIF signals (band width of 0.0013 nm) together with 3D SIF radiative budgets (RB) of the two SIF bands, allowing for spatial quantifications of the SIF balance (emitted – absorbed SIF) and the omnidirectional SIF escape factor (SIF balance/emitted SIF) within individual 20 cm thick vertical canopy layers. Spectral 3D RB was calculated also for fAPARgreen, i.e., for the fraction of photosynthetically active radiation absorbed by green canopy foliar elements.
Results revealed that the red SIF of all three species and LAI settings was strongly driven by LAD functions. Erectophile foliage of the peppermint canopies allowed for a higher red SIF scattering and reabsorption, resulting in the lowest red TOC SIF signal. The narrow needle-leaf shape and shoot structure of spruce foliage caused the lowest TOC far-red SIF values across all species and LAI categories. Virtual removal of woody elements (trunks and branches) from the DART simulations enabled us to compute the impact of wood shadowing on fAPARgreen, and the wood interactions/obstructions of both red and far-red SIF photons. The largest wood-triggered fAPARgreen decrease was found for spruce stands (45-55%), whereas the decreases in beech and peppermint canopies were much less prominent (10-25%). Similarly, significant wood obstructions, computed as a relative difference between nadir TOC SIF escape factors from canopies with and without wooden parts, appeared for far-red SIF of spruce stands (decrease by 35-45%). A smaller SIF reducing impact (5-25%) quantified for beech and peppermint stands suggests that wood structures introduce more potential uncertainty into far-red SIF TOC observations for coniferous than for broadleaf tree species. Interestingly, we found that wood elements of the two broadleaf species did not obstruct but boosted the total red SIF signal by 1-3%. Further examination of the 3D DART SIF balance profiles indicated that this positive wood/bark effect originates from top 20% of the investigated canopy relative heights. In addition, we found that SIF escaped predominantly from the top 50% of all three examined forest stands, with the relative omnidirectional escape factor increasing with the increasing forest height from 0.1 to 0.5. This suggests that the forest Cal/Val undertakings should focus on the upper halves of the canopies. However, local exceptions may occur. For instance, contributions of lower vertical layers up to 0.1 W.m-2.nm-1 were noted when modelling red SIF of the beech canopies.
Our results demonstrate that the state-of-the-art radiative transfer modelling is ready to be included in the future FLEX mission Cal/Val activities next to the field and air-/space-borne measurements. Inclusion of RTMs’ inputs as variables of interest would allow us to use RTMs as efficient tools revealing potential uncertainties of FLEX SIF products, especially when not measurable experimentally.
Trvalý link: https://hdl.handle.net/11104/0341990