Variability of sun‐induced chlorophyll fluorescence according to stand age‐related processes in a managed loblolly pine forest

Abstract

Leaf fluorescence can be used to track plant development and stress, and is considered the most direct measurement of photosynthetic activity available from remote sensing techniques. Red and far-red sun-induced chlorophyll fluorescence (SIF) maps were generated from high spatial resolution images collected with the HyPlant airborne spectrometer over even-aged loblolly pine plantations in North Carolina (United States). Canopy fluorescence yield (i.e., the fluorescence flux normalized by the light absorbed) in the red and far-red peaks was computed. This quantifies the fluorescence emission efficiencies that are more directly linked to canopy function compared to SIF radiances. Fluorescence fluxes and yields were investigated in relation to tree age to infer new insights on the potential of those measurements in better describing ecosystem processes. The results showed that red fluorescence yield varies with stand age. Young stands exhibited a nearly twofold higher red fluorescence yield than mature forest plantations, while the far-red fluorescence yield remained constant. We interpreted this finding in a context of photosynthetic stomatal limitation in aging loblolly pine stands. Current and future satellite missions provide global datasets of SIF at coarse spatial resolution, resulting in intrapixel mixture effects, which could be a confounding factor for fluorescence signal interpretation. To mitigate this effect, we propose a surrogate of the fluorescence yield, namely the Canopy Cover Fluorescence Index (CCFI) that accounts for the spatial variability in canopy structure by exploiting the vegetation fractional cover. It was found that spatial aggregation tended to mask the effective relationships, while the CCFI was still able to maintain this link. This study is a first attempt in interpreting the fluorescence variability in aging forest stands and it may open new perspectives in understanding long-term forest dynamics in response to future climatic conditions from remote sensing of SIF.

The 2013 FLEX—US Airborne Campaign at the Parker Tract Loblolly Pine Plantation in North Carolina, USA

“…However, F685 radiances (and/or F685 yields) provided a more reliable indicator of stand age, canopy chemistry, and physiological status than F740 radiances (or F740 yields), and was stable over older aged, closed canopy stands. This conclusion was also reached in a sister study for this campaign [34]. However, F685 radiances were non-linearly affected by the forest temperature difference (canopy-air, °C) throughout the diurnal cycle.…”

“…This expectation was realized, since the CHM structure parameter for canopy height derived from LiDAR returns was strongly related to LP stand age (r 2 = 0.95, 2-33 years old in 2013), and was non-linearly related to F685 radiances (Figure 8). That particular finding was further studied in a separate analysis reported from this campaign [34] that capitalized on the inherent design of this site, based on the chronosequence of age classes. That study attributed the decline in F685 radiances and F685 yields across LP age classes to developmental processes that reduce hydraulic conductance in older trees [71,72], which would certainly affect the physiological performance we identified.…”

“…The ROI cutouts were separately analyzed at the 14 m 2 spatial resolution described by the G-LIHT data products, from which a relationship between canopy height, stand age, and red fluorescence was successfully developed and reported elsewhere by Colombo et al [34]. …”

Hyplant-Derived Sun-Induced Fluorescence—A New Opportunity to Disentangle Complex Vegetation Signals from Diverse Vegetation Types

“…Particularly, the measurement of emitted Sun-Induced Fluorescence (SIF) is an emerging research field with growing interest. SIF is considered the most direct measurement of photosynthesis and facilitates the assessment of vegetation functioning [15,16]. Substantial progress has been made to assess the information content of SIF and its relation to plant photosynthesis across scales.…”

“…Data acquisition took place during the SWAMP (Spectrometry of a Wetland and Modelling of Photosynthesis) airborne campaign held on 11 July 2015 in Poland (the first HyPlant campaign taken over heterogeneous peatlands and surrounding ecosystems organized under the FLuorescence Explorer (FLEX) project supported by the European Space Agency (ESA)). Previous studies using HyPlant data have already revealed highly interesting insight into the information content of SIF and the functioning of ecosystems, mainly homogenous ones like croplands, grasslands, and forest areas [16,28,[36][37][38]. Rossini et al [37] measured the red and far-red SIF values with HyPlant over grass carpets treated with herbicides to estimate the rate of photosynthesis (P) and relationships between SIF and P. Wieneke et al [28] used the HyPlant-based SIF retrievals over sugar beet fields in the Rur river catchment to estimate GPP.…”