Spatial Variation of Leaf Optical Properties in a Boreal Forest Is Influenced by Species and Light Environment

Abstract

Leaf Optical Properties (LOPs) convey information relating to temporally dynamic photosynthetic activity and biochemistry. LOPs are also sensitive to variability in anatomically related traits such as Specific Leaf Area (SLA), via the interplay of intra-leaf light scattering and absorption processes. Therefore, variability in such traits, which may demonstrate little plasticity over time, potentially disrupts remote sensing estimates of photosynthesis or biochemistry across space. To help to disentangle the various factors that contribute to the variability of LOPs, we defined baseline variation as variation in LOPs that occurs across space, but not time. Next we hypothesized that there were two main controls of potentially disruptive baseline spatial variability of photosynthetically-related LOPs at our boreal forest site: light environment and species. We measured photosynthetically-related LOPs in conjunction with morphological, biochemical, and photosynthetic leaf traits during summer and across selected boreal tree species and vertical gradients in light environment. We then conducted a detailed correlation analysis to disentangle the spatial factors that control baseline variability of leaf traits and, resultantly, LOPs. Baseline spatial variability of the Photochemical Reflectance Index (PRI) was strongly influenced by species and to a lesser extent light environment. Baseline variability of spectral fluorescence derived LOPs was less influenced by species; however at longer near-infrared wavelengths, light environment was an important control. In summary, remote sensing of chlorophyll fluorescence has good potential to detect variation in photosynthetic performance across space in boreal forests given reduced sensitivity to species related baseline variability in comparison to the PRI. Our results also imply that spatially coarse remote sensing observations are potentially unrepresentative of the full scope of natural variation that occurs within a boreal forest.

A spectral analysis of 25 boreal tree species

“…This is in line with earlier studies that have emphasized importance of species and needle age over light environment (Knapp and Carter 1998;O'Neill et al 2002;Atherton et al 2017). On the other hand, Lukeš et al (2013) observed differences in coniferous needle reflectance between canopy positions, which were much larger than those observed in our study.…”

Spectral Properties of Coniferous Forests: A Review of In Situ and Laboratory Measurements

“…Needle age (current-year vs. previous-year needles), on the other hand, explained up to 37% of the total variance in measured transmittance for the same set of species. In addition, canopy position, and thus light environment, had a relatively small influence on needle spectra [11], which is in line with other studies that have emphasized importance of species and needle age over light environment [17,47]. On the other hand, in another study limited to three species, larger differences in coniferous needle reflectance were indeed observed between canopy positions [27].…”

“…The average lifespan, defined as the leaf age-class retaining at least 50% of leaves, varies for coniferous species from 6 to 110 months [46]. Spectral differences within coniferous crowns are mainly observed (1) between needles of different age, and (2) between vertical canopy layers with different levels of adaptation to direct/diffuse radiation conditions [11,17,27,47]. For example, current-year Norway spruce needles collected from a study site in the Czech Republic (see Appendix B for details) show the largest spectral differences between two vertical canopy levels, whereas older cohorts of needles exhibit almost no differences in their spectral properties (Figure 2).…”

Upscaling of Solar Induced Chlorophyll Fluorescence From Leaf to Canopy Using the Dart Model and a Realistic 3d Forest Scene

“…For example vertical and horizontal gradients in leaf level biophysical parameters could be assigned, which are known to occur in actual canopies in response to light environment (Atherton et al, 2017). Additionally, background and branch influences on canopy SIF simulation can also be simulated based on this method.…”

“…Although measureable from space and aircraft, SIF originates close to the site of the photosynthetic reactions, and is therefore closely linked to plant growth and physiological status. Unlike the traditional reflectance vegetation indices, such as the Normalized Different Vegetation Index (NDVI), SIF responds very rapidly to environmental stress, and therefore has the potential to track time-varying stress at (sub) diurnal scales (Amoros-Lopez et al, 2008;Yang et al, 2015;Alonso et al, 2017).…”