Foliar sampling with an unmanned aerial system (UAS) reveals spectral and functional trait differences within tree crowns

Abstract

Imaging spectroscopy is currently the best approach for continuously mapping forest canopy traits, which is important for ecosystem and biodiversity assessments. Ideally, models are trained with trait data from fully sunlit top-of-canopy leaves. However, sampling top-of-canopy leaves is often difficult and sunlit foliage from the crown periphery is collected instead, assuming minimal within-crown trait variation among sunlit leaves. We tested the degree to which crown position affects foliar traits and spectra using mixed-effects models comparing sun leaves from crown centres of mature <i>Acer saccharum</i> trees collected with DeLeaves, a novel twig-sampling Unmanned Aerial System device, to sun leaves from the crown periphery collected with a pole pruner. Sun leaves from the crown centre differed from sun leaves from the crown periphery in absorption, reflectance and transmittance, and in a series of foliar traits, including leaf thickness, leaf mass and leaf nitrogen content per unit area, pointing out differences in resource allocation depending on sun exposure. Our study highlights the importance of matching exactly the location of foliar samples and spectral data, and of sampling across gradients of intra-individual variation for accurately predicting foliar trait distributions across and within canopies with imaging spectroscopy.

The DeLeaves: a UAV device for efficient tree canopy sampling

“…The results showed significant differences in many leaf traits resulting from the sampling location within the canopy. More details are available in Schweiger et al (2020).…”

“…Although other methods are available to plan fertilization treatments (i.e., soils analysis), foliar analysis is the most direct way to monitor trees' nutritional needs (Weetman and Wells 1990). Tree canopy samples are also used to develop and calibrate models of leaf chemistry based on hyperspectral imagery, enabling large-scale ecosystem monitoring (Clevers et al 2010;Inoue et al 2016;Schweiger et al 2020). Foliar samples are particularly useful for accurately interpreting spectral data toward mapping nutrients like nitrogen and phosphorus (Clevers and Kooistra 2012;Sammons 2019).…”

Coupling spectral and resource-use complementarity in experimental grassland and forest communities

“…The spectral hypervolume occupied by an individual plant can be quantified by leaf spectra from that plant. Its size represents intra-individual spectral variation [20] and is conjectured to be indicative of the range of environmental conditions experienced by the plant, including gradients of light, wind and temperature, as well as variation in pathogen and herbivore pressure within the canopy. Greater variation in environment and in leaf spectra within a plant give rise to a larger spectral hypervolume, which is in turn, we hypothesize, positively correlated with plant growth and biomass.…”

“…Spectrally complementary species, which represent contrasting spectral types, can be assumed to use resources differently, including light, nutrients and water, because resource-use strategies of plants are reflected by their foliar chemistry, anatomy and morphology [22, 23], all of which influence the spectral response following the physics of light absorption and scattering [2]. From an individual perspective, spectral complementarity means that leaves from different parts of the canopy complement each other in terms of resource-use, for example, through foliar adaptations in response to light gradients within canopies [20, 24]. From a stand or community perspective, spectral complementarity means that different individuals or species partition resources which allows them to compete less with each other and use the total resource pool together more completely [25-27] leading to a positive relationship between spectral complementarity, total resource-use and productivity.…”

Coupling spectral and resource-use complementarity in experimental grassland and forest communities

“…This is because resource-use strategies of plants are reflected by their foliar chemistry, anatomy and morphology [22,23], all of which influence the spectral response following the physics of light absorption and scattering [2]. From an individual perspective, spectral complementarity means that leaves from different parts of the canopy complement each other in terms of resource-use, for example, through foliar adaptations in response to light gradients within canopies [20,24]. From a stand or community perspective, spectral complementarity means that different individuals or species partition resources which allows them to compete less with each other and use the total resource pool together more completely [25][26][27] leading to a positive relationship between spectral complementarity, total resource-use and productivity.…”