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Biogeosciences An interactive open-access journal of the European Geosciences Union
Biogeosciences, 14, 3371-3385, 2017
https://doi.org/10.5194/bg-14-3371-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
14 Jul 2017
On the challenges of using field spectroscopy to measure the impact of soil type on leaf traits
Matheus H. Nunes, Matthew P. Davey, and David A. Coomes Department of Plant Sciences, University of Cambridge, CB2 3EA, UK
Abstract. Understanding the causes of variation in functional plant traits is a central issue in ecology, particularly in the context of global change. Spectroscopy is increasingly used for rapid and non-destructive estimation of foliar traits, but few studies have evaluated its accuracy when assessing phenotypic variation in multiple traits. Working with 24 chemical and physical leaf traits of six European tree species growing on strongly contrasting soil types (i.e. deep alluvium versus nearby shallow chalk), we asked (i) whether variability in leaf traits is greater between tree species or soil type, and (ii) whether field spectroscopy is effective at predicting intraspecific variation in leaf traits as well as interspecific differences. Analysis of variance showed that interspecific differences in traits were generally much stronger than intraspecific differences related to soil type, accounting for 25 % versus 5 % of total trait variation, respectively. Structural traits, phenolic defences and pigments were barely affected by soil type. In contrast, foliar concentrations of rock-derived nutrients did vary: P and K concentrations were lower on chalk than alluvial soils, while Ca, Mg, B, Mn and Zn concentrations were all higher, consistent with the findings of previous ecological studies. Foliar traits were predicted from 400 to 2500 nm reflectance spectra collected by field spectroscopy using partial least square regression, a method that is commonly employed in chemometrics. Pigments were best modelled using reflectance data from the visible region (400–700 nm), while all other traits were best modelled using reflectance data from the shortwave infrared region (1100–2500 nm). Spectroscopy delivered accurate predictions of species-level variation in traits. However, it was ineffective at detecting intraspecific variation in rock-derived nutrients (with the notable exception of P). The explanation for this failure is that rock-derived elements do not have absorption features in the 400–2500 nm region, and their estimation is indirect, relying on elemental concentrations covarying with structural traits that do have absorption features in that spectral region (constellation effects). Since the structural traits did not vary with soil type, it was impossible for our regression models to predict intraspecific variation in rock-derived nutrients via constellation effects. This study demonstrates the value of spectroscopy for rapid, non-destructive estimation of foliar traits across species, but highlights problems with predicting intraspecific variation indirectly. We discuss the implications of these findings for mapping functional traits by airborne imaging spectroscopy.

Citation: Nunes, M. H., Davey, M. P., and Coomes, D. A.: On the challenges of using field spectroscopy to measure the impact of soil type on leaf traits, Biogeosciences, 14, 3371-3385, https://doi.org/10.5194/bg-14-3371-2017, 2017.
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The paper investigates whether variability in leaf traits is greater between tree species or soil type and whether field spectroscopy is effective at predicting intraspecific variation in leaf traits as well as interspecific differences. The influences of species identity on traits were generally much stronger than within-species differences related to soil type. This study demonstrates the value of spectroscopy, but highlights problems with predicting within-species variation indirectly.
The paper investigates whether variability in leaf traits is greater between tree species or...
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