Seasonal variability of multiple leaf traits captured by leaf spectroscopy at two temperate deciduous forests

Published Date

15 June 2016, Vol.179:1–12, doi:10.1016/j.rse.2016.03.026

Title 

Seasonal variability of multiple leaf traits captured by leaf spectroscopy at two temperate deciduous forests

• Author 

• Xi Yang ^a,b,,
• Jianwu Tang ^b,,
• John F. Mustard ^a
• Jin Wu ^c,e
• Kaiguang Zhao ^d
• Shawn Serbin ^e
• Jung-Eun Lee ^a

• aDepartment of Earth, Environment, and Planetary Sciences, Brown University, Providence, RI 02912, USA
• bThe Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA 02543, USA
• cDepartment of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721, USA
• dSchool of Environment and Natural Resources, Ohio Agricultural and Research Development Center, The Ohio State University, Wooster, OH 44691, USA
• eEnvironmental & Climate Sciences Department, Brookhaven National Laboratory, Upton, NY 11973, USA

Received 31 August 2015. Revised 16 March 2016. Accepted 23 March 2016. Available online 2 April 2016.

Highlights

• •
  We collected 7 leaf traits and spectra weekly for two entire growing seasons.
• •
  We observed trait-specific temporal patterns and variations in leaf spectra.
• •
  Leaf spectra analyzed using the PLSR approach track the seasonality of leaf traits.
• •
  PLSR model shows robustness across time, sites, and light environments.
• •
  We offered suggestions on field sampling interval.

Abstract

Understanding the temporal patterns of leaf traits is critical in determining the seasonality and magnitude of terrestrial carbon, water, and energy fluxes. However, we lack robust and efficient ways to monitor the temporal dynamics of leaf traits. Here we assessed the potential of leaf spectroscopy to predict and monitor leaf traits across their entire life cycle at different forest sites and light environments (sunlit vs. shaded) using a weekly sampled dataset across the entire growing season at two temperate deciduous forests. The dataset includes field measured leaf-level directional-hemispherical reflectance/transmittance together with seven important leaf traits [total chlorophyll (chlorophyll a and b), carotenoids, mass-based nitrogen concentration (N_mass), mass-based carbon concentration (C_mass), and leaf mass per area (LMA)]. All leaf traits varied significantly throughout the growing season, and displayed trait-specific temporal patterns. We used a Partial Least Square Regression (PLSR) modeling approach to estimate leaf traits from spectra, and found that PLSR was able to capture the variability across time, sites, and light environments of all leaf traits investigated (R² = 0.6–0.8 for temporal variability; R² = 0.3–0.7 for cross-site variability; R² = 0.4–0.8 for variability from light environments). We also tested alternative field sampling designs and found that for most leaf traits, biweekly leaf sampling throughout the growing season enabled accurate characterization of the seasonal patterns. Compared with the estimation of foliar pigments, the performance of N_mass, C_mass and LMA PLSR models improved more significantly with sampling frequency. Our results demonstrate that leaf spectra-trait relationships vary with time, and thus tracking the seasonality of leaf traits requires statistical models calibrated with data sampled throughout the growing season. Our results have broad implications for future research that use vegetation spectra to infer leaf traits at different growing stages.

Keywords

• Phenology
• Leaf physiology
• Foliar chemistry
• Carbon cycle
• Chlorophyll
• Carotenoids
• Nitrogen
• Leaf mass per area
• Partial least square regression (PLSR)
• Sun and shaded leaves

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• ⁎ 
  Correspondence to: X. Yang, Department of Earth, Environment, and Planetary Sciences, Brown University, Providence, RI 02912, USA.
• ⁎⁎ 
  Corresponding author.

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http://www.sciencedirect.com/science/article/pii/S0034425716301201