Variability and bias in active and passive ground-based measurements of effective plant, wood and leaf area index

Kim Calders,Niall Origo,Mathias Disney,Joanne Nightingale,William Woodgate,John Armston,Philip Lewis

doi:10.1016/j.agrformet.2018.01.029

Abstract

In situ leaf area index (LAI) measurements are essential to validate widely-used large-area or global LAI products derived, indirectly, from satellite observations. Here, we compare three common and emerging ground-based sensors for rapid LAI characterisation of large areas, namely digital hemispherical photography (DHP), two versions of a widely-used commercial LAI sensor (LiCOR LAI-2000 and 2200), and terrestrial laser scanning (TLS). The comparison is conducted during leaf-on and leaf-off conditions at an unprecedented sample size in a deciduous woodland canopy. The deviation between estimates of these three ground-based instruments yields differences greater than the 5% threshold goal set by the World Meteorological Organization. The variance at sample level is reduced when aggregated to plot scale (1 ha) or site scale (6 ha). TLS shows the lowest relative standard deviation in both leaf-on (11.78%) and leaf-off (13.02%) conditions. Whereas the relative standard deviation of effective plant area index (ePAI) derived from DHP relates closely to TLS in leaf-on conditions, it is as large as 28.14–29.74% for effective wood area index (eWAI) values in leaf-off conditions depending on the thresholding technique that was used. ePAI values of TLS and LAI-2x00 agree best in leaf-on conditions with a concordance correlation coefficient (CCC) of 0.796. In leaf-off conditions, eWAI values derived from DHP with Ridler and Calvard thresholding agrees best with TLS. Sample size analysis using Monte Carlo bootstrapping shows that TLS requires the fewest samples to achieve a precision better than 5% for the mean ± standard deviation. We therefore support earlier studies that suggest that TLS measurements are preferential to measurements from instruments that are dependent on specific illumination conditions. A key issue with validation of indirect estimates of LAI is that the true values are not known. Since we cannot know the true values of LAI, we cannot quantify the accuracy of the measurements. Our radiative transfer simulations show that ePAI estimates are, on average, 27% higher than eLAI estimates. Linear regression indicated a linear relationship between eLAI and ePAI–eWAI (R2 = 0.87), with an intercept of 0.552 and suggests that caution is required when using LAI estimates.

Highlights

Leaf area index (LAI) is an essential climate variable (ECV) that describes the amount of leaf material in an ecosystem (Nemani et al, 2003; Asner et al, 2003; Disney et al, 2016)
The three different passive methods (LAI-2x00, digital hemispherical photography (DHP)(G) & DHP(TC)) were benchmarked against the terrestrial laser scanning (TLS) measurements because the latter are insensitive to illumination conditions and inclination sensors provide accurate levelling
In leaf-off conditions, we observed the best agreement between TLS and the DHP (G) method (CCC = 0.306)

Summary

Introduction

Leaf area index (LAI) is an essential climate variable (ECV) that describes the amount of leaf material in an ecosystem (Nemani et al, 2003; Asner et al, 2003; Disney et al, 2016). To be useful for climate modelling, full end-to-end traceability and assessment of the uncertainty of the process from sensor measurement through to the generation of the ECV product and the resulting time-series is needed (Dowell et al, 2013). Spaceborne estimates of LAI are essential to provide a greater spatial and temporal coverage compared to in situ estimates, but the retrieval process is more complex due to the mixed contributions of leaves, other tree elements, understorey vegetation and soil to the measured radiation flux. We require knowledge of the measurement uncertainty and the uncertainty of the derived ECV and its time-series. It is critical to benchmark the different (global) space-derived LAI products and compare these against in situ measurements to ensure their accuracy and reliability. WMO (2012) listed different breakthrough and threshold requirements depending on the application area of LAI products

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Conclusion