Discrimination of Canopy Structural Types in the Sierra Nevada Mountains in Central California

Margarita Huesca,Susan L Ustin,Keely L Roth,Mariano García

doi:10.3390/rs11091100

Abstract

Accurate information about ecosystem structure and biogeochemical properties is essential to providing better estimates ecosystem functioning. Airborne LiDAR (light detection and ranging) is the most accurate way to retrieve canopy structure. However, accurately obtaining both biogeochemical traits and structure parameters requires concurrent measurements from imaging spectrometers and LiDARs. Our main objective was to evaluate the use of imaging spectroscopy (IS) to provide vegetation structural information. We developed models to estimate structural variables (i.e., biomass, height, vegetation heterogeneity and clumping) using IS data with a random forests model from three forest ecosystems (i.e., an oak-pine low elevation savanna, a mixed conifer/broadleaf mid-elevation forest, and a high-elevation montane conifer forest) in the Sierra Nevada Mountains, California. We developed and tested general models to estimate the four structural variables with accuracies greater than 75%, for the structurally and ecologically different forest sites, demonstrating their applicability to a diverse range of forest ecosystems. The model R2 for each structural variable was least in the conifer/broadleaf forest than either the low elevation savanna or the montane conifer forest. We then used the structural variables we derived to discriminate site-specific, ecologically meaningful descriptions of canopy structural types (CST). Our CST results demonstrate how IS data can be used to create comprehensive and easily interpretable maps of forest structural types that capture their major structural features and trends across different vegetation types in the Sierra Nevada Mountains. The mixed conifer/broadleaf forest and montane conifer forest had the most complex structures, containing six and five CSTs respectively. The identification of CSTs within a site allowed us to better identify the main drivers of structural variability in each ecosystem. CSTs in open savanna were driven mainly by differences in vegetation cover; in the mid-elevation mixed forest, by the combination of biomass and canopy height; and in the montane conifer forest, by vegetation heterogeneity and clumping.

Highlights

Terrestrial ecosystems regulate the exchange of energy and matter between the land and the atmosphere [1]
We focus on three sub-regions (49.94 km2, 24.39 km2, and 134.02 km2) located coincidentally within the National Science Foundation (NSF) National Ecological Observatory Network (NEON) terrestrial sites in Domain 17 and which overlaps with two sites and is near to the third site of the National Science Foundation’s Southern Sierra Critical Zone Observatory (CZO) (Figure 1)
The spatial patterns for the structural variables between Light Detection and Ranging (LiDAR) and optical data closely match for all four variables at all three sites, even with the banding observed in the clumping images (Figure 6)

Summary

Introduction

Terrestrial ecosystems regulate the exchange of energy and matter between the land and the atmosphere [1]. The structural and biogeochemical traits within ecosystems may promote complementarity in energy use, including higher biomass and productivity that allow greater species coexistence in space, while vertically differentiated forest light strategies promote coexistence [4]. Jetz [5] identified the lack of this information as limiting our ability to understand global terrestrial ecosystem functioning and its influence on the Earth’s climate though the carbon, water, energy and nutrient cycles. Better constraints on greenhouse gas (GHG) fluxes could be achieved with concurrently measured estimates of ecosystem functioning in terms of biogeochemical traits and three dimensional canopy structures. Measurement of horizontal and vertical canopy structure provides quantitative information on wildlife habitat [13,14,15,16], and biodiversity [17]

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Discussion

Conclusion