Hyperspectral Time Series Analysis of Native and Invasive Species in Hawaiian Rainforests

Ben Somers,Gregory P Asner

doi:10.3390/rs4092510

Abstract

The unique ecosystems of the Hawaiian Islands are progressively being threatened following the introduction of exotic species. Operational implementation of remote sensing for the detection, mapping and monitoring of these biological invasions is currently hampered by a lack of knowledge on the spectral separability between native and invasive species. We used spaceborne imaging spectroscopy to analyze the seasonal dynamics of the canopy hyperspectral reflectance properties of four tree species: (i) Metrosideros polymorpha, a keystone native Hawaiian species; (ii) Acacia koa, a native Hawaiian nitrogen fixer; (iii) the highly invasive Psidium cattleianum; and (iv) Morella faya, a highly invasive nitrogen fixer. The species specific separability of the reflectance and derivative-reflectance signatures extracted from an Earth Observing-1 Hyperion time series, composed of 22 cloud-free images spanning a period of four years and was quantitatively evaluated using the Separability Index (SI). The analysis revealed that the Hawaiian native trees were universally unique from the invasive trees in their near-infrared-1 (700–1,250 nm) reflectance (0.4 > SI > 1.4). Due to its higher leaf area index, invasive trees generally had a higher near-infrared reflectance. To a lesser extent, it could also be demonstrated that nitrogen-fixing trees were spectrally unique from non-fixing trees. The higher leaf nitrogen content of nitrogen-fixing trees was expressed through slightly increased separabilities in visible and shortwave-infrared reflectance wavebands (SI = 0.4). We also found phenology to be key to spectral separability analysis. As such, it was shown that the spectral separability in the near-infrared-1 reflectance between the native and invasive species groups was more expressed in summer (SI > 0.7) than in winter (SI < 0.7). The lowest separability was observed for March-July (SI < 0.3). This could be explained by the invasives taking advantage of the warmer summer period to expand their canopy. There was, however, no specific time window or a single spectral region that always defined the separability of all species groups, and thus intensive monitoring of plant phenology as well as the use of the full-range (400–2,500 nm) spectrum was highly advantageous in differentiating each species. These results set a basis for an operational invasive species monitoring program in Hawai’i using spaceborne imaging spectroscopy.

Highlights

Invasive species rank second only to habitat loss as a threat to biodiversity and ecosystem processes
We addressed the following research questions: Are the spectral differences between the species groups originally observed by Asner et al [2] consistent over time? Or does phenology plays a significant role in determining the spectral separability of these tree species? Which time of year is best suited for discriminating different species groups? Can we integrate spectral information from different times of year to optimize spectral separability, or is there a specific time of year that allows for systematic separation of species groups? Answering these questions is requisite to advancing invasive species mapping and monitoring efforts in Hawai’i
Asner et al [2] came to the same conclusion based on their January observation, our results indicated that the August-September period was better suited for the spectral separation, detection and monitoring of invasive species extent in Hawaiian rainforests

Summary

Introduction

Invasive species rank second only to habitat loss as a threat to biodiversity and ecosystem processes. The spectra express the biochemical and structural properties of the vegetation, but translating that to species composition requires an increased understanding of the spectral separability of species at different levels of ecological and taxonomic aggregation In this light, Asner et al [2] reported on the spectral separability of the most common native and invasive tree species found in tropical and subtropical forests of Hawai’i. Intensive monitoring of plant species dynamics allows us to capture the phenological differences among taxa, which in turn, can aid in determining the best time to discriminate between targeted species [8] Whereas such an intensive temporal spectral analysis is not generally feasible using airborne campaigns, the advent of spaceborne imaging spectrometer missions (e.g., Hyperion, EnMap, Prisma) is increasing the availability of hyperspectral time series enabling temporal analysis of vegetation dynamics. We addressed the following research questions: Are the spectral differences between the species groups originally observed by Asner et al [2] consistent over time? Or does phenology plays a significant role in determining the spectral separability of these tree species? Which time of year is best suited for discriminating different species groups? Can we integrate spectral information from different times of year to optimize spectral separability, or is there a specific time of year that allows for systematic separation of species groups? Answering these questions is requisite to advancing invasive species mapping and monitoring efforts in Hawai’i

Study Area and Remote Sensing

Spectral Separability

Species Comparisons

Conclusions