Remote Sensing Assessment of Forest Disturbance across Complex Mountainous Terrain: The Pattern and Severity of Impacts of Tropical Cyclone Yasi on Australian Rainforests

Robinson Negrón-Juárez,Stephen Goosem,Jeffrey Chambers,Stephen Cocke,Michael Stott,Bachir Annane,Daniel Metcalfe,George Hurtt,Sassan Saatchi,Mark Powell

doi:10.3390/rs6065633

Abstract

Topography affects the patterns of forest disturbance produced by tropical cyclones. It determines the degree of exposure of a surface and can alter wind characteristics. Whether multispectral remote sensing data can sense the effect of topography on disturbance is a question that deserves attention given the multi-scale spatial coverage of these data and the projected increase in intensity of the strongest cyclones. Here, multispectral satellite data, topographic maps and cyclone surface wind data were used to study the patterns of disturbance in an Australian rainforest with complex mountainous terrain produced by tropical cyclone Yasi (2011). The cyclone surface wind data (H*wind) was produced by the Hurricane Research Division of the National Oceanic and Atmospheric Administration (HRD/NOAA), and this was the first time that this data was produced for a cyclone outside of United States territory. A disturbance map was obtained by applying spectral mixture analyses on satellite data and presented a significant correlation with field-measured tree mortality. Our results showed that, consistent with cyclones in the southern hemisphere, multispectral data revealed that forest disturbance was higher on the left side of the cyclone track. The highest level of forest disturbance occurred in forests along the path of the cyclone track (±30°). Levels of forest disturbance decreased with decreasing slope and with an aspect facing off the track of the cyclone or away from the dominant surface winds. An increase in disturbance with surface elevation was also observed. However, areas affected by the same wind intensity presented increased levels of disturbance with increasing elevation suggesting that complex terrain interactions act to speed up wind at higher elevations. Yasi produced an important offset to Australia’s forest carbon sink in 2010. We concluded that multispectral data was sensitive to the main effects of complex topography on disturbance patterns. High resolution cyclone wind surface data are needed in order to quantify the effects of topographic accelerations on cyclone related forest disturbances.

Highlights

Tropical cyclones are recurrent weather phenomena responsible for fatalities, large economic losses [1] and are important drivers of the dynamics of forest ecosystems after landfall [2,3,4]
Our results shown that multispectral data was sensitive to the spectral signature of tree mortality and captured the observed effects of topography on disturbance patterns associated with tropical cyclones in a complex mountainous terrain
By using multispectral data we found that most frequent disturbance was associated with forest located along (±30 ) the path of cyclone track

Summary

Introduction

Tropical cyclones (hurricanes, typhoons, and southern hemisphere cyclones) are recurrent weather phenomena responsible for fatalities, large economic losses [1] and are important drivers of the dynamics of forest ecosystems after landfall [2,3,4]. Winds are responsible for most of the observed structural damage to forests from tropical cyclones, the severity of the impact depends on biotic (e.g., tree size, species, and stand characteristics) and abiotic factors (e.g., soil type, topography and rainfall) [2,4,5,10]. Topographic characteristics governs the degree of exposure of a forest to tropical cyclonic winds at any point [19] but can alter both wind speed and direction [11] making the study of the association between terrain features and hurricane disturbance difficult

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Discussion

Conclusion