Categorizing Wetland Vegetation by Airborne Laser Scanning on Lake Balaton and Kis-Balaton, Hungary

András Zlinszky,Norbert Pfeifer,Werner Mücke,Hubert Lehner,Christian Briese

doi:10.3390/rs4061617

Abstract

Outlining patches dominated by different plants in wetland vegetation provides information on species succession, microhabitat patterns, wetland health and ecosystem services. Aerial photogrammetry and hyperspectral imaging are the usual data acquisition methods but the application of airborne laser scanning (ALS) as a standalone tool also holds promises for this field since it can be used to quantify 3-dimensional vegetation structure. Lake Balaton is a large shallow lake in western Hungary with shore wetlands that have been in decline since the 1970s. In August 2010, an ALS survey of the shores of Lake Balaton was completed with 1 pt/m2 discrete echo recording. The resulting ALS dataset was processed to several output rasters describing vegetation and terrain properties, creating a sufficient number of independent variables for each raster cell to allow for basic multivariate classification. An expert-generated decision tree algorithm was applied to outline wetland areas, and within these, patches dominated by Typha sp. Carex sp., and Phragmites australis. Reed health was mapped into four categories: healthy, stressed, ruderal and die-back. The output map was tested against a set of 775 geo-tagged ground photographs and had a user’s accuracy of > 97% for detecting non-wetland features (trees, artificial surfaces and low density Scirpus stands), > 72% for dominant genus detection and > 80% for most reed health categories (with 62% for one category). Overall classification accuracy was 82.5%, Cohen’s Kappa 0.80, which is similar to some hyperspectral or multispectral-ALS fusion studies. Compared to hyperspectral imaging, the processing chain of ALS can be automated in a similar way but relies directly on differences in vegetation structure and actively sensed reflectance and is thus probably more robust. The data acquisition parameters are similar to the national surveys of several European countries, suggesting that these existing datasets could be used for vegetation mapping and monitoring.

Highlights

Shore wetland vegetation plays an important role in the functioning of lake systems
Multiple echoes were only recorded in trees, as the wetland vegetation canopy height rarely exceeded the minimum vertical distance needed for separation of subsequent echoes of a single pulse
The patterns of the wetland vegetation classification showed striking similarity to the patterns of vegetation patches observed in the field or on aerial photographs

Summary

Introduction

Shore wetland vegetation plays an important role in the functioning of lake systems. The ecotone between land and water creates a large variety of microhabitats and the high biomass production of wetland vegetation feeds energy into the food web [1]. Wetlands are often the least disturbed areas of a lake and can act as a refuge for wildlife in seasons when pressure of human presence on the ecosystem is intensive. Some functions of shore wetlands, such as erosion, flood protection and pollution demobilization are important from an economic point of view [3]. Pressure on shore wetlands is increasing in Europe, since shore areas are intensively used for recreation and industry, and global climate change and local pollution affect these sensitive communities [4]

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Discussion

Conclusion