Abstract
Koa (Acacia koa) forests are found across broad environmental gradients in the Hawai‘ian Islands. Previous studies have identified koa forest health problems and dieback at the plot level, but landscape level patterns remain unstudied. The availability of high-resolution satellite images from the new GeoEye1 satellite offers the opportunity to conduct landscape-level assessments of forest health. The goal of this study was to develop integrated remote sensing and geographic information systems (GIS) methodologies to characterize the health of koa forests and model the spatial distribution and variability of koa forest dieback patterns across an elevation range of 600–1,000 m asl in the island of Kaua‘i, which correspond to gradients of temperature and rainfall ranging from 17–20 °C mean annual temperature and 750–1,500 mm mean annual precipitation. GeoEye1 satellite imagery of koa stands was analyzed using supervised classification techniques based on the analysis of 0.5-m pixel multispectral bands. There was clear differentiation of native koa forest from areas dominated by introduced tree species and differentiation of healthy koa stands from those exhibiting dieback symptoms. The area ratio of healthy koa to koa dieback corresponded linearly to changes in temperature across the environmental gradient, with koa dieback at higher relative abundance in warmer areas. A landscape-scale map of healthy koa forest and dieback distribution demonstrated both the general trend with elevation and the small-scale heterogeneity that exists within particular elevations. The application of these classification techniques with fine spatial resolution imagery can improve the accuracy of koa forest inventory and mapping across the islands of Hawai‘i. Such findings should also improve ecological restoration, conservation and silviculture of this important native tree species.
Highlights
Acacia koa Gray is a large evergreen forest tree in the Fabaceae family
The red band contained the lowest spectral values for all vegetation classes and a large degree of overlap, especially among tree species. This band separated healthy from unhealthy koa stands and the grass-soil mixture, a large spectral overlap was observed between healthy koa with the other major tree species (Figure 3)
The spectral analysis and classification of GeoEye1 satellite imagery proved to be a useful methodology for the characterization of koa forest dieback across a 500-m elevation (3 °C mean annual temperature (MAT))
Summary
For established trees the true leaf is replaced a phyllode, which is the expanded rachis of the true leaf. These are thought to aid in drought tolerance [1]. Koa is an important native tree species in Hawai„i due to its high economic, ecological and cultural values. Koa serves as the preferred and critical habitat for native insects [1] and threatened and endangered bird species [2], improves soil nitrogen content [3], and creates favorable understory conditions for native plant regeneration [4]. Koa exists on most of the main Hawaiian Islands in remaining native forest areas and regenerating second-growth stands across a wide range of elevation (600–2,300 m asl), mean annual precipitation (850–5,000 mm), and soil types [5]. Koa productivity generally increases with precipitation, but nutrient availability becomes more limiting due to increased leaching and plant demand [6,7,8]
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