We lacked niche information for A. formosana, so the environmental factors (slopes and aspects) were evaluated in this study. This information is valuable for land management decisions regarding soil and water conservation and eco-technology. The slope was measured in degrees in the image. The aspect values were grouped into eight principal directions (north, northeast, east, southeast, south, southwest, west, and northwest). We used two-way ANOVA and cluster analysis in SAS to analyze and recognize the niche of A. formosana. The results indicated that slope and aspect have significant impacts on the growth of this species (Pr < 0.0001).
A. formosana grew on slopes ranging from 0.00° to 81.82°, with an average of 53.99° ± 13.45° (mean ± standard deviation). Slopes of 74-78° are the most suitable for this species. The steeper peaks, where slopes were steeper than 79°, were still covered with bare soil (Fig. 18.7a). Cluster analysis demonstrated that northeast, east, south, and southeast directions from 22.5° to 202.5° were more suitable for A. formosana growth than the other directions. According to Liu and Su (1983), the south aspect is drier than the north in the Northern Hemisphere because of the radiance effect, and this might explain the directionality of A. formosana growth (Fig. 18.7b).
Earthquake and multitemporal typhoons caused serious landslides in the Jiou-Jiou Peaks Natural Reserve. Despite these natural disasters, A. formosana can still be found growing well in rock crevices. After observing its growth characteristics, we integrated ortho-aerial photographs and DTM to estimate its niche area accurately. A textural image was created using a gamma filter, and the original photographs were enhanced with this image. It is difficult to distinguish the variance between these images and observe the benefit of the enhancement via visual observation. However, our results showed that the textural image enhancement reduced the shadow effect of the image classification. Because of the ability of A. formosana to grow in steeply sloped areas, we estimated the growth distribution and niche area of A. formosana by using normal and 3D methods. The areas of A. formosana growth were estimated to be 26.34 % and 32.86 % of the investigated area by using normal and 3D methods, respectively. The reason for this discrepancy is that A. formosana thrives in steep regions where trees cannot generate and grow. Our results also suggest that slopes ranging from 74° to 78° and southeast to southwest aspects were the most suitable places for A. formosana growth. Several kinds of rock plants grow, generate, and thrive in Taiwan; thus, the methodology that we have described here can be applied to other similar species or regions. Moreover, our niche analysis provided valuable information for land managers making decisions regarding soil and water conservation and eco-technology.
Acknowledgments We thank the Nantou Forest District Office and Taichung Working Station for providing the field survey data.
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