Evaluation of the topographic sheltering effects on the spatial pattern of Taiwan fir using aerial photography and GIS

The forests of Taiwan fir (Abies kawakamii) in the Hohuan Mountain area exhibit a two-phase mosaic, fir patches alternating with gaps. The study attempted to apply topographic variables to examine the association between topography and patch-gap pattern in this area. Topographic data layers for GIS analyses were derived from aerial photography. A topographic sheltering index, serving as a surrogate for wind, was developed according to the concepts of shelterbelt and point-in-polygon operation in GIS. Chi-square tests were performed to identify variables significantly associated with this pattern. Elevation and topographic sheltering were highly associated with this pattern, whereas the reverse held true for slope and aspect. Hence, Taiwan fir preferred a cold and humid environment at elevations above 3000 m; it also preferred sheltered sites to windswept sites. The index proved useful for discriminating this pattern in the landscape. The outcomes dovetailed the wind explanation proposed in previous studies in which wind was considered a primary factor causing this pattern. However, in situ monitoring of wind will be needed to verify the importance of wind to this pattern. The index reduced the area of future fieldwork, thereby making it more feasible. The outcomes will provide a valuable basis for modelling the potential habitat of Taiwan fir and a guideline for planning feasible fieldwork so that the model can be verified.