UBC Theses and Dissertations
A multi-scaled approach to understanding the environmental drivers of Southeast Asian forest-savanna mosaics Pletcher, Elise
Seasonally dry tropical forest in Southeast Asia is a complex mosaic of dry evergreen forest and deciduous dipterocarp “forest” (which are structurally and functionally savanna). These patchy savannas are threatened by fragmentation and forest-centric management practices. Understanding the ecological processes that govern the boundaries between forest and savanna is essential for their proper conservation. I analyzed patterns of remotely sensed tree cover and identified the key determinants of both tree cover and landscape mosaics across SE Asia. First, I examined distributions of tree cover for bimodality—evidence that forests and savanna exist as alternate stable states. I found no bimodality in tree cover regionally; however, bimodality was present within landscapes. Second, I analyzed the relative importance of environmental factors in shaping patterns of tree cover and landscape mosaics, using regression models. I found that fire was the strongest predictor of tree cover at the regional scale and that the prevalence of landscape mosaics is dependent on climate (rainfall and seasonality). These results reveal that the same environmental factors important in delineating the distribution of savannas globally drive patterns of tree cover and landscape mosaics across SE Asia. More broadly, this work shows a useful approach for parsing out which environmental factors maintain and drive forest-savanna mosaic formation. While global vegetation products (GVPs), such as those I employed in my first analysis, are powerful and easy to use, their applicability in highly heterogeneous landscapes such as savanna-forest mosaics is poorly understood. I therefore explored the limitations of global vegetation structure products, including those used in my primary analysis using Airborne Laser Scanning. I found that no GVP was a clear winner. MODIS VCF significantly underestimates tree cover in SE Asian savannas, relative to forests, and the Global Forest Change (GFC) product’s ability to accurately map tree cover within forest-savanna mosaics is limited. Overall, I argue that while datasets such as GFC allow ecologists to test theories at large spatial scales (such as alternate stable states), researchers should be mindful of where and what type of vegetation was used to validate the GVP before conducting research at regional and global scales.
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