Site index is widely used as a measure of forest site productivity, and is an essential input into growth and yield models forecasting forest stand development. However, accurate estimates of site index are frequently difficult or even impossible to obtain, particularly in the boreal mixedwoods of trembling aspen (Populus tremuloides Michx.) and white spruce [Picea glauca (Moench) Voss]. Topographic indices derived from remotely sensed digital elevation models have become increasingly available

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... and can provide information about site and soil characteristics. The depth-to-water (DTW) index measures the hydrological connectivity of any cell in the landscape with the nearest flow channel by integrating the horizontal and vertical distance between them. The central objective of this thesis was to evaluate the potential to use the DTW index to estimate site index of aspen. The first component of this study was aimed to understand the capabilities and limitations of the DTW index to map site and soil properties. The DTW index was related with soil moisture regime, drainage and depth to mottles, attributes that are closely controlled by water movement in the soil. Important edaphic characteristics, such as nutrient availability or texture, were not associated with the DTW index. The second component of this thesis examined the linkage between the DTW index and drought stress. Carbon isotope ratio, as a measure of water stress in trees, of both aspen and spruce increased with DTW until a certain point after which it levelled off, indicating that large DTW values reflect site conditions that cause water stress. Tree height and diameter followed a similar pattern with the DTW index. These first two components provided the necessary foundations to address the central objective of this thesis. Site index of aspen was significantly associated with the DTW index but model fit iii statistics indicated that predictions of aspen site index based solely on the DTW index are not reliable. Throughout the thesis I also investigated the effects of flow initiation area assumptions on relationships between DTW and other variables. For the relatively flat topography of the boreal landscape a catchment area of 2 or 4 ha is recommended, whereas a threshold value of 12 or 16 ha is suggested for the stronger topographic gradients found in the foothills region. This thesis has shown that the DTW index can capture site and soil properties closely influenced by water in the soil but fails to represent nutrient availability, which limits the extent to which the DTW index can be used to estimate site index. iv Preface The digital elevation models were provided by the Government of Alberta. The geospatial data (i.e., flow direction, flow accumulation, flow channels and depth-to-water rasters) were prepared by Jae Ogilvie of the University of New Brunswick.