| Summary: | Deforestation is recognized as a serious global problem that contributes to biodiversity loss, soil degradation and atmospheric change. This thesis is an investigation of deforestation in central Saskatchewan. The purposes are: to quantify the extent and rates of deforestation and associated changes in spatial structure for multi-jurisdictional boreal landscapes; and to estimate the magnitude of carbon losses associated with agriculture-induced deforestation at sites within one of these landscapes.<p>
Deforestation was analyzed using topographic map chronosequences for two 460 000 ha landscapes in central Saskatchewan. An estimated 16 400 ha was deforested between 1963 and 1990 within the Waskesiu Hills landscape (53° 45' N, 106° 15' W) and 371 000 ha was deforested between 1957 and 1990 within the Red Deer River landscape (52° 45' N, 103° 00' W). Federal and provincial legislation establishing publicly owned parks and forests served to inhibit deforestation within portions of these landscapes. On agricultural lands within the two landscapes, where private holdings dominate and forests are not protected under federal or provincial law, deforestation occurred at rates exceeding 1.2 % yr<sup>-1</sup> over the time periods examined even though human populations declined. Within the two study areas, extant forests that are unprotected by legislation remain vulnerable to deforestation.<p>
Spatial structure was analyzed for portions of these two landscapes using landscape metrics. A positive correlation between largest patch size index and proportion of land area wooded was evident for both 1975/76 (r<sup>2</sup> = 0.99, p < 0.01) and 1990 (r<sup>2</sup> = 0.99, p < 0.05). Since past deforestation disproportionately reduced the sizes of the largest wooded patches, future reforestation efforts should be aimed at expanding large patches. Reforestation with large patches contiguous to protected forests may initiate a reversal of the process of fragmentation that has impaired forest wildlife and ecosystem processes.<p>
Vegetation carbon densities were compared at six forest sites, six pasture sites and six cultivated sites on hummocky glacial till landforms across three townships within the Waskesiu Hills landscape. Medians for aboveground biomass (60 Mg C ha<sup>-1</sup> for forests, 1 Mg C ha<sup>-1</sup> for pastures and 4 Mg C ha<sup>-1</sup> for cultivated sites) were significantly different (p < 0.15). Including estimated losses of coarse roots, deforestation and subsequent agricultural land use led to losses of approximately 70 Mg C ha<sup>-1</sup> for vegetation.<p>
Statistically significant losses of soil organic carbon were not detected between the forest sites and the agricultural sites. The experimental design accounted for land use and topographic landform effects, but these were small (< 20 Mg C ha<sup>-1</sup>) relative to the inherent range of variation in soil organic carbon within the study area (> 50 Mg C ha<sup>-1</sup> for natural forest sites). Across all sites regardless of land use, there was a positive correlation (r<sub>s</sub> = 0.76, P <0.01) between soil organic carbon and the proportional frequency of groundwater influenced soils (Gleysols plus Woody Calcareous Chernozems).<p>
Terrestrial carbon losses due to deforestation within the Waskesiu Hills and Red Deer River landscapes prior to 1990 were crudely estimated as approximately 100 Gg C yr<sup>-1</sup>. Carbon releases due to deforestation across the Boreal Plain Ecozone of Saskatchewan prior to 1990 were estimated to be an order of magnitude greater, and may have exceeded direct carbon emissions resulting from fossil fuel burning by the provincial agricultural production sector. Reestablishing natural forests on 2 % of agricultural lands within the Boreal Plain Ecozone might offset a substantial portion of the direct carbon emissions from agricultural fossil fuel burning in Saskatchewan.
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