The future forests of Saskatchewan
Anderson, Howard George
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The southern boreal forest environment in Saskatchewan has been affected by the large-scale pulpwood harvesting operations that commenced in 1968. The present study was undertaken to assess the present environmental conditions on various terrain types with the aim of predicting the likely future forests that will develop. A biophysical inventory of approximately 5600 km2 of forest terrain was undertaken to provide the frame of reference. Forty-one land systems based on landform, topography, texture of surficial deposits, drainage and vegetation were mapped at 1:125000 from air photos at 1:77500. Standing crop and regeneration potential of each terrain type provided a basis for predicting ecosystem development. Total stand volumes decline in the following order: > 300m3 ha-l on fresh sandy loam till; 250-300m3 ha-1 on fresh outwash or moderately fine textured till; 200-250 m3 ha-l on dry sandy loam outwash and moist to very moist moderately fine textured till; 150-200 m3 ha-l on dry sands; < 150 m3 ha-l on very dry outwash sands. Advance regeneration, primarily black spruce and balsam fir, does not play a major role in stand re-establishment. Cumulative actual evapotranspiration (AE) on ten important terrain types over the 1973 growing season was much higher than in 1972 due to -warmer and wetter conditions in 1973. Cumulative curves were generally sigmoid in shape with the most rapid rise coinciding with leaf expansion of broad-leaved vegetation. A plateau was attained in early to mid-August which coincides with cessation of height and diameter growth. Curve slopes in mid-summer were correlated with soil moisture holding capacity and/or stage of vegetation development. Recent cutovers and/or dry sandy sites had flatter curve slopes than 5-year-old cutovers or moist sandy loam till sites. Daily rates of AE in 1973 (May 1 to August 31) ranged up to 5 mm. Semi-monthly estimates of AE, which were calculated from the cumulative curves, indicate moisture environemnts of significant length in determining regeneration success. Water stress reduces germination and growth of regeneration. Ratio of AE to potential evapotranspiration (PE) is one measure of stress; the magnitude is influenced by growing season weather and available soil water capacity. Ratios progressively decreased in the following order: sandy clay loam till, washed sandy loam till and sandy lacustrine or outwash terrain. Comparative ecological studies of white spruce and jack pine seedlings in greenhouse and field revealed morphological and physiological differences. Pine had deeper taproots and greater root mass than white spruce. Control of water loss and enhanced drought avoidance are suggested by pine leaf water characteristic curves. White spruce consistently developed higher stresses than pine in the field under similar weather conditions. Stresses for both species were highest on coarse textured soils. Pine strategy favours drought avoidance and that of spruce favours growth. July precipitation and temperature regimes of the previous year correlate well with current-year height increments of jack pine, white and black spruce seedlings. Wet to very wet conditions (precipitation more than 2 cm above normal) appear most favourable for growth. Integration of the various studies suggests that species conversion and/or stand degradation is unlikely except under extreme environments on moist sandy clay loam till supporting mixedwood forests or in black spruce forests on wet, organic flats. Specific silvicultural recommendations are made for major terrain types. Greatest regeneration problems occur on moist lower , slopes and very moist to wet flats of till plains. Maximum size of cutovers should be 50 and 25 ha respectively. Recommended site preparation includes scalping or trenching on the former and scarification followed by artificial reforestation on the latter.