In mature forests, gains and losses of nitrogen may be dominated by the gaseous transformations, asymbiotic nitrogen fixation and biological denitrification. Both are reduction reactions and are affected by moisture conditions, temperature, pH, supply of organic carbon and the availability of mineral nitrogen. Gaseous nitrogen inputs, due to asymbiotic nitrogen fixation, and outputs, due to biological denitrification were quantified for a mature coniferous forest in southwestern British Columbia. Forest floor material, mineral soil, decaying wood, foliage and bark were incubated in an atmosphere of 0.1 atm acetylene to allow the simultaneous measurement of N₂0 production by denitrifying bacteria and acetylene reduction by free-living bacteria and blue-green algae. Forest floor material accounted for 80% of a total annual input of 0.8 kg N ha⁻1 a⁻1. Relatively small amounts of nitrogen were fixed in mineral soil, decaying wood and foliage and no indication of nitrogen fixation activity in bark was detected. Traces of denitrification were found, but gaseous output of nitrogen was effectively 0.0 kg N ha⁻1 a⁻1. It is hypothesized that this forest may prevent nitrogen Joss by outcompeting other sinks for mineral nitrogen, thereby allowing a slow accretion of nitrogen by asymbiotic nitrogen fixation and bulk precipitation input. / Forestry, Faculty of / Graduate

Forest road building and timber harvesting operations have been recognized as principal causes of forest soil degradation. These activities can result in accelerated soil erosion, excessive scarification, and/or increased soil density, which may adversely affect site productivity. A study of landing areas emphasize the deficiencies in current rehabilitation guidelines Increased soil density on both summer and winter landings was still evident at 30 cm and the soil nutrient quality was poor. Two field tests of a green fallow system on subsoil materials exposed by erosion and landing construction proved successful in building site nutrient capital to acceptable levels. Seedling growth response to green fallow crop establishment in the coastal study verified these findings. A benefit-cost analysis of several forest soil rehabilitation scenarios demonstrated the importance of including secondary and intangible factors. From a pecuniary standpoint, based on primary benefits and costs, rehabilitation was economically feasible only when a low social discount rate (2%) and an optimistic stumpage increase projection (3% per year), were used. A discussion of some secondary and intangible benefits (i.e., harvesting rates, employment, government revenues, erosion control, and industry image) stresses the need for effective forest soil rehabilitation. / Forestry, Faculty of / Graduate

Site preparation treatments are often used prior to the planting of clearcut forest lands to improve planter access and to increase the number and quality of planting spots. Most mechanical site preparation treatments alter the configuration and material composition of surface soil materials, and can have marked effects on soil properties important to seedling survival and growth. Effects of some of these treatments on soil moisture, soil temperature, rates of nitrogen mineralization, and the establishment of Picea glauca x engelmannii seedlings were examined on fresh, moist, and wet sites in the moist cold subzone of the Sub-boreal Spruce Zone in west-central British Columbia. Four types of microsite alteration were investigated: forest floor removal (spot scalping), soil mounds over inverted sections of forest floor (inverted mounds), mineral soil mounds over a mineral soil surface, and inversion of the forest floor and mineral soil in place. Soil temperature was monitored continuously and soil moisture weekly at the 10-cm depth in 16 combinations of site and microsite treatment during two growing seasons. The response of seedling height and diameter growth was monitored for three growing seasons. Effects of altering soil temperatures through mechanical treatments on rates of nitrogen mineralization were examined by incubating a standard soil material in a range of microsites created by six combinations of site and mechanical treatment. Effects of substrate quality and soil temperature on rates of nitrogen mineralization were examined in paired mounded and untreated spots in fresh, moist, and wet sites. In all sites, early growing season soil temperatures in the seedling rooting zone were substantially warmer in inverted mounds than in other treatments. Spot scalping increased temperatures slightly relative to controls in the fresh site, but had little or no warming effect on moist and wet sites. Inverted mounds became substantially drier than other treatments during periods of low rainfall, particularly in the fresh site. After three growing seasons, seedling height growth was greatest in inverted mounds, irrespective of site. Amounts of nitrogen mineralized in a standard soil material during incubation for 77 days in the field were significantly greater for samples placed in inverted mounds than for those placed in other microsite treatments. There was a significant positive correlation between amounts of nitrogen mineralized during field incubations and degree hour sums calculated for associated microsite treatments and sites. Both substrate quality and soil thermal regime affected rates of N mineralization in samples from paired mounded and untreated spots, and an interaction was observed between the two factors. / Graduation date: 1991

Engelmann spruce-subalpine fir [Picea engelmannii Parry ex Engelm.-Abies lasiocarpa (Hook.) Nutt.] (ESSF) forests occupy large portions of western North America, and of British Columbia (B.C.) in particular. These areas represent a harsh environment for plant growth. The ESSF forests of B.C. have presented serious problems of regeneration following harvesting; several factors stimulated speculation as to whether N supply limitations were involved. This study was intended to highlight the post-harvest N dynamics of an ESSF forest, and the implications of the latter for silvicultural practices. Its general objectives included characterization of the post-harvest assart effect, and investigation of the N status and growth of advance regeneration. These were achieved by means of a comparative study of an age sequence of harvested sites. The assart effect lasted for at least eight years after harvesting, with a peak of change between years three and six. There were no major physical changes in the forest floor. Low C/N ratios between 19 and 32 were believed to have contributed to increased N availability. ESSF forests may have a generally higher level of N availability than previously supposed. The advance regeneration benefited from the assart effect. Nutrient uptake appeared to increase generally from at least three years after harvesting; increases of up to 78% were noted for N. There appeared to be no general macronutrient or micronutrient limitation to growth. However, evidence of S deficiencies was encountered in some trees. Moreover, the critical levels used for N may be in need of revision. A revised critical level of 1.40% for foliar N concentrations is proposed for subalpine fir advance regeneration. If this is accurate, regeneration may have been at least temporarily N-limited from year eight after harvesting. A more rigorous investigation of these possibilities is needed. The cutting method applied to the sites approximated a one-cut shelterwood method. The method as encountered in this study should not be considered a viable silvicultural option for similar ESSF forests. Its successful application would involve some degree of forest floor manipulation to improve seedbed conditions and soil microclimatic regimes. The findings of this study demonstrate that the environmental and biological characteristics of ESSF forests make high levels of planning and care a prerequisite for the success of silvicultural practices. The question of what comprises realistic growth and yield expectations of second-rotation stands in the ESSF zone needs to be addressed urgently. / Forestry, Faculty of / Graduate