UBC Theses and Dissertations

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UBC Theses and Dissertations

The soil water regime and growth of uneven-age interior Douglas-fir (Pseudotsuga menziesii var. glauca) stands Korol, Ronda Lee


A study was initiated in 1984 to investigate the soil water regime and growth in uneven-age interior Douglas-fir stands. Eleven 10 m x 10 m microclimate plots covering a wide range of canopy coverages and twenty-two 25 m x 25 m inventory plots were established in the IDF[sub a] and IDF[sub b] biogeoclimatic subzones in the Kamloops area. On one microclimate plot windspeed, relative humidity, air temperature, solar irradiance, net radiation and soil temperature were measured. Relative humidity, solar irradiance and air temperature were measured on six additional plots, while on all eleven plots snow water equivalence, root zone water storage and rainfall were measured. The inventory plots, which had stand densities ranging from 96 to 2,784 trees ha⁻¹, were located near the microclimate plots on areas partially logged between five and thirty years ago. There were a total of 956 trees on these plots. One hundred trees were randomly selected for stem analysis by falling and removing cross-sectional discs at ground level, breast height, base of the crown and midway through the crown. An additional 700 increment cores were taken and analyzed. Diameter at breast height was recorded for all the trees in all plots, while height and bark width was measured on 30% of the total number of trees. The number and age of the regeneration were obtained and all plots were mapped. Four permanent sample plots established by Balco Industries Ltd. were also remeasured. From the data obtained from the microclimate plots, the snow water equivalent depths, soil water matric potentials and courses of the soil water storage over the growing season were analyzed. The growing season evapotranspiration and transpiration rates, determined by the water balance method for each of the canopy coverages, were analyzed. It was found that although growing season evapotranspiration rates were roughly similar for different canopy coverages, growing season transpiration rate varied considerably. As stand density increased, growing season transpiration rate tended to decrease. This was felt to be due to the large contribution of the grass component to the transpiration at low canopy coverages. Canopy resistance increased with increasing vapour pressure deficit. Sites with higher soil water matric potentials had higher transpiration rates. Gross Interception loss for a 100% canopy coverage was about 37% of the total rainfall. Both the breast height diameter and height growth rate decreased with increasing stand density. The ratio of the diameter to height was used successfully as a variable in the local volume equation and decreased with increasing stand density. The highest growth rates (9 -11 m³ ha⁻¹ year⁻¹) were found at stand densities of 901 - 1200 trees ha⁻¹, and stand volumes of 225 to 300 m³ ha⁻¹. At lower stand densities, the annual volume growth rates tended to increase with increasing stand volumes. At stand densities of > 1500 trees ha⁻¹, which had stand volumes of between 96 and 240 m³ ha⁻¹, annual volume growth rate decreased with increasing stand volume. Furthermore, it was found that there was a wide range of annual volume growth rates that can be obtained for a given stand volume. This variation was due to different stand densities. Stand diameter distributions which favoured annual volume growth rates had q-values (5 cm diameter classes) of between 1.28 and 1.29. There was a positive correlation between annual volume growth rate and growing season transpiration for stands > 35% canopy coverage. At lower canopy coverages, the poor correlation was thought to be due to the large grass component. The reduction in the annual volume growth rates of stand with densities > 1500 trees ha⁻¹ appeared to be due to increased between-tree competition and not solely to lower transpiration rates because of interception loss.

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