| A series of experiments were established to gain a better understanding of the extent to which Douglas-fir seeding root architecture may be manipulated and subsequent influences on seedling morphological and physiological development.; The incorporation of amendments into nursery soils changed root architecture to some degree, but did not produce large differences in morphology at lifting or following two growing seasons under field-fertilized and non-fertilized conditions. The application of controlled-release fertilizers (CRF) to the planting hole, however, produced an interesting response in which aboveground growth was enhanced during the first field season but negatively affected thereafter. The resulting hypothesis was that drought stress was responsible for the growth reduction.; To investigate rooting response to locally-applied CRF, two greenhouse experiments were established. In the first experiment, differences in seedling morphological and physiological development over time were observed under two comparable CRF types and this was attributed to variations in nutrient release. Roots proliferated in the soil zone above the locally-applied CRF, though root penetration into lower soil zones was not restricted. With increasing CRF rates in the second greenhouse experiment, however, root penetration into soil zones below the CRF decreased with increasing CRF rate six months following transplant (R2 = 0.72), likely due to the creation of a toxic osmotic gradient between rhizosphere and root. It was hypothesized that this response might intensify seedling drought stress following field fertilization.; A field study investigated the influence of initial root volume and field fertilization at a relatively high rate on seedling drought resistance. Regardless of initial root volume, fertilized seedlings became more drought stressed during summer and had lower rates of stomatal conductance near the end of summer. An increase in shoot:root dry weight, which was greater for fertilized seedlings, was inversely correlated with xylem pressure potential (R 2 = 0.54). There was no distinct proliferation of roots near the CRF layer as root growth in all vertical soil zones was negatively affected for fertilized seedlings. |
