First year survival and growth of planting stock of various size on adverse sites

Abstract

First year survival of Douglas-fir seedlings outplanted in areas characterized by intense vegetative competition is heavily dependent on available soil moisture. To test this hypothesis, five distinct classes of Douglas-fir planting stock were planted on the south slope of McCulloch Peak in McDonald Forest in February of 1975. The stocking classes represented in this study are 2-2 transplants, 2-0 seedlings, one-year-old container-grown (plug) seedlings, 3-0 seedlings, and 2-1 transplants. Four treatments were applied in two replications: (1) a combination of irrigation and herbicidal control of competing vegetation; (2) irrigation; (3) herbicidal control of competing vegetation; and (4) no cultural treatment. The Scholander pressure bomb technique was used to determine the timing of the irrigation treatment. Whenever the average pre-dawn xylem pressure potential of the seedlings fell below -20 bars, irrigation was applied. The two replications corresponded to two distinct vegetative communities: a brush-dominant community and a, grass-dominant community. To eliminate the variable of wildlife pressure, every seedling was protected by a mesh animal exclosure. Seedling mortality was tallied at intervals throughout the summer, and leader elongation was measured in October of 1975. The vegetation community in which a seedling was outplanted was of overriding importance to the seedling's potential for survival. Phenological development of the constituents of the vegetation community greatly influenced the availability of soil moisture so critical to seedling establishment. In turn, community structure determined the favorability, or lack thereof, of the microenvironment in which a seedling developed. In respect to both phenology and structure, the community dominated by grasses was more adverse to the introduction of Douglas-fir seedlings than was the community dominated by brush. The importance of the type of vegetative cover was further underscored by the response to the various cultural treatments. the brush-dominant community, irrigation, herbicides, and the combination of irrigation and herbicides proved equally effective as measures of site preparation. This was in contrast to the results in the grass-dominant community which showed that irrigation alone could not ensure acceptable seedling survival. Due to their inherent ability to disrupt the normal development of established vegetation, herbicides emerged as an especially effective means of ameliorating adverse site conditions. In both communities, little additional benefit was realized by coupling irrigation to the herbicide treatment. As was expected, seedlings which received no cultural treatment performed poorly regardless of type of vegetative cover. In regards to the performance of the various age-classes, the one-year-old container-grown seedlings showed a survival rate of nearly 90% in the grass community. Unable to match this performance, the 2-0, 2-1, 3-0, and 2-2 bare-rooted stock survived at the following rates: 45%, 44%, 36%, and 33%, respectively. The container-grown seedlings were not, however, superior in the brush community. Both the 2-1 and 3-0 planting stock had higher survival, 76 and 71%, respectively, than the container-grown seedlings (70%) and the 2-2 transplants (68%). The 2-0 seedlings (56% survival) performed poorly in the brushy area; although they had the highest survival of the bare-rooted stock in the grass community. Seedling morphological characteristics were meaningful to survival in the case of the 3-0 seedlings in the grass community and the container-grown seedlings in the brush community. In terms of height, diameter, and weight, the smaller 3-0 seedlings adapted to their new environment better than did larger 3-0 seedlings. For the container-grown seedlings, larger stem diameters were correlated with increasingly better survival. As a check on the various seedlings control of stomatal aperture, leaf water conductance was measured with a null balance diffusion porometer. Small seedlings tended to have higher rates of transpiration than large seedlings, but total transpirational loss under given environmental conditions was judged to be equivalent regardless of seedling size.