Abstract:
The survival, growth, foliar nutrient status, ectomycorrhiza (EM) colonization,
and associative N-fixation of ponderosa pine seedlings (Pinus ponderosa var. ponderosa
Doug!. ex Laws.) were studied under different gap sizes in forest canopy during the first growing season. Seedling performance variables were assessed relative to gap size of the forest canopy with and without a shrub removal and trenching treatment. Percent cover of shrubs and herbaceous plants, soil nutrient status, and soil moisture concentration were also determined. The study sites were located on forested lavas in central Oregon. Seedling survival in clearcut plots (5 7%) was much lower than in patch cut plots (94%) and undisturbed forest plots (97%), probably because of increased water stress in clearcuts. Soil moisture concentration within one month after planting had declined more rapidly in clearcuts, perhaps aggravated by the abundance of sedges in clearcut plots relative to other plots. Shoot dry weight of surviving seedlings was greatest in clearcut plots, intermediate in patch cut, and lowest in undisturbed forest plots. This probably reflects the availability of light and nitrogen. Foliar nitrogen concentration and content of
seedlings in clearcut plots were higher than in patch cut and undisturbed forest plots.
Graphical analyses of element composition in needles indicated that seedlings in clearcuts
and undisturbed forest plots were nitrogen deficient, and seedlings in patch cuts were iron deficient. Thirty EM morphotypes were found on 108 seedlings. EM colonization and
morphotype riclmess were greater in patch cut and undisturbed forest plots than in
clearcut plots. The presence of active and seedling compatible EM fungi near established
seedlings. Early EM colonization in patch cuts and forest plots may have increased
seedling drought tolerance whereas delays in colonization may have aggravated seedling
mortality in clearcut plots. EM colonization may have played a role in facilitating early
seedling growth. Seedlings less than 15% colonized with EM did not grow well, and
seedlings with the best growth performance were always greater than 50% colonized. Rates of associative N-fixation in seedling rhizospheres were greatest in forest,
intermediate in patch cut, and lowest in clearcut plots. A shrub removal and trenching treatment increased seedling growth in clearcuts, patch cuts and undisturbed forest plots, probably by increasing the availability of soil moisture and nutrients. Seedling survival, EM variables, and associative N-fixation were
not affected by the treatment.
