Graduate Thesis Or Dissertation
 

The effect of plant moisture stress on the physiology and establishment of planted Douglas-fir and ponderosa pine seedlings

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  • The role of moisture stress in seedling physiology and establishment was studied in both the field environment and laboratory experiments. Plant moisture stress was measured using the pressure chamber technique. This technique was modified and tested using a fascicle of pine needles as a sample, an improvement which allows the technique to be used repeatedly in the same seedling and is equivalent to measurements on twigs. Using the pressure chamber, ponderosa pine and Douglas-fir seedlings which were planted in early spring were monitored for both seasonal and diurnal changes in moisture stress throughout the first growing season until the fall rains recharged the soil. Differences in seedling growth and-survival during the three years of measurement were related to moisture stress levels experienced by the seedlings. Needle length of ponderosa pine was a good indicator of seasonal moisture stress levels. The photosynthetic response of seedlings to different levels of moisture stress with light at saturation levels was evaluated in a series of laboratory experiments using an infra-red gas analyzer in a closed system. The relative net photosynthetic response of ponderosa pine and Douglas-fir to moisture stress showed distinct differences between the two species. Douglas-fir showed an almost linear decline in photosynthesis from a maximum when plant moisture stress (PMS) was less than 8 atm to 20% of maximum at 22 atm. In contrast, ponderosa pine showed little decline in photosynthesis until about 15 atm PMS. There the decline, probably associated with stomatal closure, occurred abruptly with no measurable photosynthesis occurring after 20 atm PMS. Seedlings which had experienced 50 atm PMS showed reduced photosynthetic rates even two weeks after the moisture stress was relieved. PMS levels of 80 atm were lethal to the needle tissue of both species. There was more variability in response within the population of pine seedlings compared to the population of Douglas-fir. The photosynthetic response of ponderosa pine seedlings to temperature and moisture stress varying independently was also evaluated. The resulting response surface showed the proportion of a given response attributable to temperature and the proportion attributable to moisture stress. A partial differential equation is suggested for use in examining how the environmental factors contribute to a given plant response. An equation to describe how the plant occupies a predominant regulatory position in the soil-plant-atmosphere continuum is proposed.
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