Physiological responses of four Douglas-fir populations in three contrasting field environments Public Deposited

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  • When trees from 16 populations of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco. var. menziesii) were grown at ten geographically diverse locations for seven years, those from a Vancouver Island source were among the top three for height growth on nine of the ten plantations (Rowe and Ching, 1974). In contrast, growth of a west central Oregon source consistently ranked only poor or fair in comparison to other sources. Although each population was planted with the others at its home site, seldom did it grow better there than most of the introduced populations. I hypothesized that observed differences in growth among Douglas-fir populations were due to inherent differences in stomatal behavior and/or allocation of growth resources. I also hypothesized that definition of seasonal contrasts between field environments would help identify periods of environmental stress which would, in turn, help determine the factors most important in limiting productivity on different sites. I attempted to link studies of genetics, physiology and land classification together to provide a better biological foundation for improving forest productivity. I observed moisture stress, stomatal aperture, leaf resistance, cambial and leader growth for four populations of Douglas-fir at a Coast Range, Willamette Valley and Cascade Mountain plantation. Climatic factors including radiation, humidity, rainfall, soil and air temperature were recorded at each location for the 1971 growing season. Stomatal behavior for the four populations at the three locations was similar. In the spring before soils had warmed, stomata were partly closed at night and nearly wide open during the day. In early summer, stomata were wide open day and night. Later, as drought stress increased, stomata were closed at night, and partly closed during the day. After fall rains, stomata were again open day and night. Leaf conductance at the peak of drought was variable during the day, suggesting cyclic opening and closing of stomata, The southern source appeared to have higher conductance during the day although this trend could not be confirmed by statistical test. Except for unusually high values in the spring, plant moisture stress followed the expected daily and seasonal pattern. There were characteristic differences between locations but all populations were similar at a given area. A simulation of photosynthesis was developed around a model which predicted daily CO2 fixation per square decimeter of foliage. Functions were included to reduce potential photosynthesis under conditions of high moisture stress, cold soils, and frost. Predicted photosynthesis for three areas revealed major differences in seasonal activity and yearly potential.. The simulation was useful in defining seasonal contributions to total production, and the effect of drought and cold stress. It also provided an index to site productivity. At the dry interior Coast Range site, 31 percent of predicted CO2 uptake occurred in the dormant season, while at the Cascade site, less than 10 percent accumulated in the same period. Almost 60 percent of growing season photosynthetic potential was not realized due to moisture stress in the interior Coast Range. At a Willamette Valley site on deep soils, less than 40 percent of growing season potential was lost because of moisture stress. The photosynthesis index, along with indexes of moisture stress and temperature, helped define more clearly the environmental restrictions on forest productivity at different sites. These methods should be useful in matching the biological potential of seed sources to suitable forest environments. Although there were accumulated differences in size of individuals from certain sources, cambial and leader growth of 311 populations were similar for the 1971 growing season. This was true for actual growth and when growth increment was normalized for initial tree size or needle mass, Partitioning of biomass into needles, branches and bole differed by area, but not by population. Trees at the Coast Range site had more needle mass per unit bole volume and grew more per unit leaf area than those in the mountain plantation. The large differences in growth between areas were interpretable through the physiological indexes which were generated from environment3l observations at each site. Analysis of growth indicated that size differences among populations were either diminishing or remaining consistent, Variations in height which prompted this study were attributed to distinctive behavior effective during establishment. Allocation of carbohydrates, photosynthetic efficiency, hormone balance, or stomatal control may be involved in explaining contrasts in early growth. Future efforts to explain genetic differences in growth of co3st3l Douglas-fir should concentrate on times when individual trees are under environmental or competitive stress.
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