Leaf resistance responses in selected conifers interpreted with a model simulating transpiration Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/1831cm94x

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  • The resistance of a leaf to molecular gaseous diffusion is an important physiological response. Control of leaf resistance enables a plant to control its transpired water loss and regulates photosynthesis. Four study sites representing a gradient in available moisture and evaporative demand, were chosen in western Oregon for the study of leaf resistance responses in five conifers. The conifers included Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), grand fir (Abies grandis, Lindl.), western hemlock (Tsuga heterophylla (Raf.) Sarg.), noble fir (Abies procera, Rehd.) and Sitka spruce (Picea sitchensis (Bong.) Carr). Study areas were located at a coastal site, in the interior Coast range, at mid elevation in the west central Cascades, and in the western Siskiyous near Oregon Caves. Environments could be characterized respectively as mild and moist, hot and dry, cool and moist, and warm and dry in reference to temperature and evaporative demand. Leaf resistance measurements were made during the period August 8 - September 24, 1972, with a modified aspirated diffusion porometer. Leaf resistance response was correlated with predawn plant moisture stress, absolute humidity deficit, and temperature. In both Douglas-fir and grand fir leaf resistance response was closely correlated with predawn plant moisture stress. As plant moisture stress increased from 4 - 18 atm, leaf resistances rose from 5 - 300 sec cm⁻¹. Leaf resistance in western hemlock ranged from 12 - 100 sec cm⁻¹ but was not closely correlated to any of the measured environmental variables. Noble fir and Sitka spruce showed little control of transpiration by leaf resistance over the range of environments measured. Leaf resistance measurements were compared to an indirect estimate of stomatal aperture obtained by a pressure infiltration test. The comparison proved poor on all species. One possible cause was that leaf resistance measurements were restricted to current foliage which are more sensitive and variable than the older foliage normally selected for the infiltration test. Under extreme evaporative demand Douglas-fir and grand fir exhibited leaf resistances ranging from 300 - 1,200 sec cm⁻¹. Infiltration tests indicated stomata were still partially open suggesting that stomatal resistance was not the cause of these extremely high leaf resistances. Rather it appears that mesophyll resistance may play an important role in reducing transpiration during times of high evaporative stress. From the knowledge of leaf resistance responses, a simulation model of transpiration was constructed. Plant moisture stress estimated by soil water status was used to control leaf resistance. Simulation runs were made for an 80 meter tree, a 10.4. hectare watershed, and a 2 meter tree. Results from the 0 meter tree simulation indicated the importance of the hydrostatic gradient in stomatal control. From the 2 meter tree simulation the importance of water storage in sapwood for meeting daily transpiration demand was emphasized.
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