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Notes: Summary The daily temperature and water relations of 7 perennial subalpine, understory species (1 shrub, 1 subshrub, 5 herbs) were compared in the Rocky Mountains of southeastern Wyoming with an emphasis on the effects of natural sun and shade exposure. Field measurements of rainfall; leaf, air, and soil temperatures; stomatal conductance to water vapor diffusion; and plant and soil water potentials were supplemented with leaf and root morphological measurements to evaluate potential adaptive patterns in understory species. Morphologically, all 7 species had relatively broad leaves that were hypostomous and bicolored with the abaxial leaf surface lighter than the abaxial surface. Root systems tended to be shallow (〈20 cm), especially for the herbaceous species. Although soil water potentials from 4 to 40 cm depths remained relatively high throughout the summer (〉-1.0 MPa), plant xylem water potentials for sunlit plants decreased to below-2.0 MPa during midday. During these sunlit periods, leaf temperatures and conductances increased substantially, leading to severe wilting for 4 of the 5 herbaceous species. Stomatal conductance and density for 6 of the 7 species monitored were much greater on abaxial compared to adaxial leaf sides and substantial stomatal closure occurred when either leaf side was oriented to receive direct sunlight. Moreover, stomatal opening on abaxial leaf sides corresponded to the amount of sunlight incident upon the adaxial rather than abaxial leaf surfaces. The 2 shrubby species did not wilt during these periods and were characterized by the highest leaf temperatures (〉30°C). These 2 species also had consistantly lower xylem water potentials throughout the summer growth period. These results are discussed in terms of the possible adaptive significance of midday wilting, leaf hypostomy and bicoloration and stomatal behavior to the water and photosynthetic relations of understory species.

Notes: Summary The influence of variations in the boundary air layer thickness on transpirtion due to changes in leaf dimension or wind speed was evaluated at a given stomatal resistance (r s) for various combinations of air temperature (T a) and total absorbed solar energy expressed as a fraction of full sunlight (S ffs). Predicted transpiration was found to either increase or decrease for increases in leaf size depending on specific combinations of T a, S ffs, and r s. Major reductions in simulated transpiration with increasing leaf size occurred for shaded, highly reflective, or specially oriented leaves (S ffs=0.1) at relatively high T a when r s was below a critical value of near 500 s m-1. Increases in S ffs and decreases in T a lowered this critical resistance to below 50 s m-1 for S ffs=0.7 and T a=20°C. In contrast, when r s was above this critical value, an increase in leaf dimension (or less wind) resulted in increases in transpiration, especially at high T a and S ffs. For several combinations of T a, S ffs, and r s, transpiration was minimal for a specific leaf size. These theoretical results were compared to field measurements on common desert, alpine, and subalpine plants to evaluate the possible interactions of leaf and environmental parameters that may serve to reduce transpiration in xeric habitats.

Notes: Summary Environmental and water relations parameters during fall were monitored for six conifer tree species common to the central Rocky Mountains growing naturally at the same location (Pinus contorta, Pinus ponderosa, Pinus flexilus, Pseudotsuga menziesii, Abies lasiocarpa, Picea engelmannii). Subsequent to what appeared to be the beginning of seasonal stomatal closure, leaf conductance to water vapor declined sharply following the onset of freezing air temperatures at night. A coincident rapid decline in morning xylem pressure potentials (ψp) also occurred which resulted in values that were considerably below afternoon ψp. Continuing decreases in maximum leaf conductance during the day were highly correlated with corresponding decreases in minimum nocturnal air temperatures of the preceding night. By mid-December, morning ψp returned to values very near afternoon ψp and were only slightly lower than before the onset of subfreezing nights. A preliminary model is proposed which interprets the qualitative interaction between air and soil temperatures, soil and plant water potentials, and leaf conductance during seasonal stomatal closure in fall.

Notes: Summary The temperature and water relations of the largleafed, high-elevation species Frasera speciosa, Balsamorhiza sagittata, and Rumex densiflorus were evaluated in the Medicine Bow Mountains of southeast Wyoming (USA) to determine the influence of leaf size, orientation, and arrangement on transpiration. These species characteristically have low minimum stomatal resistances (〈60 s m-1) and high maximum transpiration rates (〉260 mg m-2s-1 for F. speciosa). Field measurements of leaf and microclimatic parameters were incorporated into a computer simulation using standard energy balance equations which predicted leaf temperature (T leaf) and transpiration for various leaf sizes. Whole-plant transpiration during a day was simulated using field measurements for plants with natural leaf sizes and compared to transpiration rates simulated for plants having identical, but hypothetically smaller (0.5 cm) leaves during a clear day and a typically cloudy day. Although clear-day transpiration for F. speciosa plants with natural size leaves was only 2.0% less per unit leaf area than that predicted for plants with much smaller leaves, daily transpiration of B. sagittata and R. densiflorus plants with natural leaf sizes was 16.1% and 21.1% less, respectively. The predicted influence of a larger leaf size on transpiration for the cloudy day was similar to clear-day results except that F. speciosa had much greater decreases in transpiration (12.7%). The different influences of leaf size on transpiration between the three species was primarily due to major differences in leaf absorptance to solar radiation, orientation, and arrangement which caused large differences in T leaf. Also, simulated increases in leaf size above natural sizes measured in the field resulted in only small additional decreases in predicted transpiration, indicating a leaf size that was nearly optimal for reducing transpiration. These results are discussed in terms of the possible evolution of a larger leaf size in combination with specific leaf absorptances, orientations and arrangements which could act to reduce transpiration for species growing in short-season habitats where the requirement for rapid carbon fixation might necessitate low stomatal resistances.