Gender‐Specific Physiology, Carbon Isotope Discrimination, and Habitat Distribution in Boxelder, Acer Negundo

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Abstract
In the semiarid Intermountain West, boxelder, Acer negundo, var. interior, a deciduous, dieocious tree, exhibits significant habitat—specific sex ratio biases. Although the overall sex ratio (male/female) does not deviate significantly from one, the sex ratio is significantly male biased (1.62) in drought—prone habitats, while it is significantly female biased (0.65) in moist, streamside habitats. The causes underlying gender—specific habitat associations in this species are not known. We hypothesized that spatial segregation of the sexes is maintained by differences in gender—specific photosynthetic behavior, water relations characteristics, and both instantaneous and integrated water—use efficiency. Gender—specific physiological characteristics were measured and related to growth, reproduction, population age structure, and habitat distribution of male and female trees. Under both field and controlled—environment conditions, males and females differed significantly in a number of physiological traits. Males maintained lower stomatal conductance to water vapor (g), transpiration (E), net carbon assimilation (A), leaf internal CO2 concentration (ci), carbon isotope discrimination (△ an index of time—integrated ci and water—use efficiency), and higher instantaneous (A/E) and long—term (△) water—use efficiency than females. Furthermore, male trees exhibited greater stomatal sensitivity to both declining soil water content and increasing leaf—to—air vapor pressure gradients, a measure of evaporative demand. Higher rates of carbon fixation in female trees were correlated with higher g, higher leaf nitrogen concentrations, and greater stomatal densities. For females growing in both wet and dry habitats, vegetative shoots had higher growth rates than reproductive shoots, while for males, growth rates of the two shoot types did not differ. In streamside habitats, female trees exhibited significantly greater vegetative shoot growth when compared to male trees. In contrast, males showed slightly greater vegetative and much greater reproductive shoot growth in non—streamside habitats. Regardless of habitat or growing conditions, females allocated proportionately more of their aboveground biomass to reproduction than did males. These results suggest that (1) gender—specific physiological traits can help explain the maintenance of habitat—specific sex ratio biases in A. negundo along a soil moisture gradient, and (2) that the combination of the gender—specific physiology, growth, and allocation differences contribute to differences in the size (=age) structure of male and female plants within the population. Gender—specific physiological differences may have evolved as a product of selection to meet significantly different costs associated with reproduction in male and female plants.

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