The effect of sex in dioecious Juniperus monosperma on physiological response to experimental heat and drought.

Background/Question/Methods

Dioecious plants, those with male and female reproductive structures in separate individuals, have been found to exhibit differences between sexes in non-reproductive physiological function. Research has found that female plants often have lower rates of photosynthesis and stomatal conductance than males under dry or hot conditions. These differences in physiological function can drive shifts in sex ratio across landscapes, with strong impacts on plant population dynamics. An imbalance in sex ratio due to increased aridity, shifting toward male bias could threaten the viability of dioecious plant species under climate change. In earlier work, Juniperus monosperma a dioecious woody plant of the interior southwest US, had male-biased sex ratios at drier sites, and females had higher infection with parasitic mistletoe which reduced mycorrhizal colonization. We used results from the Los Alamos Survival/Mortality Experiment (SUMO), a field manipulation of drought and heat treatments on mature J. monosperma to test whether male and female trees differed physiologically. Responses assessed included photosynthesis, stomatal conductance, water potential (pre-dawn and mid-day), non-structural carbohydrate (NSC) concentrations, and metrics derived from branch hydraulic vulnerability curves. Data collected over four years at SUMO were analyzed using sign tests to evaluate male vs. female responses over time.

Results/Conclusions

Across treatments at SUMO, male J. monosperma had 10% higher photosynthetic rates and 11% higher stomatal conductance than females. Males also had higher (less negative) pre-dawn water potentials than females, indicating that male trees had more access to soil moisture than females. For NSC we saw that males had 9% higher total NSC in leaves, yet 3% lower total NSC in stem tissues than females, with both results driven by differences in starch between sexes. Although there was no difference in root total NSC storage between the sexes, females had 30% greater sugar concentrations in roots than males, possibly a higher allocation of NSC to osmoregulation. We did not observe effects of tree sex on mid-day water potential, nor on hydraulic vulnerability curve parameters for the water potential points at 50% cavitation and embolism entry. Our findings that hydraulic vulnerability curve metrics did not differ between the sexes, but that pre-dawn water potential did, indicates that females have the same vulnerability to cavitation as males, but come closer to experiencing cavitation than males do. Our results demonstrate that biased sex ratios observed in this semi-arid tree species are driven by physiological differences in resource acquisition and storage that favor males.