Contrasting responses in the growth and energy utilization properties of sympatric Populus and Salix to different altitudes: implications for sexual dimorphism in Salicaceae

Abstract

An interesting ecological and evolutionary puzzle arises from the observations of male-biased sex ratios in genus Populus, whereas in the taxonomically related Salix, females are generally more dominant. In the present study, we combined results from a field investigation into the sex ratios of the Salicaceous species along an altitudinal gradient on Gongga Mountain, and a pot experiment by monitoring growth and energy utilization properties to elucidate the mechanisms governing sexual dimorphism. At middle altitudes 2000 and 2300 m, the sex ratios were consistent with a 1:1 equilibrium in sympatric Populus purdomii and Salix magnifica. However, at the lower and higher ends of the altitudinal gradient, skewed sex ratios were observed. For example, the male:female ratios were 1.33 and 2.36 in P. purdomii at 1700 and 2600 m respectively; for S. magnifica the ratio was 0.62 at 2600 m. At 2300 m, the pot-grown seedlings of both species exhibited the highest biomass accumulation and total leaf area, simultaneously with the balanced sex ratios in the field. At 3300 m, the specific leaf area in male P. purdomii was 23.9% higher than that of females, which may be the morphological cause for the observed 19.3% higher nitrogen allocation to Rubisco, and 20.6% lower allocation to cell walls. As such, male P. purdomii showed a 32.9% higher foliar photosynthetic capacity, concomitant with a 12.0% lower construction cost. These properties resulted in higher photosynthetic nitrogen- and energy-use efficiencies, and shorter payback time (24.4 vs 40.1 days), the time span that a leaf must photosynthesize to amortize the carbon investment. Our results thus suggested that male P. purdomii evolved a quicker energy-return strategy. Consequently, these superior energy gain-cost related traits and the higher total leaf area contributed to the higher growth rate and tolerance in stress-prone environments, which might, in part, shed new light on the male-biased sex ratios in Populus. However, no significant sexual difference was observed in S. magnifica for all the above parameters, thereby implying that the female-biased sex ratios in Salix cannot be explained in terms of the energy-use properties studied here.

Sexually differential tolerance to water deficiency of Salix paraplesia—A female‐biased alpine willow

“…Our field investigation on the adult trees showed that S. paraplesia was a female-biased species in their natural population, particularly at a high altitude. This result is consistent with the species of S. magnifica along an altitude gradient in the Gongga Mountain, which is located in the eastern edge of the Tibetan Plateau (Lei, Chen, Jiang, Yu, & Duan, 2016). A previous study reported that the sex ratio of Valeriana edulis population was strongly female-biased and altered across the elevation gradient (Petry et al, 2016).…”

“…According to empirical studies, reproduction would compete with vegetation growth and defense when resources are limited (Herms & Mattson, ). Despite the greater reproductive cost for females, the sex ratio of Salix populations was usually female‐biased in their harsh habitats (Lei et al, ; Ueno et al, ). One hypothesis is that females not only allocate more carbohydrates for reproduction but also usually invest more in defense for abiotic stress only if there is a demand.…”

Sex-Related Differences in Growth, Herbivory, and Defense of Two Salix Species

“…Kaul and Kaul [16] found Salix amygdaloides has an equal number of male and female individuals, and a 1.7:1.0 male-biased sex ratio was observed in Salix exigua [35]. However, sex ratios in the genus Salix are often more skewed toward females than males [36][37][38][39]. We have similar findings, and our data shows that the sex ratios of two S. suchowensis populations are an obviously female-biased sex ratio.…”

Sex‐specific floral morphology, biomass, and phytohormones associated with altitude in dioecious Populus cathayana populations

“…In contrast, female plants allocate more biomass to growth during the early flowering period than male plants (Delph, 1990;Gross & Soule, 1981;Korpelainen, 1992) and also contribute more carbon to floral performance (Laporte & Delph, 1996). Furthermore, the male and female plants of dioecious trees exhibit significant differences in sex ratio, physiological processes, or antioxidant defense enzymes under changes in numerous environmental conditions, including altitude elevation (Lei, Chen, Jiang, Yu, & Duan, 2017;Li et al, 2007), increased temperature (Xu et al, 2008), elevated CO 2 concentration (Wang & Griffin, 2003;Zhao, Xu, Zhang, Korpelainen, & Li, 2011), enhanced UV-B radiation (Chen et al, 2016;Xu et al, 2010), nitrogen status (Chen, Dong, & Duan, 2014;Li, Dong, Guo, & Zhao, 2015;Li & Korpelainen, 2015), and competition (Chen, Duan, Wang, Korpelainen, & Li, 2014), but limited information is available on how sexual differences in floral traits in woody species are affected by altitude.…”