Sexual differences in shoot and leaf dynamics in the dioecious tree Salix sachalinensis

Abstract

In the dioecious tree Salix sachalinensis Fr. Schm., females invested a greater amount of biomass to reproductive organs compared with males, indicating a greater reproductive cost in females. To elucidate mechanisms for the compensation of reproductive costs in females, we examined differences between the sexes in resource allocation, leaf and shoot dynamics, and leaf photosynthetic ability. In individual 1-year-old shoots, greater vegetative shoot biomass was observed in females than in males, although the mean mass of individual vegetative shoots was lower in females than in males. In both sexes, vegetative shoots at the proximal end of 1-year-old shoots completed their annual leaf production earliest and began shedding leaves earliest, although the light conditions did not differ from those at the terminal end. The leaf and shoot structure and dynamics in S. sachalinensis suggest effective carbon gain for both sexes by always placing new leaves with high photosynthetic rates at terminal shoot positions. In females, in particular, the net carbon gain would be increased by reducing construction and respiration costs and tissue loss by placing inexpensive small shoots in proximal shoot positions. To investigate to what extent the photosynthetic capacity of short-lived shoots compensated for reproductive costs in females, 80% of the vegetative-shoot buds were removed from 1-year-old shoots in females and males to simulate natural shoot shedding. In the current year, shoot removal did not reduce reproductive investment, but strongly reduced the diameter growth of 1-year-old shoots, in which the reduction was much greater in females than in males. In the following year, a reduction in reproductive investment was observed in females. Females of S. sachalinensis compensated for reproductive costs, not only by investing a greater amount of biomass in vegetative shoots, but also by using a more effective system of carbon acquisition (rapid leaf turnover on inexpensive shoots) than males.