Reproductive costs in Salix planifolia ssp. planifolia in subarctic Québec, Canada

Abstract

In dioecious plant species, females generally allocate more resources to reproduction than males, essentially because of fruit production. A higher reproductive investment for females may result in lower growth, survival and/or defense, i.e., a higher cost to reproduction. We investigated differences in reproductive cost between males and females of the dioecious shrub Salix planifolia ssp. planifolia in subarctic Québec (Canada). Our aims were to test for the presence of trade-offs between current reproduction and (i) current and future growth, (ii) current defense, and (iii) future reproduction. We estimated the reproductive effort (mass, C, N, and P) of 20 male and 20 female shrubs. We also manipulated the reproductive effort of individual shoots on 20 other shrubs by removing their reproductive buds before the beginning of the flowering season. During the growing season, we counted the number of leaves on each shoot and determined the frequency of herbivory. At the end of the growing season, we estimated twig mass, vegetative bud number, and reproductive bud mass and number. More nutrients were invested in fruit production than either bud or flower production. Thus, females had a significantly greater (1.6 to 2.0 times) reproductive effort than males. Bud removal caused a decrease in the frequency of leaf herbivory and an increase in leaf number, area and mass. Bud removal also led to an increase in the following-year reproductive bud number and mass, more so in females than in males. However, bud removal did not have any effect on twig mass or vegetative bud number, neither in males nor in females. In S. planifolia, there are significant costs to reproduction which are manifested mostly by higher susceptibility to folivores and lower potential for future reproduction.