We explore the possible role of leaf size/number trade-offs for the interpretation of leaf size dimorphism in dioecious plant species.
Total above-ground biomass (both male and female) for three herbaceous dioecious species and individual shoots (from both male and female plants) for three woody dioecious species were sampled to record individual leaf dry mass, number of leaves, dry mass of residual above-ground tissue (all remaining non-leaf biomass), number of flowers/inflorescences (for herbaceous species) and number of branches.
For two out of three woody species and two out of three herbaceous species examined, male plants produced smaller leaves but with higher leafing intensity—i.e. more leaves per unit of supporting (residual) shoot tissue or plant body mass—compared with females. Male and female plants, however, did not differ in shoot or plant body mass or branching intensity. We interpret these results as possible evidence for a dimorphic leaf deployment strategy that promotes both male and female function, respectively. In male plants, capacity as a pollen donor may be favored by selection for a broadly spaced floral display, hence favoring relatively high leafing intensity because this provides more numerous axillary meristems that can be deployed for flowering, thus requiring a relatively small leaf as a trade-off. In one herbaceous species, higher leafing intensity in males was associated with greater flower production than in females. In contrast, in female plants, selection favors a relatively large leaf, we propose, because this promotes greater capacity for localized photosynthate production, thus supporting the locally high energetic cost of axillary fruit and seed development, which in turn requires a relatively low leafing intensity as a trade-off.
Six dioecious species (three herbaceous and three woody; Table 1) were sampled from large natural populations (one population per species) located in the vicinity of Kingston, Ontario (44°17′N, 76°34′0W), including at the Queen’s University Biology Station (44°33′N, 76°21′W). Within each population, transects were positioned randomly during the growing season once the herbaceous species had reached their full reproductive size and once the new (current year) shoots and leaves for woody species had reached their full sizes. Samples were collected from the nearest mature individuals (i.e. currently reproductive, in the case of herbaceous species) located at random positions (determined from a table of random numbers representing meters) along the transects (see Table 1 for sample sizes). Individuals for herbaceous species were sampled whole by clipping them at ground level. For woody species, one shoot from each individual was collected from the highest reachable point on the plant using tree clippers that extended to 5 m. Because these particular sampled shoots did not necessarily possess flowers, flower production could not be accurately measured for woody species. A shoot was defined as the current year’s growth—consisting of a leader and its subtending axillary branches, leaves and reproductive structures (if present)—emerging from both the terminal bud and its subtending axillary buds (respectively) that were produced on a single leader stem in the previous year. Herbaceous samples were frozen and woody samples stored at 4°C until processing.
From each herbaceous plant, all leaves were removed and counted. Counts of leaf scars (if present, indicating leaves that had been previously shed or consumed) were added to the existing leaf counts. Leaf tissue and residual tissue (all remaining non-leaf above-ground biomass) were dried separately at 60°C for 3 days. Leaf size was measured as average individual leaf dry mass, and leafing intensity was measured as the total number of leaves produced per unit residual dry mass (following Whitman and Aarssen 2010). Total number of branch endings and total number of flowers or inflorescences were also counted for each plant, representing the number of axillary meristems used by the plant (for branching and reproduction).
For each shoot from woody species, the number of leaves (and leaf scars if present, representing lost leaves) were counted on the terminal stem and subtending axillary branches, representing therefore the current year’s leaf production. The number of axillary branches was also counted. Leaf tissue and woody shoot tissue (together with reproductive tissue, if present) were dried separately for 3 days at 60°C. Leaf size was again measured as average individual leaf dry mass, and leafing intensity was measured as total number of leaves produced per unit of residual (remaining, non-leaf) dry mass (following Kleiman and Aarssen 2007).