Leafing intensity—number of leaves produced per unit of supporting (nonleaf, aboveground) dry mass—determines the size of a plant’s “bud bank”, i.e., the number of axillary meristems per unit plant body or shoot size. This in turn determines the plant’s capacity for flexible and economic meristem deployment strategies as vegetative or reproductive structures. From recent research, it is now widely established that leafing intensity has a strong and isometrically negative relationship with individual leaf mass at the between-species level for both woody and herbaceous species. In the present study of 24 natural populations of herbaceous angiosperms, we show that these two traits also have a general trade-off relationship at the between-plant level within a species. Smaller resident reproductive (i.e., mature) plants generally produced smaller leaves, and plants with smaller leaves generally had higher leafing intensity, in most cases involving an isometric trade-off. For several species, however, the trade-off was allometric—i.e., plants with smaller leaves, which also had generally smaller body sizes, had generally greater than proportionally higher leafing intensity. This parallels results of an earlier study at the between-species level suggesting that, when plant body size is relatively small, there may be a premium—in terms of maximizing fitness—on relatively high leafing intensity. The latter, it is proposed, may function in maximizing the capacity for “reproductive economy”, i.e., successful reproduction despite intense size suppression owing to competition.
Twenty-four herbaceous plant species, representing 13 families, were collected from abandoned old field and (or) grassland sites in the vicinities of Kingston, Ontario (44°17′N, 76°34′W), Queen’s University Biological Station (44°33′N, 76°21′W), and Whitney, Ontario (10°27′N, 29°11′W), Canada. Common species were selected based on local availability and with relatively large populations—allowing, in most cases, at least 40–50 individuals to be sampled—but were otherwise selected haphazardly (Table 1). For each population at the peak of flowering, transects were laid out within central sections of the vegetation with approximately uniform topography and unaffected by heterogeneous edge effects (e.g., avoiding shade effects from trees), but were otherwise positioned randomly. At random intervals along the transects, the nearest individual reproductive plants were located and harvested at ground level. Only plants with flowers were collected to ensure that typical adult plant sizes were represented. All samples were frozen until processing.
For each sample, individual leaves were removed and counted—including counts for leaf scars, indicating leaves that had been shed or consumed—to obtain the total number of leaves produced that year. Leaf tissue and residual tissue (all remaining, nonleaf, aboveground biomass) were separated, and dried at 80 °C for 3 days. Average individual leaf dry mass (g) was used as a measure of leaf size, and leafing intensity was measured as the total number leaves produced per unit residual dry mass (g). Total per plant leaf dry mass was adjusted to take account of lost leaves (indicated by leaf scars).