Summary
- A sample of woody angiosperm species was used to test a central prediction of the ‘leafing intensity premium’ hypothesis: higher leafing intensity (number of leaves produced per unit dry mass of shoot vegetative tissue produced in the same growing season) confers a larger bud bank (i.e. number of axillary meristems per unit shoot tissue) that can be deployed for reproduction, and thus confers generally greater fruit numbers, and hence higher potential fecundity allocation (i.e. fecundity per unit size of the supporting shoot tissue that is produced in the same growing season.
- Current‐year shoots (i.e. bearing leaves) were collected to record: shoot dry mass, total number of leaves, total number of fruits or fruit clusters (if derived from inflorescences), mean individual leaf dry mass and mean individual fruit dry mass. Sampled individuals (shrubs and trees) were also measured for body size (main stem height and circumference).
- Species with larger individual fruit (or fruit cluster) mass have generally larger leaves, but they also have a negative trade‐off relationship with ‘fruiting intensity’ – that is the total number of reproductive meristems producing fruits (or fruit clusters) per unit dry mass of shoot vegetative tissue produced in the same growing season. Variation in fruiting intensity, however, is better predicted by a positive relationship with variation in bud bank size.
- Species with smaller leaf size (dry mass) have generally higher leafing intensity; species with higher leafing intensity in turn have generally higher fruiting intensity; and species with higher fruiting intensity in turn have generally higher potential fecundity allocation (based on the typical species maximum number of seeds per fruit, obtained from published floras). Species with smaller body size have generally higher potential fecundity allocation, but body size had no significant relationships with other measured traits when controlling for phylogeny (using phylogenetically independent contrasts).
- Synthesis. Our results indicate that bud bank size is an important functional trait for defining adaptive strategy in woody angiosperms. A larger bud bank is generated by higher leafing intensity, which in turn generates higher fruiting intensity, thus generating greater potential fecundity allocation. These traits will be important for maximizing reproductive economy – that is capacity to produce offspring despite growth or body size limitation (e.g. due to crowding/competition, or because of limited time available for growth, flowering, pollination or fruit/seed maturation).
Methodology
Deciduous woody angiosperms–both trees and shrubs (Table 1)–were sampled from several locations in southern and central Ontario,Canada: lands on and in the vicinity of Queen’s University Biology Station (44°33'N, 76°21'W) north of Kingston; Lemoine Point Conservation Area near Kingston (44°13'N, 76°36'W); and South Algnquin Crown land near Whitney (45°29'N,78°14'W). Within each location, sampling was carried out within open fields or along south-facing woodland edges, in order to ensure that canopies of sampled trees and shrubs were exposed (at least in part) to direct open sky for most of the day (i.e. with a relatively high light intensity) with none are neighbours that could impose shade on the south-facing canopy edge. Particular species of interest were sampled based on their avail-ability (including with fruiting individuals) within the 2011 growing season, while maximizing as much as possible the between-species range and distribution of both leaf size and fruit size (or fruit cluster size if derived from an inflorescence).For each study species, three individual plants were selected from the same population, and three representative shoots were sampled from each individual. A‘shoot’(Fig. 1) is defined here as the cur-rent-year‘terminal’leader, the subtending previous-year leader, all current-year lateral branches emerging from the previous-year leader and all associated leaves and fruits. Note that leaves and fruits can be produced only on the current-year leader or current-year lateral branches (or, for fruits in some species–less commonly–on the previous-year leader as an axillary or branch base cluster; Fig. 1). For each species, the three sampled individuals and their shoots were selected with the following limitations (but otherwise randomly): (i)the three shoots were located on the outermost south-facing edge of the canopy at a height sufficient to avoid shading effects from neighbouring plants or from over topping shoots on the same plant; (ii) their twig material was undamaged by physical disturbance (i.e. with no breakage), and leaf material was minimally damaged by consumers or evidence of disease; (iii) they were bearing at least some mature undamaged fruits (or fruit clusters).