Summary
Larger plants of course generally produce more seed offspring than smaller plants. The vast majority of species, however (and resident reproductive plants within a given species’ population), are relatively small. To help interpret why this may be so, we controlled for effects of between-species body size variation by measuring fecundity per unit plant body size (i.e. ‘fecundity allocation’) – to test whether variation in the latter could be accounted for in part by between-species variation in two additional size metrics: seed size and leaf size. All else being equal (including body size), a plant that makes smaller seeds can be generally expected to produce more of them. Here, we explored whether the same effect on seed production may be associated with variation in leaf size, in accordance with the ‘leafing intensity premium’ hypothesis: a plant that makes smaller leaves can produce more of them per unit body size (i.e. a higher ‘leafing intensity’) – and hence more axillary meristems (i.e. a larger ‘bud bank’) per unit body size that are available, therefore, for deployment in sexual reproduction. We harvested the largest resident plant (above ground) at reproductive maturity from a natural population of each of 72 herbaceous angiosperm species ranging widely in potential body size. For each plant, we recorded total stem/shoot dry mass (representing ‘body size’), total and mean individual leaf dry mass, leafing intensity, mean individual seed mass, and potential fecundity – i.e. total number of fruits (or flower ovaries) per plant multiplied by mean number of seeds per fruit (or ovules per ovary). As expected, the majority of between-species variation in potential fecundity could be explained by variation in body size and seed size. Fecundity allocation (with body size effect removed), however, had a significant positive relationship with leafing intensity (and hence a negative relationship with leaf size). Species with higher fecundity allocation also had generally smaller seed size, and importantly, smaller body size. These results suggest that relatively high leafing intensity may be an important component in promoting the ‘reproductive economy’ of small plant species – i.e. the capacity to produce at least some offspring (despite body size limitation), especially when also suppressed in size (by larger species) within crowded vegetation.
Methodology
Study sites and field sampling protocol
A total of 72 herbaceous species (Table 1) were collected as time permitted during the growing season (May–Oct) of 2017, with a view to include as wide a range as possible in species body sizes. Species were chosen based on local availability as populations were encountered in haphazard surveys along roadsides and walking trails, and within old fields and recently disturbed habitats in the vicinity of Kingston Ontario and Queen’s University Biological Station (44°13′ N, 76°36′ W). For each species population with at least 20 resident plants, the largest individual (based on height and canopy spread) that had no, or negligible, herbivore damage was monitored and harvested at ground level once flower production had virtually finished and mature seeds were starting to disperse. Only the largest individual was harvested since our objective for data analyses required that we obtain an estimate of the range of between-species variation in potential body sizes. Harvested plants were placed in a plastic bag and transported to cold storage (−2°C) in the lab for later processing.
Lab processing
The following traits were recording for each harvested plant: number of leaves and number of leaves missing (i.e. older leaves that had already fallen in the field, evident from leaf scars), number of fruits and number of fruits missing (evident from peduncles), and number of remaining flowers (or flowers missing, evident from peduncles). The collection of all leaves and the collection of all fruits (removed and each placed in paper bags), were dried in an oven at 80°C for three days to record dry mass. Dry mass was also recorded (separately) for the remaining vegetative plant body (stems/shoots) – abbreviated here as ‘body size’ – and for any remaining flowers/inflorescences.
Seed mass data were available for some species from a lab data base compiled from previous studies. When seed mass was not available, a random sample of at least five seeds from each plant were weighed. When individual seed mass was too small to be accurately recorded, five samples of between five and twenty seeds were weighed together to determine mean mass per seed.