Leafing intensity predicts fecundity allocation in herbaceous angiosperms

Abstract

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.