No abstract

The selection consequences of competition in plants have been traditionally interpreted based on a “size-advantage” hypothesis – that is, under intense crowding/competition from neighbors, natural selection generally favors capacity for a relatively large plant body size. However, this conflicts with abundant data, showing that resident species body size distributions are usually strongly right-skewed at virtually all scales within vegetation. Using surveys within sample plots and a neighbor-removal experiment, we tested: (1) whether resident species that have a larger maximum potential body size (MAX) generally have more successful local individual recruitment, and thus greater local abundance/density (as predicted by the traditional size-advantage hypothesis); and (2) whether there is a general between-species trade-off relationship between MAX and capacity to produce offspring when body size is severely suppressed by crowding/competition – that is, whether resident species with a larger MAX generally also need to reach a larger minimum reproductive threshold size (MIN) before they can reproduce at all. The results showed that MIN had a positive relationship with MAX across resident species, and local density – as well as local density of just reproductive individuals – was generally greater for species with smaller MIN (and hence smaller MAX). In addition, the cleared neighborhoods of larger target species (which had relatively large MIN) generally had – in the following growing season – a lower ratio of conspecific recruitment within these neighborhoods relative to recruitment of other (i.e., smaller) species (which had generally smaller MIN). These data are consistent with an alternative hypothesis based on a ‘reproductive-economy-advantage’ – that is, superior fitness under competition in plants generally requires not larger potential body size, but rather superior capacity to recruit offspring that are in turn capable of producing grand-offspring – and hence transmitting genes to future generations – despite intense and persistent (cross-generational) crowding/competition from near neighbors. Selection for the latter is expected to favor relatively small minimum reproductive threshold size and hence – as a tradeoff – relatively small (not large) potential body size.

Aims Competitive ability in plants is defined traditionally by a ‘size advantage’ hypothesis – i.e. larger species are generally expected to be more successful under competition because of greater capacity for resource capture, and thus capacity to deny resources to neighbours (e.g. through shading). We therefore tested the prediction (for crowded herbaceous vegetation) that species with a larger maximum potential body size (dry mass) should: (1) have generally increased resident plant abundance (i.e. more rooted units), resulting from more successful recruitment of reproductive and/or clonal offspring; and (2) account for a relatively large proportion of the standing biomass within crowded neighbourhoods. Location Queen's University Biological Station (QUBS), Chaffey's Locks, Ontario, Canada. Methods A field experiment was used to record neighbourhood above‐ground biomass (dry mass) and resident plant abundance data for species within replicate plots in an old‐field meadow community – combined with body size metrics recorded for the same species in an earlier study at the same field site. Results The results, at both the community and plot level, showed that resident plant abundance was generally higher (as expected) for species that had more total neighbourhood (plot) biomass harvested in the previous year. However, these species tended to be relatively small, with relatively small minimum reproductive threshold size – not those with relatively large maximum potential body size. Conclusions These results suggest that in crowded vegetation, bigger species are not more successful competitors in terms of offspring recruitment/numerical abundance, nor do they have a larger contribution to neighbourhood biomass. Smaller species are contributing at least as much biomass as bigger species to their total neighbourhood biomass, in part because of their smaller minimum reproductive threshold size. These smaller species therefore have greater ‘reproductive economy’, which means that they are more likely to produce at least some offspring when conditions are especially crowded.

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).

Alternative metrics exist for representing variation in plant body size, but the vast majority of previous research for herbaceous plants has focused on dry mass. Dry mass provides a reasonably accurate and easily measured estimate for comparing relative capacity to convert solar energy into stored carbon. However, from a “plant's eye view”, its experience of its local biotic environment of immediate neighbors (especially when crowded) may be more accurately represented by measures of “space occupancy” (S–O) recorded in situ—rather than dry mass measured after storage in a drying oven. This study investigated relationships between dry mass and alternative metrics of S–O body size for resident plants sampled from natural populations of herbaceous species found in Eastern Ontario. Plant height, maximum lateral canopy extent, and estimated canopy area and volume were recorded in situ (in the field)—and both fresh and dry mass were recorded in the laboratory—for 138 species ranging widely in body size and for 20 plants ranging widely in body size within each of 10 focal species. Dry mass and fresh mass were highly correlated (r 2 > .95) and isometric, suggesting that for some studies, between‐species (or between‐plant) variation in water content may be unimportant and fresh mass can therefore substitute for dry mass. However, several relationships between dry mass and other S–O body size metrics showed allometry—that is, plants with smaller S–O body size had disproportionately less dry mass. In other words, they have higher “body mass density” (BMD) — more dry mass per unit S–O body size. These results have practical importance for experimental design and methodology as well as implications for the interpretation of “reproductive economy”—the capacity to produce offspring at small body sizes—because fecundity and dry mass (produced in the same growing season) typically have a positive, isometric relationship. Accordingly, the allometry between dry mass and S–O body size reported here suggests that plants with smaller S–O body size—because of higher BMD—may produce fewer offspring, but less than proportionately so; in other words, they may produce more offspring per unit of body size space occupancy.