1. According to the traditional “Size Advantage” (SA) hypothesis, plant species with larger body size are expected to be more successful when competition is intense, that is, within severely crowded vegetation. Recent studies in old‐field habitats, however, have shown that those species with greater numerical abundance as resident plants generally have a relatively small minimum reproductive threshold size (MIN), not a relatively large maximum potential body size (MAX).
2. In this study, we test for a size advantage in terms of species abundance representation in the soil seed bank, and we extend the SA hypothesis to include two additional size metrics: leaf size and seed size. Specifically, we ask, for resident species within a crowded old‐field meadow: is larger seed size, leaf size, and/or body size associated with greater reproductive / recruitment success (i.e. number of germinable seeds within—and establishing plants emerging from—the soil seed bank)? We collected soil cores for a greenhouse experiment to record relative species abundances of germinable seeds in the seed bank, and we used a field experiment to record local abundances of species emerging from the resident seed bank within denuded plant neighbourhoods over three subsequent field seasons.
3. We found no general support for the SA hypothesis involving any of the size metrics, and none of the latter was a strong predictor of the number of germinable seeds emerging from soil cores in the greenhouse experiment. However, for species establishing in the field experiment from the seed bank over the 3‐year survey period, more abundant species in years 2 and 3 tended to be those with smaller MIN, and thus smaller MAX. In addition, within more crowded neighbourhoods, representation of reproductive plants was generally greater for species with relatively small MIN (and hence small MAX).
4. Synthesis. Our results extend the support for the “Reproductive Economy Advantage” hypothesis in old‐field habitats, to include not just established, largely undisturbed vegetation, but also very early stages of recruitment from seed within locally crowded plant neighbourhoods. Specifically, more successful species here are not those with relatively large potential body size (MAX); they are species capable of producing at least some offspring despite severe body size suppression, because they have a relatively small MIN. We summarize our interpretation using a simple conceptual model for predicting selection effects of local neighbourhood crowding on variation in fecundity (fitness estimate) of resident plants, resulting from genetic covariation in MAX and MIN. Relatively large potential body size (MAX) should be expected to promote fitness here, not when competition from near neighbour effects is severe, but when its effects are locally more moderate, or virtually absent—and hence relatively rarely within the generally crowded vegetation of old‐field habitats.
Data were collected between 2014 and 2016 at Queen’s University Biological Station (QUBS) near Chaffey’s Locks, Ontario, Canada (44°33’N, 76°21’W), using an old‐field meadow known locally as Wire Fence Field. The field is roughly two hectares in size, irregularly‐shaped, and surrounded by mature woodland. The site was last tilled and sown about 75 years earlier with a mixture of timothy grass (Phleum pratense) and red clover (Trifolium pratense), and was periodically mown for hay until summer 2008. Since then, the field has been relatively undisturbed, with only occasional foot traffic and vegetation sampling by field researchers. Tracey and Aarssen (2011) recorded 43 resident species, ranging in MAX across two orders of magnitude; however, 46 resident species were recorded in the present study (Appendix S1).
In summer 2013, 100 experimental field plots were established at the study site, delineated by metal cylinders installed in randomly chosen locations throughout the field. The cylinders were made of aluminium flashing, held together with rivets, and measured 25 cm in height, and 60 cm in diameter (2,827 cm2). Cylinders were installed using a spade to a depth of 20 cm, leaving 5 cm visible above‐ground level. Empty spaces around cylinders (left after installation) were filled with sterile sand. Later in the summer (September 2013), Roundup (Glyphosate – Monsanto Canada)—a broad spectrum systemic herbicide—was applied to kill the plants growing inside each cylinder, and once dead, the vegetation was cut at ground level, collected, and disposed of. Lids, made of aluminium flashing edges and wire mesh covers (with 0.5 × 0.5 cm openings) were then attached (using screws) to each cylinder to protect resident seeds from granivores. These cylinders allowed us to precisely control the area of impact by the herbicide and hence the area of denuded vegetation within the experimental plots. They also prevented roots and rhizomes of nearby vegetation from encroaching on the plots, thus preventing their impact on the recruitment and establishment success of resident species from the seed bank as we monitored this over the subsequent three growing seasons.