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.
Queen's University Biological Station (QUBS), Chaffey's Locks, Ontario, Canada.
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.
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.
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.
Data were collected in 2012 and 2013 at Queen's University Biological Station (QUBS), Chaffey's Locks, Ontario, Canada (44°33′ N, 76°21′ W), using an old‐field meadow known locally as ‘Wire Fence Field’, approximately 2 ha in size, irregular in shape and surrounded on all sides by mature woodland. The field is one of several similar meadows at QUBS that were tilled about 70 yr earlier and sown with a mixture of timothy grass (Phleum pratense ) and red clover (Trifolium pratense ), and had been subsequently mown periodically for hay. Neighourhood crowding is intense in these communities. A previous study in a neighbouring field (Taylor & Aarssen 1990) showed that target plants (selected randomly) that had near neighbours left in place were 75–85% smaller in size (above‐ground dry mass) at the end of the growing season compared with target plants that had near neighbours removed.
Since 2008 the study field has not been mown and has been relatively undisturbed, except for localized vegetation sampling and occasional foot traffic. Tracey & Aarssen (2011) recorded 43 resident species, ranging in MAX (based on above‐ground dry mass) across two orders of magnitude, 39 of which were recorded in the present study (Appendix S1).
Species biomass harvest data (in 2012)
Forty plots measuring 0.5 m × 1.0 m were located along a grid of transect rows separated by 5 m, and neighbouring plots within a row were separated by 4–8 m (determined from a table of random numbers). Recognizing that species may vary in their phenology, reaching peak biomass at different times, sampling was spread out over the growing season, with two to four plots selected randomly for harvest approximately every 5 d between 1 Jun and 20 Aug 2012.
For each sample, a quadrat (0.5 m × 1.0 m) was used to outline the plot to be harvested, and all above‐ground biomass was cut, removed and transported in a plastic bag to cold storage at the lab for processing within 2–3 d. Metal spikes (15‐cm long) were positioned in the corners of each harvested plot so it could be relocated in the following year.
In the lab, each plot sample was sorted based on species identity, and the collective biomass for each species was dried in an oven at 80 °C for 72 h and then weighed.
Species abundance (recruitment/re‐establishment) data (1 yr after harvest, in 2013)
Of the 40 plots harvested in 2012, 38 were relocated in the summer of 2013. Plots were surveyed for species abundance (recruitment/re‐establishment) counts in situ, in the relative order in which they were harvested in the previous year, with about five plots processed per week on average through Jul and Aug 2013. For each plot, resident plant abundance – number of ‘rooted units’ – was recorded for each species. Counts here would have included individuals that survived and re‐established (following the 2012 biomass harvest) as well as new recruits (which could not be distinguished) resulting from sexual (seed) offspring establishment as well as (for some species) from clonal offspring establishment.