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.
Data collection – abundance
Between mid‐June and mid‐August 2010, abundance data were measured as counts of individuals (or ‘ramets’) for all vascular plant species present within 78 randomly positioned quadrats (1.0 × 1.0 m). For tufted, noncreeping sedges and grasses (e.g., Phelum pratense L.), both an individual tiller and a tuft of tillers were counted as an individual. For clonal rhizomatous species (e.g., Poa pratensis L.), just the local tiller or tuft of tillers that were visible aboveground were regarded as an individual. For stoloniferous species (Fragaria virginiana L, Trifolium repens L.), all connected tissues that could be seen aboveground were regarded as an individual. Within each 1 × 1 m quadrat, all individuals of each species were clipped at ground level and counted, with separate counts for reproductive and nonreproductive individuals. A reproductive individual was defined as one displaying flowers or indications of present or recent seed/fruit production. For each resident species, regardless of its MAX, most counted individuals were only a small fraction of MAX because of natural crowding/competition from near neighbors.
Data collection – maximum potential body size
Plant species obviously differ in maximum potential body size that can be attained by a single rooted unit (which may be an individual genet or may be an individual ramet in a clonal species), but quantifying this accurately in situ for natural vegetation is complicated by uncontrolled sources of variation in genotype, age and local environmental conditions. Accordingly, MAX, like fitness, can only be recorded as a relative estimate. To estimate relative MAX for our study species, the field was surveyed in May 2011 using visual inspection to locate the five largest individuals (target plants) of each of the 35 species studied by Tracey and Aarssen (2011). Neighboring plants within a 50 cm radius around each target plant (defined as the target plant ‘neighborhood’) were clipped at ground level (with clippings left in place). Metal wire cages 1 m in diameter and 1 m high were constructed and attached to the ground, centered on each target plant, in order to prevent herbivory by deer and rabbits and to prevent aboveground growth of plants beyond the cages from extending into the target neighborhood area. In addition, the ground around the exterior perimeter of each cage was trenched with a spade to a depth of 25 cm in order to sever any roots of outside plants that might extend into the target neighborhood area. Dried hay was added to a depth of approximately five cm on the ground surrounding the target plant within each cage to minimize re‐growth of cut vegetation and to minimize any increased moisture evaporation from the soil surface resulting from the reduced ground cover caused by removing neighboring vegetation. For species that were climbers (i.e., Vicia cracca L., Cerastium arvense L., etc.), target plants were able to climb up the side of the cage. The plots were visited weekly over the growing season to monitor the condition of target plants and to clip any emergent re‐growth of vegetation within the target neighborhood.
Later in the growing season, each target species was harvested when plants reached the reproductive stage and had stopped growing in size, while showing visible signs of senescence (e.g., withering/brown tissue, fallen leaves). The plant was cut at ground level, and all aboveground biomass (including leaves that had fallen off) for each target plant was collected and dried in a drying oven for 7 days at 70°C. The plants were then weighed to obtain their dry mass, representing their MAX (for aboveground growth). Minimum reproductive threshold sizes (MIN) were recorded during 2009 as part of an earlier study (Tracey and Aarssen 2011) for the same 35 species growing as resident plants within the same community – and, again, where a reproductive individual was defined as one displaying flowers or indications of present or recent seed/fruit production.
Data collection – recruitment
From August to October 2012 – 1 year after target plants were harvested – recruitment of individuals was recorded for the neighborhood areas within which vegetation had been removed around the target plants in the previous year. Recruitment was measured by cutting – at ground level – all vegetation growing within the 1‐m diameter target neighborhoods and counting the number of individuals present for both the target species and other (nontarget) species.