Revising traditional theory on the link between plant body size and fitness under competition: evidence from old‐field vegetation

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.

Species with larger body size do not dominate neighbourhood biomass production in old‐field vegetation

“…Importantly, counts here would have included re‐establishing survivors (following the 2012 biomass harvest) as well as new recruits, resulting from sexual (seed) offspring establishment, as well as (for some species) from clonal offspring establishment. These findings concur with previous study results collected from the same field site showing that larger species have generally lower numerical abundance (in terms of ‘rooted unit’ counts within undisturbed plots), and that conspecific recruitment around target plants had a significant negative relationship with species MAX (Tracey & Aarssen ). We note that larger species may produce larger seeds (and hence fewer seeds per unit body size becaue of a seed size/number trade‐off; Aarssen & Jordan ), but even if true for our study species, this would not necessarily account for the lower numerical abundance of larger species in our data set; under crowded conditions (as in our study site), a larger seed is traditionally expected to confer advantage under competition, and hence confer increased numerical abundance success.…”

“…Data estimates for maximum potential body size (MAX) and minimum reproductive threshold size (MIN) for the resident species were collected in earlier studies at the same field site (Tracey & Aarssen , ). MAX data were collected in summer 2011 by locating the five largest individuals of each resident species (based on visual estimation) very early on in the growing season.…”

“…in the most severely crowded vegetation, where most resident plants (regardless of their potential body sizes) are forced to remain at only a fraction of their species potential. Moreover, within such vegetation, recent research (Tracey & Aarssen , ) has shown that species with higher numerical abundance generally have a relatively small maximum potential body size (MAX) and that this is associated with a relatively small minimum reproductive threshold size (MIN). In terms of success under competition, therefore, smaller species may have a ‘reproductive economy advantage’, conferring higher numerical abundance and hence increased gene copying and transmission success to future generations (Aarssen ).…”

“…These relationships are of central importance to understanding the role of life history strategies in shaping the structure and function of plant communities, and in identifying accurate predictions for applied goals associated with habitat restoration and conservation. To address the above questions, therefore, we collected data from a two‐season field experiment in the same old‐field community used in Tracey & Aarssen (, ).…”

Different sets of belowground traits predict the ability of plant species to suppress and tolerate their competitors

“…The situation may be different in early-successional or annual communities, where size effects may dominate (Goldberg 1996). However, trade-offs with other functions, such as tolerance of herbivore damage (Rose et al 2009), timing of reproduction (Chaney and Baucom 2014;Tracey and Aarssen 2014) and resource use efficiency (Ryser 1996;Campitelli et al 2016) may still limit the selective advantage of large size. We therefore need to move beyond the paradigm of fast growth being equivalent to competitive superiority to recognize the importance of traits conferring competitive tolerance.…”