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

Abstract

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.

Being John Harper: Using evolutionary ideas to improve understanding of global patterns in plant traits

“…In the past, it was thought that large plants were responsible for most seed production and biomass accumulation (both within populations of a species and across different species in a community), because large individuals produce more seeds and more biomass per plant than do small individuals. However, the sheer numerical dominance of small individuals both within and across species (Chambers & Aarssen, ; Tracey & Aarssen, ) means that small individuals actually produce the majority of the biomass (Tracey, Irwin, Mcdonald, & Aarssen, ) and seeds (Chambers & Aarssen, ). This body of work overturns the traditional size‐advantage hypothesis (Aarssen, ) and provides a nice example of the insights that can be gleaned by combining trait ecology with a more demographic approach in which the fitness of all individuals is considered rather than selectively focussing on a few “representative” mature plants.…”

Response of bluebunch wheatgrass to invasion: Differences in competitive ability among invader‐experienced and invader‐naïve populations

“…Competitive response is a plant's tolerance of competition; tolerance can be related to either the ability to access limited resources or the ability to grow and reproduce even under reduced resource levels. Tolerance of competition can be driven by traits related to the ability to grow rapidly towards resources (Goldberg, 1996;Tabassum & Leishman, 2016) or tolerate limited resources (Keddy, Fraser, & Wisheu, 1998;Tracey, Irwin, McDonald, & Aarssen, 2017), such as relative growth rate, plant height and leaf area. The novel selective pressures exerted by invasive plant species may not only lead to trait shifts in remnant native populations but may also select for shifts in the elements of competitive ability that allows a remnant population to persist.…”

Species‐specific size vulnerabilities in a competitive arena: Nutrient heterogeneity and soil fertility alter plant competitive size asymmetries

“…Size‐asymmetric competition, in which larger individuals take up a disproportionate amount of resources relative to their size (Begon, ; Harper, ; Schwinning & Weiner, ), gives larger individuals a competitive advantage and could lead to the competitive exclusion of smaller and slow‐growing species (DeMalach et al, ). However, despite the possible exclusion of smaller individuals, they tend to have a higher abundance within communities (Moles et al, ; Niklas, Midgley, & Rand, ; Tracey, Irwin, Mcdonald, & Aarssen, ). Thus, there must be mechanisms allowing the coexistence of small and large individuals despite the occurrence of size‐asymmetric competition.…”