Are competitive effect and response two sides of the same coin, or fundamentally different?

Abstract

Summary1. The ability to suppress neighbour growth and the ability to withstand growth suppression are widely viewed as two forms of competition, competitive effect and competitive response. 2. We conducted a greenhouse experiment to determine whether these two forms of competition were functionally linked, and to determine which plant traits are associated with effect and response competitive abilities among seedlings of 22 perennial North American prairie species. We further explored the trait-function relationship by growing plants under different soil fertilities and with different neighbour species. 3. To determine competitive abilities, we used a phytometer approach with two phytometer species: Poa pratensis and Achillea millefolium grown in competition with each of the 22 focal species under low and high fertility. Root and shoot morphological traits were measured and principal component analyses were used to reduce the dimensionality of the data. Three axes were extracted, which roughly corresponded to size, root and shoot architecture. 4. The hierarchy of competitive effect ability of the target species did not vary with either soil fertility or neighbour identity, while the hierarchies of competitive response abilities were highly variable among the treatments. Competitive effect ability was closely associated with size-related traits under high nutrient conditions, and with root-related traits under low nutrient conditions. In contrast, few plant traits axes were related to competitive response. 5. These findings indicate significant differences between competitive effect and response ability. We suggest competitive effect ability is a consistent trait of a species, linked to specific plant traits. In contrast, we found little evidence to support the idea that competitive response ability is itself a species trait, and instead it appears this may be simply a collection of different ways of avoiding or tolerating competition and ⁄ or low nutrient conditions. Supporting this argument was a lack of any consistency in which traits were associated with competitive response ability. 6. We recognize the limitations of a single study of seedlings under greenhouse conditions. However, we suggest these findings indicate a need to critically examine current assumptions about plant competition, how it is defined, and the traits which control a species' competitive ability.

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

“…While some studies have reported positive correlations between competitive effect and competitive response (Novoplansky and Goldberg 2001;Wang et al 2010), others have not detected significant correlations (Cahill et al 2005;Baron et al 2015). Strong competitive effects have been linked to plant size and resource pre-emption but traits related to competitive response remain to be elucidated (Cahill et al 2005;Wang et al 2010). As root competition is more size-symmetric than shoot competition and involves multiple resources (Schwinning and Weiner 1998;Cahill and Casper 2000), competitive success in belowground interactions may be less reliant on size and more dependent on diverse traits conferring competitive tolerance (Keddy et al 1998;Belter and Cahill 2015).…”

“…the ability to minimise the negative impact of sharing resources with neighbours. While some studies have reported positive correlations between competitive effect and competitive response (Novoplansky and Goldberg 2001;Wang et al 2010), others have not detected significant correlations (Cahill et al 2005;Baron et al 2015). Strong competitive effects have been linked to plant size and resource pre-emption but traits related to competitive response remain to be elucidated (Cahill et al 2005;Wang et al 2010).…”

“…competitive effect and response, respectively) or b) allow niche differentiation (demonstrated by reduced competitive suppression between species with dissimilar traits). Competitive outcome and relationships between functional traits and competitive ability may be strongly influenced by the choice of the competitor species (Wang et al 2010;Baron et al 2015). In natural conditions, community species composition and spatial structure determine the identities of interacting individuals, and the frequency of such interactions is a factor regulating local adaptation in competing species (Turkington 1989;Grondahl and Ehlers 2008;Lankau 2012;Abakumova et al 2016).…”

“…It has been shown that higher competitive ability is associated with greater root system size, either achieved through greater allocation of biomass to roots or through higher specific root length (Wang et al 2010;Ravenek et al 2016). Specific root length and a combination of specific root length and aboveground traits have also been shown to contribute to niche differentiation (Kraft et al 2015;Bennett et al 2016).…”

Dependence of the Yield of Alfalfa on Plant Density and Diet

“…The ability to inhibit the growth of its neighbors, and the ability to resist the inhibition of growth are widely regarded as two forms of competition: competitive effect and competitive response [10]. ρ-density of standing scalar; T-temperature averaged over short intervals of time of observation; v-wind velocity directed perpendicular to the direction of the series; g-frequency precipitation averaged over short intervals direction; h-level rainfall averaged over short intervals of observation; R-soil type which is characterized by several interrelated parameters.…”