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Different sets of belowground traits predict the ability of plant species to suppress and tolerate their competitors
Different sets of belowground traits predict the ability of plant species to suppress and tolerate their competitors
Marina Semchenko
Anu Lepik
Maria Abakumova
Kristjan Zobel
Responsible Editor: Amandine Erktan.
0 ) School of Earth and Environmental Sciences, University of Manchester , M13 9PT, Manchester , UK
Background and aims Functional traits may underlie differences in niches, which promote plant species coexistence, but also differences in competitive ability, which drive competitive exclusion. Empirical evidence concerning the contribution of different traits to niche differentiation and the ability to supress and tolerate competitors is very limited, particularly when considering belowground interactions. Methods We grew 26 temperate grassland species along a density gradient of interspecific competitors to determine which belowground traits a) explain species' ability to suppress and tolerate neighbours and b) contribute to niche differentiation, such that species with dissimilar trait values experience reduced competition. Results We found that having larger root systems with extensive horizontal spread and lower root tissue density enabled efficient suppression of neighbours but did not significantly contribute to the ability to tolerate competition. Species with deeper root systems, lower specific root length and less branched roots were better at tolerating competition, but these traits did not significantly affect the ability to suppress neighbours. None of the measured traits contributed significantly to niche differentiation, either individually or in combination. Conclusions This study provides little support for belowground traits contributing to species co-existence through niche differentiation. Instead, different sets of weakly correlated traits enable plants to either suppress or tolerate their competitors.
Belowground interactions; Species coexistence; Competitive ability; Functional traits; Niche differentiation; Phenotypic plasticity; Rooting depth; Stress tolerance
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Plant functional traits have been extensively used as a
means to understand plant community assembly (Lavorel
and Garnier 2002; de Bello et al. 2010; Gotzenberger et al.
2012). Differences in functional traits have been used as a
proxy for stabilising niche differences, based on the
principle of limiting similarity, which predicts stronger
competition between species with similar trait values
(MacArthur and Levins 1967; Chesson 2000). However,
plant traits may also influence competitive ability and
contribute to competitive exclusion of species with inferior
trait values (Chesson 2000; Grime 2006; Mayfield and
Levine 2010). Indeed, an increasing number of studies
have found that commonly used functional traits are more
strongly related to competitive hierarchies than to niches
(Kunstler et al. 2012; Herben and Goldberg 2014; Kraft
et al. 2015). This apparent contradiction highlights the
current lack of a mechanistic understanding of the way in
which easily measured traits relate to plant function and the
way in which different traits contribute to competitive
hierarchies versus niche differentiation between species
(Kraft et al. 2015; Shipley et al. 2016).
The existence of competitive hierarchies among plant
species is well documented. Plant size has been
repeatedly identified as a trait that is central to competitive
success (Goldberg 1996; Keddy et al. 2002). Several
aboveground traits such as greater maximum height,
larger leaf area, higher leaf nitrogen content and lower
leaf dry matter content have also been shown to provide
an advantage in interspecific competition (Goldberg and
Landa 1991; Herben and Goldberg 2014; Bennett et al.
2016). Competitively superior species are also
frequently characterised by high specific leaf area (Kunstler et al.
2012; Fort et al. 2014; Kraft et al. 2015). These traits are
often used to describe the leaf economics spectrum and
suggest that species with strongly acquisitive traits are
good competitors, whereas species with more
conservative traits are less competitive (Wright et al. 2004; Reich
2014). Despite the importance of roots for plant
functioning and ecosystem processes (Bardgett et al. 2014)
and competition for soil-based resources being equally
or even more important than competition for light (Kiaer
et al. 2013), most trait-based studies of plant community
assembly have concentrated on aboveground traits.
Current knowledge of belowground traits and their
contribution to species co-existence still consists of too
few traits and species to draw robust generalisations. 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; (...truncated)