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Text S1 Criteria for selecting studies.
(1) The study was conducted in the field or a common garden, and individual plants were
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not isolated within containers (e.g., pot) except when containers were used to prevent
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belowground competition in reduced competition treatments.
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Examples of the studies excluded: Not a field experiment/survey: Van et al. (1999),
Holmgren et al. (2012) and Le Bagousse-Pinguet et al. (2012).
(2) The study investigated the same plant interaction(s) in at least two stress levels (the
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lowest and highest stress conditions were used if they had multiple stress levels), and quantified
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or described the stress gradient of their study. The performance of target plants without neighbors
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was reduced by at least 10% at high stress (relative to at low stress), to verify the existence of a
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functional stress gradient (Lortie & Callaway 2006; Lortie 2010) (so any arbitrarily or even
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anthropocentrically assigned high levels of stress that did not actually have reduction effects on
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plants were excluded from our analysis) and also ensure the resulting database would still be
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large enough for a powerful analysis. The stress factor can be biotic grazing or any abiotic factors.
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The stress gradient could be spatial, temporal or experimental. To reduce interdependence, we
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included studies with a temporal gradient only if they used different plots (when the response
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variables were determined on a plot basis) or different individuals (when the response variables
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were determined on a plant individual basis) in different years. Studies that investigated plant
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interactions in different seasons were excluded to reduce seasonal effects. Studies that
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manipulated stress by adding nutrients, water, shade, or excluding mammals or spraying
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insecticide were included as experimental gradients, provided that the stress manipulation was
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applied to all potentially interacting species. Studies that manipulated disturbance by varying site
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preparation methods, that artificially simulated herbivory by clipping, and that focused on the
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effects of stress history (e.g. grazing history, drought legacy) on competition were excluded.
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Examples of the studies excluded: No functional gradients, i.e. the performance of target
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plants without neighbors was reduced by < 10% at high stress: Bockelmann & Neuhaus (1999),
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Fig.3 & 5 data, and Smit et al. (2007), Fig. 2b data; Had temporal gradients but used permanent
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plots or targets: Tielbörger & Kadmon (2000) and Armas & Pugnaire (2005); Stress not
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manipulated for both targets and neighbors: Reader (1992), Erneberg (1999), and Russell &
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Spencer (2010); Seasonal gradients: Noda et al. (2003) and Kikvidze et al. (2006);
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(3) The study determined the performance of target plants with and without competition.
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Competition treatments included experimental removal, addition of neighbors, and natural
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presence or absence of neighbors. Studies that controlled competition by removing neighbors
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must have removed aboveground neighbors. Studies that controlled only belowground
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competition by trenching, that removed neighbors only at the beginning of the experiment but
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didn’t control significant re-growth, or that only removed plant litter but not live aboveground
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biomass were excluded. Studies that created neighbor effects by addition (seeding or
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transplanting) were included only if the seeded neighbors established. Natural competition
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treatments were also included (i.e. studies conducted in places naturally with and without
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neighbors). If the neighboring vegetation was manipulated using several different methods, we
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used only the one that most completely reduced competition (for neighbor removal experiments),
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or the one that created the highest level of competition (for neighbor addition experiments), to
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reduce interdependence among data in our analysis. Studies that planted a species at different
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densities or thinned a species to different densities to investigate intraspecific interactions were
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included.
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Examples of the studies excluded: Did not determine the performance of target plants with
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and without competition: Van Uytvanck et al. (2008) and Granda et al. (2012); Not the same plant-
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plant interactions at low and high stress levels (many studies have the same test species at both
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levels but different neighbors): Pennings & Callaway (1992), Hacker & Bertness (1999), Kikidze et
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al. (2005), and Smit et al. (2011); Only controlled belowground competition: Tanner & Barberis
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(2007); Focused on succession, no control of significant regrowth of neighbors: Viejo et al. (2008).
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(4) The study determined at least one of three components of fitness of target plants: (1)
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survival, (2) growth (biomass, cover, number of leaves or individuals, height, or diameter) and (3)
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reproduction (number of seeds/fruits/ inflorescences, weight of seeds/fruits/ inflorescences, and
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density, cover/proportion of flowering plants). If the experiment was conducted with both natural
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and added target plants, we preferred naturally occurring plants. For studies that investigated
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intraspecific interactions by thinning or using different plant densities, we included only studies in
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which the response variable was determined on an individual or proportional basis (e.g.,
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biomass/seed number per plant), but not on a plot/unit area basis (e.g., cover/biomass of a plot).
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The response variables (and sample sizes, their standard errors, standard deviations or confident
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intervals) were shown in a table/figure of the paper (data shown in figures were digitized using
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Engauge Digitizer). We never repeated two or more measures for the same fitness component
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from a study to reduce interdependence and pseudoreplication. If several different measures
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were presented for a given fitness component, we used the one more widely used or more directly
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related with that fitness component. For example, we used number of seeds for reproduction
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instead of weight of seeds. If in a study a fitness component was measured on several dates, we
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used the results at the end of the experiment when available. For studies published after 1995,
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we contacted authors for data if sample sizes, standard errors, or standard deviations were not
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shown, or if the authors described that they determined a response variable suitable for this study
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but did not present it, or if the data were presented as means (standard errors) pooled across
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treatments, to incorporate as comprehensive a data set as possible.
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Exclusion of studies can be due to failure to get authors’ contacts/response and authors
not having the data any more.
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(5) The study did not have any serious design problems. For example, if in a study the
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reduced competition treatments were accomplished by severe substrate disturbance like plowing,
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the study was excluded. Also for studies that artificially created communities (planting neighbors
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and targets), the design had to be an addition series. Studies that used a replacement design (i.e.,
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density of all plants were the same between competition and no competition treatments) to create
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artificial communities were excluded. We also excluded studies with a sample size < 3.
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Examples of the studies excluded: Severe substrate disturbance occurred: Scherber et al.
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(2003); Used replacement designs to create artificial communities: Fynn et al. (2005); < 3 sample
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size: Dyer et al. (1997); Cahill et al. (2002); Hwang & Lauenroth (2010).
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Studies were excluded based on the literature selection criteria for our test of the SGH,
while their own research questions were not necessarily to test the SGH.
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