Action

Increase number of species in seed mix

How is the evidence assessed?
  • Effectiveness
    not assessed
  • Certainty
    not assessed
  • Harms
    not assessed

Source countries

Key messages

  • Five studies examined the effects of increasing the number of species in a seed mix on grassland vegetation. Four studies were in the USA and one was in Germany.

VEGETATION COMMUNITY (4 STUDIES)

  • Overall richness/diversity (1 study): One replicated, site comparison study in the USA found that increasing the number of species in a seed mix did not change plant species richness.
  • Characteristic plant richness/diversity (1 study): One replicated, controlled study in Germany found that increasing the number of species in a seed mix, along with sowing seeds from a local source, increased the species richness of target plants.
  • Sown/planted species richness/diversity (2 studies): Two replicated, controlled studies in the USA (one of which was randomized) found that increasing the number species in a seed mix increased the species richness of sown plants.

VEGETATION ABUNDANCE (3 STUDIES)

  • Characteristic plant abundance (1 study): One replicated, controlled study in Germany found that increasing the number of species in a seed mix, along with sowing seeds from a local source, increased the cover of target plant species.
  • Sown/planted species abundance (2 studies): One of two replicated, controlled studies (one of which was randomized) in the USA found that increasing the number of species in a seed mix increased the cover of sown plant species. The other study found that there was no change in the cover of sown species.

VEGETATION STRUCTURE (0 STUDIES)

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A replicated, controlled study in 2000–2004 in a former arable field in Kansas, USA (Piper et al. 2007) found that increasing the number of species in a seed mix increased the species richness of sown species. Species richness of sown plant species was higher in areas sown with the seeds of eight to 16 species (4–9 species/plot) than in areas sown with one to four species (1–3 species/plot). In February 2000, soil at the site was disturbed by harrowing. Five 6 × 6 m plots were each sown with the seeds of one, two, three, four, eight, 12 or 16 plant species. Seeds were a mixture of grasses, nitrogen-fixing species and Asteraceae obtained from local or regional commercial suppliers. Plots were mowed in June 2000, April and June 2001, and November or December 2002, 2003 and 2004. In June each year, species richness and plant cover were estimated using four 0.75 × 0.75 m quadrats placed in each plot.

    Study and other actions tested
  2. A replicated, controlled study in 2004–2010 at a former mining site in Saxony-Anhalt, Germany (Kirmer et al. 2012) found that increasing the number of species in a seed mix, along with sowing seeds from a local source, led to an increase in the species richness and cover of target plants. After six years, plots sown with a high diversity local seed mix had on average a greater number and cover of target plant species (28 species, 83% cover) than plots sown with a low diversity non-local seed mix (12 species, 36% cover). In December 2004, three blocks were established on an unvegetated area (240 x 50 m) of boulder clay mixed with sand. In each block, one plot was sown with a high diversity mix of seeds from a local source (11 grass and 40 herb species, sown at 36 kg/ha), and one plot was sown with a low diversity mix of non-local seeds (three grass cultivars, sown at 100 kg/ha). Vegetation was monitored annually within a 5-m2 quadrat in each plot in 2005–2010.

    Study and other actions tested
  3. A replicated, site comparison study in 2004–2011 in 27 restored prairie sites in Michigan, USA (Grman et al. 2013) found that increasing the number of species sown led to an increase in sown species richness but did not alter overall plant species richness. Four to seven years after seeding, sites sown with seed mixes containing a greater number of species had a higher species richness of sown plants (data reported as statistical model results). However, higher numbers of species in seed mixes did not lead to higher overall plant species richness (data reported as model results). Seeds were sown at all sites in 2004–2008 following removal of all vegetation with herbicide. Information on the seed mixes used were collected from the practitioner who oversaw the restoration projects. In July–September 2011, a 50-m transect was established at each site and a 1 x 1 m quadrat placed every 5 m on the transect. Vegetation cover of all plants in each 1 x 1 m quadrat was estimated.

    Study and other actions tested
  4. A replicated, controlled study in 2006–2009 in a former arable field in Nebraska, USA (Nemec et al. 2013) found that increasing the number of species sown did not alter the cover of seeded species or cover of invasive species, but did reduce the cover of unseeded species compared to areas that were sown with a low diversity seed mix. Cover of seeded species did not differ significantly between areas sown with a high diversity seed mix (3–25 cm) and areas sown with a low diversity seed mix (3–33 cm). Cover of invasive plant species also showed no significant difference (0–1.4 cm vs 0–0.5 cm). Cover of unseeded species was lower in areas where a high diversity seed mix was sown (4–34 cm) than in areas where a low diversity seed mix was sown (12–33 cm). In March–April 2006, twelve 55 x 55 m plots were seeded with a high diversity seed mix containing 97 plant species, and twelve plots were seeded with a low diversity mix containing 15 plant species. All plots were burned in March 2008. In July 2008, invasive plants were sprayed with glyphosate herbicide. Vegetation cover was recorded using five 55-m transects in each plot in June 2007–2009. Cover was measured at six points along each transect.

    Study and other actions tested
  5. A replicated, randomized, controlled study in 2007–2012 in a former corn field in Kansas, USA (Piper 2014) found that increasing the number of species sown increased the species richness and cover of sown plant species, but reduced the number and cover of unsown species. After six years, cover of sown species was higher in areas where high diversity seed mixes were sown (98–127%) than in areas where a low diversity mix was sown (45%). The same was true for the species richness of sown plant species (7–14 species vs 3 species). After six years, cover of unsown species was lower in plots where a high diversity seed mix was sown (6–20%) than in areas where a low diversity mix was sown (55%). In three of four cases, unsown species richness was lower in plots where high diversity seed mixes were sown (6–11 species) than in plots where a low diversity seed mix was sown (5 species). In 2006, the corn field was mowed. In February 2007, twenty-five 900-m2 plots were established. In 20 plots, 8–20 species were sown, while in five plots four species were sown. All plots were mown to a height of 10 cm in June 2007 and January 2008. In July 2007–2013, twelve 75 x 75 cm quadrats/plot were used to assess vegetation cover and plant species richness.

    Study and other actions tested
Please cite as:

Martin, P.A., Ockendon, N., Berthinussen, A, Smith, R.K. and Sutherland W.J. (2021) Grassland Conservation: Global evidence for the effects of selected interventions. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

List of journals searched by synopsis

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Grassland Conservation

This Action forms part of the Action Synopsis:

Grassland Conservation
Grassland Conservation

Grassland Conservation - Published 2021

Grassland Synopsis

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