Screening of grassland plants for restoration after spotted knapweed invasion

Published source details Perry L.G., Vivanco J.M. & Paschke M.W. (2005) Screening of grassland plants for restoration after spotted knapweed invasion. Restoration Ecology, 13, 725-735.
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Summary

Some exotic invasive plant species introduce phytotoxins into invaded habitat which may impact upon native species which have little resistance to them, thus facilitating the further spread of the invasive concerned. Such toxins (allelochemicals) appear to assist invasions of North American grasslands by three Eurasian knapweeds: spotted knapweed Centaurea maculosa, diffuse knapweed Centaurea diffusa and Russian knapweed Acroptilon repens. Such plants are termed allelopathic invaders

Restoration of native plant communities in areas where such invasives have been removed is often limited by residual effects, including the persistence of residual alleochemicals (e.g. residual soil catechin produced by spotted knapweed, may interfere with reestablishment of native grassland species even after the knapweed has been removed. Restoration efforts are more likely to succeed if the species selected for revegetation are relatively resistant to residual allelochemicals. In this study, grassland plants appropriate for revegetation of areas invaded by spotted knapweed are investigated.

Test species: A mix of 23 species were tested for catechin resistance: nine grasses, seven forbs, four shrubs and three legumes (see Table 1, attached). Additionally, spotted knapweed Centaurea maculosa was included as a resistant control. Most of the species evaluated are native western North American perennials, along with six introduced species (see table). Non-perennials were: Common sunflower Helianthus annuus (an annual); alfalfa Medicago sativa (sometimes an annual); and curlycup gumweed Grindelia squarrosa, Bigelow's tansyaster Machaeranthera bigelovii and scarlet globemallow Sphaeralcea coccinea (all sometimes biennial). Of the species tested, seven have been observed growing in spotted knapweed dominated habitat: common yarrow Achillea millefolium, white sagebrush Artemisia ludoviciana, Curlycup gumweed, hairy false goldenaster Heterotheca villosa, needle and thread Hesperostipa comata, alfalfa and scarlet globemallow.

Catechin resistance: Resistance was evaluated by measuring the effects of seven catechin concentrations (0, 0.125, 0.25, 0.5, 1.0, 2.0, and 4.0 mg/mL) on seed germination, seedling root and shoot elongation, and seedling mortality. The catechin treatment concentrations were chosen to reflect the range of those found under field conditions in soil where spotted knapweed was present. Methods were derived from U.S. EPA guidelines for evaluating germination and root elongation toxicity (U.S. EPA 1996).

Catechin resistance: Inhibition of root elongation was the strongest and most common effect of catechin treatment, reducing seedling root elongation of 20 of the 23 grassland species (including spotted knapweed but only at high concentrations). Three 'highly resistant' species (unaffected at even the highest catechin concentrations tested) were identified: mountain brome Bromus marginatus, curlycup gumweed, and needle and thread.

High catechin concentrations reduced mean root lengths of five (of the species by more than 75% and another 10 species by more than 55%. Experimentally derived concentrations needed to reduce root length by 50% (i.e. EC50, an indicator of catechin resistance) ranged from 0.43 mg/mL to more than 4.0 mg/mL among species. Rocky Mountain penstemon Penstemon strictus, Sandberg bluegrass Poa secunda, Idaho fescue Festuca idahoensis, Bigelow's tansyaster, intermediate wheatgrass Thinopyrum intermedium and common yarrow.

Conclusions: This study indicates that several species may be appropriate for re-vegetation of sites with high residual soil catechin concentrations resulting from spotted knapweed invasion. In particular, germination and seedling growth of mountain brome, curlycup gumweed, and needle and thread were not inhibited even by very high catechin concentration. An additional five species (alfalfa, common blanketflower Gaillardia aristata, boreal sweetvetch Hedysarum boreale, cicer milkvetch Astragalus cicer and basin wildrye Leymus cinereus) were also identified as resistant to catechin (i.e. an EC50 greater than 3 mg/mL) and are therefore probably also suitable for restoration projects.

Reference
U.S. EPA (United States Environmental Protection Agency)(1996) OPPTS 850.4200. Seed germination/root elongation toxicity test, public draft. Ecological effects test guidelines. United States Environmental Protection Agency, Washington, D.C.

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