Long-term effects of sheep grazing on giant hogweed (Heracleum mantegazzianum)

Published source details Andersen U.V. & Calov B. (1996) Long-term effects of sheep grazing on giant hogweed (Heracleum mantegazzianum). Hydrobiologia, 340, 277-284.
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Summary

Since the 1960s giant hogweed Heracleum mantegazzianum (native to temperate Asia) has spread vigorously though Denmark and is now considered invasive. The aim of this study was to assess the effects of long-term sheep grazing on giant hogweed and to determine if seven years of grazing is sufficient to prevent re-establishment from the seed bank.

Study area: The study was undertaken 1.7 ha mesotrophic meadow next to Lake Furesø in northeast Zealand, Denmark. Prior to grazing, the meadow was dominated by a mature stand of giant hogweed Heracleum mantegazzianum (up to 4 m tall), with very few other plant species of low frequency present. The meadow had not been grazed for 25 year and no fertilisers of pesticides had been applied.

Sheep grazing: The meadow was grazed each May to October by sheep in the years 1987-1993. In 1987-89 the stocking rate was 5 sheep/ha, increasing to 10/ha during 1990-93.

Vegetation surveys: Vegetation surveys were carried out in 1989 and 1993. Ten circles (0.1 m²) were placed at random, and all plants rooted inside were recorded and the results expressed as percentage frequency. These surveys were carried out twice per growing season. Cover percentages of all species were recorded in three quadrats in each of four sites (two in the grazed field and two in an ungrazed adjacent control field also invaded by giant hogweed) on 18 April 1994. Within quadrats in the ungrazed areas the numbers of vegetatively resprouting H.mantegazzianum plants were counted and the number of seedlings estimated.

Soil samples & germination: Soil samples were collected on 5 August 1993 at a depth of 0-10 cm and transferred to shallow polystyrene trays. The trays were kept in greenhouse at 18 C and irrigated regularly for three months. All emerging seedlings were recorded. Soil sampling was repeated on 12 October 1993 and 3 March 1994, when samples from the control field were also included. Germination tests were subsequently undertaken (see original paper).

Impact of sheep grazing: After two years of grazing, giant hogweed cover had been greatly reduced. A comparison of results in May 1989 and May 1993 showed that frequency of giant hogweed had been reduced from 75 to 0% in the grazed area overall and cover from 13% to 0%. It had completely disappeared from the grazed area after 7-years of grazing except for three seedlings recorded in April 1994. Average plant species diversity and total number of species were higher at the grazed sites after grazing had eradicated giant hogweed and their percentage cover greatly increased, whereas at the ungrazed sites giant hogweed accounted for nearly 50% of the total vegetation cover, and the numbers of other species present was very low.

Germination: Soil sampled from the grazed area developed no giant hogweed seedlings in a germination test and contained no viable hogweed seeds. In contrast soil from adjacent hogweed stands produced numerous seedlings with a peak emergence from samples collected after the winter. Seeds collected in October showed a viability (from tetrazolium testing) of 88%. Germination averaged 22% after storage at room temperature and 25% following three weeks treatment at -18 °C.

Conclusions: In this Danish study sheep-grazing proved effective in eradication of giant hogweed and a typical mesotrophic meadow flora was reinstated. The persistence of giant hogweed seeds in the meadow soils appears to be less than seven years (this being about half the time of some earlier estimates).

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