Individual study: The effect of prescribed fire on insect herbivory of the rare grassland perennial, northern blazing star Liatris scariosa, Kennebunk, Maine, USA
Vickery P.D. (2002) Effects of the size of prescribed fire on insect predation of northern blazing star, a rare grassland perennial. Conservation Biology, 16, 413-421
Native sandplain grasslands have declined by over 90% in New England, USA, since the settlement of Europeans. Remaining grasslands persist as small, isolated fragments and many species have declined or become locally extinct. To ensure persistence of the remainig fragments, there has been intensive management using a variety of techniques such as prescribed burning, the effects of which have been well documented for some taxa. However, the effects of prescribed burning on native plants and invertebrates are less well understood. Of particular concern is the rare grassland perennial northern blazing star Liatris scariosa var novae-angliae, which has disappeared from 69% of sites where it formally occurred. At seven unburnt (for a minimum of five years) study sites in Maine and Massachusetts, northern blazing star suffered 65% seed loss to two species of microlepidopteran moths (Tortricidae) and there was no juvenile recruitment. In this study, the effects of the size and frequency of prescribed fire on seed predation of northern blazing star is documented.
Study site: Kennebunk, is a 240 ha sandplain grassland managed by The Nature Conservancy, in northeast USA. It is divided into 16 management units that are burned every 2-5 yrs. Burn units were 13-20 ha in area, apart from one smaller one of 6 ha in size.
Temporal effects of burning on northern blazing star flowering: In 1994, eight burn units were selected which covered a range of burn histories:
1) burned within 6 months
2) burned between 6-12 months previous
3) burned between 22-60 months previous
4) units not burned for at least 60 months (control)
One control unit was burned in November 1994. In each unit, one blazing star flowering stem was collected for each of 20 genets in October 1994 (pre-control burn) and 1995 (post-control burn). Each flower stem was examined for evidence of seed predation, and a flower head was classified as predated if there were one or more caterpillars on the flower or if seeds showed signs of being eaten.
Effect of burn size on seed predation by moths: Two widely separated burn units (>400 m apart) that had been burned within the last six months were selected. A line transect from 40 m outside the burn unit to 120 m within the burn unit was created at each site. In September 1994, 20 flowering stems of northern blazing star were collected within each of the following distances from the burn boundary: 0-40 m unburned (outside the burn unit); 0-40 m burned; 40-80 m burned, 80-120 m burned. Flowers were examined as above.
Temporal effects of burning on northern blazing star flowering: In four units that had been burned within 12 months of October 1994, there was an average seed predation of 16%, compared to an average seed predation of 89% in four units burned over 22 months previously. By October 1995, seed predation in the four units burned within 24 months was on average 83%, corresponding to an average increase in seed predation of 68%. In contrast, the three units not burned for at least 34 months showed no change in seed predation levels. Finally, seed predation in the fourth unit (control unit) that was burned in November 1994 declined by 66% from 87% in October 1994 to 22% in October 1995. These results suggest that in the year after burning there is significantly less seed predation than in subsequent years.
Effect of burn size on seed predation by moths: There was a significant reduction in seed predation with distance from the edge of the unit: outside of the unit average seed predation was 45%; at 0-40 m burned, average seed predation was 28%; at 40-80 m average seed predation was 17%; and at 80-120 m average seed predation was 5%.
Conclusions: The results suggest that burning reduces seed predation during the first post-burn year. Furthermore, the effect of distance suggests that the moths that eat the seeds are dispersal limited, which allows them to rapidly recolonise burned areas close to an unburnt area, but prevents them from efficiently spreading to patches over 80 m from the unit edge. The authors suggest therefore that burn units should be sufficiently large (>10 ha) to ensure that the moths are unable to spread to the core of the unit for at least one year.
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