Individual study: Raking after mowing as an alternative to prescribed burning promotes desirable herbaceous plant species at Murphy Lake Prairie, Scott County, Minnesota, USA
Tix D. & Charvat I. (2005) Aboveground biomass removal by burning and raking increases diversity in a reconstructed prairie. Restoration Ecology, 13, 20-28
Periodic vegetation removal, e.g. by fire, benefits North American prairie plant communities by discouraging woody plants and favouring prairie species. However, prescribed burning is impractical at many sites, especially those near urban and roadside areas. As many undesirable (non-native) grasses emerge early in the growing season, prescribed burns are often undertaken in spring to control them. Raking after mowing (and mowing without litter removal) as an alternative to burning in a reconstructed Minnesota prairie (northern USA) was tested.
Survey site: The sudy took place at the Murphy Lake Prairie, a 38 ha area of restored tallgrass prairie (dominated by big bluestem Andropogon gerardii and Indian grass Sorghastrum nutans) seeded with native species in 1995. An analysis of functional groups and diversity in remnant prairies in southeastern Minnesota established desirable restoration goals at the study site.
Experimental design: In the summer of 2000, 40 plots (10 × 10 m) were established on the prairie with 2 m buffers between plots. Three treatments and one untreated control were assigned randomly in blocks of four, giving 10 replicates. Treatments were applied on 4–9 May 2001 and 23–30 April 2002. In each block, one plot was burned, two plots were mowed of which one was raked, and one was left untreated to be the control.
Mowing: Mowing was undertaken using a high-weed mower, cutting about 5 cm above the soil surface. Within 1-2 days, litter was hand-raked to the edge of each plot.
Burning: Each fire was started with a drip torch at the downwind edge of the plot, and once the fire had moved about 1 m into the plot, a head fire was started. Each plot burned for less than 4 mins, no living green tissue was left.
Plant monitoring: Frequency, flowering stem abundance, and cover were measured for all species and five native functional groups (C4 grasses, C3 graminoids, forbs, legumes, and annual or biennial forbs) in 1 m² quadrats. Cover data were also collected from 1 m² plots evenly distributed along transects at three remnant prairies in southeastern Minnesota.
At the remnant native prairie sites, cover of C4 grasses was generally higher than at Murphy Lake. Mowing alone did not differ from the control in its effect on any functional groups of plants.
In 2002, round-headed bush clover Lespedeza capitata (a legume) and black-eyed Susan Rudbeckia hirta (a biennial), increased in frequency with treatments that removed biomass (i.e. fire or raking). Plants established well from seed, vitality of mature plants was unaltered. Raking had similar effects to burning on most functional groups, although flowering stems of C4 grasses were significantly more abundant after fire than after raking. Burning reduced some C3 forbs and grasses and favoured C4 grass dominance. Annual and biennial species, which were more frequent in both removal plots than in the control and mowed plots.
Conclusions: Burning, or mowing plus raking, benefited germination especially of legumes, annuals and biennials. Raking after mowing in the spring provides an alternative to prescribed spring burning with many of the same positive aspects as fire but not promoting aggressive C4 grasses to the same extent. Raking after mowing is less damaging than late April-early May burning to species that emerge early in the season or have seeds that are sensitive to intense heat.
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