Individual study: Effects of prescribed fire, season of burn and herbicide application in controlling invasive sulphur cinquefoil Potentilla recta at Dancing Prairie Preserve, Montana, USA
Lesica P. & Martin B. (2003) Effects of prescribed fire and season of burn on recruitment of the invasive exotic plant, Potentilla recta, in a semiarid grassland. Restoration Ecology, 11, 516-523
Sulphur cinquefoil Potentilla recta, a rosette-forming perennial, is an exotic invasive plant (native to the Mediterranean region of Europe) introduced to North America before 1900. It has subsequently spreading through much of Canada and the USA. This study investigated the effects of prescribed fire and season of burn on recruitment of sulphur cinquefoil in a semiarid grassland prairie reserve in northwest Montana.
Study site: The study was undertaken at Dancing Prairie Preserve which lies in the Tobacco Plains, a valley (825 m altitude) 6 km north of Eureka, northwest Montana, USA. Average annual precipitation at Eureka was 377 mm. The preserve consists of 250 ha of fescue–wheatgrass–needlegrass grasslands developed on rolling topography. Common grasses are rough fescue Festuca scabrella, Idaho fescue F.idahoensis and Kentucky bluegrass Poa pratensis.
Field methods: A factorial design with all combinations of spring burn, autumn burn, no burn, herbicide and no herbicide over a 5-year period was used. Burns were undertaken in early spring and late autumn to reduce damaging dominant native grasses. Vegetation is generally too wet to carry out winter burns. A total of 54 (1-m radius) circular plots were established in sulphur cinquefoil infested areas in May 1996. Each was marked with a central post. The amount of plant litter (fuel) in each plot was classified as low, medium or high. Plots were stratified by amount of litter and location and randomly assigned to one of the six treatment combinations. These were originally replicated nine times but five herbicide plots were not treated as planned, resulting in an unbalanced design: autumn burn/herbicide (7), spring burn/herbicide (7), no burn/herbicide (8), autumn burn/no herbicide (11), spring burn/no herbicide (11), and control–no burn/no herbicide (10).
Picloram herbicide was applied with a boom sprayer at the recommended rate of 1.2 L/ha around the centre post of each herbicide plot to a radius of 2–3 m on 10–11 June 1996. Picloram requires several months to kill target plants, so cinquefoil counts during the first recording period on 11–12 July 1996 were unaffected by it. Each burn plot was burned in its entirety to a radius of 3 m. Autumn burns were applied on 17 October 1996, spring burns on 7–8 April 1997; spring fires burned cooler than autumn burns.
Cinquefoil counts: In mid-July 1996–2001, the number of cinquefoil plants in a 1 m radius around the centre post were counted into one of two size classes: small (one to four leaves without a well-developed caudex); and large (at least five leaves and a woody caudex). These classes were used as at this site plants in the small class are usually less than 1 year old, whereas larger plants are older.
Pocket gophers: Pocket gophers Thomomys talpoides are common at Dancing Prairie Preserve and their digging may facilitate exotic plant invasives through soil disturbance. Therefore gopher mounds were counted in sample plots in 1997-2001 to account for variation in cinquefoil density due to this.
Cinquefoil population trends: The density of small cinquefoil plants in control (no fire, no herbicide) plots was highest in 1996 (around 39/plot), declining in 1997 (20/plot) and 1998 (5/plot) and remaining around this density throughout the rest of the study. Large plants declined from 1996 (c.13/plot) to 1997 (c.7/plot) but then increased gradually to initial levels by the end of the study.
Effect of fire: Fire had a significant effect on the density of small cinquefoil plants but only in the first year following the burn. From 1996 to 1997, average density of small plants declined by 47% in control plots (38 to 20/plot), and 26% in autumn and spring burn plots (c.33 to 24/plot). Following this in 1997 and 1998, densities declined by about the same in both burned and unburned plots. Small plants remained at low levels in both treatment and control plots for the remainder of the study. There was no detectable overall effect of fire on the density of large plants.
In the first year after burning, average density of small plants declined by 42% in spring burn but only 12% in autumn burn plots. However, this trend reversed in the final year of the study as between 2000 and 2001 small plants increased slightly in spring burn but declined by 72% in the autumn burn plots. There was no detectable effect of burn season on the density of large cinquefoil plants.
Effect of pocket gophers: Gopher diggings were observed in 17 of the 32 no-herbicide plots between 1997 and 2001. There was no effect of gophers detected on the density of cinquefoil plants, large or small. There was also no significant interaction between the presence of pocket gopher mounds and fire.
Effect of herbicide: Picloram was very effective at killing sulphur cinquefoil. In 1997 and 1998 this species was completely absent from herbicide plots (treated in 1996). Two herbicide plots (out of 22 in total) had a single cinquefoil plant in 1999. By 2001 there were five plots with cinquefoil plants, all burn plots, with an average of two plants in each. By 2001 cinquefoil density was higher in herbicide plots that had been burned compared with those that had not. Burn season had no effect on cinquefoil density in herbicide plots
Conclusions: In this study, sulphur cinquefoil recruitment was higher in burn plots compared with controls the first year after the fire. This however, did not lead to subsequent significant population growth, possibly due to drier than normal conditions prevailing in most years of the study. Effect of season of burn varied but was higher in autumn compared with spring burn plots across all years. Picloram effectively eliminated sulphur cinquefoil for 3–5 years. At the end of the study cinquefoil density was greater in herbicide plots that were burned than those that were not. Results suggest that burning will enhance sulphur cinquefoil germination. Prescribed fire may reduce the long-term efficacy of herbicide and will be most beneficial in controlling sulphur cinquefoil at Dancing Prairie when conducted in the spring rather than the autumn.
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