Effects of timing of prescribed fire on the demography of an invasive plant, spotted knapweed Centaurea maculosa

Summary

Prescribed burning is a key tool used in prairie restoration projects. The timing and frequency of fires are important as they can differentially affect the survival of individual plant species, including undesirable invasive exotics. The consequences of burning on populations of the invasive spotted knapweed Centaurea maculosa was assessed in prairie restoration experiment in Michigan, USA. C.maculosa is one of the most problematic invasive species in North America, infesting nearly 1.5 million ha of rangeland in western USA.

Spring, summer and autumn burns were compared at two frequencies: annually (2000–03) and in alternate years (2001, 2003). Their effect on survival, growth and reproduction of C.maculosa populations was examined using life-table response experiments and elasticity analyses to determine how season of fire affects population growth rates.

Study species: Most Centauria maculosa control efforts have focused on large-scale herbicide applications and biocontrol efforts. In mid-western prairie remnants, prescribed fire is often used to control C.maculosa and other non-native species while encouraging growth of native species. However, data to support fire as a successful management tool for C.maculosa are sparse.

C.maculosa is a perennial forb that can live up to 8 years. Seedlings germinate in late autumn or early spring and usually remain as juvenile rosettes for their first year. In the Michagan sites (see below) flowering individuals produced between 10 and 1,100 seeds/year. In the USA C.maculosa seeds have been shown to be capable of remaining viable in the soil for up to 8 years, in Michigan a viable seed bank persists for at least 1 year.

Study site: This experiment was established as part of a larger restoration study at the Fort Custer Training Center, a US Army National Guard base with approximately 3,000 ha of mostly forested, land near Augusta, Michigan, USA. Historically, this area was a patchwork of prairie, oak savanna and forest maintained by natural summer fires, and spring and autumn fires used by Native Americans. Currently, this site is mostly second-growth deciduous forest intermixed with small prairie and oak savanna remnants. Dominant native species in these remnants include goldenrod Solidago spp., poverty oatgrass Danthonia spicata and little bluestem Schizachyrium scoparium. Common non-native species (in addition to C. maculosa) include hawkweed Hieracium caespitosum, sheep sorrel Rumex acetosella and Kentucky bluegrass (or meadow-grass) Poa pratensis. Annual rainfall averages 86–91 cm. Soils are sand and sandy loam glacial outwash deposits.

Experimental design and treatments: From April 2000 to October 2003, replicated experiment were conducted on four prairie remnants to look at the effects of alternative fire management on C.maculosa demography. These remnants varied in primary productivity (136–508 g/m²) and C.maculosa abundance (6–25% of total biomass). Seven burn treatments were established in 5 × 5 m plots at each of the four sites. Burn regimes included:

annual spring (April) burns initiated in 2000, plots burned every year through 2003

alternate year spring (April) burns in 2001 and 2003

summer (July) burns

autumn (October) burns

a control (no burning)

A propane torch was used to start fires along one edge of the plot, which then carried across the entire plot. In some years, productivity and fuel loads were too low to carry a fire so the torch was used across the entire plot to ensure a complete burn. Pre- and post-burn biomass was collected from 0.50 × 0.25 m areas outside of the sampling plots to estimate fire intensity. All fires were of relatively low intensity (consuming 40–57% of total biomass) typically with flames 8–20 cm high. The intensities of these small experimental fires were comparable with larger scale prescribed fires in adjacent natural areas. In 2003, one area was accidentally burned, so demographic analyses for 2003 were based only on three sites.

Censuses and life-cycle components: A 1 × 0.25 m permanent census plot was established in each of the larger treatment plots. Approximately 650 C. maculosa plants were marked with an identification tag, recording life stages in the autumns of 2001, 2002 and 2003. Individuals were classified either as first-year juveniles, non-reproductive adults, one-stemmed small adults or multi-stemmed large adults. Censuses were made of newly emerged seedlings in the autumn and spring of these years. Reproduction was quantified by counting seeds produced per individual in autumn 2002; in 2001 and 2003, this was done by counting seed heads on each individual and multiplying these values by the average number of seeds per head (from the 2002 data). In 2002, recruitment from the seed bank was estimated by placing seed-exclosure cages over selected plots in early August, to prevent dispersal of seeds produced in that year into these plots. The cages (0.25 × 0.25 × 0.10 m) were constructed from 6 mm hardware cloth, lined with bridal veil to catch dispersing seeds. Cages were cleared of seeds and debris weekly so that light would be minimally obstructed. Cages were left on the ground until the October census. Seedling emergence in these plots was assumed to be from seeds in the seed bank. Seed longevity in the seed bank was not quantified in this study but published C.maculosa seed bank data indicates approximately half of seeds in the seed bank were no longer viable after 1 year.

In 2001, to see if the fire treatments affected phenology of C.maculosa populations, weekly censuses were made of the number and stage of flowers per individual in separate marked plots. Above-ground biomass was harvested in July 2001 and 2002 in 0.25 m² plots outside the census area. Samples were dried and weighed, and sorted to species to quantify the effects on the relative C.maculosa abundance.

Population structure, phenology and reproduction: The biomass harvests in 2001 and 2002 showed no significant effect of burning on the relative abundance of C. maculosa.

In 2001 there were significant differences among burn treatments upon flowering. Annual and alternate year spring burns reduced the percentage of adults that flowered by approximately 50%, while annual and alternate year summer burns reduced flowering almost entirely in a burn year. In contrast, autumn burns had no effect on flowering. Due to reduced flowering, annual summer burns in turn reduced seed production (reproduction) to almost zero. Alternate summer burns reduced seed production in the year of the burn in 2001, but not in 2002 and 2003. Spring burning only reduced reproduction in the alternate year burned plots in 2001, when seed output was about 50% of that of the control plots. There was no effect of spring burning in the years following. Autumn burning had no effect on reproduction.

Annual summer burns reduced total C.maculosa numbers by 4–8 times compared with the control, with fewer individuals than the control plots every year (2001–03). Annual summer burns significantly reduced first-year juvenile abundances in 2002 and 2003, reduced non-reproductive adult abundance in 2003 and reduced small adult abundances in 2002. Large adult abundance was not significantly reduced in any of the burn treatments. No other burn treatments led to significant reductions in C.maculosa abundance in any year.

Population growth rates: C.maculosa population growth rates were calculated for 2001–02 and 2002–03, and for the weighted mean matrices for each treatment. Annual summer burning was the only treatment that significantly reduced growth rates below 1.0 compared with the control (for 2001–02, 2002–03 and weighted matrix, respectively). The average predicted population growth rate in the control was 1.17, indicating positive population growth, while the average growth rate in the annually summer burned plots was less than half that (0.56), indicating population decline.

Conclusions: This study, using a combination of field experiments and matrix population modelling to examine the effect of season and frequency of prescribed fire on population growth of C.maculosa, indicates that a non-traditional management option, i.e. summer burning in mid-west USA prairies, may be an effective management tool for the control of this highly invasive species.


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