The effects of a spring burn on noxious weeds in a tallgrass prairieE. Vander Broek, T. Tracy, J. Boersma, and R. Weeks

Biology Department, Northwestern College, Orange City Iowa

Abstract

In an effort to control Canada thistle (Cirsium arvense), a spring burn was conducted in April 2006 Northwestern College's 20-acre tallgrass prairie near Hawarden, Iowa.  To examine the efficacy of the burn in controlling the thistle and other noxious weeds, we performed a vegetative survey of the prairie in September 2006 and compared post-burn plant composition to survey results from the same survey location in the years preceding the burn.  Our findings suggest that the spring burn actually increased the proportions of noxious weeds while decreasing proportions of several native species.  We conclude that the spring burn was counterproductive in controlling weed growth in our prairie and that further intercession with a better-timed burn, plus spraying and cutting, is warranted.

Figure 1. Location of the Northwestern College Prairie.

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Materials and Methods

Point-count surveys of a 50m x 100m section of the prairie were performed each September from 2003 to 2006. The burn in question was conducted on April 25, 2006. Proportions of total plants surveyed were determined for the aforementioned invasive and noxious native species, plus the five non-noxious native grasses in the prairie (Canada wildrye Elymus canadensis, sideoats grama Boutelova curtipendula, big bluestem Andropogon gerardii, indiangrass Sorghastrum nutans, and switchgrass Panicum virgatum) and the five most common forbs in the prairie (purple coneflower Echinacea purpurea, upright prairie coneflower Ratibida columnifera, stiff goldenrod Oligoneuron rigidum, pitcher sage Salvia azurea, and partridge pea Chamaecrista fasciculata). To determine the effects of the burn on each species, Chi-square analyses were performed on each species to compare its proportion of the total plants surveyed in 2005 (before the burn) to 2006 (after the burn). Likewise, Chi-square analyses were also performed on 2005 vs. 2006 proportions for each category of plants (invasives, noxious natives, non-noxious grasses, and non-noxious forbs).

Results

Considerable increases in proportions were observed for all four invasive species found (Fig. 2a). In fact, three of the four invasives were not detected in our survey area until after the burn. Canada thistle was first observed in the area when the path was mowed in 2004, but most of those plants were eradicated via spraying and cutting, and no thistle showed up in our survey until 2005. Compared to 2005, a 16-fold increase in thistle was seen in 2006. Of the noxious natives, horsetail appears to have been eradicated by the burn, while foxtail barley appears to have benefited by the inhibition of competition, as had also occurred in 2004 when the path was mowed (Fig. 2b). Of the five native grasses, big bluestem and sideoats grama showed significant increases, while Canada wildrye showed a significant decline (Fig. 2c). Of the forbs, pitcher sage and stiff goldenrod showed a significant increase, and no species showed a decline (Fig. 2d). Overall, our results showed significant increases in invasives, noxious natives, and non-noxious native forbs from 2005 to 2006 and a significant decrease in non-noxious native grasses (Fig. 2e).

2003-2006 Survey Results: Invasives

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Canada thistle Downy Brome Lambsquarter Dandelion

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Fig. 2a. Survey results for four invasive species found in the prairie. Chi-square results comparing 2005 to 2006 proportions are shown. ***= p<0.001.

Introduction

In 1999, students at Northwestern College began restoring a 20-acre tallgrass prairie in an agricultural field near Hawarden, Iowa (Fig. 1). Today, over 30 native species of grasses and forbs are present in the prairie. Unfortunately, three noxious native species (horseweed Conyza canadensis, foxtail barley Hordeum jubatum, and western ragweed Ambrosia psilotachya) and four noxious invasives (Canada thistle, Downy Brome Bromus tectorum, lambsquarter Chenopodium album, and dandelion Taraxacum officinale) have appeared within the prairie, initially as a result of the one-time mowing of a path through the interior of the prairie in spring 2004.

Past management efforts in prairies have generally involved burning as a means to control the reproduction and spread of unwanted species (Dornbush 2004). Our prairie was first burned in April 2003, not to control weeds, but rather to control the overabundance of Canada wildrye Elymus canadensis. Although burning may increase diversity and help control weeds in prairies, it may also inadvertently help noxious weeds when timed poorly and/or when an inappropriate weed species is targeted. In fact, several studies have shown that prescribed burning either increased or had no effect on invader abundance (e.g., Van Dyke et al. 2004, Travnicek et al. 2005).  

Burns during different seasons will affect species differently (Emery and Gross 2005). For eradicating warm season weeds such as Canada thistle, the burn should occur in the summer or fall, as spring burns may increase sprouting and reproduction in these species (Van Dyke et al. 2004, USDA). The frequency of the burn also needs to be taken into consideration. Emery and Gross (2005) found that while annual burns helped control exotic grasses, less frequent fire increased their population stability.

In an effort to control the noxious weeds (especially Canada thistle) in Northwestern’s prairie, the caretaker conducted a burn in April 2006. This study examines the results of this questionable burn to determine its effects on the noxious weeds and native grasses and forbs within the prairie.

2003-2006 Survey Results: Noxious Native Weeds

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Foxtail barley Horseweed Western ragweed

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2003-2006 Survey Results: Native Grasses

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2003-2006 Survey Results: Native Forbs

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Pitcher sage Stiff goldenrod Partridge pea Other forbsP

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2003-2006 Survey Results: Summary

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Invasives Noxious natives Non-noxious nativegrasses

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Fig. 2b. Survey results for three noxious native species found in the prairie. Chi-square results comparing 2005 to 2006 proportions are shown. ***= p<0.001.

Fig. 2c. Survey results for five native grasses found in the prairie. Chi-square results comparing 2005 to 2006 proportions are shown. **=p<0.025, ***= p<0.001.

Fig. 2d. Survey results for five native forbs found in the prairie. Chi-square results comparing 2005 to 2006 proportions are shown. *=p<0.05, **= p<0.025.

Fig. 2e. Survey results for five native grasses found in the prairie. Chi-square results comparing 2005 to 2006 proportions are shown. **=p<0.025, ***= p<0.001.

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Discussion

We found that a spring burn is not efficacious in controlling weeds such as Canada thistle and in fact appears to aid the proliferation of the species. Van Dyke (2004) demonstrated that spring burning may decrease plant diversity by enhancing the dominance of warm-weather plants via increase in rates of flowering. Canada thistle begins growth in the spring, with another spurt of growth in the fall (USDA). Because a spring burn may allow for better reproduction of the species, it is suggested that burns take place later in its life cycle (NPS). Mowing should likewise be used with caution, as thistle initially invaded the interior of our prairie as a result of a mowing event.

Forbs benefit from spring burns when they occur while plants are dormant (USGS). Correspondingly, we found that the spring burn resulted in an increase in pitcher sage and stiff goldenrod, two warm-season perennial forbs. Spring burns have been shown to inhibit the growth of Canada wildrye in the year of the burn, with the population fully recovering the following year (Howe 1994), so our observed decline in Canada wildrye in 2006 may be a short-lived phenomenon.

References:

Dornbush, M.E. 2004. American Midlands Naturalist 151:241-250.Emery, S.M. and Gross, K.L. 2005. Journal of Applied Ecology 42:60-69.Howe, H. 1994. Conserv. Biol. 8:691-704.National Park Service (NPS) and U.S. Department of Interior. 2006. http://www.nps.gov.Travnicek, A.J., Lym, R.G., & Prosser, C. 2005. Rangeland Ecology and Management

58:413-422USDA, NCRS. 2006. The PLANTS database. http://plants.usda.gov.USGS Northern Prairie Wildlife Research Center. Effects of fire on upland grasses and forbs.

http://www.npwrc.usgs.gov/resource/habitat/fire/grasforb.htmVan Dyke, F., Van Kley, S., Page, C., & Van Beek, J. 2004. Restoration Ecology 12:575-585.