Abstract
Invasive species, including the non-native forb Centaurea stoebe (spotted knapweed), constitute a threat to degraded and restored native prairies. Considering the threat that C. stoebe poses to prairie ecosystems, we examined the effectiveness of fire as a control for C. stoebe and (±)-catechin, a known allelopathic compound. We conducted an experiment in a reconstructed tallgrass prairie community at Pierce Cedar Creek Institute in Barry County, Michigan starting in May 2016. Our experiment consisted of individually burning 60 1-m2 plots with a propane torch to achieve high (316°C) and low (103°C) temperatures across spring and summer seasons over two years, then planting and seeding six native prairie plant species to monitor their establishment after burning. We compared the effects of the different burn treatments on the plant community by estimating percent cover and biomass of all species within each plot in August 2017. We also examined the effects of the simulated burn treatments on soil (±)-catechin levels, which we quantified using High Performance Liquid Chromatography. Centaurea stoebe was less dominant in burned plots than unburned plots, with summer-burned plots having the lowest biomass and cover. Differences in burn temperature failed to produce significantly different results. Planted native grasses increased more after spring burns than after summer burns. Preliminary findings suggest that high-temperature spring burns may indirectly reduce soil (±)-catechin levels. Overall, these results indicate that prescribed burning is an effective tool for controlling C. stoebe and promoting native species establishment in restored tallgrass prairies.
Restoration Recap
Burn season affects C. stoebe and native species establishment more than burn temperature.
Mid-spring and summer burns reduce C. stoebe dominance.
Summer burns are more effective at reducing C. stoebe dominance than spring burns, but may hinder native warm season grass establishment.
High temperature spring burns may reduce soil (±)-catechin levels.
Conservation and restoration of valuable or imperiled ecosystems are major foci of restoration ecology. Native grassland ecosystems have suffered serious declines in Midwestern North America since European settlement (Samson et al. 2004, Savage 2011). Despite their rarity, these grasslands provide important habitat for many plant and animal species. Nearly 260 bird species use grasslands as nesting habitat in the North American Great Plains (Savage 2011). In Michigan, nearly one-third of the state’s threatened, endangered, or special concern species find their primary habitat in grasslands (O’Connor et al. 2009). Many of these grassland species are in decline due to habitat loss and fragmentation from agricultural development (Herkert et al. 2003, Savage 2011), which strengthens the case for grassland conservation and restoration. Therefore, developing techniques to restore and manage grassland communities should be a primary concern for both ecologists and land managers. In Michigan, some communities have experienced statewide declines of nearly 99.99 percent (O’Connor et al. 2009), leading to designation of all of the state’s prairie communities as either imperiled or critically imperiled (Cohen et al. 2015). Although the majority of lost grassland communities can be associated with conversion to agriculture, invasive species threaten what little remains (D’Antonio and Meyerson 2002, Grant et al. 2009).
Centaurea stoebe (spotted knapweed) is a non-native, invasive, Eurasian forb that has infested over 2.9 million hectares of degraded and remnant grassland communities in North America (DiTomaso 2000). C. stoebe forms dense monotypic stands and may outcompete some native plant species (Tyser and Key 1988). C. stoebe succeeds as an invasive plant due to high seed production and germination (Schirman 1981), effective use of abundant resources (Knochel et al. 2010), and production of (±)-catechin (hereafter catechin). Catechin is an allelopathic chemical which C. stoebe excretes into the soil and has been shown to decrease growth of other plants in both lab and field studies (Perry et al. 2005, Thorpe et al. 2009). Catechin is thought to be a novel weapon (Callaway and Ridenour 2004, Inderjit et al. 2011). However, some studies doubt the influence of catechin in C. stoebe invasion due to low levels of catechin found in C. stoebe soils and a lack of evidence for catechin as a cause of oxidative stress in affected plants (Blair et al. 2006, Duke et al. 2009). Recent studies have answered some of this criticism by demonstrating the potential for catechin to harm beneficial soil biota as well as interact with and amplify phytotoxic metals present in the soil (Pollock et al. 2009, 2011, Wang et al. 2013). Additionally, the effect of catechin may be variable within soils at a site and catechin retention may depend on site-specific conditions such as soil type and companion compounds (Perry et al. 2007, Tharayil et al. 2008, Pollock et al. 2009).
Naturally occurring frequent fires were an important force in shaping North American grassland communities prior to European settlement (Samson et al. 2004, Allen and Palmer 2011). As such, prescribed fire is a tool used in the restoration of grassland systems and often employed to suppress invasive species (Kyser and DiTomaso 2002, DiTomaso et al. 2006, Bowles and Jones 2013). Fire can reduce the dominance of C. stoebe and recruitment by seed in infested areas (Emery and Gross 2005, MacDonald et al. 2007, Vermeire and Rinella 2009). Research has also shown that infested areas subjected to fire saw increased establishment of native prairie plants (MacDonald et al. 2007, Martin et al. 2014). Emery and Gross (2005) found burning C. stoebe in mid-summer to be most effective in reducing its biomass and number of flowering individuals, compared to early spring and mid-fall burns, although fuel loadings were quite low during some burn dates due to low productivity and warm-season grass cover. MacDonald et al. (2007) observed significant reductions in C. stoebe densities and biomass as a result of mid-spring burning in an area with high fuel loadings and dominated by warm-season grasses.
Studies have shown wide-ranging effects on the chemical properties of soil in systems that are managed with prescribed fire (Gómez-Rey et al. 2013, Pereira et al. 2017). Therefore, fire may also degrade or alter the allelopathic chemical catechin in the soil, although no research on the topic has been performed to our knowledge. Additionally, the effects of fire temperature on C. stoebe infestations is unknown, as is the optimal timing of burns for the restoration of C. stoebe-infested communities. Both mid-spring and summer burns are potentially effective control methods for C. stoebe in tallgrass prairies, but a direct comparison has yet to occur. Moreover, the response of the native plant community to summer burns in C. stoebe infestations is an important component of restoration that requires further study.
Our experiment takes into account both fire season and temperature to identify how different prescribed burning techniques affect invasive weeds, native plant communities, and catechin in the soil. Understanding these relationships can help managers better address C. stoebe control, native species establishment, and soil catechin degradation. We address several questions: 1) how does fire season and temperature affect C. stoebe dominance? 2) how does fire season and temperature affect native species establishment? and 3) does the application of prescribed fire reduce the amount of catechin present in soils? Answering these questions will advance the field of restoration ecology and inform future restoration of grassland communities.
Methods
Study Site
Our study took place at Pierce Cedar Creek Institute (PCCI) in Barry County, Michigan. PCCI operates as an environmental education center and biological field station while the 742 acres of land is managed as a public nature reserve. Soils at the site are classified as Perrinton Loam and average annual rainfall is 95.15 centimeters (Natural Resources Conservation Service 2017). The specific study area was historically farmed but was taken out of production in the 1950s. The area has been reconstructed and is now classified as mesic prairie, which is considered critically imperiled in Michigan (Cohen et al. 2015). PCCI has engaged in prairie restoration activities since 1998, but our site has received little attention aside from occasional mowing, leading to continued infestation by C. stoebe. Prior to any treatments, average C. stoebe cover at the site was consistent with greater than 20% cover observed at sites used by Perry et al. (2007) in a similar examination of C. stoebe and catechin. Other common species at the site included Bromus inermis, Poa pratensis, Rumex acetosella, Achillea millefolium, and Solidago spp.
We established 60 1-m² plots in seven parallel rows at the site, with 5–13 plots in each row, and a 0.5-m buffer between each plot. The shape and size of the study area prohibited an even number of plots in each row. We incorporated (SPHT), spring burn/low temperature (SPLT), spring control (no burning; SPC), summer burn/high temperature (SUHT), summer burn/low temperature (SULT), and a summer control (SUC). We subjected each burn plot to its specific treatment twice over the course of the study, once in 2016 and once in 2017. Treatments were randomly assigned to individual plots throughout the study area and each treatment was replicated 10 times for a total of 60 plots. We felt that a block design was unnecessary, as our study occurred in a small area with little variation in soils, topography, or plant composition.
Burn Treatments, Native Species Plantings, and Vegetation Sampling
To simulate prescribed fire, we used a propane torch to burn each plot individually. We chose the low (103°C) and high (316°C) temperatures to reflect the range of typical tallgrass prairie fire temperatures at the soil surface (Vermeire and Roth 2011, Ohrtman et al. 2015). We used Tempilaq G® heat-sensitive paint (LA-CO Industries, Grove Village IL) applied to small sheets of aluminum to determine when the plots reached the specified temperature. This paint turns to liquid when it is heated to the correct temperature. Low temperature plots required five seconds of burning to reach 103°C and high temperature plots required 15 seconds of burning to reach 316°C. Spring burns were conducted on May 19, 2016, while summer burns were conducted on June 29, 2016. We removed plant biomass in control plots using a gas-powered weed trimmer in conjunction with the 2016 burns. This removed the influence of remaining aboveground biomass on planted species establishment, without the added effects of burning. MacDonald et al. (2013) demonstrated that such single-application mowing treatments did not significantly reduce C. stoebe densities or biomass, so we feel that these plots represented an appropriate control.
Following each burn, we seeded and planted plugs of a suite of native genotype grassland species (from Hidden Savanna Nursery, Kalamazoo, Michigan) in the burned plots and their associated control plots. Seeded species included three forbs: Lupinus perennis (wild Lupine), Asclepias tuberosa (butterfly-weed), and Anemone cylindrica (thimbleweed) and three grasses: Sorghastrum nutans (Indian grass), Schizacyrium scorparium (little bluestem), and Pancium virgatum (switchgrass). We included an equal number of seeds for each species. We raked seeds into the soil at a relative abundance of 600 total seeds/m2 to a depth of approximately ¼ inch immediately after seeding in half of each plot. Prior to planting, we appropriately scarified and/or thermally stratified seeds for each species. We planted container grown plugs on the remaining side of each plot at a rate of two plugs per species for a total of 10 plugs per plot (n = 600 for experiment; 300 per burn season). Plug species included all seeded species, with the exception of A. cylindrica, which could not be obtained from the supplier. At the time of planting, seedlings were well developed with heights between 10 cm and 20 cm, depending on the species. We irrigated seeds and plugs daily during the first week following seeding and planting. When rainfall fell at least 20 percent below the weekly average (2.36 centimeters from May to August), we irrigated all plots with enough water to achieve the average when combined with observed precipitation.
We collected vegetation data for all 60 plots on May 15 and 16, 2017. Within each plot, we sampled species richness, vegetative cover, and above-ground biomass. We determined vegetative, bare ground, and litter cover using point-intercept sampling. For the point-intercept sampling, we placed a 1 × 1-m PVC frame over each plot. We used permanent marker to place equally spaced hashes on the frame (six hashes on two opposite sides, nine hashes on the other two), which lined up to create a sampling grid of 54 points. At each point, we dropped a survey pin and recorded each plant species touching the pin, with the amount of touches for each species corresponding to total percent cover of that species within the plot. After estimating cover for each plant species, we harvested all aboveground biomass in a 10 cm × 1 m strip from each sampled plot, sorted to species, and dried all biomass at 65°C to a constant mass in a drying oven. We then weighed and recorded the biomass for each species in each plot. We then burned the SPLT and SPHT plots a second time on May 19, 2017, following the same burn procedure from 2016. On June 30, 2017, we burned the SULT and SUHT plots a second time. We watered plots whenever weekly precipitation fell below average using the same procedures described for 2016. We did not seed or plant any new species following the 2017 burns. In August 2017, we collected species richness, cover, and biomass data for all 60 plots, avoiding the previous strip of biomass collection when collecting biomass for the second time. We calculated change in C. stoebe, planted grass cover, and biomass by comparing May and August vegetation sampling results.
Soil Catechin Analysis
In April 2017, we set up five additional plots adjacent to the original 60 plots to directly examine catechin levels at the site and determine the effect of the different burn treatments on soil catechin. Our catechin study only incorporated one replicate for each treatment due to the logistical constraints associated with processing a large number of soil samples, so interpretations of the data were treated with caution. We chose five mature C. stoebe plants of approximately the same size (canopy diameter roughly 21 cm) to serve as the center of each 90 cm diameter plot. We then hand-pulled all other C. stoebe individuals within one meter of each of the five center individuals in order to isolate the analysis to a single plant. When necessary, we used a trowel to assist in taproot removal. We continued to weed the plots throughout the summer as needed. Due to the relatively quick degradation of catechin in soils (Tharayil et al. 2008) and the demonstrated effectiveness of hand-pulling as a control method for C. stoebe (MacDonald et al. 2013), we are confident that no residual catechin from the pulled plants affected our analyses.
To identify the relationship between C. stoebe density and soil catechin levels, we divided each plot into three zones of 15 cm increments. Zone One was 0–15 cm from the center plant, Zone Two was 16–30 cm from the center plant, and Zone Three was 31–45 cm from the center plant. We collected a standard amount of soil from each of the three zones by filling a cylinder with the top five centimeters of soil at four points for a total of 8.84 cm3 per zone. We took our first samples immediately before burning on May 19, 2017, and continued collection once each month in June, July, and August. We randomly subjected each s C. stoebe plant to one of the different burn treatments from the vegetation survey (SPLT, SPHT, SULT, and SUHT) and included a single control plant which was not burned. We also collected soil samples for analysis immediately after each plot received its burn treatment in case there were any immediate impacts on soil catechin levels. Immediately after collection, we froze all soil samples in an on-site freezer in order to prevent catechin degradation.
To determine catechin levels in our soil samples, we used High Performance Liquid Chromatography (HPLC) available via the Grand Valley State University Chemistry Department. Our method for catechin extraction followed that of Blair et al. (2005), which identified a 75% acetone, 25% water, and 0.1% phosphoric acid extraction solvent as the most efficient for catechin recovery. We ran extracted catechin samples through a gradient system using a 90% water, 10% acetonitrile, 0.1% phosphoric acid mobile phase, which was increased after five minutes to 30% acetonitrile over 10 minutes and held at 30% for three minutes (18 minutes total). Using catechin standards, we determined that catechin appeared on the HPLC chromatograms at roughly 9.1 minutes. We quantified catechin in μg/mL by comparing peak area of soil extractions to peak areas of known concentrations of catechin in prepared standards.
Data Analysis
Our data did not meet the assumptions for parametric statistical analyses, even after using transformations. We used a non-parametric Sheirer-Ray-Hare (Scheirer et al. 1976) test to determine whether significant differences in C. stoebe and planted species biomass and cover occurred in response to our methods. We ran Sheirer-Ray-Hare tests on average C. stoebe and planted grass cover in August, change in cover, biomass in August, and change in biomass with burn season (spring, summer) and burn temperature (control, low, high) as independent factors. For comparisons of individual treatments, we used a nonparametric Mann-Whitney U test. We also ran a Sheirer-Ray-Hare test on average soil catechin with burn treatment (Control, SPLT, SPHT, SULT, SUHT) and distance from plant (0–15, 16–30, 31–45 cm) as independent factors. We used SPSS statistical software to conduct all tests, (SPSS v. 22, IBM Analytics, Armonk, NY).
Results
Plant Community Response
Across all plots and sampling dates, we encountered 55 total plant species. Of these 55 species, 25 were species native to Michigan, and 30 were non-native. C. stoebe was among the most common species, occurring in all 60 plots prior to 2017 burn treatments. Seeded grasses established in all plots, and we encountered seedlings of each planted species except A. cylindrica throughout the study site. Within the first few weeks of planting, all forbs planted as plugs were eaten by herbivores. Seeded forbs established in low numbers in 2017. We observed slightly more A. tuberosa seedlings (25) than L. perennis seedlings (17) at the end of data collection. Due to the small sample size, we were unable to perform statistical analyses on the seeded forb results.
On average, control plots contained 22 percent more C. stoebe cover and roughly five times more C. stoebe biomass when compared to all burned plots. We observed significant differences in C. stoebe cover among plots according to burn season (F1,58 = 11.01, p = 0.001) and burn temperature (F2,57 = 17.74, p < 0.001) across all treatments. However, the differences in C. stoebe cover were rarely significant between individual burn treatments (Figure 1). We also observed significant differences in C. stoebe cover change between plots according to burn season (F1,58 = 6.48, p = 0.011) and burn temperature (F2,57 = 28.24, p < 0.001) across all treatments. Again, differences were rarely significant between individual burn treatments (Figure 1B). In August, C. stoebe cover was lower in summer-burned plots than in spring-burned plots, with the lowest cover found in SUHT plots and the highest cover found in control and SPLT plots (Figure 1A). Centaurea stoebe cover increased the most in control plots from May to August, with lower increases observed in spring burn plots, and decreases observed in summer burn plots (Figure 1B).
Burning at both temperatures resulted in significantly lower C. stoebe biomass in August (F2,57 = 17.63, p < 0.001), and biomass change between May and August (F2,57 = 15.13, p = 0.001) when comparing all treatments together, although these differences were not significant between individual treatments. Burn season did not significantly affect average C. stoebe biomass or change in biomass overall, although individual treatments did significantly affect both biomass and change in biomass when compared to their respective controls in most cases (Figure 1). All burn treatments resulted in lower C. stoebe biomass in August when compared to control plots, with the lowest biomass found in summer burn plots (Figure 1C). Centaurea stoebe biomass increased in control plots but decreased in all burn plots from May to August, with the largest decreases observed in SUHT plots (Figure 1D).
Burn season affected planted grass cover (F1,58 = 9.97, p = 0.002), change in cover (F1,58 = 8.21, p = 0.004), biomass (F1,58 = 6.59, p = 0.010), and change in biomass (F1,58 = 8.69, p = 0.003). However, these overall differences seldom showed up between individual treatments (Figure 2). We did not observe any differences in planted grass response variables as a result of burn temperature. Planted grass cover was higher in spring-burned plots than in summer-burned plots, with the highest planted grass cover in SPHT plots (Figure 2A). Planted grass cover increased when exposed to all treatments, although the increases were more substantial in spring, specifically SPHT plots (Figure 2B). Planted grass biomass was higher in spring-burned plots and lower in summer-burned plots when compared to control plots at the end of the season, and biomass was again highest in SPLT plots (Figure 2C). Planted grass biomass increased slightly in control plots, with larger increases observed in spring burn plots, and almost no increases observed in summer burn plots (Figure 2D).
Soil Catechin Results
We detected catechin at least once in all five plots throughout the season, although none of our treatments significantly affected soil catechin levels. We found the highest levels of catechin in June for all distance zones and treatments, with the exception of SPHT, in which we detected no catechin in June even though catechin was present in samples taken from this plot in May. Catechin was typically present in lower levels in May, and completely absent from our soil samples in July and August (Table 1). We generally found more soil catechin in the SPLT plots, although the differences between treatments were not significant. Samples taken immediately before and after burning revealed no differences in soil catechin levels (Table 2). We found highest soil catechin levels in the zone 15–30cm away from the C. stoebe plant, and the lowest levels in the 0–15cm zone (Table 1). Catechin levels in the soil never exceeded 1μg/mL.
Discussion
Centaurea stoebe Dominance
Fire reduced C. stoebe dominance in all burn plots relative to control plots, although individual burn treatments differed in overall success. Both our study, and that of MacDonald et al. (2007) show that mid-spring burning can be an effective control for C. stoebe. Although generally effective, mid-spring burns were less successful at reducing C. stoebe cover, biomass, and growth than summer burns. Emery and Gross (2005) also found summer burns to be most successful for C. stoebe control, and they concluded that early-spring burns did not significantly reduce recruitment or biomass. However, grassy fuel loadings were much higher in the study conducted by MacDonald et al. (2007), and the study area utilized by Emery and Gross (2005) was not always able to sustain a fire. Therefore, it is important to consider the effects of both fuels and burn timing when considering the results of past studies. Summer burns are likely most effective due to the phenology of C. stoebe, which had bolted and was beginning to flower around the time of our summer burns, but was still in rosette form during spring burns at our site. Repeated burns that coincide with a target plant’s growing season may reduce root carbon reserves, thereby limiting future growth (Schutz et al. 2011). Additionally, defoliating C. stoebe during the flowering stage severely limits seed production and viability, thereby limiting reproductive capacity and contributions to the seedbank (Benzel et al. 2009). Such benefits relating to seed reduction likely were not observed during our study and may become more evident over time. Overall, summer burns were more effective for reducing C. stoebe dominance in invaded communities than spring burns.
We did not find an overall trend for the effect of burn temperature on the success of C. stoebe control. Communities that are invaded by C. stoebe often lack large amounts of native grasses, which provide the fine fuels required for high temperature fires (Bidwell and Engle 1992). Our results indicate that burning in such areas can be an effective tool for C. stoebe management, despite the lack of necessary fuels for more intense fires. However, when considering both season and temperature, high temperature summer burns (SUHT) were consistently more successful at reducing C. stoebe cover, biomass, and growth than any other burn treatment. Although successful overall, low temperature spring burns (SPLT) were the least effective treatment for reducing C. stoebe cover, biomass, and growth. Spring burns effectively reduced C. stoebe cover in high temperature burn plots only, suggesting that spring burns for C. stoebe management should be conducted at high temperatures if possible. This could explain why Emery and Gross (2005) found spring burns to be ineffective for C. stoebe control, since all their burns were reported to be of low intensity. High temperature burns are not necessary for C. stoebe cover and biomass reduction, indicating that burning can still be an effective management tool in areas with high C. stoebe densities and relatively little fine fuels. However, managers should attempt high temperature burns when feasible, either by manipulating fuels or through burn techniques.
Planted Species
We found no impact of burn temperature on patterns of planted grass establishment. We also found little impact of burn season on planted grass species, with some exceptions. Spring burn plots were very similar to control plots when measuring cover, biomass, and growth. Conversely, both MacDonald et al. (2007) and Martin et al. (2014) found increased growth of warm season grasses in C. stoebeinfested areas that were treated with mid-spring burns. However, both studies conducted their burns in areas with established warm-season grasses, while our study burned newly planted grasses that were still establishing. It is possible that our grasses would have responded more positively to mid-spring burns had they been given time to establish themselves. We also found that planted grass cover and biomass were generally higher in spring burn treatments than in summer burn treatments, although the differences were negligible. Despite similarities in final biomass levels at the end of the season, increases in planted grass biomass were reduced in summer burn plots when compared to spring burn plots, but not when compared to control plots. Our results suggest that summer burns at high or low temperatures can reduce the growth of warm season grasses as compared to spring burns. However, burning in the summer did not seem to meaningfully harm warm season grasses overall in our study. This is consistent with past research (Towne and Kemp 2008), although other studies indicate that growing season burns may reduce the flowering potential of warm season grasses in prairie restorations (Pavlovic et al. 2011). It is likely that the positive effects on our planted grasses of summer burns from removing C. stoebe outweighed the negative effects from reduced growth. Therefore, summer burns in areas of C. stoebe with establishing warm season grasses are still beneficial to the community overall and should be considered by managers.
Planted forb species did not make meaningful contributions to planted species cover or biomass. This is likely related to herbivory that occurred in our plots immediately after planting in 2016. We observed herbivory of every planted forb plug within one week of planting in both spring and summer plots, although grasses remained mostly untouched. Past research indicates that planted prairie forbs exposed to herbivory for the duration of the growing season suffer detrimental reductions to growth and reproduction (Sullivan and Howe 2009). The herbivory that we observed suggests that planting forb plugs may not be effective in the first year of planting without substantial herbivore controls. Native forbs also take a longer time to establish from seed than grasses (Hillhouse and Zedler 2011), so the effects of our burn treatments on the planted forb species may not be evident for several more growing seasons. However, past research by Towne and Kemp (2008) indicates that summer burns may benefit perennial forb species, with inconsistent effects on both annual and biennial forbs.
Soil Catechin
Our study of fire effects on soil catechin was limited, and results should be considered preliminary. However, the results do reveal interesting trends that warrant discussion. We only found catechin at very low levels during our experiment (never exceeding 1μg/mL), which is lower than levels observed to inhibit growth in nearby plants (Perry et al. 2005, Thorpe et al. 2009, May and Baldwin 2011). It is important to note that our catechin study reflected low densities of C. stoebe due to removal of all but one individual in the five experiment plots. These low densities could account for the observed low catechin levels. Perry et al. (2007) found that soil catechin levels may be highly variable within an invasion site. This variation may occur due to differences in soil pH or moisture (Blair et al. 2006), or due to the presence of certain metals in the soil (Pollock et al. 2009). Blair et al. (2006) found that catechin persisted longer in dry, acidic soils. The loamy soils at our site have high moisture-holding capacity and are very slightly acidic (pH = 6.7) (Natural Resources Conservation Service 2017), which is consistent with the low amount of catechin found in our soils. Further monitoring of soil catechin at our site could help determine the exact impact of catechin on the plant community.
Absence of soil catechin in July and August samples suggests that catechin production ceased after mid-June at our site. As a result, summer burns likely did not influence soil catechin. Total loss of soil catechin in the SPHT plot between the time of burning in May and sampling in June indicates that high temperature spring burns could reduce soil catechin levels even though we did not observe an immediate reduction in catechin after burning. None of the C. stoebe individuals in the catechin study died immediately after burning, and all survived until at least August. Therefore, any changes in soil catechin levels cannot simply be attributed to C. stoebe removal. While burning did not directly affect soil catechin, it may have indirectly lowered catechin levels over time by physiologically stressing C. stoebe. Stressed plants with limited energy and resource access often exhibit trade-offs between growth and secondary chemical production (Herms and Mattson 1992, Fine et al. 2006). Significant reductions in C. stoebe cover, biomass, and growth as a result of high temperature spring burns could have forced the plant to use energy for growth that would otherwise go towards catechin production. Therefore, in addition to reducing C. stoebe dominance, high temperature spring burns may also limit the influence of catechin in systems where it plays a major role in C. stoebe invasion. This could, in turn, promote establishment of native species by reducing the allelopathic advantage of C. stoebe. However, a more extensive study is required to further elucidate the effects of prescribed burns on soil catechin.
Our results suggest that prescribed burning can be an effective tool for restoring native grasslands by helping to control C. stoebe and by shifting the competitive advantage to native grass species. Both mid-spring and summer burns reduced C. stoebe dominance, although summer burns were clearly more effective in our study. When combined with the findings of past studies, our research indicates that prescribed fire increases in C. stoebe control effectiveness from early-spring (not effective), to mid-spring (somewhat effective), to summer (most effective). Burn season is more influential than burn temperature, but higher temperature burns typically increase the effectiveness of fires, especially in spring. Moreover, burning may have the added benefit of reducing soil catechin levels, although more study is required. While slightly less beneficial than spring burns for native grass establishment, summer burns still provide net benefit for establishing warm season grasses that are competing with C. stoebe, and overall did not prohibit their establishment. However, if establishment of warm season grasses is of more importance than C. stoebe removal, a spring burn may be more appropriate. Ultimately, management goals and site-specific conditions will determine the best management strategy for impaired grassland communities.
Acknowledgements
We thank Pierce Cedar Creek Institute for funding this project, lending equipment, and providing logistical support. We also thank the staff at PCCI for their help with various aspects of the project. Additionally, thank you to Katie Walker and members of the Aschenbach Lab at GVSU for assistance in data collection. Finally, thank you to Dr. Blair Miller and the GVSU Chemistry Department for assistance with soil analyses.
This open access article is distributed under the terms of the CC-BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0) and is freely available online at:http://jhr.uwpress.org