RESEARCH
The Replacement of Wetland Vegetation by Reed Canarygrass (Phalaris arundinacea) by Debbie A. Maurer, Roberto Lindig-Cisneros, Katherine J. Werner, Suzanne Kercher, Rebecca Miller, and Joy B. Zedler
A concerted research program discovers why reed canarygrass becomes established in degraded wetlands and suggests management options.
I
nvasions by aggressive exotic plant species are typically explained by a community’s invasibility and an invader’s characteristics; yet the near-complete replacement of native vegetation by invasive monotypes is not well understood. Invasive species management has become a large component of the restoration and conservation of native communities, and knowledge of the factors that facilitate or limit invasion is critical to the development of effective control plans. Our recent research on reed canarygrass suggests how environmental conditions and plant traits might interact to allow some invaders to displace nearly all the native species in some disturbed wetlands. We hypothesize that environmental changes that increase both community invasibility and invader aggressiveness allow the replacement of native vegetation in freshwater wetlands. We propose that sediment-rich waters associated with runoff simultaneously a) make a native wetland community more invasible by altering the biotic and physical characteristics of the community, and b) facilitate aggressive invasion by increasing the availability of nutrients.
W h y F o cu s on R eed C an ary grass?
Reed canarygrass is one invader of wetland communities that is able to create monotypic stands and eliminate many native species. Although reed canarygrass is native to Wisconsin, aggressive ecotypes have been introduced from Europe for agricultural use. It is believed that the aggressive form found in wetlands is the non-native Eurasian ecotype or a hybrid of the native and non-native ecotypes. The aggressive form of reed canarygrass has many attributes of a typical invasive species: high allocation to reproduction, clonal growth, long growing period, rapid growth, high productivity, and a broad tolerance to environmental variability (Marten and Heath 1985, Apfelbaum and Sams 1987). Reed canarygrass dominates many wetland plant communities in Wisconsin, including marshes, fens, stream banks, sedge meadows, and wet prairies. It is problematic in native and restored wetland communities across northern North America (Galatowitsch and others 1999). For example, in a Wisconsin sedge meadow, Werner and
Ecological Restoration, Vol. 21, No. 2, 2003 ISSN 1522-4740 E-ISSN 1543-4079 © 2003 by the Board of Regents of the University of Wisconsin System.
116
ECOLOGICAL RESTORATION
21:2
n
JUNE 2003
W h at L im its R eed C an ary grass E stab lish m en t?
Light quality and quantity limit reed canarygrass establishment. In the laboratory, Lindig-Cisneros and Zedler (2001) found that reed canarygrass seeds exposed to 14 hours of red and white light have a 40 to 50 percent higher germination rate than those exposed to far-red light. Light transmitted through leafy canopies is known to have a lower red:far-red light ratio than direct sunlight. When reed canarygrass seeds were exposed to lower red:far-red ratios, germination decreased by nearly 30 percent (Lindig-Cisneros and Zedler 2001). Reed canarygrass also establishes from vegetative propagules. In a field experiment conducted in three wetland communities, Maurer and Zedler (2002) found that reed canarygrass establishment from rhizome fragments (underground stems) is also limited by the light environment. Reed canarygrass rhizomes planted in a dense canopy with low light availability at ground level had lower survival than those planted in a canopy with high light transmission. This trend was
ECOLOGICAL RESTORATION
21:2
n
JUNE 2003
100% –
3 spp.
80% – 29 spp.
P. arundinacea
28 spp.
60% –
20% –
Typha sp.
40% – C. stricta
Mean relative cover
Zedler (2002) found that a dense stand of reed canarygrass supported one-ninth of the species found in a dense stand of cattails (Typha spp.) and a tussock sedge (Carex stricta) meadow, and the average relative cover of reed canarygrass was twice that of cattails and tussock sedge (Figure 1). Reed canarygrass reduces native species richness and is a threat to the persistence and development of natural and restored wetland communities. This species is so widespread that restorationists and managers should anticipate that their lands will be invaded (Antieau 2002), and landowners should act aggressively to control existing populations and reduce community invasibility. Invasion requires dispersal of the invader to the site, followed by establishment and spread. Dispersal is often uncontrollable; however, establishment (that is, germination and sprouting of vegetative propagules) and survival can be prevented.
Carex stricta meadow
Typha spp. stand
0% – P. arundinacea stand
Community type Figure 1. Species richness and mean relative cover in a tussock sedge (Carex s tricta) meadow, a cattail (Typha spp.) stand, and reed canarygrass (Phalaris arundinacea stand (n = 1). More species occurred in the sedge meadow community and cattail stand than in the reed canarygrass stand. Data of Werner and Zedler 2002
supported with a greenhouse experiment that tested rhizome establishment under four levels of shading: heavy shade reduced reed canarygrass survival by 25 percent and growth by about 95 percent. Because the light environment limits reed canarygrass establishment, a closed canopy should be less susceptible to invasion than an open canopy. After an invader establishes, the shade cast by neighboring plants might no longer inhibit growth or vegetative spread. In a greenhouse experiment with three-month-old reed canarygrass clones, Maurer and Zedler (2002) tested the effects of shading (47, 67, and 86 percent treatments) on the expansion of new tillers. When tillers were attached to unshaded parent clones new growth was not significantly affected. Tillering of established clones was not limited by heavy shade. This suggests that established parent clones subsidize the expansion of new tillers into heavily shaded microsites that are otherwise unsuitable for seed germination or establishment by rhizome fragments. Because initial establishment is limited by the shade of plant canopies, a native plant community should become
more invasible if it has canopy gaps or a canopy that transmits a high percentage of light. Lindig-Cisneros and Zedler (2002) tested this hypothesis in peat-filled mesocosms planted with native species from a local fen community (Figure 2). The mesocosms were planted to have a matrix of fowl manna grass (Glyceria striata) with 1, 6, and 15 other species. They were then sown with reed canarygrass seeds. Multiple gaps were created in half of each canopy by clipping. The two-year study showed that canopy gaps increased invasibility; reed canarygrass did not germinate in no-gap treatments, regardless of species richness. Reed canarygrass only germinated in gap treatments, and then only under the 1-species canopies. Reed canarygrass germination in the speciesrich treatments was 43 percent lower than that of the 1-species canopy. This pattern of reed canary establishment was supported by a greenhouse experiment where 1-species canopies were more invasible, that is, 1-species canopies had greater than 60 percent more seedlings than multi-species canopies (Lindig-Cisneros and Zedler 2002). Disturbances that directly or indirectly open up plant canopies should
117
Figure 2. Roberto Lindig-Cisneros in 2000 with experimental mesocosms at the University of Wisconsin-Madison Arboretum. Photo by Bill Arthur
make the community more invasible, and if the environmental change also increases invader aggressiveness, a monotype could prevail.
W h at D istu rban ce S im u ltan eou sly In creases C om m u n ity In vasib ility an d In vader A ggressiven ess?
Several community characteristics might enhance species richness or the continuity of the plant canopy—and thus reduce invasibility. For example, tussock sedge meadows support a rich diversity of plant taxa; an individual tussock supports up to ten species (Werner and Zedler 2002). The number of species per tussock correlates with the surface area of the tussock. Thus, disturbances that eliminate the tussock form or reduce microtopography could reduce species richness and increase invasibility. Sediment-rich stormwater inflow is such a disturbance. When stormwater enters a sedge meadow, sediment can fill in depressions and low spots between tussocks, reducing the surface area of the tussocks and lowering the number of species per tussock. Werner and Zedler (2002)
118
calculated that a 13-in (33-cm) tall tussock would lose 1.5 species with every 4 inches (10 cm) of sediment accretion. With fewer species in the canopy, more light would be transmitted and invasibility would increase, according to LindigCisneros and Zedler (2002). In one Wisconsin wet prairie (Greene Prairie at the University of Wisconsin-Madison Arboretum), a steady inflow of urban runoff has deposited more than 3 feet (1 m) of sediment (Werner and Zedler 2002). Today, the area supports a 7-acre (2.8-ha) monotype of reed canarygrass. Simultaneously with sediment inflows, stormwater brings nutrients, which increase invader aggressiveness. Kercher and Zedler (2001) compared the growth of native fen species, forbs, grasses, and reed canarygrass under nutrient-rich conditions and four hydroperiods. They found that reed canarygrass out-grew all other groups by 20 to 130 percent, regardless of hydroperiod. Reed canarygrass had significantly lower growth in only in one of four hydroperiods. Maurer and Zedler (2002) tested the response of reed canarygrass clones to lowand high-nutrient conditions that differed by a factor of five in a greenhouse experiment. After seven weeks, nutrients increased the number of tillers and total
stem length by 56 percent and doubled the distance of lateral spread, even under heavy shade. Moreover, under high nutrient conditions, reed canarygrass allocated more biomass to aboveground growth and rhizome production than to roots. Green and Galatowitsch (2001) and their colleagues found the same pattern in a mesocosm study where reed canarygrass was grown with competitors (sedge meadow species) in soil that varied in nitrate-N levels. High nutrient conditions allow this plant to grow taller and allocate more energy to vegetative reproduction and spread, thus increasing invader aggressiveness. Reed canarygrass is not only able to increase its growth and spread under high nutrient conditions, it also tolerates a variety of hydroperiods. In a mesocosm experiment, Miller and Zedler (in press) tested the effects of two water depths and four hydroperiods on reed canarygrass growth and found no significant effects on reed canarygrass aboveground growth, a finding supported by data of Kercher and Zedler (2001). Compared to a native grass, prairie cordgrass (Spartina pectinata), the hollow stems of reed canarygrass produced twice the stem length per unit biomass, and reed canarygrass stem length was 20 percent greater when grown with a competitor than when grown alone (Miller and Zedler in press). These traits likely increase the competitive ability of reed canarygrass, especially under high light and high nutrient conditions.
S u ggestio n s for M an agin g R eed C an arygrass In vasion
1. Maintain or encourage rapid development of vegetation with a dense canopy. 2. Plant native species that can compete with reed canarygrass, such as widespread non-invasive graminoids and forbs. 3. Integrate microtopography into restoration sites to facilitate the development of species-rich vegetation. 4. Quickly remove new invader populations to prevent their spread. 5. Monitor and control sedimentation and nutrient loading.
ECOLOGICAL RESTORATION
21:2
n
JUNE 2003
Figure 3. Illustration of a sedge meadow: (left) tall, robust tussock sedge (Carex s tricta) community occurs with high microtopographic variation, high species richness, low invasibility, and minimal sediment accumulation; (center) partial smothering of sedges by nutrient-rich sediment not only kills some plant but decreases microtopographic variation, reduces species richness, creates canopy gaps, and increases community invasibility; and (right) sediments bury and destroy sedge meadow community, reed canarygrass invades the nutrient- and light-rich environment and becomes a monotype. Modified from Werner and Zedler 2002
C on clu sio n
Our findings suggest that influxes of sediment-rich stormwater are capable of simultaneously making native vegetation more invasible by decreasing microtopography, species richness, and plant canopy cover (Figure 3), and making reed canarygrass more aggressive by increasing nutrient resources. This may well explain the outcome of reed canarygrass invasion in many wetlands, namely the replacement of species-rich native plant communities by monotypic stands of reed canarygrass.
ACKNOWLEDGMENTS We thank John O. Jackson and the Wisconsin Department of Transportation, Bureau of Environment for encouraging the preparation of Fact Sheet FS-1-02, the precursor to this article. The research was facilitated by the University of Wisconsin-Madison Arboretum, the Friends of Pheasant Branch, and the University of Wisconsin-Madison Walnut Street Greenhouse. Funding was provided by
ECOLOGICAL RESTORATION
21:2
n
JUNE 2003
an EPA Water and Watersheds Project (Grant # R-82801001-0), the Wisconsin Academy of Sciences, the University of WisconsinMadison Arboretum, the University of Wisconsin-Madison Academy of Science, Arts, and Letters Lois Almon Small Grants Program, and the University of Wisconsin Botany Department Davis Fund.
REFERENCES Antieau, C.J. 2002. Biology and management of reed canarygrass, and implications for ecological restoration. Society for Ecological Restoration, Northwest Chapter. http://www.halcyon.com/sernw/rcgrass/rc_ docs.htm (January 2002). Apfelbaum, S.I. and C.E. Sams. 1987. Ecology and control of reed canarygrass. Natural Areas Journal 7:69-74. Galatowitsch, S.M., N.O. Anderson and P.D. Ascher. 1999. Invasiveness in wetland plants in temperate North America. Wetlands 19(4):733-755. Green, E.K. and S.M. Galatowitsch. 2001. Difference in wetland plant community establishment with additions of nitrate-N and invasive species (Phalaris arundinacea
and Typha x glauca). Canadian Journal of Botany 79:170-178. Kercher, S. and J.B. Zedler. 2001. Comparative responses of 16 wetland plant species to four hydrologic regimes. Paper presented at the Society of Wetland Scientists, 22nd Annual Meeting, Chicago, Illinois. Lindig-Cisneros, R.A. and J.B. Zedler. 2001. Effects of light on Phalaris arundinacea L. germination. Plant Ecology 155:75-78. __. 2002. Species-rich canopies limit the germination microsites for Phalaris arundinacea L. Oecologia 133:159-167. Marten, G.C. and M.E. Heath. 1985. Reed canarygrass. Pages 207-215 in M.E. Heath, R.F. Barnes and D.S. Metcalfe (eds.), Forages: The science of grassland agriculture. Ames: Iowa State University Press. Maurer, D.A. and J.B. Zedler. 2002. Differential invasion of a wetland grass explained by tests of nutrients and light availability on establishment and clonal growth. Oecologia 131:279-288. Miller, R.C. and J.B. Zedler. In press. Responses of native and invasive wetland plants to hydroperiod and water depth. Plant Ecology. Werner, K.J. and J.B. Zedler. 2002. How sedge meadow soils, microtopography, and vegetation respond to sedimentation. Wetlands 22(3):451-466.
Debbie Maurer is a restoration ecologist with the Lake County Forest Preserve District, Natural Resources Division, 2000 North Milwaukee Ave., Libertyville, IL 60048-1199, 847/968-3285, Fax: 847/680-5062,
[email protected]. Roberto Lindig-Cisneros is a professor and restoration ecologist at the University of Michoacán, Morelia, Michoacán, Mexico. Katy Werner is an ecologist with Applied Ecological Services, Broadhead, WI. Suzanne Kercher is a doctoral student in the University of Wisconsin-Madison Botany Department, Madison, WI. Rebecca Miller is a restoration ecologist with Wetland Research Associates, San Rafael, CA. Joy Zedler is the Aldo Leopold Professor of Restoration Ecology at the University of WisconsinMadison Botany Department and Arboretum, Madison, WI.
119
