Leaf Litter as an Indicator of Woody Plant Diversity in Invaded Habitats

1. Leaf Litter as an Indicator of Woody Plant Diversity in Invaded Habitats Angela Tomars, Apryl Smith, Ben Bosman, Biology Department, University of Wisconsin-River Falls Conclusion The conclusion of statistical analysis of data collected is that the hypotheses must be rejected on the grounds of inconclusive and inaccurate data. Reasons for inconclusiveness may include lack of reliability of leaf litter due to wind and other weather effects, high variability of sampling sites due to small size and quantity compared to total area of park that was sampled, or possibly inaccuracy due to exclusion of seedlings under 12” in height for ease of identification. Another possible confounding factor may be that the areas considered as "uninvaded" did contain a small presence of Buckthorn. Although our hypothesis must be rejected many studies have successfully shown a relationship between species diversity in forests and buckthorn presence. These studies concluded that buckthorn density was inversely related to seedling density, herb cover, and ground level plant species richness(Callaway)(Knight). In an effort to accomplish the same results as these published studies the use of leaf litter as an quantitative measurement would need to be replaced by the meticulous counting of each species of seedling, tree, herb, and vine as well as estimations of percent cover of each. Most importantly, sample sites would need to be greater in size and in number in order to reach the accuracy of above studies, increasing our 26 in diameter circles to a minimum of 1m x 1m plots or even 12m in diameter circular plots. The leaf litter collection method for data was chosen for the time of year. Seedling sampling would have better shown the future impact of Buckthorn on the study site, however, seedling identification was impossible at the time of the study due to leaf drop. Buckthorn, as shown in the photos, does not drop its leaves until much later in the season compared to the native woody species in the study area. Also, coniferous woody plants tend not to drop leaves annually. Therefore, leaf litter collection during most times of the year would produce unreliable results. Materials and Methods The study site was selected because of its high concentration of Buckthorn, as well for its location within the River Falls Park System. An area of gradient consisting of non-invaded to completely invaded habitat was selected on the ridge of the Western side of Mound Park near a city water reservoir. Random sampling was achieved using the quadrat method. First, a baseline running lengthwise along the gradient of invasiveness was determined. Using a random number table, we established points on this baseline where we measured into the sampling area a random distance (<100ft.) and placed our quadrat. Introduction Results Morphotypes Present Fig. 5-6 Processing leaf litter Study Sites Fig. 1 Sample area as viewed from the northern trail. It is estimated that invasive species have an economic cost of 34 billion dollars annually in the U.S. alone. Aside from the drastic economic cost, invasive species create a myriad of environmental problems (Dukes). Common Buckthorn (Rhamnus cathartica) and Glossy Buckthorn (Rhamnus frangula) are two such species with great local significance. These species are of Eurasian origin and were purposely introduced into the U.S. as an ornamental. These plants form small trees or shrubs sometimes reaching 20-25 feet in height. There are separate male and female plants; the latter producing numerous dark purple berries that are consumed heavily by birds, thus perpetuating dispersal of seeds. Within a few years, one female Buckthorn plant can create thousands of seedlings (IPAW). To further complicate this issue, these seeds can survive for several years in the soil. Therefore, long after Buckthorn plants are physically removed, Buckthorn seedlings may still present themselves (IPAW). One study showed that species richness in a habitat where Buckthorn had been newly removed took an average of two years before improvement was noted; perhaps because Buckthorn inhibits the establishment of native tree seedlings even after these plants are removed (Frappier). The environmental impacts of these species are widespread. Buckthorn species replace native vegetation by disrupting the growth of native seedlings and creating a thick understory growth shading out those species requiring sunlight. These plants destroy wildlife habitat as well, completely upsetting the natural balance of local flora and fauna. In areas heavily disrupted by Buckthorn, few native trees may be found at all (IPAW). Despite these disturbing facts, Buckthorn is still legally sold in the U.S. as an ornamental (IPAW). Fig. 4 Sampling in the field Sorted by morphotype Table 1. Statistical results. Fig. 3 Mound Park with baseline and sample sites The quadrat consisted of a hoop 26 inches in diameter. All leaf litter within the quadrat was collected for processing. Where woody stems compromised sampling (trees in the way), the quadrat was adjusted to obtain a suitable sampling site. Additionally, a digital camera and garbage bags were used for image collection and leaf litter transportation and storage. Trees of North Americawas utilized for species identification. A measuring tape was used for baseline measurement. SPSS statistical software was used for data analysis. Fig. 9 Looking South from the sampling baseline: River Falls References Callaway R.M., Aschehoug E.T. 2000. Invasive Plants Versus Their New and Old Neighbors: A Mechanism for Exotic Invasion. Science. 290: p521- 3. Dukes Jeffrey S., Theoharides Kathleen A. 2007. Plant Invasion Across Space and Time: Factors Affecting Non-indigenous Species Success During Four Stages of Invasion. New Phytologist. 176(2): p256-73. Invasive Plants Association of Wisconsin(IPAW). 2007. Buckthorn Identification and Control. www.ipaw.org Nov. 21, 2007. Frappier Brian, Eckert Robert T, Lee Thomas D. 2004. Experimental Removal of the Non-indigenous Shrub Rhamnus frangula (Glossy Buckthorn): Effects of Native Herbs and Woody Seedlings. Northeastern Naturalist. 11(3): p333-42. Frappier Brian, Eckert Robert T, Lee Thomas D. 2003. Potential Impacts of an Invasive Exotic Shrub Rhamnus frangula L. (Glossy Buckthorn) on Forests of Southern New Hampshire. Northeastern Naturalist. 10(3): p277-96 Knight Kathleen S., Reich Peter B. 2004. Opposite Relationships Between Invasibility and Native Species Richness at Patch Versus landscape Scales. Oikos. 109(1): p81-8. Lavergne Sébastien, Molofsky Jane. 2007. Increased Genetic Variation and Evolutionary Potential Drive the Success of an Invasive Grass. Ecology. 104(10). In the lab, each collection site was processed individually by sorting the leaves. Morphotype groups were determined by leaf size, shape, color, edge type and vein characteristics. I.e.: Oak, Maple, Ash, etc. The number of morphotypes present in each sample represented our data for statistical analysis. Fig. 2 Young female buckthorn branch with fruit Traditional methods of control have included controlled burning (which must be performed annually until native plants re- establish themselves), basal bark treatments by means of chemicals(for heavily invaded areas), the cutting of and treating of stumps(for mildly infected areas), and the use of “weed wrenches” to remove smaller plants (IPAW). These methods have been met with varying degrees of success with the emphasis being on continuous monitoring and treatment of the problem. We chose to study the effect that Buckthorn species have on tree diversity locally; narrowing our study to Mound Park in River Falls because this park offers areas heavily infested with Buckthorn as well as habitat that has only been marginally disrupted for comparison. Our hypothesis was that areas of the park that are relatively undisturbed by Buckthorn would have a greater diversity of tree species than those areas where Buckthorn is present. Analysis of leaf litter, as well as a record of woody species present in the randomly selected areas of the study were used to measure for woody plant diversity. Fig. 6-8 Three study sites. Top: Semi-invaded. Lower Left: Relatively Non-invaded. Bottom Right: completely invaded. Discussion Non parametric statistical tests were used to analyze the data, including: a One Way Anova test and the Kruskal-Wallis test. The results of analysis of the data show a larger abundance of species as well as a greater diversity of species in buckthorn invaded areas while the uninvaded areas show a smaller abundance as well as a much more limited species diversity (table 1). Using a significance level of 0.05, our p-value of 0.303 and a significance level of 0.402 showed no significant statistical difference between the woody plant diversity of the two study sites. Leaf litter pre-sort Acknowledgements We would like to thank Dr. John Wheeler and Dr. John Ford for their advice and expertise.