The European Buckthorn Project

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What Factors Influence the Invasion of Rhamnus cathartica?

INTRODUCTION
Biological invasions present the single greatest threat to North American eastern deciduous forests (Vitousek et al. 1996) and the second leading threat to biodiversity across all ecosystems in the United States (Wilcove et al. 1998). Annually, invasive species cost the U.S. an estimated $138 billion (Pimentel 2000). A better understanding of factors that affect biological invasions is important for better prediction and prevention of these costly phenomena.

The importance and success of buckthorn (Rhamnus cathartica) make it an interesting species with which to study the factors that influence invasion. R. cathartica is a shrub or tree that is native in many areas of Europe (Godwin 1943). It was introduced to North America through planting as an ornamental shrub beginning in the mid-1800s (Possessky et al. 2000) and became naturalized in many areas (Gourley 1985). It invades forests, becoming the dominant understory vegetation in some cases, and it has been identified as a major threat to native biodiversity (Catling 1997).

OBJECTIVE
My research focuses on three factors that influence invasion, using a combination of field observations and controlled experiments, which will allow me to determine causal relationships between factors and apply them to current landscape patterns of invasion.

1. Does native species richness affect invasion of R. cathartica?
2. Does light, a limiting resource in forest understories, affect invasion of R. cathartica?
3. Does R. cathartica facilitate its own invasion through interactions with the soil microbial community and effects on abiotic soil characteristics? Could this facilitation allow R. cathartica to tolerate lower light conditions than it does in its native range?

1. EFFECTS OF SPECIES RICHNESS AND LIGHT
To observe the relationship between invasion, species richness, and light in a forest system, I first conducted a survey of Minnesota forests invaded by R. cathartica. This survey revealed a scale-dependent relationship: within small areas (1m2) species richness and R. cathartica invasion were negatively related, while across large areas, a positive relationship existed (Figure 1) (Lacasse, submitted to Oikos, February 2004). These results are consistent with results of other surveys (Stohlgren et al. 1998; Stohlgren et al. 1999; Brown & Peet 2003), small-scale experiments (Tilman 1997; Knops et al. 1999; Dukes 2000; Naeem et al. 2000; Kennedy et al. 2002), and theory (Shea & Chesson 2002). The survey also revealed a positive relationship between R. cathartica invasion and light within small areas (consistent with results of small-scale experiments of Burke & Grime 1996; Davis & Pelsor 2001), and a negative relationship between R. cathartica invasion and light heterogeneity across large areas. This survey clearly showed that landscape patterns of invasion, species richness, and light are correlated, but as a survey, it could not reveal causal relationships.

To quantify the effect of species richness and light on the invasion of R. cathartica, I am conducting a field experiment in which R. cathartica seedlings are grown with native species in four species richness levels across varying light levels. This experiment tests for causal relationships among invasion success, resource levels, and species richness, which will complete the answer to my first two research questions. Invasion success will be measured as R. cathartica germination, survival, cover, and biomass, as well as the biomass and number of species of “weeds” (other forest plants) that invade my plots. The experiment partitions competition effects into aboveground and belowground components, which will aid in the identification of limiting resources. It also tests for influences of mature trees of R. cathartica on young buckthorn plants, addressing my fourth major research question.

The experiment is being conducted at the Warner Nature Center in Minnesota, where R. cathartica is invading native oak forest. I located 24 plots with varying light levels, and removed existing plants from the plots. In each of 18 of the plots, all > 10m from mature trees of R. cathartica, six subplots were created and subjected to the following treatments:

1. High diversity: 10 species (10 plants) were planted
2. Medium diversity: 6 species (10 plants) were planted
3. Low diversity: 3 species (10 plants) were planted
4. No competition: no plants were planted
5. Shade: no plants were planted, plot shaded with 50% shadecloth
6. Belowground competition: 10 species (10 plants) were planted, shoots were tied back

The remaining six plots were located <5m from a mature R. cathartica tree, and received only the high diversity and no competition treatments in their subplots. The species planted were all native perennial forest herbs. Seeds of R. cathartica were planted in the center of each subplot. The experiment will continue through summer 2005, and I will monitor R. cathartica germination and growth, weed invasion, soil moisture, soil fertility and light levels each summer. Because R. cathartica’s phenology allows it to gain carbon while other plants are leafless, light, moisture, and percent cover of native plants will be measured early in the spring when R. cathartica leaves are first present, in mid-summer when all plants have leaves, and late in the fall when only R. cathartica’s leaves remain. At the end of the experiment, all R. cathartica plants will be harvested, dried and weighed. A MANCOVA analysis will be used to show whether species richness, resource levels, and large buckthorn shrubs contribute to invasibility.

Predictors Responses (measurements of invasibility)
light above subplot – 3 seasons (continuous) buckthorn germination rate
light to buckthorn – 3 seasons (continuous) buckthorn height
presence of mature buckthorn (categorical) buckthorn number of leaves
treatment (categorical) buckthorn biomass
soil moisture – 3 seasons (continuous) weed biomass
soil fertility (continuous) weed species richness
cover of native plants – 3 seasons (continuous)
block by plot (categorical)

Data from the first summer of this experiment showed that the R. cathartica seeds I planted had a higher germination rate in the plots near mature R. cathartica trees (Figure 2) (p<0.001 controlled for propagule pressure from the mature R. cathartica). Perhaps the high leaf nitrogen of R. cathartica leaves (Reich, unpublished) is elevating soil fertility. My measurements of soil fertility next summer will show whether this hypothesis is plausible. Or perhaps the soil microbial community that builds up near R. cathartica trees is beneficial for their seedlings.

2. EFFECTS OF LIVING ORGANISMS
The diverse community of soil microbes can have positive or negative net effects on plants. Different species of plants build up different microbial communities, consisting of pathogens and mutualists (Lovelock and Miller 2002; Bever 1994; Klironomos 2002). Invasive plants may escape their host-specific pathogens, yet reap the benefits of associating with generalist mutualists, such as AM fungi. Evidence for this process comes from an experiment with Prunus serotina, which is native to North America and invasive in Europe. Reinhart et al. (2003) found that in North America, P. serotina develops a soil community that inhibits conspecific seedlings, while in Europe, it develops a beneficial soil community that facilitates the growth of seedlings.

I hypothesize that invasive plants can tolerate lower-resource conditions due to this positive soil feedback in their invaded ranges. Specifically, I will test whether R. cathartica seedlings can survive in lower-light conditions due to soil microbial communities that adult trees develop in their invaded ranges. Although R. cathartica invades forest interiors in North America, it is almost exclusively found along forest edges in Europe (Luke Skinner, pers. comm.)

I will locate three forested areas in Europe and Minnesota with similar soil characteristics. Soil near the base of 10 mature R. cathartica trees will be collected and combined for each forest. A control treatment will be created by pasteurizing soil to kill any soil organisms. The soil will be used to inoculate pasteurized soil collected from a Minnesota site (this will eliminate differences in fertility and other abiotic soil characteristics), and placed in cone-tainers. R. cathartica seeds from North American and European populations will be planted, and the cone-tainers will be placed in growth chambers. Three light-level treatments, 2%, 6%, and 10% of full sunlight, will be applied in a factorial design. The lowest and highest light levels mimic light levels found in maple and oak forests, respectively. Each treatment will be replicated 10 times, for a total of 720 cone-tainers. The germination, height, number of leaves, and biomass of the plant will be recorded. After these measurements are made, plants will be examined for infection by soil symbionts.

ANOVA (see summary below) will be used to determine whether R. cathartica exhibits negative soil feedback in its home ranges and positive soil feedback in its invaded ranges, and whether this feedback allows them to tolerate lower light levels in invaded ranges. Better growth in live North American soil than sterilized soil will indicate a positive net effect of the soil microbial community in R. cathartica’s invaded range, and better growth in sterilized soil than live European soil will indicate a negative net effect of the soil microbial community in R. cathartica’s native range. A comparison of growth between North American and European live soil at each shade level will show whether R. cathartica can grow at lower light levels in North America due to interactions with soil microbes. The alternative hypothesis, that genetic differences between North American and European R. cathartica populations account for differences in shade tolerance, will be tested by comparing the performance of these two populations in different shade levels.

ANOVA Predictors
Continent of soil sample (North America or Europe)
Site of Collection (three forests on each continent)
Light Level (2%, 6%, or 10% of full light)
R. cathartica genotype (North American or European)
Soil Treatment (live or pasteurized)

I have obtained the permit application for importing European soil for this experiment and spoken with USDA officials about regulations for the use of this soil. I have contacted taxonomists at the Institute of Dendrology in Poland and identified potential sites in Poland for sampling soil from European forests.

WORK CITED
Bever, J. D. 1994. Feedback between plants and their soil communities in an old field community. Ecology 75(7):1965-1977.

Brown, R. L., and R. K. Peet. 2003. Diversity and invasibility of southern Appalachian plant
communities. Ecology 84(1):32-39.

Burke, M. J. W., and J. P. Grime. 1996. An experimental study of plant community invasibility.  Ecology 77(3):776-790.

Catling, P. M. 1997. The problem of invading alien trees and shrubs: some observations in Ontario and a Canadian checklist. Canadian Field Naturalist 111:338-342.

Davis, M. A., and M. Pelsor. 2001. Experimental support for a resource-based mechanistic model of invasibility.  Ecology Letters 4:421-428.

Dukes, J. S. 2001. Biodiversity and invasibility in grassland microcosms. Oecologia 126:563-568.

Godwin, H. 1943. Biological Flora of the British Isles: Rhamnaceae. Journal of Ecology 31:66-92.

Gourley, L. C. 1985. A Study of the Ecology and Spread of Buckthorn (Rhamnus cathartica L.) with Particular Reference to the University of Wisconsin Arboretum. Madison, WI: Dept. of Landscape Architecture, University of Wisconsin Madison, USA.

Kennedy, T. A., S. Naeem, K. M. Howe, J. M. H. Knops, D. Tilman, and P. Reich. 2002. Biodiversity as a barrier to ecological invasion. Nature 417:636-638.

Klironomos, J. N. 2002. Feedback with soil biota contributes to plant rarity and invasiveness in communities. Nature 417:67-70.

Knops, J. M. H., D. Tilman, N. M. Haddad, S. Naeem, C. E. Mitchell, J. Haarstad, M. E. Ritchie, K. M. Howe, P. B. Reich, E. Siemann, and J. Groth. 1999. Effects of plant species richness on invasion dynamics, disease outbreaks, insect abundances and diversity. Ecology Letters 2:286-293.

Lovelock, C. E., and R. Miller. 2002. Heterogeneity in inoculum potential and effectiveness of arbuscular mycorrhizal fungi. Ecology 83(3):823-832.

Naeem, S., J. M. H. Knops, D. Tilman, K. M. Howe, T. Kennedy, and S. Gale. 2000. Plant diversity increases resistance to invasion in the absence of covarying extrinsic factors. Oikos 91(8):97-108.

Pimentel, D., L. Lach, R. Zuniga, and D. Morrison. 2000. Environmental and economic costs of nonindigenous species in the United States. BioScience 50(1):53-65.

Possessky, S. L, C. E. Williams, and W. J. Moriarty. 2000. Glossy Buckthorn, Rhamnus frangula L.: A threat to riparian plant communities of the northern allegheny plateau (USA). Natural Areas Journal 20(3):290-292.

Reinhart, K. O., A. Packer, W. H. Van der Putten, and K. Clay. 2003. Plant-soil biota interactions and spatial distribution of black cherry in its native and invasive ranges. Ecology Letters 6:1046-1050.

Shea, K., and P. Chesson. 2002. Community ecology theory as a framework for biological invasions. TREE 17(4):170-176.

Stohlgren, T. J., D. Binkley, G. W. Chong, M. A. Kalkhan, L. D. Schell, K. A. Bull, Y. Otsuki, G. Newman, M. Bashkin, and Y. Son. 1999. Exotic plant species invade hot spots of native plant diversity. Ecological Monographs 69(1):25-46.

Stohlgren, T. J., K. A. Bull, Y. Otsuki, C. A. Villa, and M. Lee. 1998. Riparian zones as havens for exotic plant species in the central grasslands. Plant Ecology 138:113-125.

Tilman, D. 1997. Community invasibility, recruitment limitation, and grassland biodiversity. Ecology 78(1):81-92.

Vitousek, P. M., C. M. D’Antonio, L. L. Loope, and R. Westbrooks. 1996. Biological invasions as global environmental change. American Scientist 84:468-478.

Wilcove, D. S., D. Rothstein, J. Dubow, A. Phillips, and E. Losos. 1998. Quantifying threats to imperiled species in the United States. BioScience 48(8):607-615.

FIGURES

Figure 1. Multiple regression analysis revealed a negative relationship between species richness and R. cathartica invasion at the small scale and a positive relationship at the large scale. These are the leverage plots from those analyses.

Figure 2. R. cathartica germination was higher in plots near mature R. cathartica trees (p<0.001).

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