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Kate Bradley Do herbivory rates differ for oak
seedlings in There are many ecological factors that are likely to influence the success of tree invasion, but few experimental studies involving trees. I examined herbivory as a potential mechanism limiting pin and bur oak invasion at a site in Minnesota, where trees have been slow to invade abandoned fields. There is substantial evidence that deer [1, 2] and gophers [3] are limiting tree recruitment, but insects have not been examined in this context. These old fields are primarily composed of C4 grasses, which have a lower nitrogen and higher lignin content compared to seedlings, whereas in the oak-savanna there are more plant species with higher tissue quality. This lead me to hypothesize that herbivory rates on seedlings could be higher in old fields than in oak-savannas. Alternatively, the presence of C4 grasses might reduce overall herbivore load and reduce herbivory on seedlings. I wanted to know if herbivory rates differed for oak seedlings in different plant neighborhoods and if the rates differed between pin oaks (Quercus ellipsoidalis) and bur oaks (Q.macrocarpa). Methodology: I surveyed seedlings in old fields & adjacent oak-savannas, at 4 sites across Cedar Creek. Two areas had never been burned, and two were burned periodically. After determining seedling density, 20 seedlings of each species were marked in the old fields and oak savannas. Leaf number, number of leaves damaged, percent leaf area damaged, height, and stem width were measured 3 times over the summer. Canopy density, soil moisture, and the percent water in the leaf tissue were also measured. We also determined the dominant vegetation types in each old field and oak savanna. Preliminary Results: Basically I have found that pin oaks experience twice as much herbivory in the oak savanna (15 % leaf area damaged) as they do in the old fields (7% leaf area damaged). Burr oaks experience about the same amount of herbivory in either location. This pattern is consistent across the sampling dates. Future work: I do not know what is causing this pattern. Is it driven by one group of trees? How does their location in relation to each other affect herbivory? Are their specific differences between the 4 sites? Spatial distribution seems to be an important factor to consider for herbivory, since insects are not out their operating haphazardly. I plan to establish X-Y coordinates for each tree and look at their spatial distribution in relation to each other and the edges of the fields. Also, there may be an important pattern of the abiotic variables that is influencing the pattern of herbivory. 1. Inouye, R.S., et al., Pocket gophers (Geomys bursarius), vegetation, and soil nitrogen along a successional sere in east-central Minnesota. Oecologia, 1987. 72: p. 178-184. 2. Lawson, D., et al., Patterns of woody plant abundance, recruitment, mortality, and growth in a 65 year chronosequence of old-fields. Plant Ecology, 1999. 145: p. 267-279. 3. Inouye, R.S., T.D. Allison, and N.C. Johnson, Old field succession on a Minnesota sand plain: Effects of deer and other factors on invasion by trees. Bulletin of the Torrey Botanical Club, 1994. 121: p. 266-276. 4. Tilman, D., et al., Biodiversity and ecosystem properties. Science, 1997. 278: p. 1866-1867.
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