1. The Effect of a Nitrogen-Fixing Tree (Robinia pseudoacacia) on the Productivity and Diversity of Understory Shrub Communities. Redding, K. Michael* and Karl W. Kleiner, Department of Biological Sciences, York College of Pennsylvania, York, PA 17405. INTRODUCTION Species diversity in plant communities has traditionally been identified with the availability of abiotic resources and the competition for those resources (Tilman and Pacala 1993). Nitrogen is one resource that has been demonstrated to alter competitive interactions and thus the composition of plant communities, often by reducing species diversity (Wilson and Tilman 1991). Black Locust (Robinia pseudo-acacia) is one of a few leguminous nitrogen fixing tree species that has been demonstrated to return substantial quantities of nitrogen to the soil (Boring et al. 1981; Kleiner et al. 2000) and therefore increase the growth of neighboring tree species (Boring and Swank 1984). Our hypothesis was: Since black locust trees add nitrogen to the soil, there should be greater productivity and lower diversity of shrubs around black locust trees than those around yellow popular (Liriodendron tulipifera), a non-nitrogen-fixing tree species. ABSTRACT The availability of soil nitrogen has been demonstrated to alter competitive interactions among plants and thus, the composition of a plant community. The black locust tree (Robinia pseudoacacia) is a leguminous nitrogen-fixing tree species that contributes nitrogen to the soil. We hypothesized that the shrub communities growing near black locust trees would have greater growth productivity and lower diversity than those growing around a non-nitrogen fixing tree species, yellow poplar (Liriodendron tulipifera). Shrub density around black locust trees did not differ from those around yellow poplar trees (P = 0.22). Additionally, there was no difference in shrub basal area (P = 0.29). The Shannon diversity index was slightly lower for shrubs around black locust trees (P = 0.15). These results are consistent with the known effects of anthropogenically added nitrogen to forested communities. ACKNOWLEDGEMENTS Tina Redding – Assisted with data collection METHODS We identified and recorded all shrub individuals with a dbh of less than 5 cm around 11 black locust trees (37-70 years old) and 11 yellow poplar trees (Liriodendron tulipifera) (39-65 years old), located throughout a 500+ acre forested area in northern York County, PA. Yellow poplar trees, the most abundant non-nitrogen fixing tree species in the forest, were at least 10 meters from any black locust tree. Rectangular transects (15 meters long by 5 meters wide) were established to the northeast and southwest of each tree in the study (Figure 1). Diameter of each individual shrub was measured using a Vernier Caliper. Shrubs consisting of many stems were measured and counted as one individual. Distance from the tree was recorded for each shrub. 1)Shrub density was compared between black locust and yellow poplar using an unpaired t-test (Graph Pad Prism ver. 3.02). 2)To determine if the amount of shrub coverage differed between black locust and yellow poplar, we compared the basal area (cm 2 /m 2 ) using an unpaired t-test (Graph Pad Prism ver. 3.02). 3)To determine if shrub diversity differed between black locust and yellow poplar, we compared Shannon Diversity Indices using an unpaired t-test (Brower et al. 1997). We graphically evaluated species diversity by constructing relative abundance-curves for shrubs surrounding black locust and yellow poplar. The log of mean relative abundance was plotted on the ordinate and species rank was plotted on the abscissa. Communities with low species diversity will have steeper slopes than those with greater diversity. 15 m 5 m N Figure 1. Northeast and southwest establishment of rectangular transects. DISCUSSION Our hypothesis that the nitrogen fixing capabilities associated with black locust might influence diversity and productivity in shrub communities was not supported. There was no overall difference between diversity and productivity of shrub communities surrounding black locust and yellow poplar trees. The lack of significant differences in shrub communities may be attributed to one or more of the following possibilities: 1)The amount of nitrogen that black locust adds to the soil may not be enough to affect community composition. As a stand ages, nitrogen is increasingly removed from the soil and stored in the biomass, perhaps making it unavailable to influence the vegetative dynamics (Boring and Swank 1984). 2)Nitrogen additions may have greater influence over herbaceous communities compared to woodland communities. Moreover, nitrogen additions may have greater influences over annual plants than perennial plants (Wilson and Tilman 1991). 3) Some shrub species are more likely to respond to nitrogen additions than others (Wilson and Tilman 1991; Falkengren-Grerup 1993; and Hurd et al. 1998). In this regard, the relative abundance of spicebush and mapleleaf viburnum do differ substantially. LITERATURE CITED Boring, L.R., Monk, C.D., Swank, W.T. 1981. Early regeneration of a clear-cut southern Appalachian forest. Ecology 62:1244-1253. Boring, L.R., Swank, W.T. 1984. The role of black locust (Robinia pseudo-acacia) in forest succession. Ecology 72:749-766. Brower, James E., Zar, Jerrold H., and von Ende, Carl N. 1997. Field and Laboratory Methods for General Ecology. 4 th ed. WCB McGraw- Hill, Boston, MA. Falken-Grerup, Ursula. 1993. Effects on beech forest species of experimentally enhanced nitrogen deposition. Flora. 188:85-91. Hurd, T.M., Brach, A.R., Raynal, D.J. 1998. Response of understory vegetation of Adirondack forests to nitrogen additions. Canadian Journal of Forest Research. 28:799-807. Kleiner, K. W., J. M. Mann and W.D. Eaton. 2000. Does it matter who your neighbor is? The effect of black locust (Robinia pseudoacacia) on the growth of neighboring trees. Journal of the Pennsylvania Academy of Science 73:163 (abstract) Tilman, D., Pacala, S. 1993. The maintenance of species richness in plant communities. Species Diversity in Ecological Communities: Historical and Geographical Perspectives. University of Chicago Press, Chicago, Illinios. Wilson, S.D. and Tilman, D. 1991. Interactive effects of fertilization and disturbance on community structure and resource availability in an old-field plant community. Oecologia 88:61-71. RESULTS 1)Mean density of shrubs did not differ between black locust (1.832 stems/m 2 ) and yellow poplar (1.457 stems/m 2 ) (Figure 2; P = 0.22) 2)Mean basal area of shrubs did not differ between black locust (438.7 cm 2 /m 2 ) and yellow poplar (341.3 cm 2 /m 2 ) (Figure 3; P = 0.29). 3)Mean Shannon Diversity Index was slightly lower for black locust (H’ = 1.662) plots compared to plots around yellow poplar (H’ = 1.953) (Figure 4; P = 0.15). The slope of the relative-abundance curve for black locust was slightly steeper and shorter than that of yellow poplar (Figure 5). This result is indicative of communities with lower species equitability and lower species richness. Less common species were more equitable in abundance around yellow poplar than around black locust. The most abundant shrub species differed between black locust and yellow poplar. Spicebush was the most abundant species around black locust, whereas the most abundant species around yellow poplar was mapleleaf viburnum (Table 1). The mean relative density of spicebush was nearly twice as great around black locust than around yellow poplar. However, the mean relative density of mapleleaf viburnum was nearly one-tenth as great around black locust as around yellow poplar.