Tree Species' Fruit Production Respond Differently along Soil Resource Gradients in Northern Hardwood Forests

David M. Minor* and Richard K. Kobe
Michigan State University
Department of Plant Biology

Fruit production is critical to tree species composition, presenting one of the first bottlenecks to regeneration. Reproductive output may not only be affected by the size of the individual but also by abiotic and biotic factors. Among these factors is the soil nutrient environment, which is changed by anthropogenic nutrient addition, such as nitrogen (N) deposition, having the potential to alter reproductive output. The goal of this study is to investigate how soil nutrients, along with characteristics of individual trees and their neighbors, influence fruit production among 11 northern hardwood forest species. To examine these factors, I visually measured fruit production in the crowns of approximately 1700 trees located across a natural fertility/productivity gradient in northwest lower Michigan over two growing seasons. This method allows for a measure of fruit production on individual trees prior to fruit dispersal and predation, more closely approximating reproductive effort than seed rain or seedling density. Influences on fruit production were tested by calibrating individual-based models of fruit production as functions of tree size, neighborhood crowding, local conspecific dominance, and soil resource availability (N, base cations, and phosphorus). The smallest diameter at which fruit production occurred varied by species, ranging from 10.2 cm in Acer rubrum to 28.6 cm in Fraxinus americana, but was not related to species shade tolerance or soil fertility association. For the seven species with substantial reproductive activity, diameter was a significant predictor of individual two-year fruit production, with the relationship being strongest in Fagus grandifolia. However, a great deal of the variation in fruit production remained unexplained, and many individuals did not reach the production level that would be predicted by diameter. In Quercus rubra and Q. velutina, which were treated together since they naturally hybridize, conspecific relative basal area, soil nitrate, N mineralization rate, and soil ammonium were positively correlated with fruit production, suggesting that pollination efficiency and N soil resources are important for reproduction. In four other species fruit production was weakly correlated with at least one soil resource, but no species were correlated with neighborhood density (the number of trees within 5 m of each individual). These results demonstrate that soil resources, especially N, are more strongly associated with fruit production in certain tree species. Because species have different fruit production responses across a soil fertility gradient, changing nutrient levels due to N deposition may cause different responses among species, potentially altering forest community composition.

* Corresponding Author: 3328 Trappers Cove Trail, Apt. 1C, Lansing, MI 48910 Phone: (734) 748-5114 Email: