Soil nutrient and neighborhood competition influences on northern hardwood reproduction

Background/Question/Methods 

Fruit production is essential for forest regeneration, and ultimately helps shape future community composition. Reproductive output may be affected by many factors, including the species and size of the individual, soil nutrient availability, and local competition. If reproduction is limited by the same nutrient across species, then competition for this resource may constrain the amount of fruit produced. However, fruit production may be limited by different nutrients in each species, thus reducing heterospecific competition. To examine the effect of these factors on tree reproduction, we visually measured fruit production in crowns of approximately 1700 trees of ten northern hardwood species located across a natural fertility/productivity gradient in northwest lower Michigan over four years. Fruit production for individual trees was measured prior to fruit dispersal, thus more closely approximating reproductive effort than seed rain or seedling density. Influences of tree size, neighborhood crowding, and soil resource availability on fruit production were tested by fitting individual-based models for each species.

Results/Conclusions

For all species, larger trees produced more fruit, and were more likely to produce fruit. Soil resources were also related to fruit production for each species. The species with the strongest relationships with soil resources were Acer rubrum, A. saccharum, Fraxinus americana, Quercus rubra/velutina, and Tilia americana, in which higher calcium (Ca) levels were associated with greater fruit production. In F. americana fruit production was also positively correlated with potassium (K). For Fagus grandifolia and T. americana reproduction was positively correlated with ammonium (NH₄) availability, but more weakly than other species' relationships with Ca. These associations suggest that base cations may be a more limiting resource to reproduction than nitrogen (N), which is typically a limiting nutrient for growth in temperate forests. Furthermore, these species may have different nutrient requirements for producing fruit. Six species also had weak negative relationships with neighborhood density. Overall, our results support that both species-specific nutrient requirements and competition (for light or shared limiting nutrients) influence fruit production in temperate forest species.