Masting synchrony in northern hardwood forests: Super-producers dominate population fruit production

Background/Question/Methods

Peaks in fruit production via interannual masting cycles are a common phenomena in tree populations. Masting cycles require some level of synchronization in fruit production among individuals. The level of synchrony is largely unknown because most studies of masting measure fruit production at a population level, rather than for individuals. By measuring individual fruit production, I investigated the degree to which individuals were synchronized, and how much fruit each individual contributed to a mast event. Through direct canopy counts, I estimated the number of fruit produced by 1635 individuals of 10 temperate tree species across 4 years in northern lower Michigan. I also used a longterm dataset of seed and seedling density on the forest floor to place the individual tree data into a broader context of population mast cycles. 

Results/Conclusions

Three species had mast events during the 4-year period of the study, in addition to some minor peaks in fruit production in these and other species. In Acer saccharum and Fagus grandifolia, which had multiple peaks in reproduction, a small subset of the population reproduced more often and was responsible for a large portion of the total number of fruit produced in the population. In Tilia americana, different subsets of individuals were responsible for multiple peaks in reproduction. This suggests that it is important to incorporate individual measures of fruit production when investigating mast cycles, as small subsets of trees may dominate population patterns. Larger trees tended to produce more fruit in all species and were more likely to be reproductively mature, although even large trees often produced zero or few fruit. There were no direct effects of nutrient availability or competition on fruit production. Although the cause is unclear, in some species a small number of trees dominate the number of fruit produced and the frequency of mast cycles. By producing more fruit, more often than their conspecifics, these individuals increase their chance of successfully producing offspring over time.