Cone maturation phenology, seed dispersal, and eastern red cedar range expansion: It's all about timing!

Background/Question/Methods

Eastern red cedar (Juniperus virginiana; ERC) is native to eastern North America, but is expanding into grassland and prairie ecosystems of the midwestern United States. ERC produces small light-green cones during spring that mature over the next 6-8 months. As cones mature, they become deep-purple or blue, indicating changes in seed viability and nutrient content. Ultimately, these changes act as a visual signal to attract foraging birds and mammals, driving ERC range expansion through seed dispersal in late-fall and winter when other food sources are scarce. After fecal deposition, these seeds may germinate, likely allowing colonization of new sites. Thus, successful seed dispersal will depend upon temporal overlap between foraging and viable seeds. Individual foraging groups differ in when they are present and feeding, with resident birds and mammals present year-round, nomadic foragers occasionally present as they track food availability, and migratory species present only during spring/fall migrations. To understand these interactions, we characterized ERC cone phenology and seed availability/viability monthly while simultaneously documenting foraging by birds and mammals using wildlife cameras. Using these data, we seek to understand ERC range expansion by defining the relationship between plant phenology and the timing of animal-mediated seed dispersal.

Results/Conclusions

For successful seed dispersal, temporal overlap of dispersers and ERC trees that have cones containing mature/viable seeds is critical. Cones form in April, shifting from green to purple during August-October. At this point, seeds are viable and have desirable proteins and sugars/starches for dispersers. While sugar/starch levels are relatively consistent regardless of cone color, protein content declines significantly from early green cones to purple/blue cones. Once cones are blue, maturation is complete and seeds are viable. Given these data, foragers gain similar rewards whenever cones are available. However, based upon wildlife camera data, bird foraging is most common when cones are purple/blue, and thus, most visible (September-February). Given this, all three feeding groups (residents, migrants, nomads) have potential to disperse viable seeds, with resident species foraging for the entirety of cone presence, migratory species foraging during fall and spring, and nomadic species foraging sporadically as resource availability changes (November-February). By understanding the importance of temporal overlap between disperser presence (and visual cues they use for foraging) and seed viability, mitigation of ERC range expansion into new areas may become possible. Ultimately, it is critical to reduce establishment of ERC to avoid negative impacts on native plants.