Recruitment networks in expanding forest populations mediated by dispersal mutualisms

Background/Question/Methods

The persistence and expansion of remnant plant populations ultimately depends on their ability to disperse their propagules and the genes they harbour. This represents a formidable challenge to forest species that frequently rely on frugivorous animals to disperse their seeds. The arrival of molecular markers to plant ecology unfold the role of dispersal mutualisms in shaping dispersal kernels and promoting long distance dispersal events. Yet, if dispersal mutualisms influence population dynamics and evolutionary trajectories of plant populations they should influence both the early and the late demographic stages. Still, with most empirical work confined to the earliest demographic stages, our appreciation of the dynamic and genetic outcomes of dispersal mutualisms remains partial.

Here I investigate the colonization dynamics of an endozoochorous species, Phoenicean juniper (Juniperus phoenicea subsp. turbinata), by depicting recruitment networks, i.e., groups of related adult individuals and their dispersed offspring across the colonization front in an expanding population. Specifically I pursue to: (i) Asses the effective dispersal distances; (ii) Depict recruitment networks entailing remnant patches and recently colonized patches; and (iii)Decipher the role of different plant traits and landscape features in assembling recruitment networks.

Results/Conclusions

Preliminary results suggest that recruitment networks comprise non-random subsets of reproductive individuals across the landscape.  These are few large and highly fecund individuals located at the core of remnant patches. Additionally, we found a sizeable contribution of young and peripheral individuals located nearby perch structures, such stone pines or conspicuous dead trees. Both results related to the foraging preferences of the medium-sized frugivorous birds providing dispersal services. Adult trees contributed progeny from various locations and I assessed a high frequency of long distance effective dispersal events (> 40%) recently colonized sites. Consequently, I detected increased genetic diversity at the colonization front compared to sites located at remnant fragments. Yet, stronger spatial genetic structure emerged at colonized sites compared to remnant sites, likely because a severe self-thinning (non-random mortality of juveniles) in mature forests erases the effect of group dispersal (i.e., the observed trend of maternal progeny to be jointly dispersed). These findings pointed out that dispersal mutualisms had lasting and measurable spatial, demographic, and genetic effects at the late recruitment stages, as expected. Yet, the action of post-dispersal processes produced contrasting spatial genetic patterns compared to those frequently described at the early demographic stages.