The continuum of masting behavior and its relation to seed predation and survival in temperate trees

Background/Question/Methods

Temperate tree species exhibit interannual and spatial variation in fecundity, and this variation has important consequences for forest dynamics. A continuum of masting responses can exist, from relatively constant annual fecundity rates to highly variable and synchronous fecundity. A high degree of masting could provide seed predator avoidance through predator satiation. In this study, we quantified the degree of reproductive variation in 38 tree species to test whether variation is as predicted by the predator satiation hypothesis. For a subset of large-seeded species in the genera Carya, Cornus, Nyssa, and Quercus, we tested whether variation in seed predation and survival rates vary with fecundity. We used data from 19 years of forest inventory and seed collection at 13 experimental forest plots in North Carolina to estimate individual tree fecundities. We then calculated population-level coefficients of variation (CV_p) and indices of synchrony in fecundity, quantified as population Pearson correlation between individuals (r). Finally, we used a logistic regression to test whether seed predation rates by insects, birds, or rodents declined with site-level seed count data, as predicted by the predator satiation hypothesis, and whether this leads to greater seed survival through initial dispersal to seed baskets.

Results/Conclusions

Preliminary results were consistent a continuum of masting responses, and we found limited support for predator satiation. Both CV_p and r differed by species and site, ranging from 0.01 to 3.34 and from 0.01 to 0.83, respectively. For large-seeded species included in the predation analysis, CV_p was often greater than 1.  Individual correlations r for Carya, Cornus, and Nyssa species were high (0.27 to 0.59) and still positive, but of lower magnitude for Quercus (0.13 to 0.19). Thus, these species exhibited variation consistent with higher degrees of masting, though synchrony in Quercus species was unexpectedly low. Seed predation rates by birds and rodents increased in masting years for most species, but not for C. florida. By contrast, all Quercus species except Q. velutina showed weak negative trends in insect seed predation rates in masting years. Survival of mature seed through initial dispersal declined 10 to 40% in masting years for most species. Conversely, Q. alba showed 20% increase in seed survival in masting years. Still unclear is the degree to which other factors interact with variations in fecundity to influence seed survival and how these rates differ in relation to CV_p and  for each population.