Spatio-temporal analyses of seed production and resource depletion in three forest tree species
Mast-seeding is the synchronous production of seeds by plants in a population, at supra-annual intervals. Although its’ effects on fitness have been well-documented, the proximate physiological mechanisms of mast-seeding are less well understood. One hypothesis (known as the Resource Budget Model, RBM) is that mast seeding arises as a consequence of resource allocation within individual plants: If seed production depletes a plant’s stored resources below the threshold needed for seed production in the next year, then reproduction by individual plants is cyclical or mathematically chaotic. If seed production by individuals is cyclical or chaotic, populations may be synchronized by environmental drivers or density dependent pollen limitation. Although this phenomenon is now well-established in theoretical models, the model has only been used to analyze data for a handful of plant species. Here, we analyze support for the RBM using time series of seed production for two oak species, red oak and white oak (Quercus rubra and Q. alba) in Pennsylvania (USA), and of seed production and sap sugar (a metric of stored resources) for a third tree species, sugar maple (Acer saccharum) in central Massachusetts (USA).
We analyzed a 15-year time series of seed production for red and white oaks at three sites, using methods adapted from Rees’ et al. (2002; AmNat 160: 44–59), to include variation among sites and individual trees. Results indicate that resource depletion coefficients in red oaks do not differ among sites, and do not fall in the cyclical or chaotic regions of parameter space. However, white oak dynamics differed among sites, and resource depletion coefficients for 22% of individual trees fell within the cyclical/chaotic region of parameter space. This pattern is consistent with the general notion that white oak acorns benefit more from mast-seeding because they are more palatable to granivores, and less likely to be cached because of their early germination, compared to red oaks, which have an extended period of seed dormancy. Sugar maple seed and sugar time series were consistent with the general notion that high seed years are followed by lower sap sugar. However, sugar maple dynamics did not fall in the cyclical or chaotic region of parameter space, based on monitoring of seeds and sap from 2011-2014. As time permits, we will update the sugar maple analysis with 2015 sap data, and explore the potential roles of individual and environmental variation for seed production dynamics of all three tree species.