Ecological sources of variation among individuals in cone production in Pinus palustris Mill. (Pinaceae)

Background/Question/Methods Reproductive output varies considerably among individuals within plant populations, and this is especially so in cone production of conifers. While this variation can have substantial effects on population growth and effective population size, little is known about its magnitude or causes. Moreover, while reproductive output typically is related to plant size, the strength of this relationship is understudied. We studied variation in cone production for 2008 and 2009 within a population of Pinus palustris Mill. (longleaf pine; Pinaceae) in Tampa, FL. We counted mature cones on trees of varying size (DBH), Canopy Status (open, dominant, subordinate), number of conspecific neighbors within 4 m (N4), and Locations with differing long-term fire histories. Using a mixed model approach, we evaluated the importance of Location, DBH, Canopy Status, number of conspecific neighbors within 4 m (N4), and Year.

Results/Conclusions Larger trees tend to produce more cones, but there are substantial interactions between DBH, Canopy Status, and N4.  Among trees in the open, DBH has little explanatory power. Among trees with neighbors, the effect of DBH depends on Canopy Status and N4. Subordinate trees with three neighbors produced no cones. DBH alone is a weak predictor of cone production. Interactions with neighbors play an important role in generating reproductive heterogeneity, and must be accounted for when relating cone production to size. Cone production of individuals was strongly correlated between years. Perhaps more importantly, the residuals (after fitting the best-supported model for cone production) were correlated between years. Trees producing more cones than expected (given their DBH, Canopy Status, N4, and Location) in 2008 also produced more than expected in 2009, suggesting that factors like genetic or microsite variation may play an important role in variation in reproductive output in this population.