Pollination limitation, sex allocation, and masting in whitebark pine

Background/Question/Methods

Masting, the synchronous and episodic production of seed crops, is thought to benefit plant fitness through positive density dependent effects on pollination, dispersal, and seed survival. Of these three, only increased pollination efficiency in mast years can be a proximate mechanism for masting by synchronizing reproductive effort across individuals within a population under conditions hypothesized by the resource budget model of masting. This will occur most efficiently if investment in male and female function increases synchronously during mast years. Sex allocation theory, however, predicts that allocation to male versus female function should depend on total resources invested in reproduction, because  negative density dependent factors are expected to reduce the fitness gain from investment in male and female sexual function under conditions of local mate competition and local resource competition, respectively. We documented patterns of sex allocation versus total reproductive effort within and among individuals of whitebark pine (Pinus albicaulis), a masting species of high mountain environments in western North America. We tested whether seed cone maturation was dependent on site-level pollen cone production, and whether observed pollination limitation and synchrony between pollen and seed cone production provided a sufficient fitness benefit to explain masting in this species.

Results/Conclusions

For 29 trees across 7 sites, we found that whitebark pine was pollen limited (p < 0.001) and pollen and seed cone production were positively correlated (p = 0.05). However, in simulations based on observed seed cones initiated for 84 trees at 14 sites over 15 years, average mature seed cone production was not higher for the observed synchrony than for scenarios of no synchrony or negative correlation between pollen and seed cone production. Pollination limitation in synchronous populations increased mature cone production in mast years, so pollination limitation could be a proximate mechanism for masting if another fitness benefit existed for higher cone production in mast years. For whitebark pine, this benefit is likely greater seed dispersal in mast years, accomplished by attracting its primary seed disperser, Clark’s nutcracker (Nucifraga columbiana), which previous studies showed functions as a seed disperser only when cone crops are large. If production of large cone crops each year is not energetically feasible, masting provides a way to increase cone production in some years to facilitate dispersal. If true, positive density-dependent fitness benefits may influence patterns of sex allocation in relation to total resources invested in reproduction, a possibility heretofore rarely considered in sex allocation theory.