Sex allocation, pollen limitation and masting in whitebark pine

1. Masting, the synchronous and episodic production of seed crops, is thought to benefit plant reproductive success through positive density-dependent effects on pollination, dispersal and seed survival. Of these, only increased pollination efficiency in mast years can be a proximate mechanism for masting by synchronizing reproductive effort across individuals through pollen coupling. Increased pollination efficiency requires synchronous investment in male and female function during mast years. Sex allocation theory, however, predicts a trade-off in investment between male and female reproductive allocation dependent on total resources invested in reproduction. We describe patterns of sex allocation in Pinus albicaulis (whitebark pine), using data on pollen and seed cone counts over 5years for 29 trees across 7 sites in Montana, U.S.A. Whitebark pine seed cone maturation increased with site pollen cone production, indicating pollen limitation, and pollen and seed cone production were positively correlated across years. Simulating mature seed cone production from these empirical relationships resulted in greater average mature seed cone production than alternative scenarios of (i) no synchrony between pollen cone production and pollen cone initiation, (ii) negative correlation (trade-off) between seed cone initiation and pollen cone production or (iii) no masting. Synthesis. Our data support a role for pollination efficiency in both increasing long-term seed production and as a proximate mechanism for synchronizing masting in Pinus albicaulis. Increased pollination efficiency joins greater seed dispersal and survival in mast years seen in other studies, as an additional positive density-dependent benefit of masting. Positive density-dependent fitness benefits may therefore influence patterns of sex allocation in relation to total resources invested in reproduction. The pollen limitation found here combined with stand isolation and reduced tree density due to mortality from forest pests and other environmental stressors may lead to reduced seed cone maturation and changes in masting patterns.