High nutrient costs of seed production in whitebark pine (Pinus albicaulis)

Whitebark pine (Pinus albicaulis), a keystone species of high elevation forests of the Rocky Mountains, produces large, nutritious seeds which are used by many animals as a food source. Whitebark pine is a masting species whose seeds are thought to require substantial amounts of resources stored in trees. We examined physiological costs of seed production in whitebark pine during a masting year. We compared leaf nitrogen and phosphorous concentrations before, during and after seed filling, and photosynthetic rates during seed filling in cone-bearing branches, branches with no cones, branches with cones removed early in the season and branches partially defoliated. Nitrogen concentrations of one year old needles decreased during the season in cone-bearing branches and in defoliated branches, but increased in no cone-bearing branches and branches with cones revoved. Photosynthetic rates of cone-bearing branches were significantly lower than in non-bearing branches and than in branches with cones removed, suggesting N-limited photosynthesis. Phosphorous concentrations decreased significantly in four year old needles during seed ripening coinciding with an increase in seed P.  Seed mass and N and P concentrations did not change significantly between non-defoliated and partially defoliated branches, indicating that nutrients were reallocated from significant distances. We estimated that seed production during masting years requires up to 10% of N stored in foliage and up to 24% of P. Our results suggest that masting in whitebark pine may be partially related to the high nutrient costs of seed production and that older foliage may serve as a phosphorous storage.