Do these cones make me look fat? Unexpected patterns of resource allocation in whitebark pine

Strong, synchronized fluctuations in reproductive output in plants are known as mast-seeding. While the ultimate, evolutionary advantages of mast-seeding have received much attention, empirical data of the proximate causes of mast-seeding are clearly lacking. Years of low seed production following masting years are thought to be necessary to replenish resources devoted to reproduction during mast years.  We studied resource dynamics in whitebark pine (Pinus albicaulis), a masting, keystone species in the subalpine ecosystems of the Rocky Mountains that is currently declining at alarming rates due to changes in climate and invasive pathogens. Whitebark pine seeds have very high phosphorous (P) and fat content and provide a critical food source for many animals during masting years. Such nutritious seeds are expected to be a strong string sink for resources. We tested the hypothesis that seed production incurs a substantial resource cost and depletes stored resources at the branch level. We measured stored mobile carbon pools (MCP, lipids and non-structural carbohydrates) and P in foliage and sapwood from branches with cones, reproductive branches without cones, and branches with cones removed. Contrary to our expectations, cone production and seed filling did not deplete resources in foliage and sapwood to a greater extent in branches with cones than those without cones or with cones removed. From early to late summer, all branches exhibited a decrease of foliar carbohydrate reserves and an increase of MCP and phosphorous in sapwood. Our results show no resource depletion at the branch level and suggest that the production of nutritious seeds in whitebark pine is an integrated, whole-tree strategy, with no branch-level reproductive autonomy. Ongoing analyses of sapwood stored resources at the base of the tree will help elucidate overall fluctuations in stored resources and their role in masting patterns.