Seed output fluctuates dramatically over time in many plant populations. These fluctuations, often called mast seeding, directly affect numerous plant and animal species. Nonetheless, empirical tests of the proximate causes of mast seeding are virtually nonexistent. Crone and Sala will test proximate mechanisms of mast seeding in two plant species: Astragalus scaphoides (bitterroot milkvetch), a bee-pollinated wildflower, and Pinus albicaulis (whitebark pine), a wind-pollinated tree. For each species, Crone and Sala will test whether pollen availability limits seed production, and the effect of seed production on stored resources. These experiments test the assumptions of "pollen coupling" a recently proposed, but untested, hypothesis for how mast-seeding occurs. Crone and Sala will also evaluate predictions of pollen coupling models, using wide-scale geographic patterns of synchrony in whitebark pine and whole-plant manipulations of fruit set and pollen availability in bitterroot milkvetch.
Fluctuations in the fruit or seed output of mast-seeding plants are a key factor regulating economically important wildlife and pest species. Whitebark pine exemplifies the importance of understanding mast seeding for management. In Yellowstone National Park, whitebark cone crop failures cause human-grizzly conflicts; managers need to predict non-mast years in order to deal with these conflicts and associated public relations. In Glacier National Park, whitebark pine has been decimated by an introduced pathogen, white pine blister rust. Predicting mast years will help managers locate and collect seeds of resistant trees, and design restoration strategies that maintain key elements of mast-seeding. In addition to direct application of research results, this project will provide educational opportunities for undergraduate and graduate students, and integrate with ongoing programs that provide research opportunities for inner-city and Native American undergraduate students.
Some species of plants produce large seed crops in some years, and virtually no seeds in other years. This phenomenon, known as mast-seeding, is most well-known in trees such as oaks, and orchard crops such as mandarin oranges, but it is not restricted to these species. Many past studies have tested reasons why mast-seeding increases lifetime reproduction of plants. Our project was the first to document how plants become synchronized among individual plants, so that they all produce seeds in the same years. We tested how reproduction becomes synchronized in two species of plants: bitterroot milkvetch, a perennial wildflower, and whitebark pine, a long-lived tree. We learned that individual plants deplete stored resources (sugar and phosphorous) after reproduction. Therefore, they are not able to produce seed crops in the next year. Individuals that flower in the "off" years are less likely to be pollinated and therefore make fewer seeds. This does not deplete resources, so they make seeds the next year, and they become synchronized with other plants in the population. In the process of collecting these data, we also studied relationships between insects and plants. We learned that native pollinators are affected by fluctuations in flowering by the milkvetch. We also collected data on reproduction of whitebark pine trees throughout western Montana in 2005 and 2006, just before the major pine beetle attack in 2007-2008. We are revisiting these sites to find out whether susceptibility beetle attack depends on growth or reproduction prior to the outbreak, and if and how the performance of the survivng trees has changed.
