Climatic and biotic co-limitation of conifer establishment at treelines: addressing uncertainty in bio-climatic model forecasts of forest change

Principle Investigator: Matthew Germino, Idaho State University
Co-Principle Investigators:
Lisa Gramlich, University of Arizona
Nathan Mantua, University of Washington
Jeremy Littell, University of Washington

Objectives: To reduce the considerable predictive uncertainty of bioclimatic models for dominant conifer species, by examining how tree establishment at forest-treeline boundaries is influenced by decadal climate patterns and biotic interactions.

Location: Upper and lower treeline ecotones of nine mountain sites from the Cascade rainshadow mountains of Washington to the CO/WY front Range.

Hypotheses: At upper and lower treeline, we hypothesize that: (1) climate variability, particularly at decadal scales, is important in structuring broad pulses of tree establishment; (2) that local biotic and biophysical environment act to modulate the role of regional climate in limiting/facilitating tree population growth at treelines; (3) bioclimatic models that incorporate the climatic and biotic factors limiting tree establishment result in more realistic predictions of the realized niche dimension of a species. We hypothesize that herbaceous communities, presence of other tree species, and tree community/landscape structure will be key factors that influence establishment and the presence of a given tree species at treelines.

Methods: Hypotheses will be tested through a combination of tree-ring analysis of establishment patterns, experimental manipulations of seedling growth conditions at treelines, and geographic analysis of recent changes in the area/density of trees in treelines. These empirical data will be used to adjust statistical modeling approaches for the contingencies introduced by climate and ecological conditions. In each replicate treeline, we will use dendrochronology to estimate the population age structure and historic establishment patterns of trees, relative to annual and decadal patters of temperature and precipitation, and relative to local biotic factors such as neighboring tree and herb species. At lower and upper treeline, we will examine the growth and survivorship of newly-germinated seedlings of each of tree species (eg. Picea, Abies, Pinus, and others) to experimental manipulations of climate and biotic factors. In a three-way, split-plot experimental design, we will have seedlings growing near and away from resident trees in the treeline, with and without herb cover. Nested into these vegetation treatments will be manipulations of temperature made possible with passive warming devices, and also manipulations of precipitation made possible with supplemental watering and modification of snowcover.

Deliverables: We will compare traditional bioclimatic models with models that are modified by tree-ring and experimental findings. The net outcome will be refined statistical bioclimatic models that predict species responses given the co-limitation of climatic and biotic influences on tree establishment. In addition to a graduate thesis and a suite of publications and meeting presentations, we will make our tree ring data and biogeographic predictions available online.

last updated: 15 April 2007 PLH