ABSTRACT

The position of the Arctic treeline is an important regulator of climate and subsistence resources. Recent research by the principal investigators (PIs) suggests the importance of winter snow depth as a control on tree growth. They now propose to experimentally isolate the importance of snow depth and soil nutrient availability for tree growth. This novel and interdisciplinary proposal will link the ecology of microbes to large-scale landscape patterns. If their hypotheses are confirmed, the findings will contradict the prevailing theory of the cause of treeline location.

This project will contribute to the development of the science workforce by supporting the training of three graduate students and the entrainment of numerous undergraduate students into the research activities. Outreach to the predominantly Alaskan Native community of Kotzebue will take different forms. The PIs will arrange with the local radio station, a primary means of media communication for the local region, to describe their research. They will visit the local high school to discuss the role of vegetation in climate and to share the results of their research. They will provide opportunities for outstanding students from the local high school to participate in their field research program. They will participate in the Bureau of Land Management?s Campbell Creek Science Center Fireside Chat series to promote outreach to the more urban community in and around Anchorage, AK. They will enhance the existing Interactive Model of Leaf Decomposition (IMOLD), a series of animated lessons and activities about decomposition and nutrient cycling developed under a previous award, to include examples and teaching activities derived from this work at the Arctic treeline.

It has long been thought that temperature exerts a direct control on growth of treeline trees and the position of the treeline. However, the PIs? recent work in the Arctic with white spruce suggests that indirect effects of temperature on soil nutrient availability may be of equal or greater importance. They hypothesize that cold soils at the treeline, particularly during winter, limit microbial activity and nutrient availability to the point where trees are barely able to survive and grow. Measurements made during winter have revealed that Arctic forests maintain snowpacks that are much deeper than observed at treeline. Trees are thought to trap snow and lead to a deeper snowpack, insulating the soil from cold air and allowing for greater overwinter microbial activity and greater nutrient mineralization. Indeed, the PIs found a strong positive correlation between white spruce growth and winter snow depth. They propose to isolate the mechanisms underlying this correlation by using snowfences to manipulate winter snow depth and fertilizer to increase soil nutrient availability at three treelines that differ in soil moisture. To provide an experimental test of the importance of temperature as a direct control on treeline tree growth, they propose to incorporate experimental shoot warming into their snowfence experiment in a factorial design. They predict that both experimental snow and nutrient additions will lead to large increases in microbial activity, photosynthesis, tree growth, seed quality, seed production, seedling establishment and recruitment of new trees. They expect to observe the greatest positive responses where soils are wet and cold. Meanwhile, they predict that shoot warming will lead to negligible changes in growth.

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