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Title: Woody plant encroachment into desert grasslands: an experimental assessment of the critical establishment phase
Funding Agency: NSF Ecology Program
PI: Steve Archer, School of Natural Resources, University of Arizona
Co-PIs: Susan Schwinning,Texas State University; Larry Howery, School of Natural Resources, University of Arizona
Graduate Student: Steve Woods
Research Associate: Nate Pierce

Project Summary:

The proliferation of woody shrubs and trees in arid/semiarid grasslands and savannas in recent decades has been widely documented. This worldwide phenomenon threatens the livelihood of rural economies, and is thought to have had a significant impact on the North American terrestrial carbon cycle. The ability of woody plants to encroach into grasslands varies widely among species. Why are some so successful and others not? Extensively researched case studies focusing on the most aggressive woody invaders reveal an extraordinary robustness of seedling recruitment to a variety of adverse biotic and abiotic factors, including drought, defoliation and competition from grasses. Does this mean lack of encroachment potential in other woody species is linked to greater vulnerability in the early seedling stages? By comparing the survivorship and growth of more and less successful woody encroachers under a range of edaphic conditions, we will determine a) which species characteristics are reliably associated with encroachment potential; and b) how climate x edaphic interactions affect encroachment potential. The proposed study will focus on the critical first 12 months of seedling growth where mortality rates are highest. Through a series of manipulative experiments we will test the hypothesis that an encroacher species excels in escaping lethal water stress under a wide range of ecological conditions.

With this research, we seek to improve our mechanistic understanding of woody plant recruitment into grasslands, ascertain how geological landscape factors constrain local abundance and determine with more confidence which species characteristics are reliably associated with encroachment potential. Thus, the study will provide guidelines for the identification and management of potential problem species and provide information for the development of recruitment models, filling an important need for modeling vegetation dynamics.

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