COS 91-8 - Responses of twig growth and canopy leaf area in Pinus edulis to experimental manipulations of water availability

Thursday, August 11, 2011: 10:30 AM
6B, Austin Convention Center
Amanda L. Boutz, Biology, University of New Mexico, Albuquerque, NM, William T. Pockman, Department of Biology, University of New Mexico, Albuquerque, NM and Nathan G. McDowell, Earth and Environmental Sciences, Los Alamos National Laboratory, Los Alamos, NM

Increased incidence of large-scale forest die-off attributed to drought has been observed globally over the past decade, increasing concern about the future stability of forests as carbon sinks.  To understand the mechanistic basis of semi-arid woodland responses to drought, we measured annual increment growth and leaf area on branches of Pinus edulis in a rainfall manipulation experiment at the Sevilleta National Wildlife Refuge and LTER site in central New Mexico, USA. We collected 2-4 twigs from each of five trees growing in drought, irrigation, and ambient control plots at a site in the Los Pinos Mountains.  We measured annual branch elongation and, since trees in this population retain needles for many years, the leaf area remaining in each growth increment.  We also estimated annual needle production using yearly fascicle counts.   We compared these structural data to fluctuations in annual precipitation across treatments to understand how such variation in available water influence branch growth and leaf area production.


Rainfall manipulation produced clear differences among treatment groups, with drought trees exhibiting smaller growth increments and decreased leaf area, and irrigated trees greater twig elongation and increased leaf area relative to ambient controls.  Preliminary results indicate that drought trees retain more needles from older age cohorts and produce fewer lateral shoots than irrigation trees.  The net effect of these responses is a likely shift in the allometric relationships between canopy leaf area, hydroactive xylem and absorbing root area.

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