Piñon Pine (Pinus edulis) Responses of Annual Growth to Water Availability in a Pinyon-Juniper Forest at the Sevilleta National Wildlife Refuge, New Mexico

Increased incidence of large-scale forest die-off attributed to drought has been observed globally over the past decade, raising 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 from 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 4 branches from each of five trees growing in drought, irrigation, cover control, and ambient control plots at a site in the Los Pinos Mountains.  We measured annual branch elongation, stem diameter, sapwood area, and leaf area.  We compared these structural data to fluctuations in annual precipitation across treatments to understand how such variation in available water influence branch growth.  Rainfall manipulation produced clear differences among treatment groups, with drought trees exhibiting shorter stem lengths, decreased stem and sapwood diameters, and decreased leaf area production than control treatments.  Irrigated trees displayed increased stem length, stem diameter, sapwood diameter, and leaf area production relative to ambient controls.  The net effect of these responses is a likely shift in the allometric relationships, such as hydroactive xylem and absorbing root area.