U.S. Department of Energy, Office of Science

Program for Ecosystem Research

Research Project   Hydraulic mechanisms of survival and mortality during drought in pinyon-juniper woodlands of the southwestern United States

Principal investigators:   Nate G. McDowell and William T. Pockman

Project goal

Determine the mechanisms underlying effects of altered precipitation on growth, survival, and mortality of pinyon pine (Pinus edulis) and juniper (Juniperus monosperma) in the southwestern United States.

Pinyon-juniper research site

The experimental area [photo courtesy of Neil Cobb].

Ecosystem being studied

A semiarid pinyon-juniper woodland in central New Mexico (1900 m above sea level, 34 degrees 23.16' N, 106 degrees 31.51' W) in the Sevilleta LTER. The ecosystem corresponds to the "juniper-pinyon woodland (Juniperus-Pinus)" vegetation type of Kuchler (1964), which covers 22-30 million ha of Nevada, Utah, Arizona, New Mexico, and Colorado.

Initial results

Effects of the precipitation manipulations on sap flow and pre-dawn leaf water potential.

Results

Construction of the experimental infrastucture was completed in September, 2007. Since then, the experimental treatments have affected soil water content and altered tree water potential and sap flow. For both species the drought treatments are causing reduced sap flow and more negative pre-dawn leaf water potential (see figure). The infrastructure diverting precipitation is having a small effect on plot microclimate; the ground surface is 1-2 degrees C warmer under the troughs being used.

The sprinkler irrigation treatments will be fully functioning by summer of 2008. During the coming years, water transport limits for each species will be determined to understand how each treatment-species combination affects transpiration and the consequences of altered precipitation for growth and survival in pinyon-juniper woodlands.

Why this is important

Recent drought in the southwestern United States was associated with an estimated 40-95% mortality of pinyon pine, 2-25% mortality of juniper, and 10-60% mortality of ponderosa pine, Douglas-fir, and white fir (Breshears et al. 2005; McDowell et al. 2008). This mortality significantly decreased woody plant cover and altered species distributions throughout the region. Although the final cause of mortality is attributed to bark beetle infestation (e.g., Ips confuses), the common belief is that a hydraulic mechanism underlies susceptibility to insect attack. Most climate models agree that future droughts may become more frequent and severe in the southwest (the winter of 2005/2006 was the driest and warmest on record in northern New Mexico). Thus, there is high likelihood of future mortality events of similar or greater magnitude than the recent event. Unfortunately, mechanistic knowledge required to predict effects of any future droughts on woodland ecosystems is limited. This project will shed light on fundamental physiological processes regulating ecosystem response to drought and facilitate improved models of future effects of climatic change on terrestrial ecosystems.

Plot types

The four plot types in each experimental block. In the "cover plots" (i.e., panel "d"), the troughs are inverted to intercept precipitation but then drain it to the soil surface immediately below. The inverted troughs are meant to affect radiation, mass, and momentum exchanges about equally to their affects in the "drought plots".

Methods

The project includes four 40 x 40 m research plots replicated three times (three blocks) for a total of 12 measured plots covering approximately 2 ha of pinyon-juniper woodland. One block has a southern aspect, one block has a northern aspect, and one block is relatively flat. Each block includes (a) a control plot with no water manipulation; (b) a drought plot designed to prevent 50% of incident precipitation from reaching the soil (using passive, gravity-driven removal with aboveground troughs); (c) an irrigated plot designed to receive a 50% increase in incident precipitation (applied by overhead sprinklers); and (d) a plot with precipitation-removal troughs inverted (so that intercepted precipitation does reach the soil) to assess effects of the rain collection troughs on plot microclimate. Five pinyon trees and five juniper trees are monitored within each of the 12 plots (120 total study trees). Measurements include temperature around each tree (air and soil), plant water potential, soil water potential at three depths, surface soil water content, sap flow, tree allometrics, leaf-level gas exchange, hydraulic conductivity of stems and roots, naturally occurring isotopes of water and carbon in plants and soil, soil respiration, stand characteristics, and the meteorological characteristics of the site.

Further information is available at the University of New Mexico's project website.

Personnel

Nate G. McDowell, Los Alamos National Laboratory

William T. Pockman, University of New Mexico

Enrico A. Yepez, University of New Mexico and Los Alamos National Laboratory

Judson Hill, University of New Mexico

Jen Plaut, University of New Mexico

Sandra White, University of New Mexico

References

Breshears DD, Cobb NS, Rich PM, Price KP, Allen CD, Balice RG, Romme WH, Kastens JH, Floyd ML, Belnap J, Anderson JJ, Myers OB, Meyer FW (2005) Regional vegetation die-off in response to global-change type drought. Proceedings of the National Academy of Sciences, USA 102:15144-15148

Kuchler AW (1964) Manual to Accompany the Map: Potential Natural Vegetation of the Conterminous United States. American Geological Society (Special publication No. 36), New York.

McDowell NG, Pockman WT, Allen C, Breshears DD, Cobb N, Kolb T, Plaut J, Sperry J, West A, Williams D, Yepez EA (2008) Mechanisms of plant survival and mortality during drought: why do some plants survive while others succumb to drought? New Phytologist 178:in press

Funding period:   January 2006 to present