McDowell Lab Research

Described below are the six focus areas in our research goals. Funding and support are from the LANL Laboratory Directed Research and Development (LDRD) Program and by the Institute of Geophysics and Planetary Physics and the US Department of Energy, Office of Science, Program for Ecosystem Research.

1) Continuous Monitoring of a Piñon-Juniper Ecosystem: Intra- and Interannual Response to Climate

This project was initiated in 1987 with monthly measurements of soil and plant pre-dawn water potential. In 2002 the piñon component of the ecosystem died in response to prolonged drought and bark beetle outbreak. Questions being addressed in this project include assessment of overstory and understory vegetation response to climatic variation and forest mortality, ecosystem water and carbon balance responses, and stable isotope exchanges.

2) Biosphere-Atmosphere Exchange of CO₂ and H₂O

We have monitored the stable isotope exchange from our long-term piñon-juniper field site since May 2004 to provide insight into the mechanisms regulating ecosystem carbon and water exchange with the atmosphere. The carbon and oxygen isotope composition of ecosystem CO₂ flux is monitored continuously at this site using a novel new method, Tunable Diode Laser Spectroscopy. We have over two years of continuous data from this site that allows insight into the mechanisms of response of the vegetation and soils to climatic variation.*

3) Tunable Diode Laser Spectroscopy

This novel tool allows continuous monitoring of the carbon and oxygen isotope composition of CO2. Current applications of the tool include...

• assessment of ecosystem isotopic exchange,
• assessment of ecosystem isotope model performance,
• determination of mesophyll conductance controls over carbon and water exchange at the leaf level,
• quantification of leaf-level isotopic discrimination,
• quantification of leaf level respiratory fractionation,
• quantification of the isotopic signature of soil respiration,
• examination of bryophyte discrimination for use in paleo-climate reconstructions using bryophyte fossils, and
• insect and bird metabolism.

Research is done in collaboration with David Hanson's Plant Physiology Lab at UNM.

4) Tree Ring Analyses

We utilize the isotopic composition of wood formed in the annual rings of temperate forest trees to recreate past physiological conditions. Recent projects include...

• assessment of stand density and forest management on whole-tree gas exchange and growth,
• assessment of climate variation on gas exchange and growth,
• assessment of nutrient availability on gas exchange and growth, and
• assessment of elevation on climatic sensitivity of gas exchange and growth.

5) Mechanisms Of Survival And Mortality During Drought

The aim of this study is to quantify the mechanisms of survival and mortality using ecosystem scale manipulations of water availability in a piñon-juniper woodland to simulate future climate scenarios. In this study, four treatments were replicated with three 1600 m2 plots each to represent 60% water reduction, 60% water addition above the 30-year mean, water reduction and re-application control (via overturned gutters), and ambient control. The field site is located in a woodland at the Sevilleta Long-Term Ecological Research site in central New Mexico. Detailed empirical measurements of growth and water relations are ongoing with application of a theoretical model of plant hydraulics.

Collaborators include the US DOE Program for Ecosystem Research and the Pockman Laboratory.

6) Consequences Of Mortality On Ecosystem Carbon Balance

This project is quantifying the response of ecosystem carbon balance to woody plant mortality using the ecosystem manipulations project described in Tree Ring Analysis (above) along with sites distributed throughout New Mexico and Arizona. At the ecosystem manipulation site, we are measuring soil respiration through automated and spot measurements, along with spot measurements of leaf and wood respiration. We are also inventorying soil and plant carbon to test the hypothesis that ecosystems subject to mortality are a net source of carbon to the atmosphere. An ecosystem process model 3-PG is being used to determine the impacts of mortality on carbon balance of the larger landscape.