Ecosystem consequences of precipitation variability and extremes in semiarid grassland and shrubland.

Principle Investigator: William Pockman, University of New Mexico, pockman@unm.edu

Abstract:: William Pockman and his lab group seek to understand changes in ecosystem structure and function of semiarid grassland and shrubland caused by extended periods of severe drought or above-normal precipitation and by precipitation variability.  The research directly funded by this proposal will be compared with data from two additional rainfall manipulation experiments (winter addition in grassland, annual rainfall in piñon-juniper woodland), building a unique long term database of different rainfall manipulations. This long-term project will contribute to a detailed understanding of the ecosystem responses to precipitation extremes and variability.

Location: Grass, mixed grass-shrub and shrub communities at the Sevilleta LTER in central New Mexico. The research directly funded by this proposal will be compared with data from two additional rainfall manipulation experiments at Sevilleta (winter addition in grassland, annual rainfall in piñonjuniper woodland), building a unique long term database of different rainfall manipulations.

Hypotheses: Multi-year precipitation extremes: Grass cover will be more responsive to precipitation forcing than creosotebush and grass cover changes will alter the horizontal redistribution of precipitation by changing connectivity among bare soil patches. Prolonged drought will accelerate the conversion of mixed grass-shrub to shrubland while abundant water will increase grass cover. Within-season precipitation variability: Ecosystem C fluxes and the rate of shrub encroachment into grassland will change with event size and frequency. Frequent small rain events will (1) increase soil CO2 fluxes without affecting grass growth rates, and (2) stimulate Larrea seed germination but not provide sufficient soil moisture for survival. Infrequent large events will (1) increase CO2 fluxes and stimulate grass growth, and (2) enhance Larrea seedling germination and establishment. The net result may be enhanced rate of shrub encroachment.

Methods: Long term extremes are imposed by excluding ambient precipitation with moveable shelters to impose drought or supplementing it using overhead irrigation. NICCR funds would support monitoring recovery after five years of drought treatment, the final two years of water addition treatment (now in year 3) and three years of monsoon manipulations. Long term mesic conditions re imposed by adding six large storms (20 mm each) per year. Monsoon precipitation variability is manipulated in separate plots by adding either one large (20 mm) or four small (5 mm) events monthly during July - September. Both studies measure plant water relations, NPP, and species composition and abundance to document changes in ecosystem structure and function caused by the treatment.

Deliverables: This long-term project will contribute to a detailed understanding of the ecosystem responses to precipitation extremes and variability. Deliverables include presentations and peer-reviewed publications as well as treated plots suitable for new manipulations after multi-year precipitation forcing.


last updated: 28 February 2008 PLH