Impact of climate variability and woody encroachment on productivity in a semiarid grassland in New Mexico

P.I./Institution: William T. Pockman, University of New Mexico

Co-PI/Institution: Eric E. Small, University of Mexico

Summarize in two to four sentences what activities will be (are being) carried out in the project: We will construct water addition plots in grassland, shrubland and mixed grass-shrub communities at the Sevilleta National Wildlife Refuge and L TER for use with existing rainout shelters and control plots to determine the effects of extended drought and extended mesic periods on grass and shrub productivity .After imposing rainfall manipulation treatments, we will measure the growth and physiological responses of grasses and shrubs in drought and water-addition plots relative to untreated controls to assess the effect of extended climate extremes on ecosystem productivity and encroachment of woody shrubs into semiarid grassland. Furthermore, we will determine whether the response is determined by immediate differences in grass and shrub structural and functional characteristics or whether the response develops over time as ecosystem modifications accumulate.

Project Abstract: The link between grassland response to climate forcing and the trajectory of woody encroachment of grasslands are key to our understanding of grassland responses to climate changes resulting from energy production. Understanding the basis of this dynamic is essential for realistic predictions of both vegetation change under future climate and related responses such as the net change in carbon storage in these systems under altered climate. The research proposed here will test the hypothesis that structural and functional differences between shrubs and grasses drive short term and long term physiological and growth responses that lead to vegetation changes in these ecosystems. Two questions are addressed: 1.) Do the structural and functional differences between grasses and shrubs lead immediately to differential physiological and growth responses to climate forcing in the form of extended periods of drought or above-average precipitation? 2) Are the immediate responses of grass and shrub to climate forcing maintained as treatment continues or do grass and shrub physiological and growth responses continue to diverge over time as the direct and indirect effects of climate forcing accumulate? If grass productivity declines more than shrub productivity during drought and increases more than shrub productivity during periods of above-average precipitation, these differential responses may control the process of woody encroachment in semiarid systems. Existing rainout shelters and proposed water addition treatments will be used to impose the extremes of the historical climate record, 3 years of 50% of average precipitation like the 1950's drought and 3 years of 150% of average precipitation as occurred during the 1990's. Because our manipulations include grassland, shrub-invaded grassland and shrubland we will be able to assess the differences in productivity that we may expect from these three points that span the spectrum from grassland to the shrubland that may ultimately replace it as encroachment occurs.

Summary of Proposed Work: Energy production by humans has increased atmospheric carbon dioxide concentration and is likely to lead to warmer climate with more extremes (e.g. drought and wet periods). Anticipating the effect of these changes requires understanding the carbon balance of the earth (including carbon stored in plants and soils vs. CO2 in the atmosphere) and how existing ecosystems are likely to respond to changing climate. Semiarid grasslands are an important rangeland resource. Woody shrub encroachment of grasslands in the last 150 years has reduced rangeland quality but has been identified as a potentially important site of carbon storage. This study will modify the water reaching plots in grassland, shrub land and mixed grass-shrub vegetation to understand how future climate changes may influence grass and shrub productivity and the process of woody encroachment. We expect to identify short and long term responses, initiated by structural and functional differences between grasses and shrubs that control the change in productivity and the ability of shrubs to invade an established grassland ecosystem. The results will provide improved understanding of the factors responsible for woody encroachment and the carbon storage that may be associated with it.