Impact of rainfall variability and woody encroachment on productivity in semiarid grassland in New Mexico. (with Eric Small and Scott Collins)

Supported by National Institute for Global Environmental Change (past) and National Institute for Climate Change Research (current)

Location of study site (Google Earth/Maps)

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.