Controls on plant gas exchange across a grassland to shrubland gradient in Owens Valley, California

Woody encroachment, or the conversion of grasslands to shrublands, is occurring in many areas around the world and has been attributed to a number of possible mechanisms, one of which is changes in water availability. It has been suggested that woody encroachment is occurring in Owens Valley, California due to water stress caused by groundwater pumping and redistribution, which has been occurring in the valley for almost a century. However, this linkage has not been solidly demonstrated. We evaluated seasonal patterns of plant transpiration and photosynthesis  along a vegetational gradient, from grassland to predominately shrubland. Gas exchange of the grass species Distichlis spicata at the grassland site was high at the beginning of the season, while at the intermediate and shrubland sites gas exchange of Distichlis remained consistently low throughout the season. d18O measurements indicate that shrubs accessed groundwater throughout the season, while grasses utilized isotopically enriched water in the surface soil, particularly following monsoonal rains. There was a trend toward isotopic enrichment of plant water in grasses across the grassland to shrubland gradient. However, plant water sources and predawn water potentials did not explain the seasonal patterns of plant gas exchange. In contrast, leaf nitrogen showed a seasonal pattern with declining leaf N throughout the season in all species, suggesting that N becomes more limiting for all species at all sites as soil moisture declines. Inorganic N availability was highest at the meadow and intermediate sites and lowest at the shrubland, indicating less plant available N at the shrubland. Our results suggest that plant N uptake and availability as well as water availability play an important role in community composition and gas exchange in these desert phreatophytic ecosystems.