The role of changing hydrology and plant water stress adaptations on nitrogen availability and gas exchange of semi-arid plants in California.

摘要

Human population growth in arid and semi-arid regions has the potential to alter ecosystem hydrology. Because water is a determining factor for ecosystem function, it is important to understand the mechanisms by which water availability influences ecosystem processes such as nitrogen (N) availability and plant gas exchange. I investigated the influence of groundwater depth on grass and shrub active rooting depth, vulnerability to cavitation, and soil N availability in Owens Valley, California, which is experiencing water withdrawals and changes in hydrology. I found that while grasses were more resistant to cavitation than shrub species, grass cover declined with decreasing groundwater depth and shrub cover remained relatively constant. Surprisingly, I did not find intraspecific variation in vulnerability to cavitation with changes in water availability. Sites with lower grass cover also had lower N availability. These results indicate that grass cover is limited by access to soil water through rooting depth, and that grass cover influences soil N availability. In a separate study, I found that in areas of shallow watertable depth, grass photosynthesis declined seasonally with declining soil and leaf N; however, photosynthesis in shrub species remained constant. These results suggest that when water availability is not limiting, grass photosynthesis is influenced by ecosystem nitrogen availability. Finally, I studied plant water relations in an irrigated horticultural setting of southern California, where species are imported from many regions and are generally well-watered. Specifically, I investigated the role of native biome on plant adaptations to water stress and gas exchange for species growing in common garden conditions. I found large variability in vulnerability to cavitation and maximum conductivity in arid, temperate and tropical species; however, while arid species had greater transpiration, temperate species had lower transpiration and greater water use efficiency (WUE). These results suggest that under irrigated conditions, arid species do not necessarily use less water than mesic species, and may in fact have higher transpiration rates than temperate species. Additionally, species that were more vulnerable to cavitation were also more water use efficient, indicating that there may be a tradeoff between water stress resistance and water use under well-watered conditions.