Examining California Chaparral and Coastal Sage Scrub Responses to Environmental Change: A Hydraulics Approach

Environmental change is occurring at an unprecedented rate. Being sessile and unable to escape rapidly changing conditions, many plant species will be unable to cope with the increased abiotic stress. This may lead to decreased physiological functioning and/or mortality. Drought and nitrogen deposition are two major disturbances that affect vegetation in California’s Mediterranean-type climate region. This region is a biodiversity hotspot and conservation priority. This research examined: 1) How water movement is coordinated among plant organs and the role this plays in drought adaptation; 2) The effects of long-term nitrogen deposition on plant hydraulic and drought adaptation traits; and 3) The mechanism underlying the open-vessel artifact when determining xylem vulnerability to cavitation using the centrifuge-based Cavitron. These results demonstrated that leaves act as hydraulic bottlenecks for vulnerable species, supporting the hydraulic segmentation hypothesis. In addition, Artemisia californica was highly responsive to alleviation of nitrogen limitation, displaying increasing gas exchange and stem water transport, and altered functional traits. Finally, the Champagne effect was confirmed as a culprit behind the early loss of conductance in ‘r’ shaped vulnerability curves measured using the Cavitron. Xylem water extraction curves compared with vulnerability curves showed when true cavitation was occurring. Overall, this research shows different species have different suites of traits that result in drought adaptation, indicating that not all species will respond equally to disturbance. Additionally, proper techniques and equipment are essential to correctly quantifying these traits