PARENT SESSION
Oral Session #63: Plant Ecology: Water Relations. Presiding: J. Cleverly.
Thursday, August 9, 2001. 8:00 AM to 12:00 PM. Hall of Ideas H.
Summer water relations and photosynthesis of oak seedlings in grazed/ungrazed woodlands in Southern California.
MATA, CATARINA1,2, MAHALL, BRUCE1, 1 2
ABSTRACT- To gain insight into the possible role of water stress in oak recruitment, we studied the effects of summer drought on oak seedlings in an oak savannah in the Santa Inez Valley, CA. Predawn leaf water potentials (PDLWPs) were followed from June to November 2000 in two-year old Quercus lobata and Q. agrifolia seedlings in grazed (G) and ungrazed (NG) plots. Photosynthetic rates, stomatal conductances and chlorophyll fluorescence characteristics were followed in day cycles. After June Q. lobata seedlings had lower PDLWPs than Q. agrifolia seedlings. Lowest seedling PDLWPs occurred in September: −1.72±0.19 (G) and −1.17±0.03 MPa (NG) for Q. agrifolia and −2.67±0.25 (G) and −1.93±0.30 MPa (NG) for Q. lobata. In both species seedlings consistently had lower PDLWPs during the summer in grazed areas than in matched ungrazed areas. Maximum photosynthetic rates were highest for both species in early mornings, tended to be higher in Q. lobata than in Q. agrifolia, and decreased during the summer as seedlings' water potentials decreased. There was no cumulative decrease in Fv/Fm for either species until September, when Fv/Fm declined in Q. agrifolia seedlings.
KEY WORDS: drought, Quercus agrifolia, Quercus lobata, grazing