Physiological Response to Summer Drought: a Long-term Comparative Study of an Evergreen and a Deciduous Oak Species
Claudia Tyler, University of California, Santa Barbara
Shelly Cole Moritz, University of California, Santa Barbara
Bruce E. Mahall, University of California, Santa Barbara
Summer drought poses a potential barrier to natural oak seedling recruitment and the transition from seedling to sapling stages. To evaluate this barrier, we conducted a long-term study of two oak species, Quercus lobata (a deciduous species) and Q. agrifolia (an evergreen species), measuring growth and physiological parameters. Within our 13-year study period was a 7-year drought, which included some of the driest years on record. Our understanding of the performance of these oaks may be useful in guiding predictions of the impacts of a future drier climate.
We conducted our studies on seedlings/saplings and neighboring adult trees at the UC Sedgwick Reserve in Santa Barbara county. Seedlings were established from acorns planted in winter 1998. From 2002 to 2015, in late summer, we assessed water relations, gas exchange and chlorophyll fluorescence.
Over the 13-year period, patterns remained surprisingly constant. Young individuals of both species had significantly lower predawn xylem pressure potentials (PDXPPs) than nearby adult trees in nearly all years. Young individuals of Q. lobota reached lower PDXPPs than Q. agrifolia in many years, while trees of the two species did not differ.
In both species young individuals had lower maximum rates of gas exchange per unit leaf area (Amax) than trees in nearly all years. Gas exchange rates also varied between species: Amax was higher in Q. lobata than in Q. agrifolia in both adults and seedlings.
Chlorophyll fluorescence (Fv/Fm) suggest that leaves of seedlings and trees of both species are protected from photoinhibitory damage during summers. Although in the earliest years (2002, 2003) seedlings had lower Fv/Fm values than conspecific adults, subsequently, there were no differences between the age classes. However, in most years, lower values for Fv/Fm in young Q. agrifolia compared to Q. lobata suggest that the former were under more stress in late summer.