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AJB Advance Access Published online ahead of print March 20, 2009; doi:10.3732/ajb.0800247 American Journal of Botany © 2009 Botanical Society of America, Inc.

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A comparative study of oak (Quercus, Fagaceae) seedling physiology during summer drought in southern California¹

² Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, California 93106 USA ³ Environmental Sciences, University of California, Berkeley, California 94720 USA ⁴ Science Department, Borough of Manhattan Community College, CUNY, New York, New York 10007 USA

ABSTRACT

Natural recruitment of oaks appears to be declining throughout the northern hemisphere. Summer drought poses a potentially important barrier to oak recruitment in southern California. To evaluate this barrier, we grew evergreen Quercus agrifolia and deciduous Q. lobata from seeds near parental trees. We measured water relations, chlorophyll fluorescence, and gas exchange during these seedlings’ fourth and fifth summers and compared them to neighboring adults. Most seedlings had substantially lower values for predawn xylem pressure potential (_pd), minimum photosystem II (PSII) quantum efficiency (_PSIIMIN), maximum quantum efficiency for PSII under dark-adapted leaf conditions (Fv/Fm), and maximum photosynthetic assimilation (Amax), and higher values for maximum nonphotochemical quenching (NPQmax) than did conspecific adults. The high, unvarying _pd values of the adults suggest they use perennially available groundwater. Quercus lobata seedlings commonly had lower values for _pd than did Q. agrifolia, and values for _pd and _PSIImin were significantly related to size in Q. lobata but not in Q. agrifolia. These data suggest important interspecific differences in root architecture. Lower values for _PSIImin, Fv/Fm, and higher NPQmax in Q. agrifolia indicate that Q. agrifolia seedlings were usually under more stress than Q. lobata, which typically had higher Amax rates than did Q. agrifolia seedlings. Diurnal photosynthesis curves were quite flat for Q. agrifolia, but they peaked in the morning for Q. lobata. Established seedlings appeared to be under more stress than adults, but this stress did not appear severe enough to cause death. Access to perennially available groundwater may be crucial for the seedling to sapling transition.

Key Words: chlorophyll fluorescence • drought stress • Fagaceae • leaf gas exchange • Quercus agrifolia • Quercus lobata • root architecture • seedling/sapling transition • tree recruitment • water relations

Received for publication July 18, 2008. Accepted for publication December 22, 2008.

FOOTNOTES

¹ The authors thank the Andrew W. Mellon Foundation and the Santa Barbara County Oak Restoration Program for financial support, M. Williams and the U. C. Sedgwick Reserve for providing a superb field site, F. Davis and R. Callaway for numerous, valuable conversations and reviewing the manuscript, J. King for reviewing the manuscript, and J. Kim, J. Gremer, and C. Peace for field assistance.

⁵ Author for correspondence (e-mail: mahall{at}lifesci.ucsb.edu)

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