Mk. Owens et Mc. Schreiber, SEASONAL GAS-EXCHANGE CHARACTERISTICS OF 2 EVERGREEN TREES IN A SEMIARID ENVIRONMENT, Photosynthetica, 26(3), 1992, pp. 389-398
The patterns of net photosynthesis (P(N)), transpiration (E) and condu
ctance (g) of a native broadleaf (Quercus virginiana Mill.) and needle
-like (Juniperus ashei Buchholz) evergreen tree were evaluated to dete
rmine the extent of seasonal variation in gas exchange and the respons
e of both species to drought. Gas exchange was evaluated on intact lea
ves at monthly intervals from April 1988 through March 1990. Q. virgin
iana had higher P(N), E, g and water-use efficiency than J. ashei thro
ughout both years. Maximum average P(N) for Q. virginiana occurred in
July 1988 (9.58 mumol m-2 s-1) whereas maximum average P(N) for J. ash
ei occurred in October 1988 (4.59 mumol m-2 s-1). P(N), E and g increa
sed linearly with precipitation for Q. virginiana but were unrelated t
o precipitation for J. ashei leaves. Q. virginiana exhibited a 6-fold
increase in P(N) and E between dry and wet months whereas J. ashei exh
ibited very little change. The ability of Q. virginiana to increase P(
N) and E in response to erratic precipitation and to actively photosyn
thesize during the winter may provide a competitive advantage over J.
ashei and native grasses.