© 2000 Heron Publishing—Victoria, Canada
Differences in leaf gas exchange and water relations among species and tree sizes in an Arizona pine–oak forest
T. E. Kolb (1) and J. E. Stone (1)
1. School of Forestry, College of Ecosystem Science and Management, Northern Arizona University, Flagstaff, Arizona 86011-5018,
USA / Received April 9, 1999
Summary
We compared leaf gas exchange and water potential among the dominant tree species and major size classes of trees in an upland,
pine–oak forest in northern Arizona. The study included old-growth Gambel oak (Quercus gambelii Nutt.), and sapling, pole, and old-growth ponderosa pines (Pinus ponderosa var. scopulorum Dougl. ex Laws.). Old-growth oak had higher predawn leaf water potential (Ψleaf) than old-growth pine, indicating greater avoidance of soil water stress by oak. Old-growth oak had higher stomatal conductance
(Gw), net photosynthetic rate (Pn), and leaf nitrogen concentration, and lower daytime Ψleaf than old-growth pine. Stomatal closure started at a daytime Ψleaf of about –1.9 MPa for pine, whereas old-growth oak showed no obvious reduction in Gw at Ψleaf values greater than –2.5 MPa. In ponderosa pine, Pn and Gw were highly sensitive to seasonal and diurnal variations in vapor pressure deficit (VPD), with similar sensitivity for sapling,
pole, and old-growth trees. In contrast, Pn and Gw were less sensitive to VPD in Gambel oak than in ponderosa pine, suggesting greater tolerance of oak to atmospheric water
stress. Compared with sapling pine, old-growth pine had lower morning and afternoon Pn and Gw, predawn Ψleaf, daytime Ψleaf, and soil-to-leaf hydraulic conductance (Kl), and higher foliar nitrogen concentration. Pole pine values were intermediate between sapling and old-growth pine values
for morning Gw and daytime Ψleaf, similar to sapling pine for predawn Ψleaf, and similar to old-growth pine for morning and afternoon Pn, afternoon Gw, Kl, and foliar nitrogen concentration. For the pines, low predawn Ψleaf, daytime Ψleaf, and Kl were associated with low Pn and Gw. Our data suggest that hydraulic limitations are important in reducing Pn in old-growth ponderosa pine in northern Arizona, and indicate greater avoidance of soil water stress and greater tolerance
of atmospheric water stress by old-growth Gambel oak than by old-growth ponderosa pine.
Keywords:
Gambel oak, hydraulic conductance, nitrogen, photosynthesis, Pinus ponderosa, ponderosa pine, productivity, Quercus gambelii, stomatal conductance, vapor pressure deficit, water stress.
