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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 (Ψ leaf ) than old-growth pine, indicating greater avoidance of soil water stress by oak. Old-growth oak had higher stomatal conductance ( G  w ), net photosynthetic rate ( P  n ), and leaf nitrogen concentration, and lower daytime Ψ leaf  than old-growth pine. Stomatal closure started at a daytime Ψ leaf  of about –1.9 MPa for pine, whereas old-growth oak showed no obvious reduction in  G  w  at Ψ leaf  values greater than –2.5 MPa. In ponderosa pine,  P  n  and  G  w  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,  P  n  and  G  w  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  P  n  and  G  w , predawn Ψ leaf , daytime Ψ leaf , and soil-to-leaf hydraulic conductance ( K  l ), and higher foliar nitrogen concentration. Pole pine values were intermediate between sapling and old-growth pine values for morning  G  w  and daytime Ψ leaf , similar to sapling pine for predawn Ψ leaf , and similar to old-growth pine for morning and afternoon  P  n , afternoon  G  w ,  K  l , and foliar nitrogen concentration. For the pines, low predawn Ψ leaf , daytime Ψ leaf , and  K  l  were associated with low  P  n  and  G  w . Our data suggest that hydraulic limitations are important in reducing  P  n  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.

Differences in leaf gas exchange and water relations among species and tree sizes in an Arizona pine–oak forest