Water Relations and Succession in Subalpine Conifers in Southeastern Wyoming

The water relations of three successional conifers, Pinus contorta Dougl. var. latifolia Engelm., Picea engelmannii Parry, and Abies lasiocarpa (Hook.) Nutt., were monitored at early successional (open) and late successional (understory) sites through the summer of 1980 in southeastern Wyoming. Maximum leaf conductance to water vapor diffusion (gwv) at the open site was greatest in A. lasiocarpa (6.1 mm s-1), intermediate in P. contorta (3.9 mm s-1), and least in P. engelmannii (1.5 mm s-1). Although values of maximum gwv were lower in all three conifers at the understory site than at the open site, the decrease was much greater for P. contorta (64%) versus A. lasiocarpa (39%) and P. engelmannii (13%). Minimum xylem pressure potentials were usually greater in P. contorta (-1.55 MPa) than in A. lasiocarpa (-1.9 MPa) and P. engelmannii (-2.0 MPa) at both sites. Reduction in the water vapor concentration difference between the leaf and air increased maximum gwv considerably in P. engelmannii (230%) and to a lesser extent in P. contorta (115%) and A. lasiocarpa (50%). Maximum transpiration flux density was greatest in A. lasiocarpa (83 mg m-2 s-1), followed by P. contorta (73 mg m-2 s-1) and P. engelmannii (30 mg m-2 s-1) throughout most of the summer, with consistently greater values recorded at the open site. Differences in water relations among the three conifers may contribute to the observed early successional status of P. contorta compared with the late successional conifers (P. engelmannii and A. lasiocarpa) in the subalpine zone of the Rocky Mountains.