The Water Relations of Thuja occidentalis L. from Two Sites of Contrasting Moisture Availability

Tissue water relations of Thuja occidentalis representing natural populations from two different moisture habitats were analyzed from pressure-volume curves. Xylem water potential, osmotic potential, and the water potential at incipient plasmolysis of mature trees in the field were more negative in dry upland populations than in wet lowland populations. Analyses of seedlings grown from seed collected at each site, and grown in both wet and dry conditions, indicated that the site of seed origin did not affect the water relations of these seedlings. Seedlings grown under xeric conditions, however, developed a more negative water potential, osmotic potential, and water potential at incipient plasmolysis than did seedlings grown under wet conditions. The ability of T. occidentalis seedlings to acclimate as a response to variations in moisture availability, rather than ecotypic differences, probably plays an important role in allowing growth in both wet and dry sites.