Water relations of Quercus palustris: field measurements on an experimentally flooded stand.

Field measurements of water relations were used to examine the response of mature Quercus palustris (20 m tall), a flood tolerant tree, to experimental flooding treatments. Flooding treatments included: 1) flooding in the dormant season, 2) short-term flooding in the growing season and 3) long term (<2 years) continuous flooding. Dormant season flooding had no apparent effect on phenology or physiology during the following growing season. Short term flooding in the growing season caused immediate but reversible stomatal closure without significant development of water stresses. Within 10 days after the growing season flooding treatment water relations measurements were not significantly different from pre-treatment measurements or controls. Although no significant differences in water relations were found in growing season measurements on continuously flooded trees, continuous flooding reduced reproductive fitness and caused premature autumn coloration and leafabscission.Unlike upland Quercus species, Quercus palustris does not show evidence of water limitation late in the growing season. Stomatal conductance increased early in the growing season but showed no clear mid- to late-growing season trends. No evidence of mid-day stomatal closure was found throughout the growing season. Stomatal conductance was correlated to both xylem pressure potential and photosynthetically active radiation. Combined osmotic and matric potentials decreased to-2.43 MPa by Julian day 140 and remained constant throughout the growing season. Predawn xylem pressure potentials exceeded-0.4 MPa throughout the growing season.