INTERACTIONS BETWEEN DROUGHT AND ELEVATED CO2 ON OSMOTIC ADJUSTMENT AND SOLUTE CONCENTRATIONS OF TREE SEEDLINGS

Although drought tolerance of tree species is a critical determinant o f forest composition, how elevated CO2 affects drought tolerance is un certain. Interactions between elevated CO2 and drought on osmotic pote ntial and osmotic adjustment of American sycamore (Platanus occidental is L.), sweetgum (Liquidambar styraciflua L.), and sugar maple (Acer s accharum Marsh.) were investigated using l-yr-old seedlings, planted i n 81 pots and grown in four open-top chambers, containing either ambie nt air or ambient air enriched with 300 mu mol mol(-1) CO2. A well-wat ered treatment with plants watered daily and a droughted treatment in which plants were subjected to a series of drought cycles were include d within each chamber. Sugar maple and sweetgum seedlings completed a total of seven drying cycles, whereas sycamore seedlings, because of t heir greater leaf area and plant size, completed 11 cycles. The mean s oil water potential at re-watering for droughted seedlings in ambient CO2 was -0.5, -0.7, and -1.8 MPa for sugar maple, sweetgum and sycamor e, respectively, compared with -0.2, -0.7, and -1.2 MPa, respectively, under elevated CO2. By contrast, all well-watered plants were maintai ned at soil water potential >-0.1 MPa. Drought under ambient CO2 reduc ed osmotic potential at saturation for leaves of sycamore and sweetgum by 0.30 MPa and 0.61 MPa, respectively, but leaves of sugar maple did not display osmotic adjustment to drought. Elevated CO2 increased osm otic potential at turgor loss for leaves of sugar maple by 0.33 MPa un der well-watered conditions, and 0.48 MPa under drought. This response was not evident in the other species and might be related to the rapi d growth of sugar maple causing a depletion of solutes. Whereas drough t reduced the total solute concentration in roots of sugar maple, prim arily the result of a reduction in K, elevated CO2 did not alter the c oncentration of total solutes in roots of any of the three species. El evated CO2 has differing effects on drought tolerance among tree speci es, and thus might alter the competitive relations between species.