Interactions between drought and elevated CO on growth and gas exchange of seedlings of three deciduous tree species.

Interactions between elevated atmospheric CO and drought on growth and gas exchange of American sycamore (Platanus occidentalis L.), sweetgum (Liquidambar styraciflua L.) and sugar maple (Acer saccharum Marsh.) were investigated using 1-yr-old seedlings, planted in 8 1 pots and grown in four open-top chambers, containing either ambient air or ambient air enriched with 300 μmol mol CO . Two soil moisture regimes were included within each chamber: a 'well-watered' treatment with plants watered daily and a 'drought' treatment in which plants were subjected to a series of drought cycles. Duration and depth of the drought cycles were determined by soil matric potential. Mean soil water potential at rewatering for the water-stressed seedlings under ambient CO for sugar maple, sweetgum and sycamore was -0.5, -0.7 and -1.8 MPa, respectively, compared with > -0.1 MPa for all well-watered plants. Elevated CO increased relative growth rate of well-watered sugar maple by 181%, resulting in a 4.3-fold increase in total plant dry weight after 81 d, compared with 1.4 and 1.6-fold increases for sweetgum and sycamore, respectively, after 69 d. Although elevated CO increased net CO assimilation rate of sugar maple by 115%, there was a 10-fold increase in leaf area which contributed to the growth response. Although drought did not eliminate a growth response of sugar maple to elevated CO it greatly reduced the elevated CO -induced enhancement of relative growth rate. In contrast, relative growth rates of sweetgum and sycamore were not significantly increased by elevated CO . Drought, under elevated CO , reduced leaf area of all three species to a greater extent than it reduced net CO assimilation rate. The response ranged from no effect in sugar maple to a 40 % reduction in sycamore, with sweetgum exhibiting an intermediate response. Results indicate that drought may alter the growth response, gas exchange and water relations of tree species growing in an elevated CO atmosphere. Under high nutrient and water availability, sugar maple may benefit the most (of the three species studied) from a CO - enriched atmosphere, but productivity gains will be limited if frequent drought is prevalent.