COPPICING ALTERS ECOPHYSIOLOGY OF QUERCUS-RUBRA SAPLINGS IN WISCONSINFOREST OPENINGS

In the spring of 1987, entire shoots were removed from Quercus rubra L . saplings in two southwestern Wisconsin forest openings. Shoots posse ssed newly expanding leaves at the time of coppicing. All coppiced ind ividuals sprouted from dormant stem buds near the root collar. Leaf ga s exchange and water potential were monitored on these sprouts and on untreated (control) e, rubra saplings throughout several clear warm da ys during the 1987 growing season. Daily maxima and averages for sprou t leaf photosynthesis and stomatal conductance generally exceeded thos e of controls. On average, treatment differences in daily maximum phot osynthetic rate were modest (11-14%) and were attributed primarily to a 30-38% enhancement of sprout leaf stomatal conductance. Relative dif ferences in daily average photosynthetic rate (29-39%) were substantia lly larger than those in daily maximum photosynthesis, owing to the fa ct that sprouts and controls exhibited distinct diurnal gas exchange p atterns. Photosynthetic rate and stomatal conductance of control leave s typically declined during the day following a mid-morning maximum. S prout leaves, on the other hand, tended to maintain gas exchange rates nearer to their morning maxima throughout the day. This difference in diurnal gas exchange pattern was associated with an apparent differen tial leaf sensitivity to leaf-to-air vapor pressure gradient (VPG). Th e relative decline in sprout leaf gas exchange rates with increasing V PG was less than that of controls. Treatment differences in gas exchan ge did not appear to be related to leaf water potential or tissue wate r relations, but sprouts had a higher soil-to-leaf hydraulic conductiv ity than controls.