Studies on the Status of Unburnt Eucalyptus Woodland at Ocean Grove, Victoria. V. The Interactive Effects of Droughting and Shading on Seedlings Under Competition.

Abstract

Casuarina littoralis seedlings are inherently more drought-resistant than Eucalyptus ovata seedlings over a wide range of environmental conditions. Moderate shade pre-treatment (30% of full daylight) decreased the drought resistance of seedlings of E. ovata, Acacia pycnantha and C. stricta but not that of C. littoralis seedlings. Deep shade pre-treatment (8 % of full daylight) decreased the drought resistance of all species and was associated with decreased rootlshoot ratios. Both shaded and non-shaded C. littoralis seedlings closed stomata at higher relative water contents (about 80% and 88 % respectively) than did E. ovata seedlings (about 36 % and 63 % respectively). Shading decreased the relative water content at which E. ovata closed stomata and reduced the relative decrease in water potential which occurred with unit decreases in relative water content. When E. ovata and C. littoralis seedlings were grown in competition, the larger E. ovata dominated the drought response of plants under both high and low light conditions. E. ovata rapidly depleted moisture supplies thereby subjecting C. littoralis to greater stress and earlier death than it experienced in monoculture. C. littoralis seedlings grown and droughted in competition with E. ovata exhibited smaIIer decreases in water potential per unit decrease in relative water content than seedlings grown in monoculture. The height growth of E. ovata grown in monoculture and in competition with C. littoralis was reduced for at least 10-15 weeks after the wilting treatment, but height growth of C. littoralis was not affected. Eucalypts wilted at higher water potentials (-4.3 MPa) than did C. littoralis seedlings (- 6.3 MPa). It is suggested that the replacement of E. ovata by C. littoralis at Ocean Grove, Vic. may be partly due to the differential effects of shading on the drought resistance of seedlings which become established in the grass sward of canopy gaps.