Studies on the Status of Unburnt Eucalyptus Woodland at Ocean Grove, Victoria. IV. The Effect of Shading on Seedling Establishment

Abstract

The invasion of unburnt Eucalyptus woodland by Casuarina scrub has greatly reduced light penetration, hence seedlings encounter intense shading beneath the scrub and within the grass sward. Artificial shading experiments showed that Acacia pycnantha seedlings were the most shade-tolerant, followed by C. littoralis and C. stricta, while Eucalyptus ovata seedlings all died under prolonged intense shading, apparently owing to Botrytis cinerea. Shading (30% of full daylight) for 9 months (autumn-spring) increased the yield of A. pycnantha, C. littoralis and C. stricta seedlings compared with non-shaded seedlings. Intense shading (less than 8% of daylight) caused etiolation and decreased yields in all species. Chlorophyll a/b ratios of E. ovata and C. littoralis were decreased by shading. The total chlorophyll content of E. ovata plants was increased by shading, but that of C. littoralis was not affected. Shade tolerance of A. pycnantha, C. littoralis and C. stricta was partly morphogenetic in origin (increased leaf area ratios with shading) and partly due to resistance to fungal attack. All species had low compensation points. Shaded A. pycnantha seedlings retained pinnae for much longer than non-shaded seedlings. The differential survival of shaded E. ovata and C. littoralis seedlings is apparently not the result of differences in their photosynthetic efficiency at low light intensities, as sun-grown seedlings of both species had very similar light response curves. Prolonged shading significantly decreased the root/shoot ratios of all species except C. littoralis; hence shading will reduce the drought resistance of seedlings. The results from these experiments support the idea that the differential establishment of E. ovata and C. littoralis at Ocean Grove is partly due to differences in their shade tolerance.