GROWTH OF EUCALYPTUS-MARGINATA (JARRAH) SEEDLINGS IN A GREENHOUSE IN RESPONSE TO SHADE AND SOIL-TEMPERATURE

The effects of shade and soil temperature on growth of Eucalyptus marg inata Donn ex Sm (jarrah) seedlings were studied in greenhouse experim ents. Plant dry weight and that of all plant parts declined in respons e to shade, as did root/shoot ratio. Plant leaf area was less in unsha ded plants than in plants grown in shade, and specific leaf area incre ased with shade. Unshaded seedlings had a higher light-saturated rate of photosynthesis, a higher light compensation point and a higher ligh t saturation point than seedlings grown in 70% shade. The relationship between plant dry weight and leaf dry weight was independent of shadi ng, whereas the relationship between plant dry weight and plant leaf a rea was dependent on shading. Therefore, leaf dry weight may be a bett er predictor of biomass production than leaf area in forest stands whe re shade is likely to affect growth significantly. Soil temperature ha d a significant effect on the growth of all plant parts except cotyled ons. Total plant growth and shoot growth were maximal at a soil temper ature of 30-degrees-C, but root growth had a slightly lower temperatur e optimum such that the root/shoot ratio was highest at 20-degrees-C. Roots grown at 15-degrees-C were about 30% shorter per unit of dry wei ght than roots grown at 20 to 35-degrees-C. We conclude that increases in irradiance and soil temperature as a result of overstory removal i n the forest will cause significant increases in growth of E. marginat a seedlings, but these increases represent a relatively small componen t of the growth response to overstory removal.