ROOT-GROWTH OF SUB-ALPINE AND MONTANE EUCALYPTUS SEEDLINGS AT LOW SOIL TEMPERATURES

This study examines the effect of different soil temperatures on root growth in seedlings of Eucalyptus pauciflora Sieber ex Sprengel subsp. pauciflora and Eucalyptus nitens (Deane & Maiden) Maiden. Seedlings w ere grown in a glasshouse in pots containing soil. Pots were held in w ater baths maintained at 3, 7 or 13 degrees C, whilst shoots were expo sed to ambient glasshouse temperatures. The experiments were designed to separate direct effects of soil temperature from effects due to dif ferences in seedling size. In the first experiment, seedlings were gro wn to constant height (25 cm for both species), in the second to const ant time (100 days for E. pauciflora and 64 days for E. nitens) and in the third experiment seedlings were transferred between soil temperat ures. The rate of growth of both species increased with increasing soi l temperature. E. nitens grew faster than E. pauciflora at 7 and 13 de grees C, but E. pauciflora grew faster than E. nitens at 3 degrees C. The rate of browning of roots increased with decreasing soil temperatu re and at a faster rate in E. nitens than E. pauciflora. Root length w as highly correlated to root mass within diameter and colour classes ( r(2) > 0.7). However, brown roots were heavier than white roots. Conse quently, changes in root mass did not reflect changes in root length w hen the proportion of brown to white root also changed. For example, a t a constant height of 25 cm at 3 degrees C, E. nitens had greater roo t mass but lesser root length than E. pauciflora. E. pauciflora at 3 d egrees C grew faster, and had more root length and less brown roots th an E. nitens. This supports the argument that E. pauciflora is better adapted than E. nitens to survive and grow at lower soil temperatures.