MICROCLIMATE ABOVE GRASS ADVERSELY AFFECTS SPRING GROWTH OF SEEDLING SNOW GUM (EUCALYPTUS-PAUCIFLORA)

Growth of snow gum seedlings (Eucalyptus pauciflora Sieb. ex Spreng.) was studied in response to differences in microclimate caused by diffe rential heat exchange between seedlings, grass and bare, moist soil du ring winter and spring. Seedlings were planted in a pasture either dir ectly into grassy groundcover or in circular patches of bare soil of 3 0, 60 or 120 cm in diameter, There were no differences in maximum air temperatures at seedling leaf height between treatments. However, mini mum air temperature increased by 2 degrees C with increase in patch di ameter from 0 to 120 cm such that seedlings surrounded by grass experi enced lower minimum temperatures with more frequent and more severe fr osts than seedlings growing in large patches of bare soil These small- scale differences in minimum temperature affected both photosynthetic and growth processes. Over winter, seedlings were photoinhibited, with depression in midday F-v/F-m linearly related to minimum temperatures . In spring, repeated frosts and lower minimum temperatures led to a d elay in the recovery of F-v/F-m, a delay in bud-break, damage to elong ating stems and developing leaves, lower rates of stem elongation, and ultimately a shorter growing season for seedlings in grass compared t o those in bare soil patches. Thus, microclimate above grass adversely affects spring growth of juvenile Eucalyptus pauciflora and may accou nt for much of the competitive inhibition of tree seedling growth by g rass during spring.