GROWTH-RESPONSE TO THINNING AND ITS RELATION TO SITE RESOURCES IN EUCALYPTUS-REGNANS

The factors determining individual tree growth response are examined d uring the 4 years following thinning in experiments in even-aged, 8- o r 12-year-old regrowth Eucalyptus regnans F. Muell, forest at two site s in southern Australia. At one site, a vigorous understorey dominated by a sedge developed after the thinning. At that site, light-use effi ciency by the trees was unaffected by thinning and the aboveground bio mass production by the trees in the thinned stand was substantially le ss than that in the unthinned stand. At the other site, little underst orey developed, light-use efficiency by trees in the thinned stand was greater than that in the unthinned stand, and aboveground biomass pro duction was unaffected by thinning even though the leaf weight of the thinned stand was far below that of the unthinned stand. Where the und erstorey developed, it was concluded that it competed successfully wit h the trees for water, thereby reducing production in the thinned stan d when compared with the unthinned stand. The individual tree growth r esponse that occurred in the thinned stand at that site appeared to be due soley to the extra light available to individual trees following the canopy opening. Where the understorey did not develop, it was conc luded that individual tree growth response was due not only to the ext ra light available to individual trees but also to the increased avail ability of belowground resources, most probably soil water. Applicatio n of a pre-existing stand growth model suggested that at that site the tendency for increased growth resulting from extra water availability in the thinned stand was just balanced by decreased growth due to low er radiation absorption by the reduced canopy, so that net production was unaffected by thinning.