Physiological basis of spacing effects on tree growth and form in Eucalyptus globulus

We examine the effects of spacing and layout on the growth and form of 3- t o 4-year-old Eucalyptus globulus in a farm forestry context. Four planting layouts were chosen. These represented the range commonly in use in farm fo restry: block plantings (2x4 in), triple rows (2x4 in) at 10-m intervals, s ingle rows (2x10 in) and isolated trees (10x10 in). The physiological signi ficance of key results is interpreted in terms of changes in the parameters of a simple plantation growth model. Under conditions where levels of dire ct light are high, for example during summer, block-planted trees intercept ed only 38% of the light intercepted by isolated trees. On a stand basis, h owever, the combination of incident radiation and ground coverage declined with lower stand densities. While stand leaf area index declined from aroun d 6 to 1 with increased spacing, individual tree leaf areas rose from aroun d 50 m(2) in block plantings to 150 m(2) in isolated trees. The proportion of above-ground biomass found in stems declined with increasing spacing as the mass in foliage and branches increased. Stems accounted for 65% of abov e-ground biomass in block-planted trees but only 35% in isolated trees. The contributions of leaves and branches correspondingly rose from 19% to 35% and from 16% to 29%, respectively. Changes in biomass distribution were acc ompanied by increasing branch number, branch thickness, flatter branch angl es and the longer retention of lower branches with greater spacing. These c hanges have implications for the merchantability of the timber. The efficie ncy of above-round radiation conversion was constant at 0.67 g MJ(-1) irres pective of spacing. We estimated that foliar maintenance respiration (R-m) accounted for about 90% of above-ground R-m. On a stand basis R-m costs blo ck plantings 23.90 t DM ha(-1) year(-1) (50% annual above-ground photosynth etic production) compared with 6.22 t DM ha(-1) year(-1) (40% annual above- ground photosynthetic production) in stands of isolated trees.