Photosynthetic capacity increases in Acacia melanoxylon following form pruning in a two-species plantation

The photosynthetic responses to partial defoliation through pruning were examined in a 5-year-old Acacia melanoxylon plantation grown with a Pinus radiata nurse crop. Form pruning, the removal of large branches or competing leaders, was used to remove either 25 or 50% of tree foliage from throughout individual tree crowns and compare responses to unpruned trees. Photosynthetic capacity, expressed as light-saturated photosynthetic rate (Amax), was measured periodically over a 12-month period. An increase in photosynthetic capacity of phyllodes was observed between 2 and 6 weeks after pruning and the magnitude of the increase was greater in the 50% pruning treatment. The increase in photosynthetic capacity was confined to the upper two-thirds of the crown. While maximum stomatal conductance (gs) values were generally greater following 50% pruning, there were no pruning-induced changes in photosynthetic water-use efficiency (WUE) (Amax/gs). Foliar nutrient levels were not altered by pruning treatment. Changes in Amax were strongly correlated with phyllode phosphorus content and only weakly correlated with phyllode nitrogen content. The absence of corresponding changes in water-use efficiency, phyllode nutrient content and photosynthetic efficiency suggests that the observed increases in photosynthetic capacity were driven by changes in the carbon source:sink ratio. Form pruning that removes foliage solely from the upper reaches of the crown is likely to reduce the overall magnitude of photosynthetic enhancement in the crown and have the greatest negative effect on the growth of A. melanoxylon.