Pinus radiata in a sub-humid temperate silvopastoral system: modelling of seasonal root growth

Abstract

The seasonal patterns of initiation of fine root tip and fine root elongation were studied for a period of 12 months for two genotypes of Pinus radiata (D. Don) planted with and without lucerne understorey, using rhizotron boxes. Periodic in situ growth of coarse root (52 mm diameter) diameter was also recorded over the same period. The maximum rates and timing of fine root tip initiation, fine root elongation, and coarse root cross-sectional growth were estimated by fitting the Gompertz equation to those growth data. Fine root tip initiation and elongation, and cross-sectional area (CSA) growth of coarse roots were mainly influenced by differences between the two genotypes and soil moisture stress caused by lucerne competition. Greater soil moisture content at the 10–30 cm depth in any of the treatments resulted in significantly greater fine root tip initiation and elongation compared with the 0–10 cm depth. The maximum rate of fine root tip initiation occurred earlier in the 0–10 cm than in the 10–30 cm depth, whereas the maximum rate of fine root elongation occurred earlier in the latter. The maximum rate of fine root elongation for clonal trees was 35% greater in the no understorey than in the lucerne treatment due to greater soil moisture availability in the former; whereas with the seedling trees it was not different between the understorey treatments. The occurrence of the rate maximum of fine root elongation was 8 weeks later for the seedling trees than that of the clonal trees in the no-understorey treatment, whereas in the lucerne treatment the difference was only 4 weeks. Coarse root CSA growth patterns were similar to that of the fine roots. The maximum growth rate of coarse root CSA occurred in early April (autumn) 10–12 weeks earlier in the no understorey than in the lucerne treatment.