Nutrient accumulation by trees and understorey shrubs in an age-series of Eucalyptus diversicolor F. Muell. stands

Abstract

Nutrient composition of aboveground components of overstorey trees and understorey shrubs were measured in four even-aged stands of E. diversicolor (4-, 8-, 11- and 36-years-old) and in a mature stand. The even-aged stands had regenerated from seed after cleafelling and slash-burning of native forest, and understories in the younger stands were the same age as trees. However in the 36-year-old and mature stands understories had regenerated from seed after prescribed burning and were 9 and 14 years of age, respectively. Trymalium spathulatum and the legume Bossiaea laidlawiana were the dominant understorey species in all stands. Aboveground nutrient pools varied less than biomass with stand age. This is because much of the increase in biomass as stands develop is due to production of heartwood, in which concentrations of nutrients are low. Understories contained a large proportion of the nutrients in all stands. In the 8- and 11-year-old stands about 46% of the aboveground biomass was understorey and this contained 49–56% of the N and S. In the 36-year-old stand, understorey contained 35% of the N and S although it was only 10% of the aboveground biomass. There was relatively more of the nutrient elements other than N and S in E. diversicolor than in understorey species in the 4-, 8-, and 11-year-old stands. In all five stands B. laidlawiana was an important component of the understorey, amounting to 51–93% of the biomass and containing 72–97% of the N. The leaves of B. laidlawiana contained about 40% of the P, and 31–36% of the N, S, Mg, Zn and Mn but comprised only 6–8% of the aboveground weight. Average rates of nutrient accumulation in aboveground biomass decreased with increasing stand age. This decrease was generally balanced by an increase in the amount of nutrients in litterfall so that, with the exception of mobile nutrients, the annual uptake of nutrients (accumulation plus litterfall) by vegetation was similar for each of the even-aged stands. Understorey accounted for a large proportion of the nutrients accumulating in aboveground biomass, particularly in the 36-year-old stand, where N, K, S, Zn and Cu accumulated more rapidly in understorey shrubs than in trees. The study suggests that the understorey plays a significant role in the nutrition of E. diversicolor stands.