Contribution of acacias to the growth and nutrient status of eucalypts in mixed-species stands at Ratchaburi, Thailand

The long-term sustainability of eucalypt and acacia plantations (monocultures) is being questioned because of possible adverse effects on nutrient cycling and nutrition of trees leading to decreased productivity. To test whether plantations with mixed species may offer an alternative, Eucalyptus camaldulensis and Acacia auriculiformis were planted in mixtures at Ratchaburi Experimental Station, Thailand. The plantation contained five treatments at two tree densities. Above-ground growth was monitored periodically for four years. Height of eucalypts was not affected by acacia in mixtures. Acacia bad higher tree sectional area at breast height and stem volume than eucalypts. pointing to greater competition for soil resources by acacia. Despite this, mean sectional area, annual increments in the sectional area and stem volume of eucalypts 48 months after planting showed major differences when growing in combination with acacia. Higher values of the above parameters were observed when the proportions of eucalypt and acacia were equal. Above-ground biomass and nutrient accumulation were measured at the lime of first thinning of high-density plots when the trees were 28 months old. By then the amount of N accumulated in various tree components on a plot depended on the proportion of acacia in the mixture, whereas accumulation of other nutrients depended more on the species and their relative growth rates (i.e. the amount of biomass produced). The approximate amount of N fixed by acacia after 48 months (calculated by using total N difference method) ranged from 57 (25% acacia) to 201 kg/ha (75% acacia). The positive effect of acacia on the growth of eucalypts was probably related to improved N nutrition. The merits of growing acacia in mixtures on the growth of eucalypts are discussed.