Accelerated nutrient cycling via leaf litter, and not root interaction, increases growth of Eucalyptus in mixed-species plantations with Leucaena

摘要

In the farmland/forest ecotone of southwest China, many areas are experiencing afforestation or reforestation by Eucalyptus in pure or mixed-species plantations for economic and ecological objectives. Compared with Eucalyptus monocultures, introducing nitrogen-fixing trees to Eucalyptus plantations with appropriate mixture proportions have the potential to increase productivity while maintaining soil fertility. Two mechanisms account for the improved nutrient availability: litter decomposition and root interaction. We hypothesise that the former mechanism may primarily contribute to the improvements in some arid regions where the soil is highly rocky. To test our hypothesis, the effects of admixed litter decomposition and root interaction on Eucalyptus camaldulensis and Leucaena leucocephala growth were investigated in a valley-type savanna. After 10years, E. camaldulensis growth in the mixed-species plantations was unaffected by root overlapping with L. leucocephala. E. camaldulensis grew significantly faster in the plots with admixture of litters in the mixed-species plantations than those without admixture. The admixed litter decomposition, rather than root interaction, is a primary contributor to E. camaldulensis growth in plantations with L. leucocephala in the valley-type savanna. The rocky soils in the savanna may block root overlapping by greatly inhibiting root extension of both tree species. The admixture of litters produces an increase in the supply and availability of soil nutrients that are returned to the E. camaldulensis rows via the admixed litters, due to the enhancement of the quantity, quality and decomposition rates of the leaf mixtures. These factors promote the growth of E. camaldulensis, and this effect may have implications for the necessity of litter management for Eucalyptus plantations in certain regions with arid climate and rocky soil.