Eucalyptus and Acacia tree growth over entire rotation in single- and mixed-species plantations across five sites in Brazil and Congo

Abstract The association of N 2 -fixing species (NFS) could be an attractive option for achieving a sustainable increase of Eucalyptusplantations (EP) productivity through a positive balance between facilitative effects and competition between species. A randomised block design was replicated at four sites (Cenibra, USP, Suzano and IP) in Southern Brazil and at one site in Congo. The development of mono-specific stands of Acacia mangium(100A) and Eucalyptusgrandis or urophylla × grandis (100E), was compared with N fertilisation treatment (100E + N) and with mixed-species plantations in a 1:1 ratio (50A:50E), and in an additive series with varying densities of acaciafor the same density of eucalypt (25A:100E, 50A:100E, 100A:100E). The objectives were to assess the effect of mixtures on tree growth and stand production, and the behaviour of the two species in contrasting soil and climatic conditions. Tree growth was monitored over stand rotation and the biomass of aboveground tree components estimated at mid-rotation and at harvesting age. Eucalyptusheight was 13% higher in Brazil than in Congo. Favourable ecological conditions in Congo and Cenibra led to 50% higher Acaciatree height than at the other sites. A depressive effect of Eucalyptusneighbour trees on Acaciaheight and circumference growth, lower in Congo than in Brazil, was observed in the mixtures from age 1–2 years onwards. Depressive effects of acaciaon eucalypt height and circumference growth were low in USP, Suzano and IP, high in Cenibra, and not observed in Congo, in 50A:50E and 25A:100E. A positive though insignificant response to N fertilisation was only found in USP and Congo. Complementarity for light and soil resource capture between Eucalyptusand Acaciatrees resulted in mean annual incrementsin total stand stemwood biomass (MAI) that were 7–15%, 6–12%, and 40% higher in the additive series than for 100E in Cenibra, USP and Congo, respectively at mid-rotation. Whilst lasting complementarity and facilitation in Congo led to 17–34% higher MAI in mixtures than for 100E at harvesting age, MAIs were not significantly higher in mixtures than for 100E in Brazil. Mixed-species plantations of Eucalyptusand A. mangium might enhance aboveground stand production on poor nutrient soils in warm and humid tropical climateswith low water limitations.