On the success and failure of mixed-species tree plantations: lessons learned from a model system of 'Eucalyptus globulus' and 'Acacia mearnsii'

Abstract

Mixed plantations of a 'Eucalyptus' species with a nitrogen-fixing tree species can produce significantly higher quantities of aboveground biomass than monocultures. However, if species or sites are not chosen correctly, one species may suppress the growth of the other and mixtures may be less productive than monocultures. Based on a study of 'Eucalyptus globulus' and 'Acacia mearnsii', this paper discusses the species attributes and site factors that should be considered to improve the probability of increasing biomass production using mixed-species plantations. In an 11-year-old mixed-species trial of 'E. globulus' and 'A. mearnsii' in southeastern Australia aboveground biomass production was twice as high in mixtures containing 50% 'E. globulus' and 50% 'A. mearnsii' than in 'E. globulus' monocultures. There are three main types of interactions that led to this growth outcome: competition, competitive reduction and facilitation. Facilitation occurred as 'A. mearnsii' fixed significant quantities of N, both in monoculture and when mixed with 'E. globulus'. In addition, not only rates of N but also those of P cycling through litterfall were significantly higher in mixed stands than 'E. globulus' monocultures, pointing to the importance of selecting a nitrogen-fixing species that is capable of N fixation and subsequent fast nutrient cycling through litterfall. Mixed stands developed stratified canopies, such that 'E. globulus' eventually overtopped 'A. mearnsii' after 9 years. This resulted in an increase in light capture at the stand level and a reduction in competition for light for 'E. globulus', a relatively shade intolerant species. This illustrates the importance of selecting species based on their height growth dynamics and relative shade tolerances, to ensure that neither species is suppressed by the other and that the less tolerant species is not overtopped by the more shade tolerant species. In addition to species attributes, site factors, such as soil nitrogen, phosphorus and water availability, play an important role in the interactions and processes occurring in mixtures. In a pot trial containing monocultures and mixtures of 'E. globulus' and 'A. mearnsii', mixtures produced more biomass than monocultures of either species at low levels of N fertiliser. However, at high levels of N fertiliser 'E. globulus' suppressed 'A. mearnsii' and the biomass production of mixtures was not significantly different to that of 'E. globulus' monocultures. This suggests that mixtures should only be planted on sites where the processes and interactions between species will increase the availability of, or reduce competition for, a major limiting resource for growth at that site. The accurate prediction of successful mixed-species combinations and sites is difficult due to the limited number of studies on mixtures. A mechanistic approach is required to examine the interactions and processes that occur in mixtures and to demonstrate why certain combinations are successful on some sites and not others.