Twenty years of stand development in pure and mixed stands of Eucalyptus saligna and nitrogen-fixing Facaltaria moluccana

The vast majority of tropical tree plantations are monocultures, and only a few case studies have examined growth of mixed-species plantations. Mixtures that contain nitrogen (N)-fixing species may have the potential to increase overall plantation yields, but too few case studies are available to provide general insights. We extended previous measurements of 10 years of stand development in a mixed-species experiment in Hawaii through age 20 years. The plots had a constant total number of stems initially (2500 ha−1), but varying ratios of Eucalyptus saligna (Sm.) to nitrogen-fixing Facaltaria moluccana (Miquel) Barneby & Grimes (=Albizia Facaltaria (L.) Fosberg, and Paraserianthes Facaltaria (L.) I. Nielsen). Eucalyptus stem growth declined strongly in monocultures after age of 7 years, but showed no decline in mixtures. After 10 years of stand development, Eucalyptus stem mass was greatest in the monoculture, averaging about 155 Mg/ha (versus 140 Mg/ha in the 0.5 Facaltaria treatment); total (combined) stem mass for both species was almost twice as great in mixtures as in monocultures. By the age of 20 years, Eucalyptus stem mass was within a 20% range across treatments (260 Mg/ha), and total stem mass ranged up to 430 Mg/ha in the 0.75 Facaltaria treatment. Losses of stem mass from mortality were moderate for Eucalyptus between 18 and 20 years, averaging 20–25% of gross stem growth in mixed-species treatments, but 60% in the Eucalyptus monoculture. Mortality losses of stem mass were extremely high for Facaltaria in all plots, matching or exceeding gross stem growth from age 18 to 20 years. The size of individual Eucalyptus trees was much greater in the mixed plots than in the monoculture plots, so any short-term economic gain from the mixed-species plots may derive from larger tree sizes rather than from more wood mass per hectare. The longer term benefits of mixed plots may include the effects of Facaltaria on soil fertility and community diversity. Overall, these very large changes in ecosystem production in mixtures underscores the need for a large number of additional studies to examine other species combinations, sites, and silvicultural systems.