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The balance between tree competition and facilitation may shift in relation to gradients in soil resource supplies, thereby determining patterns in overall stand-level production and community structure. We tested for variation in the nature and intensity of interactions between Eucalyptus saligna and nitrogen-fixing Falcataria mollucana along a gradient of soil nitrogen and phosphorus supply in Hawaii, USA. Neighborhood indexes were used to describe competitive and facilitative interactions between trees, and spatially explicit models of individual tree growth and survival were developed to look at the effects of soil nutrient supply on those interactions. The dynamics between neighboring trees depended strongly on soil nutrient supply. Large trees generally grew faster than smaller trees, but this pattern was moderated by interactions between nutrient availability and the intensity of competition and facilitation. Falcataria survival decreased by up to 30% with increasing Eucalyptus on low-nitrogen soils, but increased by nearly 70% on soils with higher nitrogen levels. Intraspecific interactions reduced Falcataria growth by up to 100% on high-phosphorus soils, yet facilitated the growth of focal trees growing on low-phosphorus soils by up to 60%. The effect of Falcataria neighbors on Eucalyptus growth also depended on phosphorus supply: increased Falcataria reduced predicted growth of the focal tree by 20% on low-phosphorus soils, but increased growth by 8% on high-phosphorus soils. Relationships between species were more dynamic at the scale of individual trees than previously indicated by stand-level assessments. The important role of soil nutrient supply in moderating both the intensity and nature of tree interactions implies that competition and facilitation studies need to consider a wide range of soil conditions to be generalizable.

Tree plantations are an important component of tropical landscapes, providing wood, fuel, and perhaps carbon (C) sequestration. Primary production in wet tropical plantations is typically nutrient limited. In some Hawaiian Eucalyptus plantations, nitrogen (N) limitations to production are alleviated by intercropping N-fixing Albizia trees that may decrease available phosphorus (P). Thus, sustainable productivity and C sequestration may depend on species composition. We measured soil N and P availability and ecosystem N and C sequestration in a 17-yr-old replicated replacement series of Eucalyptus and Albizia in Hawaii. Species composition included pure plots of each species and four proportions of mixtures. Soil N availability increased with the proportion of Albizia in the plot, but soil P availability declined. Aboveground tree C accumulation showed a synergistic response to increasing percentage of Albizia, with the mixed stands having more tree C than pure stands of Eucalyptus or Albizia. In the top 50 cm of soil, total N and C increased linearly with percentage of Albizia. Stands with the highest percentage of Albizia had 230 g/m2 more soil N and 2000 g/m2 more soil C than stands without Albizia. Stable C isotope analyses showed that increased soil C resulted from differences in both tree-derived C and “old” sugarcane-derived C. Deeper soil C (50–100 cm) was a substantial fraction (0.36) of total soil C but did not vary among treatments. Our results demonstrate that tree species effects on nutrient and C dynamics are not as simple as monocultures suggest. Mixed-species afforestation increased tree and soil C accrual over 17 years, and N inputs may increase soil C storage by decreasing decomposition.

Afforestation in the tropics may sequester soil C and has been proposed as a management tool to aid in controlling rising levels of atmospheric CO2. We measured changes in soil C following afforestation of sugarcane fields with fast-growing Eucalyptus saligna (Sm.) plantations in Hawaii. Using stable C isotopes, we estimated the contributions to changes in total soil C that were due to the loss of C from the prior cane cultivation, and to the gain of C from the new Eucalyptus plantations. Total soil C 10–13 yr after afforestation was 114 and 113 Mg/ha, respectively, in the Eucalyptus and cane plantation. Eucalyptus increased total soil C in the 0–10 cm layer by 11.5 Mg/ha, but that was offset by a loss of 10.1 Mg/ha of cane-derived C from the 10–55 cm layer. The net effect on soil C of afforestation of cultivated lands depends not only on new C gained, but also on C lost from the previous management.