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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. |
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