Productivity, resource use, and competitive interactions of Fraxinus uhdeiin Hawaii uplands

We examined stand growth, canopy development, and resource use of Fraxinus uhdei (Wenzig) Lingelsh, a nonindigenous tree grown in Hawaii, and its inte ractions with the native, N-fixing tree Acacia koa Gray. Along a gradient o f decreasing rainfall with elevation, on Histosols, F. uhdei had decreased stand basal area, productivity, and canopy development. At high-elevation s ites, productivity of F. uhdei was limited by N, and F. uhdei benefitted fr om association with A. koa, as (i) foliar N content of F. uhdei was positiv ely related to aboveground net primary productivity (ANPP), (ii) leaf area index, biomass increment, and ANPP of F. uhdei increased in a single-specie s stand after N additions, but there was no response by either F. uhdei or A. koa in a mixed stand, and (iii) productivity of F. uhdei in mixed stands with A. koa at high-elevation sites was greater than in single-species sta nds, and F. uhdei foliage was enriched with N in proportion to the fraction of stand basal area in A. koa. Seemingly, growth of F. uhdei on Histosols was also limited by water availability, as an index of carbon isotope compo sition of leaves (delta C-13), and, therefore, intrinsic water-use efficien cy (WUE) increased with elevation. Biomass production of F. uhdei stands pe r unit leaf area and per unit intercepted radiation (epsilon) decreased wit h increasing elevation on Histosols. Decreased nitrogen-use efficiency and epsilon of F. uhdei on Histosols were both traded off against increased WUE .