Productivity, resource use, and competitive interactions of Fraxinus uhdei in Hawaii uplands

We examined stand growth, canopy development, and resource use of Fraxinus uhdei (Wenzig) Lingelsh, a nonindigenous tree grown in Hawaii, and its interactions with the native, N-fixing tree Acacia koa Gray. Along a gradient of decreasing rainfall with elevation, on Histosols, F. uhdei had decreased stand basal area, productivity, and canopy development. At high-elevation sites, productivity of F. uhdei was limited by N, and F. uhdei benefitted from association with A. koa, as (i) foliar N content of F. uhdei was positively related to aboveground net primary productivity (ANPP), (ii) leaf area index, biomass increment, and ANPP of F. uhdei increased in a single-species 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 stands, 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 composition of leaves (d13C), and, therefore, intrinsic water-use efficiency (WUE) increased with elevation. Biomass production of F. uhdei stands per unit leaf area and per unit intercepted radiation (e) decreased with increasing elevation on Histosols. Decreased nitrogen-use efficiency and e of F. uhdei on Histosols were both traded off against increased WUE.