BOTH NITROGEN AND PHOSPHORUS LIMIT PLANT-PRODUCTION ON YOUNG HAWAIIANLAVA FLOWS

We applied fertilizers in a 2(3) complete factorial design to determin e the effects of nutrient amendments on plant growth in Hawaiian monta ne forests growing on two different volcanic substrates:'a'(a) over ba r and pahoehoe lava. Both sites were about 140 years old and their ove rstories were nearly monospecific stands of Metrosideros polymorpha. F ertilizer applications included N, P, a mixture of essential macro- an d micronutrients excepting P and N, and all combinations thereof in ea ch of four blocks. Additions of nutrients other than N or P had no sig nificant effects on measured plant-growth variables. In contrast, addi tions of either N or P significantly increased tree height growth, dia meter increments, biomass growth, and height growth of the understory fern Dicranopteris linearis in both sites. The effect of N was greater than that of P. Greatest growth rates occurred in plots receiving bot h N and P, and signficant NP interactions occurred in several cases, suggesting a synergistic effect between these two elements. Plant grow th on these young, poorly weathered, basaltic lavas is colimited by N and P availability. Growth in a similar-aged stand rowing on a mixture of volcanic ash and cinders is N but not P limited, indicating that t he texture of the parent material influences nutrient-availability pat terns during early primary succession.