Production and resource use efficiencies in N- and P-limited tropical forests: A comparison of responses to long-term fertilization

At two sites at the extreme ends of a soil development chronosequence in Ha waii, we investigated whether forest responses to fertilization on young so ils were similar to those on highly weathered soils and whether the initial responses were maintained after 6-11 years of fertilization. Aboveground n et primary production (ANPP) was increased by nitrogen (N) application at t he 300-year-old site and phosphorus (P) application at the 4.1-million-year -old site, thus confirming earlier results and their designations as N- and P-limited forests. Along with ANPP, application of the limiting element co nsistently increased leaf area index (LAI), radiation conversion efficiency (RCE), and foliar and litter nutrient concentrations. Fertilization did no t consistently alter N or P retranslocation from senescent leaves at either site, but a comparison with other sites on the chronosequence and with a c ommon-garden study suggests that there is a genetic basis for low foliar an d litter nutrients and higher retranslocation at infertile sites vs more fe rtile sites. N limitation appears to be expressed as limitation to carbon g ain, with long leaf lifespans and high leaf mass per area. P limitation res ults in high P-use efficiency and disproportionally large increases in P up take after fertilization; a comparison with other studies indicates large i nvestments in acquiring and storing P. Although the general responses of AN PP, LAI, and RCE were similar for the two sites, other aspects of nutrient use differ in relation to the physiological and biogeochemical roles of the two elements.