Plant-soil nutrient relationships in North Queensland wet tropical rain forests\ud

This study investigated plant-soil nutrient relationships at seven sites in the wet tropical rain forests of north Queensland, Australia. Biomass and nutrient standing stocks were determined for 24-year-old successional vegetation recolonizing a disturbed site (280 m²). This data was used to generate biomass regressions to estimate above-ground biomass at three undisturbed sites where the influence of soil fertility on fine root dynamics and resource allocation were examined. A further three undisturbed sites were sampled to examine the relationship between fine root biomass and soil fertility.\ud \ud The dry weight of 150 trees was measured, yielding an above-ground biomass of 53.7 t ha⁻¹. Foliage comprised 7.0%, branches 9.6%, stems 83.0% and reproductive structures 0.4% of the total. The total nutrient standing stocks at the site were 158.8 kg ha⁻¹ nitrogen, 7.53 kg ha⁻¹ phosphorus, 172.7 kg he potassium, 129.0 kg ha⁻¹ calcium, and 43.2 kg ha⁻¹ magnesium. Most of the nutrients were contained in the stems of the vegetation. Nutrient concentrations were found to vary within species, between species and between components. Foliage had the highest concentrations of all nutrients and the greatest variability between species. The lowest foliar concentrations were found in the late secondary species, Darlingia darlingiana, while the highest were found in the early secondary species. Foliar nutrient concentrations were found to decrease with increasing foliage biomass for Alphitonia petriei, however, this trend was not evident for D. darlingiana.\ud \ud Thirteen biomass regressions, based on three allometric models and an exponential model, were tested. Regressions were determined for total above-ground biomass and foliage biomass against diameter at breast height (dbh), alone and in combination with tree height and wood density. The exponential regressions were found to be unsuitable due to increasing variance with increasing tree size, while all allometric regressions gave highly significant correlations. The equations utilising both dbh and tree height yielded better estimates than equations using dbh or tree height alone. The inclusion of wood density into the regressions did not significantly improve the estimate of biomass. Biasing induced by the logarithmic transformation varied from 11 to 51%.\ud \ud Estimates of above-ground biomass did not vary substantially at three sites on soils of differing fertility. Fine root biomass was greatest on nutrient-poor sites, and lowest on more fertile sites. However, it is unknown whether this is due to differences in available phosphorus or some other, as yet, undetermined factor. Total fine root biomass in the top 50 cm of soil was found to range from 7.7 to 27.0 t ha⁻¹. The greatest concentration of fine roots at all sites was in the top 10 cm of soil, and this decreased rapidly with depth. Fine root production differed significantly between the sites, with the highest rate of production (3.28 t ha⁻¹ y⁻¹) occurring on the nutrient-rich site. Fine root production showed distinct seasonal variations, with peak production occurring during the wet season. Fine root proliferation coincided with increased phosphorus availability at all sites. The results suggest that fine roots may be as important as litterfall in returning nutrients to the soil in tropical forests. Annual nutrient turnover in the top 10 cm of soil by fine roots varied between nutrients and between sites, from 11% of the total magnesium at Mt Spec, to 192% of the phosphorus at Mt Fox.\ud \ud The results of this study demonstrate the influence of soil fertility on fine root dynamics and resource allocation within tropical rain forests