Photosynthesis and reflectance indices for rainforest species in ecosystems undergoing progression and retrogression along a soil fertility chronosequence in New Zealand.

ABSTRACT

Measurements of photosynthesis at saturating irradiance and CO2 partial pressure, Amax, "adjusted" normalised difference vegetation index, RaNDVI, and photochemical reflectance index, RPRI, were made on trees sampled along a soil chronosequence to investigate the relationship between carbon uptake and ecosystem development in relation to nutrient availability. Measurements were made on the three most dominant species at six sites along the sequence in South Westland, New Zealand with soil age ranging from < 6 to 120,000 years resulting from the retreat of the Franz Josef glacier. The decrease in soil phosphorus availability with increasing soil age and high soil nitrogen availability at the two youngest sites, due to the presence of a nitrogen-fixing species, provided marked differences in nutrient availability. Mean Amax was high at the two youngest sites, then decreased markedly with increasing site age. Analysis of the data for individual species within sites revealed separation of groups of species in the response of Amax to Nm and Pm, suggesting complex interactions between the two nutrients. There were strong linear relationships for leaf-level RaNDVI and RPRI with Amax, at high irradiance, showing that measurements of reflectance indices can be used to estimate Amax for foliage with a range in morphology and nutrient concentrations. Notwithstanding the change in species composition from angiosperms to conifers with increasing site age, the presence of nitrogen-fixing species, the variability in foliage morphology from flat leaves to imbricate scales and a wide range in foliar nitrogen and phosphorus concentrations, there were strong positive linear relationships between site average Amax and foliage nitrogen, Nm, and phosphorus, Pm, concentrations on a foliage mass basis. The results provide insights to interpret the regulation of photosynthesis across natural ecosystems with marked gradients in nitrogen and phosphorus availability.