Altitudinal Variation in the Photosynthetic Characteristics of Snow Gum, Eucalyptus pauciflora Sieb. Ex Spreng. I. Seasonal Changes Under Field Conditions in the Snowy Mountains Area of South-Eastern Australia

Abstract

A field study of seasonal changes in the photosynthetic characteristics of three altitudinal populations of the snow gum (Eucalyptus pauciflora Sieb. ex. Spreng) was conducted at elevations of 915, 1215 and 1645 m in the Snowy Mountains area of south-eastern Australia.

At optimal temperatures in midsummer, peak rates of photosynthesis under CO₂-saturating conditions (P_sub) reached levels of 125-135 ng cm^-2 sec^-1, and under ambient CO₂ conditions (P_amb) reached levels of 45-55 ng cm^-2 sec^-1. Corresponding values of the intracellular resistance to CO₂ transfer (ri) were 5-6 sec cm^-1, and of the gas phase resistance to water-vapour transfer (r,) 1.2-2 sec cm^-1.

Measured at temperatures of 15, 20 and 25°C, the peak seasonal values of P_max showed no significant difference between sites, but at 5° and 10° peak values were highest at the highest elevation (coldest site), and at 30° and 35° peak values were highest at the lowest elevation (warmest site). Generally similar patterns applied to Pa_amb. These features are consistent with the view that E. Pauciflora shows continuous variation in many physiological characteristics over its altitudinal range.

At each site the temperature optimum for photosynthesis changed markedly during the season, and was closely correlated both with the long-term maximum air temperature and with the mean maximum temperature of the 10 days prior to the date of measurement. This appeared to reflect long-term adaptation of the photosynthetic apparatus of each population to the general temperature conditions at each site, combined with a short-term acclimation to the prevailing seasonal temperature regime.