Photosynthesis and Transpiration of Trees in a Eucalypt Forest Stand: CO₂, Light and Humidity Responses

Abstract

Exchange of water vapour and CO₂ was measured on a small group of trees in a 12-year-old regenerating forest dominated by Eucalyptus spp. in Kioloa State Forest, south-eastern New South Wales. The trees were growing in a weighing lysimeter (10.3 m² ground area) with the dominant tree about 10 m high. A 12 m high enclosure made of polyethylene film was erected to enclose the trees on the lysimeter. Air was impelled in from the bottom of the enclosure by four fans at a total flow rate of 4.6 m³ s^-1. Air samples for infrared gas analysers were taken 1, 4, 7 and 10 m above the ground. Pure CO₂ (up to 100 litres min^-1) was injected to obtain the response of rates of photosynthesis and transpiration at various levels of CO₂ in the enclosure environment.

At normal ambient partial pressure of CO₂, the photosynthetic rate of the canopy (28 µmol m^-2 ground area s^-1) was found to be saturated at about half of full sunlight. At midday, the foliage in the layers 1-4 m, 4-7 m and 7-10 m from the ground level contributed 20.7, 44.3 and 35% of the total carbon assimilated by the canopy, respectively. Canopy conductance was reduced with increasing vapour pressure difference between leaves and the air on the basis that internal partial pressure of CO₂ was decreased. Light intensity required to saturate photosynthesis increased with increasing ambient partial pressure of CO₂. At 680 µbar ambient partial pressure of CO₂ and at saturating light intensity, there was a 50% increase in photosynthetic rate and a 30% reduction in transpiration rate, resulting in a reduction in transpiration ratio of about 80%. The apparent quantum yield derived at 340 and 680 µbar CO₂ was 0.049 and 0.071, respectively. The light compensation point also decreased at higher ambient partial pressure of CO₂.