The Photosynthetic Regime of Some Southern Arizona Ponderosa Pine
|The Photosynthetic Regime of Some Southern Arizona Ponderosa Pine
|Year of Publication
|University of Arizona
The climate and CO2 exchange rates of some semi-arid site ponderosa pine were measured over a four-year period. Upper crown branches were placed in polyethylene enclosures and the CO2 exchange measured by an infrared gas analyser. These enclosures had little effect upon the natural environment, except through the maintenance of a constant wind speed. In this study the wind speed was low. Needle temperatures of unenclosed seedlings departed significantly from air temperature, and daytime needle temperatures of an enclosed seedling were similar to those of unenclosed seedling in low wind conditions. Nighttime needle temperatures of the enclosed seedling were lower than those of enclosed seedlings. An energy balance analysis was successfully applied to the needle temperature measurements of enclosed seedlings. When applied to enclosed seedlings this analysis sowed a considerable decrease in the amount of radiant energy transmitted by the polyethylene, primarily due to the reflection of 13% of the long wave radiation. The absorption by the seedling of emitted long wave radiation reflected by the enclosing polyethylene made it impossible to accurately predict the needle temperature of enclosed branches. A distinct, climatically influenced annual CO2 exchange regime was found with high rates of net CO2 absorption during non-freezing winter periods, and with low net CO2 absorption or net CO2 evolution during the hot, dry summer. With favorable environmental conditions high rates of net CO2 absorption were also measured during the spring and autumn seasons. Distinct daily regimes of CO2 exchange were found associated with specific environmental conditions. The measurements of CO2 exchange obtained from the enclosed branch were confirmed as representative by occasional enclosure and monitoring of the entire tree.