Variation in transpiration and its relationship with growth for Pinus ponderosa Lawson in southern Arizona.

Abstract

The climate, water loss, arid certain aspects of growth in semiarid-site ponderosa pine were measured from 1 February 1966 to 10 March 1967 to determine the environmental relationships governing transpiration, internal water balance, and tracheid production. Water loss from branches and from entire trees was measured by means of polyethylene enclosures. Changes In internal water status of the tree were ascertained from seasonal trends in loss of water from branches and diurnal as well as seasonal changes in the radius of the bole. The elongation of branches and needles, and the production of tracheids and their size and wall thickness were related to the coincident internal water status of the tree. The branch enclosure technique was found to be a convenient method by which the water loss process could be studied. The seasonal changes in transpiration undoubtedly were influenced by the elevated temperatures that developed within the enclosure during the day. However, the elevated temperatures did not appear to adversely influence branch and needle elongation within the chamber. Warm air temperatures and low soil moisture during June and early July increased internal water stress, reduced daytime water loss, and reduced elongation of branches and needles. Fewer tracheids were produced in the upper bole. They were small and had thick walls, Moisture from rains in July replenished soil moisture and larger, thin-walled tracheids were differentiated so that the intra-anntjlar band of small, thick-walled cells became apparent. Gradual shrinkage of the upper bole in June implied that the above changes in growth were due to changes in internal water deficits. There were no marked changes in tracheid sizes and wall thickness in the lower bole during 1966. Intra-annular bands of narrow, thick-walled tracheids occurred times during the last 10 years in the upper bole in the last 50 years in the lower bole. Graphical analysis showed that midday reductions in rates of xi transpiration during periods of low soil moisture and high air temperatures occurred in spite of steepening water vapor gradients and before stomates were closed. This evidence supports the hypothesis that reductions in water loss during midday may result directly from internal water deficits. Soil moisture, air temperature, and incident radiation appeared to influence transpiration by altering the availability of water, the vapor pressure gradient, and the energy available for evaporation.