|Schaeffer, S. - UNIVERSITY OF ARKANSAS|
|Williams, D. - UNIVERSITY OF ARIZONA|
Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 11, 2000
Publication Date: December 13, 2000
Interpretive Summary: INTERPRETIVE SUMMARY Arid and semi-arid regions account for approximately one-third of the land mass of earth. These regions are experiencing continued pressure from population growth in many parts of the world. Water is a critical resource in these regions and is often in short supply. Within arid and semiarid regions free-flowing, perennial streams, with hardy stream side (riparian) vegetation are often rare but support a very large number of plants and animals. The amount of water used by the stream side vegetation can be very large but is difficult to determine. In this study on the San Pedro River in southeastern Arizona a method is used to measure the amount of water flowing through the trucks of individual cottonwood and willow trees. It was found that there was a significant difference in the water use between trees on the main flowing channel and non-flowing secondary channels with older but fewer trees. This is critical information needed to estimate overall riparian vegetation water use along the entire San Pedro as the character and age of the riparian forests must be accounted for in these estimates.
Technical Abstract: ABSTRACT Cottonwood/willow forests in the American Southwest consist of discrete, vegetation patches arranged in narrow strips along active and abandoned stream channels. We used the heat-pulse velocity technique in this study to estimate transpiration in 12 such forest patches along a perennial reach hof the San Pedro River in SE Arizona, USA during five periods in 1997. Transpiration per unit sapwood area was consistently higher for the larger cottonwood trees found on outer secondary channels compared to that of smaller cottonwood trees along the active channel, but statistically significant differences were found only in August and Oct. Conversely, transpiration per unit sapwood area in willows was markedly higher for trees along the primary channel than for those few larger trees on the outer margins of the forest. Average daily transpiration at the canopy scale among the patches in July was 4.8+/-0.7mm/day and ranged from 5.7+/- 0.6mm/day in young forest patches adjacent to the primary stream channel to 3.1+/-0.6mm/day in the older patches on secondary channels. Differences in transpiration between patches along primary and secondary channels were related to differences in the ration of sapwood area to ground area and leaf area index. Estimates of transpiration from this forest type, and projections of transpiration and groundwater flux over larger areas on the San Pedro River, should take into account structural variation in these forests that relate to population dynamics of dominant trees.