Transpiration of cottonwood/willow forest estimated from sap flux

Cottonwood/willow forests in the American Southwest consist of discrete, ev en-aged vegetation patches arranged in narrow strips along active and aband oned stream channels of alluvial flood plains. We used the heat-pulse veloc ity technique in this study to estimate transpiration in 12 such forest pat ches along a perennially flowing reach of the San Pedro River in southeaste rn Arizona, USA during five periods from April to October 1997. Transpirati on per unit sapwood area was consistently higher for the larger cottonwood trees found on outer secondary channels compared to that of smaller cottonw ood trees along the active channel, but statistically significant differenc es were found only in August and October. Conversely, transpiration per uni t sapwood area in willow was markedly higher for trees along the primary ch annel than for those few larger trees that were sampled on the outer margin s of the forest. Average daily transpiration at the canopy scale among the patches in July was 4.8 +/- 0.7 mm per day and ranged from 5.7 +/- 0.6 mm p er day in young forest patches adjacent to the primary stream channel to 3. 1 +/- 0.6 mm per day in more successionally advanced patches on secondary c hannels. Differences in our estimates of transpiration between forest patch es along primary and secondary stream channels were related to differences in the ratio of sapwood area to ground area of the forest patches, and leaf area index. Estimates of transpiration from this forest type, and projecti ons of transpiration and groundwater flux over larger areas on the San Pedr o River, should take into account structural variation in these forests tha t relate to population dynamics of dominant trees. (C) 2000 Published by El sevier Science B.V.