PARENT SESSION
Poster Session #9: Water Relations.
Monday, August 6, 2001. Presentation from 3:30 PM to 5:00 PM. Exhibition Hall

Xylem sap flow in a northern Utah Juniperus osteosperma.

LEFFLER, A. JOSHUA¹, CALDWELL, MARTYN¹, ¹

ABSTRACT- Leaf-level gas exchange measurements on Utah juniper from several studies suggest that transpiration rates are extremely low, however, the total foliage area can be high resulting in substantial amounts of water removed by these trees. We quantified the total water used with Granier-type xylem sap flux sensors for 9 juniper trees in a northern Utah juniper woodland and examined the effects of daily variations in weather and spatial distribution of individuals on transpiration from late June through October 2000. We calculate that juniper transpiration accounts for, on average, ~0.216 mol m^-2 s^-1 or ~0.69 mm d^-1 of the total water loss from this woodland. Transpiration rates varied through the season (highest in June and October) and were primarily correlated with soil water potential although VPD appears to influence transpiration in trees with low flux rates. Additionally, nocturnal transpiration was observed to account for ~ 35% of total transpiration. Variation among individuals was high and, despite expectations to the contrary, could not be explained by size of individuals or distance among individual trees. Our data indicate that large amounts of water can be transpired by juniper trees despite low leaf-level transpiration rates and that soil moisture primarily contributes to temporal water use variation in this population.

KEY WORDS: transpiration, water relations, Juniperus osteosperma, sap flux