Thomas E. Kolb, Ph.D.
Associate Professor Forestry Ecophysiology
School of Forestry
Northern Arizona University

Abstract
WATER USE OF TAMARIX AND NATIVE RIPARIAN TREES

My talk reviews information about water sources and water use of saltcedar (Tamarix sp.) and native riparian trees of the southwestern U.S., especially Fremont cottonwood (Populus fremontii) and willows (Salix sp.). Tamarix, Populus, and Salix all can use water from unsaturated, surface soil after rains when groundwater is deep. Strictly speaking, all these trees should be considered as "facultative" rather than "obligate" phreatophytes. However, groundwater is the dominant water source of mature trees of these species, whereas surface soil water is used heavily by young seedlings because of their shallow roots. Inter-annual variations in weather, especially the El Nino southern oscillation, can influence water source use of riparian trees. Total water use by riparian ecosystems containing Tamarix is influenced by interactions among stand structure, leaf area, plant species composition, weather, and soil salinity. Limited evidence suggests similar leaf-area based transpiration rate for Tamarix and selected native trees and shrubs, which suggests that differences in stand transpiration between Tamarix thickets and stands of native shrubs and trees are more strongly influenced by leaf area than leaf physiological characteristics. Tamarix water uptake, photosynthesis, and canopy dieback appear to be less sensitive to deep groundwater than native Populus and Salix. The high tolerance of Tamarix for deep groundwater is facilitated by high tolerance of photosynthesis to high temperature, high vapor pressure deficit, and negative xylem tensions, as well as salt accumulation that promotes water uptake. Moreover, Tamarix photosynthesis shows a larger positive response of photosynthesis between dry and wet conditions than Populus and Salix. Tamarix's combination of high stress tolerance and large photosynthetic response to improved water availability is unusual for plants. Tamarix also differs from native Salix gooddingii in seedling response to drops in groundwater level. Tamarix seedling survival was not affected by rates of groundwater decline between 0 and 4 cm/d, whereas Salix seedling survival declined sharply at rates > 1 cm/d. Tamarix seedlings grew tallest at a rate of 1 cm/d, and Salix seedlings grew tallest at a rate of 0 cm/d. Tamarix seedlings were considerably taller than Salix seedlings at rates between 2 and 4 cm/d. Thus, constant groundwater should favor Salix regeneration, and dropping groundwater should favor Tamarix regeneration.