M. Keith Owens

The proliferation of saltcedar (Tamarix spp.) along regulated rivers in the western United States has transformed riparian plant communities. It is commonly assumed that transpiration by these alien plants has led to large losses of water that would otherwise contribute to streamflow. Control of saltcedar, therefore, has been considered a viable strategy for conserving water and increasing streamflow in these regions. In an effort to better understand the linkage between transpiration by saltcedar and streamflow, we monitored transpiration, stream stage, and groundwater elevations within a saltcedar stand along the Pecos River during June 2004. Transpiration, as determined by sap flow measurements, exhibited a strong diel pattern; stream stage did not. Diel fluctuations in groundwater levels were observed, but only in one well, which was located in the center of the saltcedar stand. In that well, the correlation between maximal transpiration and minimal groundwater elevation was weak (R² = 0.16). No effects of transpiration were detected in other wells within the saltcedar stand, nor in the stream stage. The primary reason, we believe, is that the saltcedar stand along this reach of the Pecos River has relatively low sapwood area and a limited spatial extent resulting in very low transpiration compared with the stream discharge. Our results are important because they provide a mechanistic explanation for the lack of increase in streamflow following large‐scale control of invasive trees along semiarid rivers.

Ashe juniper (Juniperus asheiBuchholz) has encroached into the historical grasslands of the Edwards Plateau, an environmentally sensitive area that serves as the recharge zone for the Edwards Aquifer. An understanding of gas exchange and water relations of the two dominant tree species, Ashe juniper and live oak (Quercus virginianaP. Mill. var.fusiformis), is important to assess the trade‐offs between potential aquifer recharge and carbon storage during woody plant encroachment. Over a 12‐month period, juniper consistently had lower instantaneous carbon assimilation, transpiration rates and stomatal conductance than live oak. Oak exhibited greater seasonal variation and a greater correlation with antecedent precipitation in gas exchange measurements, whereas juniper did not respond to precipitation events. The consistent low rates of gas exchange and stomatal responses in juniper could indicate shallower rooting structure and/or limitation of hydraulic conductivity, as well as lower photosynthetic capacity. The higher rates of photosynthesis, transpiration and stomatal conductance exhibited by live oak during drought suggest alternate sources of soil moisture and possibly a deeper rooting pattern than Ashe juniper. Water‐use efficiency was greater for juniper than for live oak suggesting that juniper may sequester more carbon than oak trees for a given amount of water. This finding implies that Ashe juniper may not be the extravagant water user previously reported but may serve an important role in carbon sequestration. Copyright © 2009 John Wiley & Sons, Ltd.