Carbon exchange and water loss from two evergreen trees in a semiarid woodland
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Ashe juniper (Juniperus ashei Buchholz) 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 virginiana P. 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. 2009 John Wiley & Sons, Ltd.
