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Whole-Canopy Transpiration in CO2-Enriched and Warmed Pine Trees
Wang, K.-Y., Kellomaki, S., Zha, T. and Peltola, H.  2005.  Annual and seasonal variation of sap flow and conductance of pine trees grown in elevated carbon dioxide and temperature.  Journal of Experimental Botany 56: 155-165.

What was done
Wang et al. measured sap flow, crown structure and microclimatic parameters in order to calculate the transpiration rates of individual 30-year-old Scots pine (Pinus sylvestris L.) trees that were maintained for a period of three years in ambient air and air enriched with an extra 350 ppm of CO2 and/or warmed by 2 to 6C in closed-top chambers constructed within a naturally-seeded stand of the trees located near the University of Joensuu in eastern Finland.

What was learned
In the words of the scientists who conducted the work, "(i) elevated CO2 significantly enhanced whole-tree transpiration rate during the first measuring year [by 14%] due to a large increase in whole-tree foliage area, 1998, but reduced it in the subsequent years of 1999 and 2000 [by 13% and 16%, respectively] as a consequence of a greater decrease in crown conductance which off-set the increase in foliage area per tree; (ii) trees growing in elevated temperature always had higher sap flow rates throughout three measuring years [by 54%, 45% and 57%, respectively]; and (iii) the response of sap flow to the combination of elevated temperature and CO2 was similar to that of elevated temperature alone, indicating a dominant role for temperature and a lack of interaction between elevated CO2 and temperature."

What it means
As the air's CO2 content continues to rise, we probably can expect to see a decrease in evaporative water loss rates from naturally-occurring stands of Scots pine trees ... unless there is a concurrent substantial increase in air temperature.  As demonstrated in various places throughout our website, however, there is good reason to believe we will never see global, or even regional, temperature increases of the magnitude employed in this study.  Hence, our worst-case scenario would be for little to no change to occur in whole-tree Scots pine water loss rates in a high-CO2 world of the future.

Reviewed 20 April 2005