The key finding of the work is that water use efficiency has shown "an increase of nearly 40% over the past four decades". The explanation for this increase is that rising levels of atmospheric CO2 are allowing for a reduction in the stomatal conductance. At higher levels of CO2, the plants relax a bit, the CO2 is easier to extract from the atmosphere, the pores in the leaves through which transpiration takes place are smaller in diameter, and far less water escapes from the plants. The trees grow at the same rate with far less water! They did observe increasing wet season temperatures and an increase in cloud cover, but neither seemed to explain the increase in water use efficiency.
The facts seem inescapable—extra atmospheric CO2 is our gift to the trees growing throughout tropical regions!
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