CO2 -> flooding. New variable

BBC NEWS | Science/Nature | Climate flooding risk 'misjudged'

Apparently, leaves reduce the stomata size with increasing levels of CO2 in the atmosphere - and hence release less water into the air. Lesser groundwater gets pumped into the air this way, and with a severe rain - this can cause more flooding since the ground's closer to saturation already.

But is this an isolated cause-effect relationship ? Aren't there so many more factors in play ?

• there just are fewer trees
• groundwater's getting pumped out pretty fast on most of the planet
  
• the rains themselves are more severe (and if the trees releasing less water is a major enough amount, won't the rains reduce too?)
• This is likely to be a localized cause-effect, depending on soil composition, tree cover. Is this significant enough relative to the concreting/paving of large urban spaces ?
  

Of course, this may have an impact, but one gets the sense that this may be a little bit of barking up the wrong tree :)