Here is recent lesson from the city of Melbourne, Australia, regarding drought and tree cover:
* the city experienced a drought from 1999 - 2007 [1] * as a short-term measure to conserve water, a decision was made to stop watering the city's trees [2] * consequently, ~40% of the city's trees are dead, or are in decline. The council now expects that 27% of the trees will die in the next 10 years, and 44% of the trees will die in the next 20 years.
Melbourne city now is aiming to improve tree canopy cover from 22% to 40% by 2040 [3], as a means to reduce the city's temperature by 4 degrees C. There is also work to capture storm water run-off from the city to use for irrigation.
Note that this is only talking about Melbourne's city centre (0.12m residential population of 4.2m total) [4].
As an aside, outside of cities, there is a recent Nature Geoscience paper that links lethal tree water stress thresholds to long-term climate models, and forecasts tree deaths due to drought in the southwestern United States by ~2050, assuming the world follows a high-emissions business-as-usual scenario (RCP 8.5, i.e. uncontrolled emissions, which tracks reality to date) [5].
[1] https://www.melbourne.vic.gov.au/Sustainability/AdaptingClim... [2] http://citiscope.org/story/2015/can-melbourne-lower-its-temp... [3] https://www.melbourne.vic.gov.au/Sustainability/UrbanForest/... [4] https://www.melbourne.vic.gov.au/AboutMelbourne/Statistics/P... [5] http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo24...