That place is a desert for a reason other than "nobody got around to planting trees". I really wonder what they need to do in order to start and maintain such a large forest. Will it ever become self sustainable?
Sudden precipitation is —oddly— somewhat common in Arabia and usually trickle through ergs, river beds empty most of the year, and sometimes captured by water walls, feeding oasis. The last few years have seen more violent incidents, overflowing ergs leading to floods, changes in temperature in the Indian Ocean, and more water in the monsoon-like pattern, I’m not sure.
It might make sense to adapt to the changing climate than the one they’ve known until now. If tree-supported water basins can handle more water than ergs, that might be relevant terraforming.
It’s the same in “Northern” Europe: Paris, Berlin, London, Amsterdam. Those cities had hot summers and needed fountains, but not so hot that they needed adaptations like shading all plazas with trees, having every building extend with parapets to shade the inside, etc. Paris has Venetian blinds (meant to let the air through but keep the sun out in summer), but they are not common in London, for instance. All those cities are now thinking of having gree water-culvert to complement storm drains.
Changing what we think of as normal might be necessary.
The same place was a rainforest once