You’ve learned that the trees here use rain water as their only water source and that the roots of the trees do not go very deep into the soil. Water accessibility might therefore very well be a limiting factor to tree growth in this forest. Dig a bit into the soil and have a look at the soil material. Is it fine sand? Coarse sand? Loamy sand? And is there a lot of humus mixed into this or is it not that dark? These are factors that determine the capacity of the soil to hold water. What kind of sandy soil do you think holds most water? Psst! if you want to check if the soil is loamy, you get a bit of soil material with a light colour (not with much humus in it) and you rub it on the back of your hand, when your hand gets beige/brown after you’ve scraped the soil off, there is loam present! See the picture!