In soil there are millions of vertical channels, pipes if you will. These are called ”capillary tubes.“ Whenever there is a downpour, the excess water runs underground through the capillary tubes. When it is dry, the same tubes transport the water to the surface. Trees have their roots in these capillary tubes - which also contain threads of fungi that are hygroscopic (= attracting water) - and with their lateral roots they soak up the capillary water when it is hot and dry. This is how a tree survives heat. In rocks, minuscule invisible fissures function as capillary tubes.

Even in less mild climates there is more than enough water supply in the soil for a tree to survive and grow. There is more rainfall in deserts than we think: often between 150 and 250 millimetres a year. This is equal to 150 to 250 liter per m² because 1 millimeter (mm) of rainfall is equal to 1 liter of water per m². That is 1.5 to 2.5 million liter per hectare. In many deserts there is 500 mm of precipitation a year. Some deserts even get up to 1000 mm annually. The Netherlands get around 700 mm a year. The problem is not the lack of precipitation, but the period during which it falls. In some locations it rains for one month and it remains dry for eleven. If the wet period is too short to allow the roots to reach the capillary water, the sapling (young tree) dies. If the wet period is long enough and the roots reach the capillary water, the sapling will survive. If we look at rocks (the Alps, the Rocky Mountains) or at savannas (Mali, Mauritania) then we see that enormous trees can easily grow there. Even above entrances to tunnels, for example, the Dutch Coentunnel, the Dordtse tunnel, and the Maastunnel, enormous poplars can easily survive prolonged periods of drought, like during the summer of 2006. The soil is only about six meters deep and there is a tunnel beneath it. Rocks or temporary drought are no problem for trees that use their leaves to absorb condensation water. This is especially no problem when the roots have already grown to the depth of the capillary water.

This picture shows clearly the difference between groundwater and capillary water. On some days the drought is so high that the humid capillary gets visible

If we supply the water requirement during the early stages – when a tree is still young and its roots have not grown deep enough to reach capillary water – then trees can grow virtually anywhere. In addition to that, a forest creates by itself the right conditions to stimulate and aid growth. Trees cause a different microclimate to develop where they grow. They provide shadow in which other plants can grow. Animals are attracted that distribute seeds. The soil becomes looser and richer and it receives organic matter which is able to hold water and fertilizers. An environment slowly forms in which a number of plants can grow. Nature has always done this without our help, changing bare rock into rich soil.