I think... it depends.
I'm going to discuss it all from theory, not practiced myself, so there's that.
The point about making any plant drought resistant is allowing it to get water when it is not available. Otherwise you don't need to
make it tolerant when the plant is already resistant to drought.
So, where's water in drought climates? Usually there's some water below the
roots, dipping slowly onto the water table. There's the water table. There's the occasional irrigation by the gardener. There's some humidity captured by the organic matter in the soil. There can be mist and dew that some plants are able to catch.
And water is lost through evapotranspiration: The heated ground, the more thirsty plants taking more water for standing against the scorching sun or just using water for the photosynthesis process.
Trees are good at throwing their roots deep into the ground and reaching the more humid layers and the water table. Therefore, the faster they extend their roots below, the more resistant they get to drought. This is true even for
native plants already adapted to drought.
One reliable method is to deep irrigate the trees during the first year. Deep irrigation is watering in large quantities when the soil is dry and the plant is active. That can be 30 litres of water per plant every 3 weeks during the dry season. It is helpful to irrigate directly at the depth of the roots and not watering the surface where water is lost very fast.
But maybe the
land has not
enough dirt for plants to develop
deep roots, or maybe you don't have enough water for irrigating all the plants. In these cases, adding extra organic matter and mulching can increase the humidity in the upper layers, and while it will slow down the production of deep roots, it may save the life of the sapplings. Letting the plant lose some leaves thus reducing the water needs, also helps, but don't prune them during drought. If you are blessed with weeds that capture mist and dew, you can use them as an extra water source for your other plants.
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