How many space is required to plant trees?
It is not possible to answer this question. I will give you various details that can help you determine the planting distance between the trees. All this information is based on planting in hot areas:
1. The annual rainfall defines the species that you can use (as each species has a minimum water requirement). The first thing to do is look for traditional plantations. In general, traditional growers know what is the optimum planting distance is for their trees.
2. If the rainfall is just enough for the minimum requirement, you can plant less trees per hectare/acre. The size of the tree also defines spacing, if you prune them smaller they evaporate less, so you can plant more trees per hectare/acre. If the soil is sandy, it is dry and poor in minerals. So you plant less trees per hectare/acre compared to when your soil consists of clay and is rich in minerals.
3. Do temperatures go to extreme heights (up to + 45⁰C) or do they remain moderate (up to + 35⁰C). Extreme heights means less trees per hectare/acre. The reason is that for extreme high temperatures trees need water to evaporate, so you give a tree more surface to grow on, this way they have less competition with the other trees, for water.
4. The same argument is valid for a sunny climate (less trees per hectare/acre) or a cloudy climate (more trees per hectare/acre). Or a southern slope (less trees per hectare/acre) or a northern slope of a mountain or hill (more trees per hectare/acre).
For ecosystem restoration:
I advise to only plant pioneer trees and give Mother Nature time to sow seeds and have other trees colonize the area afterwards. So plant only 200 Groasis Waterboxx® plant cocoons with each one tree and one bush. This way you plant 400 pioneer trees per hectare (160 pioneer trees per acre). That is sufficient. The fact that you only use 200 Groasis Waterboxx® plant cocoons makes this ecosystem restoration model very cheap to do.