To be honest, it depends.
We work with tree-partners that plant trees in a way, that the tree keeps growing for a long time like in a forest garden or a conservation area.
Most of the trees we plant are not meant to be cut and sold for a direct material gain. The indirect value for the people and the environment is complicated to determine; that is why we claim "positive" instead of an immediate value.
Scientists from Oxford University have said that planting trees is the best method for taking carbon dioxide out of the atmosphere and for combatting global warming. Trees are elementary to plant and, as years go by, they take CO2 out of the atmosphere and into long-term storage.
On average, we calculate that each tree stores 10 kg of CO2 each year. In the tropics, this figure is many times higher (source).
That’s why compensate our environmental impact with accountable projects and we plant trees.