The most valuable benefit is air quality. In this paper, London gained £375m of value thanks to trees filtering the air of pollution from things such as diesel cars and buses. This would increase if more trees were planted, especially if they were placed next to busy main roads.
Trees are living things and as such they grow. To grow they suck carbon dioxide out of the air and convert it into bigger trunks, stronger branches, and more leaves.
The trees in London sequester CO₂, which has a benefit of £11.9m per annum. But trees grow year on year, so the value of those trees, just from the value of the CO₂ that has been stored, is now £364m.
Trees provide additional shade, and therefore the amount of energy used to cool buildings in the summer is reduced if there are trees, a benefit of £0.7m. Trees also drink plenty of water, helping to prevent localised flooding especially if it rains a lot over a few days. This was assessed to have a value of £8.4m a year.
All these values are for the whole of London, which 2906 square kilometers of land area. This means that the total value that trees bring to London is £184,000 per square kilometer, or about £37 per person.
Planting a thousand trees would require a budget of £400,000, but after 30 years it would result in benefits of £27m. Clearly, a valuable, worth while investment.