~12,5kg of CO2 per beech tree and year

So, obviously no two trees are the same – therefore this is only an indication. But certainly better than nothing:

How fast and how quickly a tree stores CO2 depends on many factors such as type of tree, its age, density of the wood or the rate of growth. Also external factors such as the climate, the quality of soil or availability of water play an important role.
Consequently, there is no universal answer to the question on how many trees it takes to absorb a ton of CO2. However, here an example that helps to get an idea:
Think of a normally grown beech tree, about 23 metres high (about a ten storey building) that at the altitude of 1.3metres has a trunk diameter of approximately 30cm (so a decent pizza size). This trees stores about 550kg of dry matter with its leaves, branches and trunk. You can add an extra ~10% of matter in the roots, so overall we are talking about 600kg of matter. This can bind a ton of CO2. The weight of the dry matter is less than the amount of absorbed CO2 as the process of photosynthesis also emits oxygen.
To become a tree sufficiently big to bind this ton of CO2, the tree needs to grow for about 80 years. This means that on average and per year the tree binds about 12,5 kg of CO2. You therefore need to plant about 80 trees to absorb about a ton of CO2 per year. Consider, that in the first years after planting them, trees only develop a limited amount of CO2 and the older they grow , the more CO2 is being absorbed.

Translated from
http://www.klima-sucht-schutz.de/mitmachen/beitrag/article/wie-viele-baeume-braucht-es-um-eine-tonne-co2-zu-binden.html