I, Gabriel, have researched this quite a lot. The fact is that the number varies. And that’s because the amount of carbon a tree fixes is…. naturally variable depending on a host of variables. Trees do not all grow to the same height and density for example: it is widely variable among species. Growth rates among trees also vary: some grow at a slow rate (oak for example) while other grow rapidly (conifers). Climate and temperature are an important factors: a tree will grow a lot more rapidly in a warm climate than in a cold climate. Rain, soil condition, and sun exposure are important determinants of growth rates as well.
The recognised standard in the UK is the woodland carbon code (WCC), which has produced tools (models and spreadsheets) to predict carbon sequestration in tonnes CO2 equivalent.
Generally WCC recommends to think in terms of area rather than individual trees, but the WCC models suggest that the verifiable carbon uptake from native woodland at the standard 1600 trees/ha is between 320 – 400 tonnes of CO2, depending on soil type and method of ground preparation – this equates to 0.25 tonne of CO2 per tree. Critically, this is calculated based on a tree lifetime of 100 years.