Diverse forests absorb more CO2
It's been calculated that each additional species of tree could add up to 6% to carbon stocks and diverse forests are 'faster' at storing CO2.
With increased species richness, more carbon is stored above and in the ground; in the trunks, roots, leaf litter, mould, soil and even in canopy ecosystems. The increased species leads to a denser multi-level canopy that has a much larger surface area able to act like a photosynthetic sponge.
Secondly, more species equals more biodiversity and more pollinators - this leads to higher levels of plant reproduction and growth.
There is also the fact diversity of tree species leads to diversity of soil biology and fertility, which allows the soil to sequester and cycle CO2 and other greenhouse gases. A minimum figure of around 20 species of multi-strata layered trees is said to be where the process can really start to develop.