Why do diverse forests store more carbon?
Forests containing multiple tree species could store at least twice as much carbon as the average monoculture plantation. A recent study also concluded that each native tree species introduced into a plantation could increase carbon stocks by 6% (per additional species).
From a basic perspective this implies that mono plantations purely planted to capture Co2 out of the atmosphere would function better as diverse native plantations that would capture dramatically higher amounts of Co2. This is without taking into account the innumerable other beneficial aspects native diversity offers - such as biodiversity formation, sustainability and resilience.
Diversity of species within a forest creates multiple dense canopy layers that maximise photosynthesis potential and carbon capture -the greater the leaf to sunlight ratio the greater the photosynthesis.
Combine this with the magic of the living forest floor as a carbon sponge and carbon cycler plus increased pollination due to greater wildlife and pollinators and suddenly the ecosystem is functionally somewhere near its happy potential.