The theory is that multiple co-existing tree species can have complementary use of resources (e.g. of light along the gradient from the canopy down to the understory, of water by rooting at different depths, and of phosphorous by using different mechanisms to mobilize recalcitrant forms in the soil), and so make fuller use of them than any one species could do. This is expected to increase productivity and 'all else being equal' (particularly carbon residence time) biomass. I don't think anyone has suggested that this relationship would be linear and an asymptotic relationship would seem more likely. In any case, observations suggest that reality is more complex than this simple theory.

The theory is that multiple co-existing tree species can have complementary use of resources (e.g. of light along the gradient from the canopy down to the understory, of water by rooting at different depths, and of phosphorous by using different mechanisms to mobilize recalcitrant forms in the soil), and so make fuller use of them than any one species could do. This is expected to increase productivity and 'all else being equal' (particularly carbon residence time) biomass. I don't think anyone has suggested that this relationship would be linear and an asymptotic relationship would seem more likely. In any case, observations suggest that reality is more complex than this simple theory.

I think what Dr. Cortlett said is generally true about biodiversity-ecosystem function relationships. Times you might observe a negative relationship between diversity and [ insert ecosystem function here ] is when there's some kind of antagonistic relationship / antibiosis among taxa that use similar resources. This should be true of trees and carbon storage, fungi and decomposition rate, etc. E.g., https://www.frontiersin.org/articles/10.3389/fmicb.2014.00251/full

https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1461-0248.2012.01759.x

Thank you, R. T. Corlett and Jesse Czekanski-Moir

Tropical forests are the most productive ... that results in most biomass (carbon) ... the more biomass there is ; the greater the chances for it to become partitioned amongst more different kinds of structures. Structural complexity creates more different kinds of niches and those various niches can be filled by a greater variety of species - especially as species are specialised and niches tend to be narrow in the tropics. Sounds good in theory but we found no relationship between carbon and diversity when we looked for it in various forest types and plantations on Doi Mae Salong ...

Remember, also, that productivity and biomass are also only rather weakly related, since biomass also depends on the turnover rate, which varies widely even in tropical forests. Redwood forests in California have more biomass than lowland dipterocarp forests in Borneo, but they have much lower productivity and diversity.