Research by the Global Forest Biodiversity Initiative shows that a “positive and consistent relationship” can be discerned between tree diversity and ecosystem productivity.

The GFBI states:

“The biodiversity–productivity relationship (BPR) is foundational to our understanding of the global extinction crisis and its impacts on ecosystem functioning. Understanding BPR is critical for the accurate valuation and effective conservation of biodiversity. Using ground-sourced data from 777,126 permanent plots, spanning 44 countries and most terrestrial biomes, we reveal a globally consistent positive concave-down BPR, whereby a continued biodiversity loss would result in an accelerating decline in forest productivity worldwide. The value of biodiversity in maintaining commercial forest productivity alone—US$166–490 billion per year according to our estimation—is by itself over two to six times the total estimated cost that would be necessary for effective global conservation. This highlights the need for a worldwide re-assessment of biodiversity values, forest management strategies, and conservation priorities.”

Source: http://www.gfbinitiative.org/research