What I don’t understand is that if biomass production is your only goal, it depends on the species, surely? Plant a field of bamboo and I bet you would get a very different story.
That said, it has been well established that declining biodiversity leads to a decrease in ecosystem functioning. Here’s a similar paper from 7 years ago, which used natural systems rather than the tightly controlled experimental plots in the article above.
Here’s the recent Science journal paper that is used in the article linked, but the full text is behind a paywall unfortunately.