Like siblings said : it's not a good benchmark, as forests were not exploited as much before.

In fact, almost all forests are exploited in some way or another. Some recently protected primal old growth will take a few centuries to recover.

https://en.m.wikipedia.org/wiki/Old-growth_forest

"The world has 1.11 billion ha of primary forest remaining.[when?] Combined, three countries (Brazil, Canada, and Russia) host more than half (61 percent) of the world's primary forest. The area of primary forest has decreased by 81 million ha since 1990, but the rate of loss more than halved in 2010–2020 compared with the previous decade."

I would say there's a difference between a fir Douglas plantation and a multi-species forest. The two are not ecologically equivalent. At least around where I live (department 71 in France), there are less and less forests and more and more tree plantations.

I think it would be interesting to look at how much biodiverse forests we have now compared to 100 years ago. I might be proven wrong.

What makes 100 years ago a good point of reference? as opposed to a date pre-industrial revolution maybe.