Under conditions of increased CO2 availability, plants tend to decrease waster usage and sequester more carbon. Growth rates are faster, but require increased mineral availability. Unfortunately, nitrogen levels in plants decrease in high-atmospheric CO2 conditions, which leads to lower protein concentrations. As a result, consumption crops provide less energy and nutrition when consumed.

So, all in all, the downsides outweigh the upsides -- faster growth, lower nutrition, more aggressive soil depletion, and increased water runoff. On the bright side, carbon sequestration increases up to 40%, so the efficacy of planting forests as a sequestration measure will increase.

As for biodiversity -- well, dieoffs are already outpacing speciation. Unless you believe in abiogenesis, the trendline is moving sharply downward.