If the diversity of tree species in a forest is very rich, then the loss of one species at this time will not have much impact on the productivity of the entire forest; but when the species diversity is scarcer, the number of tree species will continue to decrease, which will affect the productivity of the entire forest. The hit to the productivity of the entire forest will be bigger and bigger.
Forest-wide productivity won't be affected unless tree species diversity plummets can be inferred.