Biodiversity is undergoing a rapid loss in ecosystems worldwide, endangering ecosystem services crucial for socio-economic development and poverty reduction. In the global campaign against biodiversity loss in forest ecosystems, land management parties need to be informed of the forest status in terms of biodiversity, but practical guidelines are lacking. A major obstacle is the incomplete understanding of the relationship between site productivity and plant diversity, due to insufficient ecosystem-wide evidence, especially from taxonomically and structurally diverse forest ecosystems. Here, we presented a continental-scale analysis of massive (approx. 2.1×1010 bytes) ground-measured forest inventory data, with which we investigated the effects of productivity on biodiversity (in terms of tree species richness) across 19 types of forest ecosystems in North America and China. It was shown that all the forest types conformed to a consistent and highly significant (p<0.001) hump-shaped unimodal relationship, of which the generalized coefficients of determination ranged from 6.2% (Chinese northeastern mixed forests) to 45.2% (U.S. western hardwoods), with an average of 20.5% over all the forest types. Our findings shed lights on the complicated mechanisms that underlie the unimodal effects of productivity on biodiversity. The consistent productivity— biodiversity relationships described here can facilitate ongoing efforts to prioritize conservation efforts, and provide a foundation for integrating biological conservation in forestry practices.This article is protected by copyright. All rights reserved