Diversity anomalies are differences in species richness between areas that belong to different regions but have similar environments. Some hypotheses addressing the origin of well-known anomalies in plant diversity propose that regions with higher environmental spatial heterogeneity have higher diversity because heterogeneity fosters diversification or coexistence. Arguments supporting these hypotheses emphasize inter-regional comparisons of diversity and assume that spatial environmental heterogeneity is higher in: (1) eastern Asia (EA) than in eastern North America (ENA), (2) western North America (WNA) than in ENA, and (3) the Cape Floristic Region in southern Africa (CFR) than in the Southwest Australian Floristic Region (SWA). Here, we evaluate these assumptions by measuring two kinds of environmental heterogeneity – spatially implicit and explicit – each thought to affect diversity via different mechanisms. The former refers to environmental variation among sites within a region, regardless of site location. The latter refers to the spatial pattern of environmental variation across a region (e.g., monotonic or undulating).