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Abstracts of comments on the article "Global Resilience of Tropical Forest and Savanna to Critical Transitions," by Egbert H. Van Nes, Milena Holmgren, Marina Hirota, and Marten Scheffer that appeared in the October 14, 2011 issue of "Science" are presented.

Hirota et al. (Reports, 14 October 2011, p. 232) used spatial data to show that grasslands, savannas, and forests represent opposing stable states. Reanalyzing their data and drawing from temporal studies, we argue that spatial analyses underestimate the bistability of grasslands and savannas due to limitations of substituting space for time. We propose that temporal and spatial data are needed to predict critical transitions between grasslands and savannas.