Abstract
Aim Broad‐scale diversity gradients are strongly correlated with climate. This correlation may reflect direct control of richness by contemporaneous climate. Alternatively, factors that were collinear with contemporaneous climate may have affected richness (e.g. temperature during the L ast G lacial M aximum, LGM ). We tested the hypothesis that contemporaneous climate directly controls broad‐scale patterns of richness. If climate directly (i.e. causally) controls richness, then when climate changes, richness should vary with climate through time in the same way that richness varies with climate through space. We also tested hypotheses that suggest that patterns of richness are primarily determined by processes associated with climate at the LGM . Location N orth A merica, north of M exico. Methods We used temperature changes over the last 14,000 years in N orth A merica as a ‘natural experiment’. We compared a proxy of woody plant family richness (based on fossilized pollen) with two reconstructions of the spatio‐temporal variation of temperature across N orth A merica at 1000‐year intervals. Results We found that the spatial and temporal regional variations in richness of woody plant families, observed at 1000‐year intervals and expressed as a function of temperature, remained quite congruent during the changing climate since the end of the Pleistocene (the last c . 11,000 years). Between 14,000 and 11,000 yr bp , lags occurred during cooling periods, when richness remained anomalously high; however, richness was not anomalously low during the rapid warming in the late P leistocene. Richness was significantly less strongly related to climate change since the LGM . Main conclusions Our results provide a strong test of the hypothesis that, in ecological time, contemporaneous climate controls most of the broad‐scale variation in patterns of richness. Richness, independently of the responses of individual species, is likely to track current climate change, except possibly when temperature changes are very rapid.