2263 Special Feature Ecology, 86(9), 2005, pp. 2263–2277  2005 by the Ecological Society of America GLOBAL MODELS FOR PREDICTING WOODY PLANT RICHNESS FROM CLIMATE: DEVELOPMENT AND EVALUATION RICHARD FIELD, 1,4 EILEEN M. O’BRIEN, 2 AND ROBERT J. WHITTAKER 3 1 School of Geography, University of Nottingham, NG7 2RD UK 2 Gainesville College, Watkinsville, Georgia 30677 USA, and Institute of Ecology, University of Georgia, Athens, Georgia 30602 USA 3 Biodiversity Research Group, School of Geography and the Environment, University of Oxford, Mansfield Road, Oxford OX1 3TB UK Abstract. There have been few attempts to generate global models of climate–richness relationships, and fewer still that aim to predict richness rather than fitting a model to data. One such model, grounded on theory (biological relativity to water–energy dynamics) is the interim general model (IGM1) of the climatic potential for woody plant richness. Here we present a second-generation model (IGM2), and genus and family versions of both models. IGM1 describes horizontal climate–richness relationships based on climate station data and systematic species range maps, with IGM2 additionally incorporating vertical changes in climate due to topographic relief. The IGMs are mathematical transformations of empirical relationships obtained for the southern subcontinent of Africa, whereby the re-described regression models apply to the full range of global variation in all independent climate parameters. We undertake preliminary validation of the new IGMs, first by mapping the distribution and relative spatial variation in forecasted richness (per 25 000 km 2 ) across the continent of Africa, then by evaluating the precision of forecasted values (actual vs. predicted) for an independent study system, the woody plants of Kenya. We also compare the IGMs with a recent example of purely statistical regression models of climate–richness relationships; namely, the ‘‘global’’ model of A. P. Francis and D. J. Currie for angiosperm family richness. We conclude that the IGMs are globally applicable and can provide a fundamental baseline for systematically estimating differences in (woody) plant richness and for exploring the hierarchy of subordinate relationships that should also contribute to differences in realized richness (mostly at more discrete scales of analysis). Further, we found that the model of Francis and Currie is useful for predicting angiosperm richness in Africa, on a conditional basis (somewhere, sometime); we examined the relationship that it describes between climate and richness. Lastly, we found that indices of available soil water used in ‘‘water-budget’’ or ‘‘water-balance’’ analyses are not proxies for available liquid water as a function of climatological dynamics. Key words: Africa; climate; climatic potential for richness; diversity gradients; interim general model; Kenya; model evaluation; species richness; water–energy dynamics; woody plant diversity. INTRODUCTION It has long been known that global gradients in rich- ness covary with global gradients in climate. The de- velopment of statistical models of this relationship that apply globally is an important but elusive goal for ecol- ogists. In addition to improving our understanding of diversity patterns, such models could prove useful for predicting reasonable values of plant or animal richness where actual values are unknown, and for modeling how changes in climate could alter the richness (and vegetation) patterns we see today. Attempts to apply models, developed in particular regions, to other regions have produced some success Manuscript received and accepted 16 December 2004; final version received 1 February 2005. Corresponding Editor: B. A. Hawkins. For reprints of this Special Feature, see footnote 1, p. 2261. 4 E-mail: richard.field@nottingham.ac.uk but have tended not to result in globally applicable models (e.g., Currie and Paquin 1987). Such work tends to focus on mid to high latitudes; regions for which data availability is better, but which contain relatively few of the world’s species. An exception is an interim general model (IGM) of the ‘‘climatic potential for (woody) plant richness’’ (O’Brien 1998). Rather than using a purely statistical approach, O’Brien worked from first principles to develop a first-order mechanistic explanation for covariation between climate and rich- ness globally: biological relativity to water–energy dy- namics (O’Brien 1989, 1993, 1998). This idea effec- tively links water–energy dynamics (work done by wa- ter) to fundamental parameters of both climatological and biological dynamics, at all scales of analysis: liquid water and solar energy (e.g., hydrologic cycle and pho- tosynthesis, respectively). In accord with energy’s dy- namic relationship with the state of water, the model describes the relationship of climate to woody plant