Ecology, 93(9), 2012, pp. 2061–2072 Ó 2012 by the Ecological Society of America Intra- and interspecific tree growth across a long altitudinal gradient in the Peruvian Andes JOSHUA M. RAPP, 1,5 MILES R. SILMAN, 1 JAMES S. CLARK, 2 CECILE A. J. GIRARDIN, 3 DARCY GALIANO, 4 AND RICHARD TITO 4 1 Department of Biology, Wake Forest University, Winston-Salem, North Carolina 27106 USA 2 Department of Biology and Nicholas School of the Environment, Duke University, Durham, North Carolina 27708 USA 3 Environmental Change Institute, School of Geography and the Environment, Oxford University, South Parks Road, Oxford, OX1 3QY England, United Kingdom 4 Universidad Nacional de San Antonio Abad del Cusco, Peru ´ Abstract. Tree growth response across environmental gradients is fundamental to understanding species distributional ecology and forest ecosystem ecology and to predict future ecosystem services. Cross-sectional patterns of ecosystem properties with respect to climatic gradients are often used to predict ecosystem responses to global change. Across sites in the tropics, primary productivity increases with temperature, suggesting that forest ecosystems will become more productive as temperature rises. However, this trend is confounded with a shift in species composition and so may not reflect the response of in situ forests to warming. In this study, we simultaneously studied tree diameter growth across the altitudinal ranges of species within a single genus across a geographically compact temperature gradient, to separate the direct effect of temperature on tree growth from that of species compositional turnover. Using a Bayesian state space modeling framework we combined data from repeated diameter censuses and dendrometer measurements from across a 1700-m altitudinal gradient collected over six years on over 2400 trees in Weinmannia, a dominant and widespread genus of cloud forest trees in the Andes. Within species, growth showed no consistent trend with altitude, but higher-elevation species had lower growth rates than lower- elevation species, suggesting that species turnover is largely responsible for the positive correlation between productivity and temperature in tropical forests. Our results may indicate a significant difference in how low- and high-latitude forests will respond to climate change, since temperate and boreal tree species are consistently observed to have a positive relationship between growth and temperature. If our results hold for other tropical species, a positive response in ecosystem productivity to increasing temperatures in the Andes will depend on the altitudinal migration of tree species. The rapid pace of climate change, and slow observed rates of migration, suggest a slow, or even initially negative response of ecosystem productivity to warming. Finally, this study shows how the observed scale of biological organization can affect conclusions drawn from studies of ecological phenomena across environmental gradients, and calls into question the common practice in tropical ecology of lumping species at higher taxonomic levels. Key words: altitudinal gradient; Andes; climate change; species migration; temperature gradient; tree diameter growth; tropical montane cloud forest; Weinmannia spp. INTRODUCTION Tropical forests contain ;25% of the carbon in the terrestrial biosphere, and account for ;33% of global terrestrial net primary productivity (Bonan 2008). Understanding the response of tree diameter growth to temperature is important for predicting forest carbon dynamics under climate change, but studies examining recent trends in tree growth in the tropics have yielded conflicting results (e.g., Phillips et al. 1998, Clark et al. 2003, 2010a, Baker et al. 2004, Feeley et al. 2007, Chave et al. 2008, Lewis et al. 2009). While there are multiple interacting factors that determine tree growth and forest productivity, temperature has a strong influence on tree growth (Clark et al. 2010a), and has been increasing at a rate of 0.268C per decade in the tropics since the 1960s (Malhi and Wright 2004). As warming is expected to accelerate (Christensen et al. 2007, Urrutia and Vuille 2009), understanding the response of tree growth to temperature will aid in predicting whether tropical forests will be sources or sinks of carbon in the future. In this study of productivity along a tropical altitudinal gradient we examine patterns of tree growth rate at species and genus levels across a mean annual temper- Manuscript received 21 September 2011; revised 22 March 2012; accepted 26 March 2012. Corresponding Editor: D. F. Doak. 5 Present address: Harvard Forest, Harvard University, Petersham, Massachusetts 01366 USA. E-mail: rapp.joshua@gmail.com 2061