Influence of Seasonal Variations in Soil Water Availability on Gas Exchange of Tropical Canopy Trees Cle ´ ment Stahl 1 , Benoit Burban 1 , Fabien Wagner 2 , Jean-Yves Goret 1 , Fe ´lix Bompy 1 , and Damien Bonal 1,3,4 1 INRA, UMR 745 ‘Ecologie des Fore ˆts de Guyane’, Campus Agronomique BP 709, 97387, Kourou Cedex, French Guiana 2 Universite ´ des Antilles et de la Guyane, UMR 745 ‘Ecologie des Fore ˆ ts de Guyane’, BP 709, 97387, Kourou Cedex, French Guiana 3 INRA, UMR 1137 Universite ´ de Lorraine/INRA Nancy ‘Ecologie et Ecophysiologie Forestie ` re’, 54280, Champenoux, France ABSTRACT Seasonal variations in environmental conditions influence the functioning of the whole ecosystem of tropical rain forests, but as yet little is known about how such variations directly influence the leaf gas exchange and transpiration of individual canopy tree species. We examined the influence of seasonal variations in relative extractable water in the upper soil layers on predawn leaf water potential, satu- rated net photosynthesis, leaf dark respiration, stomatal conductance, and tree transpiration of 13 tropical rain forest canopy trees (eight species) over 2 yr in French Guiana. The canopies were accessed by climbing ropes attached to the trees and to a tower. Our results indicate that a small proportion of the studied trees were unaffected by soil water depletion during seasonal dry periods, probably thanks to efficient deep root systems. The trees showing decreased tree water status (i.e., predawn leaf water potential) displayed a wide range of leaf gas exchange responses. Some trees strongly regulated photosynthesis and transpiration when relative extractable water decreased drastically. In contrast, other trees showed little variation, thus indicating good adaptation to soil drought conditions. These results have important applications to modeling approaches: indeed, precise evaluation and grouping of these response patterns are required before any tree-based functional models can efficiently describe the response of tropical rain forest ecosystems to future changes in environ- mental conditions. Key words : French Guiana; photosynthesis; predawn leaf water potential; rain forest; relative extractable water; respiration; soil drought; transpiration. SEASONAL VARIATIONS IN ENVIRONMENTAL CONDITIONS (i.e., solar radiation, rainfall, soil water availability, vapor pressure deficit) influence tropical rain forest ecosystem functioning (Goulden et al. 2004, Hutyra et al. 2007, Bonal et al. 2008). For instance, a decrease in ecosystem respiration and gross primary productivity has been observed during dry periods at different sites in Amazonia (Hutyra et al. 2007, Bonal et al. 2008). Evapotranspira- tion at the ecosystem scale usually displays the lowest values dur- ing rainy seasons and the highest values at the onset of the dry season (Hasler & Avissar 2007, Rollenbeck & Anhuf 2007). Vari- ations in solar radiation are one of the main causes of seasonal variations in photosynthesis and evapotranspiration (Hasler & Avissar 2007, Bonal et al. 2008, Renninger et al. 2010), but a decrease in soil water availability or an increase in atmospheric vapor pressure deficit during dry periods also induces a regula- tion of most CO 2 and H 2 O fluxes between the different ecosys- tem compartments and the atmosphere (Bonal et al. 2000b, Renninger et al. 2010, Stahl et al. 2011). Periods of highly reduced soil water availability such as the exceptional droughts that occur during El Niño Southern Oscilla- tion events (Malhi et al. 2008) not only influence ecosystem CO 2 and H 2 O fluxes, but can also cause widespread tree mortality at the regional scale (Phillips et al. 2009). In the context of global climatic changes, these patterns could result in dramatic changes in tree species composition and in the carbon and water balance of tropical rain forest ecosystems. To predict these changes and to simulate their effects, functional models have been developed (e.g., Williams et al. 1998, Fisher et al. 2006). These models require detailed information on the growth and functional charac- teristics of the tree species present in the ecosystems, as well as on their potential responses to changes in environmental condi- tions. A negative effect of the seasonal decrease in relative extractable water (REW) on canopy tree growth has recently been demonstrated for tropical rain forest trees (Wagner et al. 2012). Furthermore, there is huge variability among tropical rain forest tree species in functional traits related to carbon processes (net photosynthesis, leaf or trunk respiration) and water flux (transpi- ration), as well as in the trade-off between these fluxes (water use efficiency) (Bonal et al. 2000b, Meir et al. 2001, Cavaleri et al. 2008, Baraloto et al. 2010, Stahl et al. 2011). Little information exists to date, however, on the range of the response of tropical rain forest canopy trees to seasonal variations in environmental conditions. Most studies published so far have been conducted on seedlings in common gardens or on 10–20-yr-old trees in plantations (Bonal et al. 2000b, Engelbrecht & Kursar 2003, Baraloto et al. 2007, Kursar et al. 2009), and to our knowledge, very few studies have measured and evaluated the direct influence of seasonal variations in soil water availability on canopy trees. A decrease in tree transpiration during dry periods has been Received 18 October 2011; revision accepted 19 April 2012. 4 Corresponding author; e-mail: bonal@nancy.inra.fr ª 2012 The Author(s) 155 Journal compilation ª 2012 by The Association for Tropical Biology and Conservation BIOTROPICA 45(2): 155–164 2013 10.1111/j.1744-7429.2012.00902.x