Phenology, Lignotubers, and Water Relations of Cochlospermum vitifolium, a Pioneer Tropical Dry Forest Tree in Costa Rica Lottie Fallas-Cede ˜ no 1 , N. Michele Holbrook 2 , Oscar J. Rocha 3 , Nelly Va ´ squez 4 , and Marco V. Gutie ´ rrez-Soto 1,5 1 Estaci ´ on Experimental Fabio Baudrit, Universidad de Costa Rica, Apartado 183-4050, Alajuela, Costa Rica 2 Department of Organismic and Evolutionary Biology, Harvard University, 16 Divinity Avenue, Cambridge, MA 02138, U.S.A. 3 Department of Biological Sciences, Kent State University, Kent, OH 44242, U.S.A. 4 Laboratorio de Histolog´ ıa, Centro Agron´ omico Tropical de Investigaci´ on y Ense ˜ nanza, Apartado 7170 CATIE, Turrialba, Cartago 30501, Costa Rica ABSTRACT We examined structural and physiological traits relevant to the phenology of the tropical dry forest (TDF) pioneer tree Cochlospermum vitifolium. Despite marked seasonality in rainfall, meristem activity occurred throughout the year. Leaves were produced almost continuously during the rainy season, while leaf shedding started early during drought, before changes in soil water content were observed. Phenological activity under drought included flowering and fruiting of leafless trees; bud break and shoot extension took place before the end of the dry season. Low wood density of C. vitifolium stems (0.17 g/cm 3 ) and lignotubers (0.14 g/cm 3 ) provided water and starch storage needed to support phenological events such as branch extension, leaf flushing, and reproduction during the dry season, and probably also contributed to survival following mechanical damage and fire, typical of early TDF successional stages. Lignotuber water and starch contents showed substantial seasonal variation, declining from the beginning of the dry season to their lowest levels at the time of reproduction and dry-season flushing. Stems progressively replaced lignotubers as main storage organs as tree size increased. Evidence for a role of water stores in buffering daily water deficits was weak. Leaf water potentials remained above  1.2 MPa and stomatal conductance below 350 mmol/m 2 /s, suggesting that gas exchange during the rainy season was limited to prevent xylem cavitation. Leaf shedding occurred when early-morning and mid-day C L converged at the rainy–dry season transition, without changes in lignotuber or soil water content, suggesting that leaves of C. vitifolium are closely tuned to atmospheric drought. Abstract in Spanish is available at http://www.blackwell-synergy.com/loi/btp Key words: dry-season flushing; fire; pioneer species; reserve storage; stem-succulent; stomatal conductance; succession. TROPICAL DRY FORESTS (TDFs) are among the most threatened biomes in the world ( Janzen 1988b). Compared with their tropical rain forest counterparts, TDFs contain a wealth of unique species that exhibit higher structural and functional complexity, higher flo- ristic endemism, slower growth and reproductive activity, higher phenological diversity, and higher resilience to frequent distur- bances (Murphy & Lugo 1986, Holbrook et al. 1995). Efforts to restore these ecosystems will succeed if the underlying physiological traits that influence successional patterns are understood. However, in contrast to tropical rain forests (Bazzaz & Pickett 1980, Mulkey et al. 1996), TDF succession remains little studied from an eco- physiological perspective (see Leiva et al. in press-a, b). As a consequence of human intervention, recurrent fire events, windstorms, and other disturbances (Rawitscher 1948, Vareschi 1962, Molina & Lugo 2006, Otterstrom et al. 2006), most TDF remnants are mosaics of forest patches at different stages of succession (Kramer 1997, Otterstrom et al. 2006). Many TDF tree species are deciduous, drought-tolerant, and include some of the most valuable timber, fruit, and ornamental shrubs and trees in the world (Record & Hess 1949). In this seasonally harsh environment, asynchrony of resource supply and demand is the rule (Chapin et al. 1990), and thus resource storage may play an important role among deciduous tree species. In the TDF of northwestern Costa Rica, recently burned, early successional sites are dominated by a small number of light- demanding, wind-dispersed species whose regeneration may be en- hanced by fire (Ewel 1977, Otterstrom et al. 2006). Of the 215 tree species reported at Santa Rosa National Park, 25 percent have wind-dispersed seeds and are considered pioneer species ( Janzen 1988a). In an area of Santa Rosa last affected by fire in 1979, 12 species of trees constituted at least 90 percent of the vegetative cover in 1986. Most of the wind-dispersed initial colonizers were large trees, attaining heights of 15–25 m, and life spans of 50 to several hundred years ( Janzen 1988a). Cochlospermum vitifolium (Wildenow) Sprengel (Cochlosper- maceae), a stem-succulent, dry-season deciduous tree, is distributed from Mexico through northern South America. It is one of the most abundant early successional species in TDFs, where it typically grows as a small tree, but may reach up to 20 m in height in mature forests. Cochlospermum vitifolium is wind-dispersed, reproduces throughout the dry season when leafless, with the onset of repro- ductive maturity occurring as early as the first year following ger- mination (Bawa & Frankie 1983). Cochlospermum vitifolium is often described as a dry-season flushing species because it initiates leaf flushing before the onset of the rainy season (Borchert & Rivera 2001, Rojas-Jime ´nez et al. 2007). Nonannual reproduction is com- mon in this species, the fruits develop rapidly throughout the dry season, and variations in reproductive frequency among individuals is high (Bullock 1995). Received 1 August 2008; revision accepted 12 March 2009. 5 Corresponding author; e-mail: surdo26@racsa.co.cr BIOTROPICA 42(1): 104–111 2010 10.1111/j.1744-7429.2009.00539.x 104 r 2009 The Author(s) Journal compilation r 2009 by The Association for Tropical Biology and Conservation