Phenology, Lignotubers, and Water Relations of Cochlospermum vitifolium, a Pioneer Tropical Dry Forest Tree in Costa Rica

Abstract

ABSTRACTWe 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/cm3) and lignotubers (0.14 g/cm3) 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/m2/s, suggesting that gas exchange during the rainy season was limited to prevent xylem cavitation. Leaf shedding occurred when early‐morning and mid‐day ΨL 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.