Desiccation Tolerance of Five Tropical Seedlings in Panama. Relationship to a Field Assessment of Drought Performance

Abstract

(G.V., T.A.K.) Studies of the desiccation tolerance of the seedlings of five tropical trees were made on potted plants growing in a greenhouse. Pots were watered to field capacity and then dehydrated for 3 to 9 weeks to reach various visual wilting stages, from slightly wilted to dead. Saturated root hydraulic conductance was measured with a high-pressure flowmeter, and whole-stem hydraulic conductance was measured by a vacuum chamber method. Leaf punches (5.6-mm diameter) were harvested for measurement of leaf water potential by a thermocouple psychrometer method and for measurement of fresh and dry weight. In a parallel study, the same five species were studied in a field experiment in the understory of a tropical forest, where these species frequently germinate. Control seedlings were maintained in irrigated plots during a dry season, and experimental plants were grown in similar plots with rain exclusion shelters. Every 2 to 4 weeks, the seedlings were scored for wilt state and survivorship. After a 22-week drought, the dry plots were irrigated for several weeks to verify visual symptoms of death. The field trials were used to rank drought performance of species, and the greenhouse desiccation studies were used to determine the conditions of moribund plants. Our conclusion is that the desiccation tolerance of moribund plants correlated with field assessment of drought-performance for the five species (r 2 Ͼ 0.94).Spatial and temporal variation in the composition of tropical forest plant communities has fascinated researchers for a long time, and understanding the causes of this variation remains a prominent topic of study. The factors that best correlate with the distribution of individual species and with diversity gradients in tropical forests are the annual amount and seasonality of rainfall (e.g. Gentry, 1988;Condit, 1998;Veenendaal and Swaine, 1998;Bongers et al., 1999). Wet forests typically have many more species than dry forests, and most species occur only over a limited range of rainfall. Tropical forests also undergo dramatic temporal changes in species abundance and distribution, on scales of decades as well as thousands of years (e.g. Flenley, 1998). Again, these shifts in species composition seem to be associated with changes in moisture availability. Because tropical forest plants experience drought and water stress (Tobin et al., 1999;Walsh and Newbery, 1999;Nakagawa et al., 2000), moisture availability in tropical forest soils may be one of the main factors influencing plant growth (Chandrashekar et al., 1998;Ito et al., 2000;Poorter and Hayashida-Oliver, 2000), mortality (Condit et al., 1995; Veenendaal et al., 1995), and habitat associations (Sollins, 1998;Webb and Peart, 2000). Thus, although tropical rainforests receive substantial rainfall, the available data suggest that water availability is the most important determinant of species' distributions.To understand how rainfall and seasonality affect the spatial and temporal changes in tropical forest composition and diversity, we n...

Unique competitive effects of lianas and trees in a tropical forest understory

“…3). Consequently, D. oleifera was the only drought-tolerant seedling in our study (Tyree 2003), and the only species that was competing with neighboring lianas for similar belowground resources during the dry season. During the wet season, however, D. oleifera seedlings were most limited by competition from trees (Fig.…”

“…In March 2008, we collected seeds from three tree species on Gigante peninsula: Lacmellea panamensis, Apeiba membranaceae, and Dipteryx oleifera. The three species represented distinct life-history strategies: L. panamensis is a canopy tree species that has medium to large sized seeds and is moderately shade-tolerant (Myers and Kitajima 2007), A. membranacea is a pioneer species with small seeds that usually only regenerates in high light environments, such as treefall gaps (Tyree et al 1998), and D. oleifera is a large emergent tree that has large seeds and is relatively drought-tolerant (Tyree 2003). We germinated the seeds in sterile soil in a shade-house and allowed them to grow for 2 months prior to transplanting them into the plots.…”

Short dry spells in the wet season increase mortality of tropical pioneer seedlings

“…Physiological traits at the cell level allowing for the tolerance of low water potentials may be more important for drought survival in seedlings of tropical pioneer species than desiccation avoidance through different biomass allocation patterns. In seedlings of tropical nonpioneer species, lethal leaf water potentials have been shown to be highly correlated with drought survival (Tyree et al 2003). For 3-week-old Ochroma seedlings surprisingly low lethal water potentials of À5 MPa were reported (Tyree and Zimmermann 2002), and preliminary data suggest that lethal water potentials for our remaining species are at a similarly low range (B. Engelbrecht, unpublished data).…”

Are Mixed Tropical Tree Plantations More Resistant to Drought than Monocultures?

“…The potential natural vegetation in Panama is relatively diverse but the majority of the area was probably covered by a tropical moist lowland forest or a tropical moist transitional forest annually experiencing a distinct dry season [45]. The lowland forest in Panama is characterized by a wide range of tree species with differing leaf phenology and adaptations to deal with drought [46,47]. This ecosystem is one of the best studied tropical forests worldwide and has, with the research site at Barro Colorado Island (BCI), one of the oldest permanent forest inventory plots in the tropics.…”