N. Michele Holbrook

Negative Xylem Pressures in Plants: A Test of the Balancing Pressure Technique

Xylem tension was experimentally imposed by centrifugal force to assess the stability of negative pressures within the xylem and the estimation of those pressures with a pressure chamber. Balancing pressure measurements of leaves attached to a spinning branch at the axis of rotation closely agreed with tensions calculated from the rotational velocity. This agreement demonstrates that the xylem is capable of sustaining large negative pressures and directly validates the balancing pressure technique.

Tropical deciduous forest: Death of a biome

Tropical Deciduous Forest: Death of a Biome THE CALAMITY that is befalling the tropical rain forests has overshadowed land-use disasters occurring in other ecosystems of the world. A meeting was held in March at the Estaci6n de Biologia, Chamela, of the Universidad National Aut6noma de MBxico (UNAM) in Jalisco, Mexico to examine the composition, structure, function, utilization and conservation of the world’s tropical deciduous forests, with a focus on the neotropics. The discussions were diverse, ranging from biogeochemical cycling to ethnobotany. Tropical deciduous forests have species-diversity levels between those of tropical wet and temperate forests. Although there is a strong correlation across forest types between rainfall and biodiversity, within tropical deciduous forest systems the diversity of plants and animals shows no correlation with the amount of rainfall the forest receives. Alwyn Gentry (Missouri Botanical Garden, St Louis, USA) reported that deciduous tropical forests can have up to 94 woody species per 0.1 hectare. Gentry also showed that the area of greatest diversity in neotropical deciduous forests is MBxico, in contrast to Amazonia for wet forests. This suggests that the evolutionary controls on biodiversity for tropical deciduous forests are different from those for moist forests. Gerard0 Ceballos (UNAM) noted that the number of vertebrate species using neotropical deciduous forests is as great as that in moist forests. He showed how this diversity is due both +o the large number of native mammals, particularly rodents and bats, and to the many migrant bird species from North America that overwinter in the tropical deciduous forests in western M6xico and Central America. Ceballos stressed the importance of adjacent aseasonal forests, such as those along rivers that pass through deciduous forest regions, as sources of vertebrate diversity. Tropical deciduous forest physiognomy, in contrast to species diversity, depends on both the amount and the timing of rainfall. Ernest0 Medina (Instituto Venezolano de lnvestigaciones Cientificas, Caracas, Venezuela) discussed the fact that forest structure is most closely correlated to potential evapotranspiration and the length of the dry period. He illustrated how the degree of deciduousness in the forest canopy and the diversity of plant growth forms in-

Physiology of Tropical Vines and Hemiepiphytes: Plants that Climb Up and Plants that Climb Down

Vines climb up. From a starting point in the forest understory, vines use the mechanical competence of neighboring plants in their ascent towards the bright sun of the forest canopy. Hemiepiphytes climb down. They begin life in the treetops and extend earthward to form permanent and often substantial connection with the soil (Figure 13.1). Vines and hemiepiphytes hold in common a period of mechanical dependence, yet the physiological consequences and constraints associated with this reliance upon external support differs substantially between the two groups. In vines, adaptations essential for effective climbing constrain their outward form and thus influence the uptake, transport, and storage of energy and materials. For hemiepiphytes, in contrast, the major constraints may not be the particular rigors associated with life as either an epiphyte or a tree, but rather in the plasticity required to succeed as both. Our discussion of tropical vine physiology is morphologically organized, considering in turn the function of stems, leaves, and roots. With hemiepiphytes, we focus primarily on the nature of the transition between the epiphytic and tree growth forms. We draw upon the roles of vines and hemiepiphytes in tropical forest communities only as such discussion provides insights into their physiology.

Seasonal patterns of acid fluctuations and resource storage in the arborescent cactus Opuntia excelsa in relation to light availability and size

SummaryWe investigated relationships between light availability, diel acid fluctuation, and resource storage in the arborescent cactus Opuntia excelsa growing in western Mexico. We compared canopy and understory individuals from a deciduous forest as well as open-grown plants of the same approximate size as those in the understory. During the wet season light availability and daily fluctuations in titratable acidity (an index of carbon uptake) were lower in the understory than in unshaded habitats. In the dry season all plants had reduced levels of acid fluctuation, with the smallest individuals, regardless of habitat, showing the greatest reduction. These data suggest that light availability in the forest understory constrains carbon assimilation during the wet season, but that a factor associated with plant size, possibly water status, limits carbon gain during the dry season. Plants in all habitats remained physiologically active for at least five months into the dry season. We suggest that this was possible due to the maintenance of constant concentrations of water and nitrogen in the photosynthetically active chlorenchyma. Parenchyma in terminal cladodes showed a different seasonal pattern of resource storage; water content and nitrogen concentration were reduced from the wet to the dry season in the parenchyma. Using the parenchyma to supply photosynthetic tissues during times of reduced resource availability allows O. excelsa to assimilate carbon during times of the year when most other trees in the forest are leafless.

Water relations of epiphytic and terrestrially-rooted strangler figs in a Venezuelan palm savanna

Water use patterns of two species of strangler fig, Ficus pertusa and F. trigonata, growing in a Venezuelan palm savanna were contrasted in terms of growth phase (epiphyte and tree) and season (dry and wet). The study was motivated by the question of how C3 hemiepiphytes accommodate the marked change in rooting environment associated with a life history of epiphytic establishment followed by substantial root development in the soil. During the dry season, stomatal opening in epiphytic plants occurred only during the early morning, maximum stomatal conductances were 5 to 10-fold lower, and midday leaf water potentials were 0.5–0.8 MPa higher (less negative) than in conspecific trees. Watering epiphytes of F. pertusa during the dry season led to stomatal conductances comparable to those exhibited by conspecific trees, but midday leaf water potentials were unchanged. During the rainy season, epiphytes had lower stomatal conductances than conspecific trees, but leaf water potentials were similar between the two growth phases. There were no differences in ∂13C between the two growth phases for leaves produced in either season. Substrate water availability differed between growth phases; tree roots extended down to the permanent water table, while roots of epiphytic plants were restricted to material accumulated behind the persistent leaf bases of their host palm tree, Copernicia tectorum. Epiphytic substrate moisture contents were variable during both seasons, indicating both the availability of some moisture during the dry season and the possibility of intermittent depletion during the rainy season. Epiphytic strangler figs appear to rely on a combination of strong stomatal control, maintenance of high leaf water potentials, and perhaps some degree of stem water storage to cope with the fluctuating water regime of the epiphytic environment.