G. Goldstein

Mist and fog interception in elfin cloud forests in Colombia and Venezuela

Fog interception and rainfall were measured during one year in three elfin cloud forests on small mountains along the Caribbean coast of South America and one in the Venezuelan Andes. (1) While rainfall increases from west to east in the small mountains along the coast, fog interception decreases. In 1985, the total annual rainfall and fog interception were 853 mm and 796 mm in the cloud forest of Serrania de Macuira, 1630 mm and 518 mm in Cerro Santa Ana and, 4461 mm and 480 mm in Cerro Copey. In the Andean forest of El Zum-bador, the 1985 rainfall was 1983 mm and the annual fog interception was only 72 mm. (2) Fog interception seems to be an important source of water to the elfin cloud forests of the small mountains which are surrounded by dry vegetation types and where the rainfall regime is highly seasonal. (3) Fog interception increases with altitude (in the same mountain), exposure (windward slopes) and leaf inclination. These variations of fog interception could partially explain the observed distribution of epiphytic flora in some of these cloud forests.

ADAPTIVE STRATEGIES OF WOODY SPECIES IN NEOTROPICAL SAVANNAS

1. In this review we discuss the adaptive strategy of woody species in tropical savannas. The low, evergreen, broadleaved, sclerophyllous tree is considered as the typical woody representative in these ecosystems. The discussion is largely based on data concerning four widespread neotropical species: Curatella americana, Byrsonima crassifolia, Bowdichia virgilioides and Casearia sylvestris, together with more fragmentary information available on other American and African savanna woody species.

Cold hardiness and supercooling along an altitudinal gradient in andean giant rosette species

SummaryFactors affecting supercooling capacity and cold hardiness were investigated in leaves of ten giant rosette species of the genus Espeletia (Compositae). These species grow along a 2,800–4,200 m elevation gradient in the Venezuelan Andes. In this high tropical environment, freezing frequently occurs every night, particularly above 3,300 m, but lasts for only a few hours. Supercooling capacty is linearly related to leaf water potential (ΨL) in all species; however supercooling is more responsive to ΨL changes in Espeletia species from high paramos. The rate of change in the species-specific supercooling point and the rate of change of average annual minimum temperature along the elevation and climatic gradient follow the same trend (approximately -0.6 K per 100 m elevation). At a given elevation, the expanded leaves of the different species tend to supercool 8–10 K below minimum air temperatures. Experimentally-induced freezing was accompanied by the formation of intracellular ice and tissue damage. The relative apoplastic water content (RAWC) of the leaves, which may influence the ice nucleation rate or the facility by which ice propagates, was determined by pressure-volume methods. Species from higher sites tend to exhibit lower RAWC (2%–7%) than species from lower sites (7%–36%). A causal relationship between supercooling capacity and RAWC is suggested. In the high tropical Andes, the temperature oxotherm plateau of Espeletia leaves seems to be sufficiently fow to avoid freezing.

Gas exchange and water balance of a mistletoe species and its mangrove hosts

SummaryThe gas exchange and water relations of the hemiparasite Pthirusa maritima and two its mangrove host species, Conocarpus erectus and Coccoloba uvifera, were studied in an intertidal zone of the Venezuelan coast. Carbon uptake and transpiration, leaf osmotic and total water potential, as well as nutrient content in the xylem sap and leaves of mistletoes and hosts were followed through the dry and wet season. In addition, carbon isotope ratios of leaf tissue were measured to further evaluate water use efficiency. Under similar light and humidity conditions, mistletoes had higher transpiration rates, lower leaf water potentials, and lower water use efficiencies than their hosts. Potassium content was much higher in mistletoes than in host leaves, but mineral nutrient content in the xylem sap of mistletoes was relatively low. The resistance of the liquid pathway from the soil to the leaf surface of mistletoes was larger than the total liquid flow resistance of host plants. Differences in the daily cycles of osmotic potential of the xylem sap also indicate the existence of a high resistance pathway along the vascular connection between the parasite pathway along the vascular connection between the parasite and its host. P. maritima mistletoes adjust to the different physiological characteristics of the host species which it parasitizes, thus ensuring an adequate water and carbon balance.

Phosotynthesis in hemiepiphytic species of Clusia and Ficus

SummaryHemiepiphytic species in the genera Clusia and Ficus were investigated to study their mode of photosynthetic metabolism when growing under natural conditions. Despite growing sympatrically in many areas and having the same growth habit, some Clusia species show Crassulacean acid metabolism (CAM) whereas all species of Ficus investigated are C3. This conclusion is based on diurnal CO2 fixation patterns, diurnal stomatal conductances, diurnal titratable acidity fluctuations, and δ13C isotope ratios. Clusia minor, growing in the savannas adjacent to Barinas, Venezuela, shows all aspects of Crassulacean acid metabolism (CAM) on the basis of nocturnal gas exchange, stomatal conductance, total titratable acidity, and carbon isotope composition when measured during the dry season (February 1986). During the wet season (June 1986), the plants shifted to C3-type gas exchange with all CO2 uptake occurring during the daylight hours. The carbon isotope composition of new growth was-28 to-29‰ typical of C3 plants.

Morphological changes along an altitude gradient and their consequences for an andean giant rosette plant

SummarySelected morphological features were measured in five populations of the giant rosette plant Espeletia schultzii occurring along an elevation gradient from 2600 to 4200 m in the Venezuelan Andes. Pith volume per amount of leaf area increases with elevation resulting in significantly larger water storage capacity at higher elevations. Thickness of leaf pubescence and, therefore, leaf boundary layer resistance, also increases with elevation resulting in both potentially higher leaf temperatures relative to air temperature and higher leaf to air vapor pressure gradients. The net effect on transpiration rate would depend on ratios of stomatal to boundary layer resistance and leaf energy balance. At higher elevations the central rosette leaves are more vertically oriented and the leaf bases show a pronounced curvature as the intersection with the main axis is approached. This gives these rosettes a distinctly paraboloid appearance and probably enhances capture and retention of incident long and shortwave radiation by the apical bud and expanding leaves. Features which result in enhanced water storage capacity and higher plant temperatures relative to air temperature without greatly increasing water loss are adaptive in high altitude paramo habitats where water availability and growth are limited by year round low temperatures (mean 2–3° C).

Freezing tolerance in Draba chionophila, a ‘miniature’ caulescent rosette species

SummaryFreezing tolerance as a cold resistance mechanism is described for the first time in a plant growing in the tropical range of the Andean high mountains. Draba chionophila, the plant in which freezing tolerance was found, is the vascular plant which reaches the highest altitudes in the Venezuelan Andes (approximately 4700m). Night cycles of air and leaf temperature were studied in the field to determine the temperature at which leaf freezing began. In the laboratory, thermal analysis and freezing injury determinations were also carried out. From both field and laboratory experiments, it was determined that freezing of the leaf tissue, as well as root and pith tissue, initiated at a temperature of approximately-5.0°C, while freezing injury occurred at approximately-12.0°C for the pith, and below-14.0°C for roots and leaves. This difference in temperature suggests that the plant still survives freezing in the-5.0 to-14.0°C range. Daily cycles of leaf osmotic potential and soluble carbohydrate concentration were also determined in an attempt to explain some of the changes occurring in this species during the nighttime temperature period. A comparison between Andean and African high mountain plants from the point of view of cold resistance mechanisms is made.

Water balance in developing leaves of four tropical savanna woody species

SummaryWater balance was studied in developing leaves of Curatella americana, Byrsonima crassifolia, Bowdichia virgilioides and Casearia sylvestris, four widespread tropical savanna evergreen woody species that renew their leaves during the dry season. Water potential components of leaves of different ages were estimated in the laboratory by determination of pressure-volume (PV) curves. Data from PV curves were used to help interpret the significance of variations in leaf water potential (ΨL) and stomatal conductance observed in leaves of different ages in the field.Initial osmotic potential at full turgidity as well as osmotic potential at the turgor loss point changed considerably during leaf development. Values of these water potential components for mature leaves were similar to those reported for temperate mesophytic woody species. Passive osmotic adjustment defined as the change in osmotic potential between full turgidity and turgor loss averaged only 0.3 MPa, much smaller than values for temperate mesophytic and drought hardy woody species. Since environmental conditions, especially soil moisture in the rooting zone, were essentially constant during leaf development, changes in leaf water potential components were not seasonal, but rather ontogenetic responses to diurnal water stress.In all species except C. sylvestris there were differences in ΨL between adjacent leaves exposed to the same environmental conditions, with immature leaves generally maintaining higher ΨL than mature leaves. The probable causes for this were 1) lower liquid flow resistance and 2) lower transpiration rates in younger leaves. At low transpiration rates liquid flow resistance was flux-dependent in all species, but became nearly independent of transpiration flux density (E) as E increased. This and their lower flow resistance permit high E to be sustained in developing leaves without excessive ψ drop across the leaf.During two typical dry season days E was high and even though complete or nearly complete turgor loss was sustained, these water deficits were not severe enough to cause complete stomatal closure. The possible relationship between a high stomatal conductance and the maintenance of a favorable carbon balance in these species is discussed.All of the water relations features observed suggested that in spite of their scleromorphic appearance, leaves of the four species under the conditions studied were adapted only to short term diurnal water deficits.