José Luis Andrade

Partitioning of soil water among canopy trees in a seasonally dry tropical forest

Abstract Little is known about partitioning of soil water resources in species-rich, seasonally dry tropical forests. We assessed spatial and temporal patterns of soil water utilization in several canopy tree species on Barro Colorado Island, Panama, during the 1997 dry season. Stable hydrogen isotope composition (δD) of xylem and soil water, soil volumetric water content (θv), and sap flow were measured concurrently. Evaporative fractionation near the soil surface caused soil water δD to decrease from about –15‰ at 0.1 m to –50 to –55‰ at 1.2 m depth. Groundwater sampled at the sources of nearby springs during this period yielded an average δD value of –60‰. θv increased sharply and nearly linearly with depth to 0.7 m, then increased more slowly between 0.7 and 1.05 m. Based on xylem δD values, water uptake in some individual plants appeared to be restricted largely to the upper 20 cm of the soil profile where θv dropped below 20% during the dry season. In contrast, other individuals appeared to have access to water at depths greater than 1 m where θv remained above 45% throughout the dry season. The depths of water sources for trees with intermediate xylem δD values were less certain because variation in soil water δD between 20 and 70 cm was relatively small. Xylem water δD was also strongly dependent on tree size (diameter at breast height), with smaller trees appearing to preferentially tap deeper sources of soil water than larger trees. This relationship appeared to be species independent. Trees able to exploit progressively deeper sources of soil water during the dry season, as indicated by increasingly negative xylem δD values, were also able to maintain constant or even increase rates of water use. Seasonal courses of water use and soil water partitioning were associated with leaf phenology. Species with the smallest seasonal variability in leaf fall were also able to tap increasingly deep sources of soil water as the dry season progressed. Comparison of xylem, soil, and groundwater δD values thus pointed to spatial and temporal partitioning of water resources among several tropical forest canopy tree species during the dry season.

Regulation of water flux through trunks, branches, and leaves in trees of a lowland tropical forest

Abstract We studied regulation of whole-tree water use in individuals of five diverse canopy tree species growing in a Panamanian seasonal forest. A construction crane equipped with a gondola was used to access the upper crowns and points along the branches and trunks of the study trees for making concurrent measurements of sap flow at the whole-tree and branch levels, and vapor phase conductances and water status at the leaf level. These measurements were integrated to assess physiological regulation of water use from the whole-tree to the single-leaf scale. Whole-tree water use ranged from 379 kg day−1 in a 35 m-tall Anacardium excelsum tree to 46 kg day−1 in an 18 m-tall Cecropia longipes tree. The dependence of whole-tree and branch sap velocity and sap flow on sapwood area was essentially identical in the five trees studied. However, large differences in transpiration per unit leaf area (E) among individuals and among branches on the same individual were observed. These differences were substantially reduced when E was normalized by the corresponding branch leaf area:sapwood area ratio (LA/SA). Variation in stomatal conductance (gs) and crown conductance (gc), a total vapor phase conductance that includes stomatal and boundary layer components, was closely associated with variation in the leaf area-specific total hydraulic conductance of the soil/leaf pathway (Gt). Vapor phase conductance in all five trees responded similarly to variation in Gt. Large diurnal variations in Gt were associated with diurnal variation in exchange of water between the transpiration stream and internal stem storage compartments. Differences in stomatal regulation of transpiration on a leaf area basis appeared to be governed largely by tree size and hydraulic architectural features rather than physiological differences in the responsiveness of stomata. We suggest that reliance on measurements gathered at a single scale or inadequate range of scale may result in misleading conclusions concerning physiological differences in regulation of transpiration.

Water uptake and transport in lianas and co-occurring trees of a seasonally dry tropical forest

Water uptake and transport were studied in eight liana species in a seasonally dry tropical forest on Barro Colorado Island, Panama. Stable hydrogen isotope composition (δD) of xylem and soil water, soil volumetric water content (θv), and basal sap flow were measured during the 1997 and 1998 dry seasons. Sap flow of several neighboring trees was measured to assess differences between lianas and trees in magnitudes and patterns of daily sap flow. Little seasonal change in θv was observed at 90–120 cm depth in both years. Mean soil water δD during the dry season was −19‰ at 0–30 cm, −34‰ at 30–60 cm, and −50‰ at 90–120 cm. Average values of xylem δD among the liana species ranged from –28‰ to –44‰ during the middle of the dry season, suggesting that water uptake was restricted to intermediate soil layers (30–60 cm). By the end of the dry season, all species exhibited more negative xylem δD values (–41‰ to –62‰), suggesting that they shifted to deeper water sources. Maximum sap flux density in co-occurring lianas and trees were comparable at similar stem diameter (DBH). Furthermore, lianas and trees conformed to the same linear relationship between daily sap flow and DBH. Our observations that lianas tap shallow sources of soil water at the beginning of the dry season and that sap flow is similar in lianas and trees of equivalent stem diameter do not support the common assumptions that lianas rely primarily on deep soil water and that they have higher rates of sap flow than co-occurring trees of similar stem size.

Dew deposition on epiphytic bromeliad leaves: an important event in a Mexican tropical dry deciduous forest

Dew deposition was calculated for leaves of the epiphytic bromeliads Tillandsia brachycaulos and T. elongata in a tropical dry deciduous forest in Yucatan, Mexico, over one year. Plant characteristics and water relations were also measured. Estimations were made by using energy balance analysis and the Penman-Monteith equation. Dew accumulation was also quantified using a cloth-plate method and by change in mass of whole plants. Tissue water content and total leaf area were similar in the two species, but maximum tank water content was three-fold greater for T. elongata than for T. brachycaulos, which had a lower degree of tissue desiccation tolerance than T. elongata. Succulence for T. brachycaulos changed little during most of the year, decreasing significantly only during one month in the dry season. Leaf water potential and osmotic pressure did not change significantly after a dew-deposition event. Theoretical dew per night on leaves was greatest during the early dry season (about 0.72 mm) compared with 0.34 mm in the rainy season and 0.008 mm in the late dry season. Dew amounts alone are not adequate to support growth but may maintain a favourable water balance for epiphytic bromeliads during the driest months of the year.

Habitat, CO2 uptake and growth for the CAM epiphytic cactus Epiphyllum phyllanthus in a Panamanian tropical forest

In the tropical forest of Barro Colorado Island, habitat characteristics, diel acidity changes, CO 2 uptake and growth were investigated for the epiphytic cactus Epiphyllum phyllanthus (L.) Haw. It occurred most frequently in tree cavities with its roots in canopy soil and was especially abundant on two tree species: Platypodium elegans J. Vogel and Tabebuia guayacan (Seem.) Hemsl. Its maximum net CO 2 uptake rates were low under natural conditions (1.4 μmol m −1 ) but were comparable to those of other CAM and C 3 epiphytes under wet conditions in a screenhouse. Under both natural conditions and in the screenhouse, partial shade enhanced growth and CAM activity. When plants grew under a photosynthetic photon flux of c. 4 mol m −2 d −1 , their nocturnal acidity increase and total net CO 2 uptake were twice as much as for plants growing at lower (an average of 2.4 mol m −2 d −1 ) and higher (7.7 mol m −2 d −1 ) photosynthetic photon fluxes. Stem elongation was 27% greater at the intermediate photosynthetic photon flux. Seedlings of E. phyllanthus survived three months of drought and responded rapidly to rewetting, recovering fully within three days. Transpiration rates and nocturnal acidity increases also recovered to the values of well-watered plants a few days after rewetting, indicating that this species can take advantage of episodic rainfall during the dry season.

Climatic and Structural Factors Influencing Epiphytic Bromeliad Community Assemblage along a Gradient of Water-Limited Environments in the Yucatan Peninsula, Mexico

There is a lack of knowledge on the factors driving epiphytic community assemblage along water-limited environments. Epiphytic bromeliad and host communities were analyzed in a range of vegetation types, following a precipitation gradient from 500 to 1500 mm y−1. Plots were set up in scrub mangrove, coastal sand dune scrub, deciduous, semi-deciduous and sub-perennial forests within the Yucatan Peninsula in Mexico. Identity and abundance of hosts and epiphytes, as well as host height and seasonal microenvironmental variables, were recorded at each vegetation type. The study found epiphytic bromeliads to be mainly located in the lower canopy strata of all but the wettest site (sub-peren nial forest). Total epiphyte richness (estimated using species accumulation curves) increased with annual precipitation. Bromeliad species density decreased with vapor pressure deficit and increased with host species density. Semi-variograms and kriging analysis showed a high spatial correlation in bromeliad and host species density. The species composition, however, was unrelated to space, according to a Mantel test, but related to host species composition. The current study shows that diversity and structural integrity of the canopy may be as important as climate in the conservation of epiphytic composition in water-limited environments, where epiphytes are found in sheltered, lower canopy strata.

Water table depth may influence the asymmetric arrangement of epiphytic bromeliads in a tropical dry forest

Studies of metapopulations of epiphytic bromeliads have been useful to test dispersal limitations and niche effects on the distribution of species, but most studies have been done in wet forests. To advance understanding of these processes in dry environments and using additional microclimatic data, we examined the horizontal distribution, vertical stratification and abundance of epiphytic bromeliads in trees from a dry forest and tested whether water table depth (indicated by the distance to an exposed underground lake, cenote) influenced the microenvironment or structure of the vegetation. Seasonal microenvironmental variables (relative humidity, temperature and light) were characterized among sites, two tree species and three vertical canopy strata. Epiphytes and trees were surveyed in plots, and various tree traits were recorded. Epiphyte density decreased with distance from the cenote (water hole) as the water table became farther from the ground surface and thus unavailable to trees. Sites close to the cenote had higher nighttime humidity, irrespective of the tree species identity. Within the same tree species, the base of the canopy was lower at sites close to the cenote, denoting that the cenote influenced the structure of the canopy. These factors may determine the distribution of the epiphytes by positively influencing the water status of these nocturnally transpiring plants with Crassulacean acid metabolism. Epiphytes were distributed randomly among all the species of trees close to the cenote, but they exhibited a species-specific vertical stratification, which may reflect physiological differences among the epiphyte species.

Pollinators of an Endemic and Endangered Species, Mammillaria gaumeri (Cactaceae), in Its Natural Habitat (Coastal Dune) and in a Botanical Garden

ABSTRACT Mammillaria gaumeri (Britton & Ross) Orcutt (Cactaceae), an endemic plant of the Yucatan Peninsula, is included by the Mexican government in the list of species that require special protection. Its natural habitat is now restricted to fragmented areas and protection programs involve botanical gardens in growing individuals rescued from disturbed areas. Little information is available on the reproductive characteristics of this species and nothing is known of its pollinators. We investigated the visitors of M. gaumeri flowers, collecting and observing bee species in its natural habitat (i.e., coastal dune) and in a botanical garden, where coastal dune vegetation had been created. Observations were made on plants whose density was artificially increased by grouping flowering individuals. At each site, we: 1) collected insects visiting the flowers; 2) recorded number of visits; and 3) video-recorded bee movements on the flowers. As expected, the number of bee species and visitation frequency were higher at the botanical garden than at the coastal dune. After landing on a flower, bees either inspected the anthers or dived among them. These behaviors, carried out by all observed species, seemed related to the state of the anthers (full or empty of pollen) and stigma lobes (opened or closed). Specifically, visits lasted longer when anthers were full of pollen and stigma lobes were opened. The same bee species recorded on the dune were also recorded at the botanical garden, suggesting that the artificial dune at the botanical garden offered suitable conditions for the natural pollinators of this endangered cactus.

Physiological Ecology of Vascular Plants

The Yucatan Peninsula has opposing gradients of precipitation and evaporation from north to south: as precipitation increases, evaporation rates decrease. Also, due to bedrock composed primarily of limestone, the area presents high porosity and rainfall infiltrates quickly, resulting in no superficial runoff. Natural disturbances such as hurricanes and fires are also common. The interaction of these factors has created a mosaic of environmental conditions that has given rise to a series of physiological adaptations in the plant species of the area. This chapter focuses mainly on the morphophysiological responses and adaptations of native plant species of the Yucatan to natural conditions, and on how plants respond to environmental factors at the level of the individual, species, population and/or functional groups. In this chapter, species were grouped according to their metabolic pathway (C3 species or crassulacean acid metabolism species (CAM)), and discussed based on adaptations to limiting resources: water, nutrients, temperature, and light. For C3 species only trees were included, and studies incorporated in this section spanned from plant establishment, growth, water use and water relations, alternative water sources, and carbon and nutrient flow. For the section on CAM the focus was on the factors that affect CAM plants in the microenvironments in which they occur. Other issues analyzed include plant physiological responses to natural disturbances, the potential impact of climate change on plant populations, and gaps in information as well as additional perspectives of study.

Morphophysiological Plasticity in Epiphytic Bromeliads Across a Precipitation Gradient in the Yucatan Peninsula, Mexico

Plasticity may be a key factor to determine plant survival under a changing environment as a result of climate change or land use modification. Plasticity in physiological and morphological traits was evaluated in seven epiphytic Tillandsia species (Bromeliaceae) from six vegetation communities along a precipitation gradient in the Yucatan Peninsula, Mexico. Microenvironmental conditions (air temperature and humidity, light, and vapor pressure deficit), as well as Δ titratable acidity, osmotic potential, relative water content, and succulence were characterized during wet, early dry, and dry seasons. We calculated the relative distances plasticity index using physiological data from the wet and dry seasons; morphological plasticity was also calculated for foliar trichome and stomatal traits from previously published data. We found high variation in microenvironmental conditions between seasons, particularly for the tropical dry deciduous forest. The dry season had a negative effect in all physiological variables (decrease from 40% to 59% for Δ titratable acidity and 10% to 38% for relative water content). The highest plasticity was registered for T. balbisiana (physiological: 0.29, anatomical: 0.18) and the lowest for T. fasciculata and T. yucatana. Nonmetric multidimensional scaling analysis separated individuals distributed in the wettest vegetation types from those distributed in the driest vegetation types, irrespective of the species, showing convergent physiological strategies to confront environmental variation. We found higher plasticity in water use traits in atmospheric species, compared to tanks and higher plasticity in general in species with wide distribution compared to those with small distribution ranges.