Water vapour isotopic exchange by epiphytic bromeliads in tropical dry forests reflects niche differentiation and climatic signals

The 18O signals in leaf water (delta18O(lw)) and organic material were dominated by atmospheric water vapour 18O signals (delta18O(vap)) in tank and atmospheric life forms of epiphytic bromeliads with crassulacean acid metabolism (CAM), from a seasonally dry forest in Mexico. Under field conditions, the mean delta18O(lw) for all species was constant during the course of the day and systematically increased from wet to dry seasons (from 0 to +6 per thousand), when relative water content (RWC) diminished from 70 to 30%. In the greenhouse, progressive enrichment from base to leaf tip was observed at low night-time humidity; under high humidity, the leaf tip equilibrated faster with delta18O(vap) than the other leaf sections. Laboratory manipulations using an isotopically depleted water source showed that delta18O(vap) was more rapidly incorporated than liquid water. Our data were consistent with a Craig-Gordon (C-G) model as modified by Helliker and Griffiths predicting that the influx and exchange of delta18O(vap) control delta18O(lw) in certain epiphytic life forms, despite progressive tissue water loss. We use delta18O(lw) signals to define water-use strategies for the coexisting species which are consistent with habitat preference under natural conditions and life form. Bulk organic matter (delta18O(org)) is used to predict the deltaO18(vap) signal at the time of leaf expansion.     

Diversity and distribution of herbaceous vascular epiphytes in a tropical evergreen forest at Varagalaiar, Western Ghats, India

Herbaceous vascular epiphytes were screened in a total sample of 13 445 trees (in 153 species) and 348 lianas (in 30 species) 30 cm girth at breast height in a 30 ha plot of tropical evergreen forest at Varagalaiar, Indian Western Ghats. Of these, 4.3% of trees and 3.7% of lianas were infested with epiphytes. Epiphyte diversity totaled 26 species in 19 genera and 10 families. Sixteen species were angiosperms in three families (Orchidaceae 54%; Piperaceae and Araceae 8%) and 10 species (38%) were pteridophytes in seven families. The orchid, Pholidota pallida was most abundant and occurred on 178 (29.6%) stems. Asplenium nidus occurred on the maximum of 62 host species. The species richness estimators employed for species accumulation curves after 100 times randomization of sample order have stabilized the curve at 18th and 19th hectares respectively for Incidence-based Coverage Estimator and Chao2. A total of 588 trees and 13 liana stems lodged epiphytes, 74% of which were evergreen species and 26% deciduous. Epiphyte density was greater (56%) in deciduous species (Bischofia javanica 30% and Vitex altissima 8%). A significant positive relationship was found between trunk size and epiphyte association. Larger epiphyte species occurred mostly on middle and larger stems and smaller epiphyte species occurred on smaller stems. The majority of epiphytes (92%) were of autochorous dispersal type, bearing capsule or dust diaspores, while the remaining species with berries and nutlets are dispersed by small vertebrates.     

Comparative ecophysiology of CAM and C3 bromeliads. II. Field measurements of gas exchange of CAM bromeliads in the humid tropics

Abstract The results described represent the first detailed measurements of gas exchange of epiphytic plants with crassulacean acid metabolism (CAM) in the humid tropics. A portable steady-state CO₂ and H₂O porometer was used to measure net exchange rates of CO₂ and H₂O vapour (J_CO2, J_H2O), leaf temperature (T₁), air temperature (T_A), air relative humidity (RH) and photosynthetically active radiation (PAR) for bromeliads in the field during the dry season in February and March 1983 on the tropical island of Trinidad. Different lengths of tubing (up to 25 m) were used so that the gas exchange could be measured of bromeliads in situ in their epiphytic habitats. Derived parameters such as leaf-air water-vapour-concentration difference (Δ_w), water-vapour conductance of leaves (g) and internal CO₂ partial pressure (pⁱ_CO2) could be calculated. The particular problems of making such measurements in the humid tropics due to high relative humidities and high dew-point temperatures are discussed. The long and often broad, strap-like leaves of bromeliads are well suited for measurements with the steady-state porometer. It is shown that CAM activity varies along the length of individual leaves, and variability between different leaves is also demonstrated. The major phases of CAM, i.e. nocturnal stomalal opening, CO₂ uptake and dark fixation as malic acid (Phase I), daytime stomatal closure and light-dependent assimilation of CO₂ derived from decarboxylation of the malic acid (Phase III), and late-afternoon stomatal opening with direct light-dependent assimilation of atmospheric CO₂ (Phase IV) were all clearly shown by CAM bromeliads in situ. Their expression and magnitude depended on the environmental conditions. An early-morning peak of CO₂ uptake as is characteristic of Phase II of CAM was not detected during the night-day transition. A bromeliad intermediate between C₃ and CAM, Guzmania monostachia, showed substantial net CO₂ uptake in the early morning but no net uptake integrated over the whole of the night.     

Comparative ecophysiology of CAM and C3 bromeliads. IV. Plant water relations

Abstract An investigation was carried out into the water relations of CAM and C₃ bromeliads in their natural habitat during the dry season in Trinidad. Measurements were made of xylem tension with the pressure chamber and of cell-sap osmotic pressure and titratable acidity on crushed leaf samples. A steady-state CO₂ and H₂O-vapour porometer was also used so that changes in leaf water relations during individual day-night cycles could be directly related to gas-exchange patterns in situ. Xylem tension changed in parallel with transpiration rate and in general reached its maximum value in CAM bromeliads at night and in C₃ bromeliads during the day. In addition, large nocturnal increases in cell-sap osmotic pressure and titratable acidity (ΔH⁺) typically occurred in the CAM bromeliads. The C₃-CAM intermediate Guzmania monostachia showed slight nocturnal acidification, but had higher values of xylem tension during the day. Very high values of AH+ were observed in the CAM species when the tanks of the epiphytic bromeliads contained water: Aechmea nudicaulis showed a mean maximum ΔH⁺ of 474 mol m^?3, the highest value so far observed for CAM plants. On some nights dew formed on the leaf surfaces of the epiphytes, partially curtailing gas exchange and leading to a marked decrease in xylem tension in both C₃ and CAM species. Between-site comparisons were also made for a wide range of habitats from arid coastal scrub to montane rain forest. Compared with values characteristic of other life-forms, xylem tension and cell-sap osmotic pressure were low for all bromeliads, and did not differ significantly in co-occurring CAM and C₃ bromeliads. Mean maximum xylem tension (10 species in total) ranged from 0.29 M Pa at the montane sites to 0.67 MPa at the most arid site, and mean minimum osmotic pressure (17 species) from 0.51 to 0.97 MPa. At the arid sites the bromeliads were exclusively CAM species, two of which (Aechmea aquilega and Bromelia plumieri) grew terrestrially in the undergrowth of the coastal scrub. Xylem tension in these species was low enough to indicate that they must be functionally independent of the substratum during the dry season. In the wetter part of Trinidad, no between-site differences in leaf water relations were found along an altitudinal gradient in the Northern Mountain Range; seasonal differences in this area were also small. Overall, leaf water relations and gas exchange in the bromeliads were strongly affected both by short-term changes in water availability and by longer-term climatic differences in the various regions of the island.     

Root surface acid phosphatase activities of vascular epiphytes of a Costa Rican rain forest

The objective of this work was to compare root surface phosphatase activities of vascular epiphytes typical of a lowland tropical forest. Acid phophatase, measured at pH 5.0 with the substrate p-nitrophenyl phosphate, was detected in 22 species distributed within 10 plant families. Epiphytes were classified as trunk, canopy-mat or bare-limb species based upon their usual occurrence. Phosphatase activity was not significantly correlated with plant occurrence. However, phosphatase activity was generally highest in trunk occurring and canopy-mat epiphytes rooted in mosses and humus-like accumulations, and lowest in species restricted to bare limbs. Epiphyte shoot phosphorus and chlorophyll content were correlated with species occurrence, with phosphatase being positively correlated with plant P content. The observed changes in acid phosphatase production among habitats were consistent with predicted changes in the availability of organic P sources. However, observed changes also reflect accompanying shifts in root structure including: the occurrence of velamen or waxy layers, changes in root diameter, branching and root hair density.     

Protozoan communities in chalk streams

This study assessed the individual effects of three mayflies (Paraleptophlebia sp., Ephemerella subvaria McDunnough and Epeorus sp.) and one caddisfly (Psilotreta sp.) on periphyton communities associated with clay tiles and leaves. Algal densities were estimated for leaf discs and tiles from experimental chambers (with individual grazers) and control chambers (i.e., no grazers). Scanning electron micrographs (SEM) of leaf discs and tiles also were taken for all mayfly grazing experiments. Densities of algae on leaf discs were two to five times lower than on tiles. Mouthpart morphology influenced how different insects grazed the periphyton community. Paraleptophlebia had typical collector-gatherer mouthparts and had no effect on diatom densities associated with leaves whereas diatom densities on grazed tiles were higher than densities on tiles from control chambers. Epeorus had brusher mouthparts and had little impact on diatom densities regardless of substratum type. The other two grazers had the blade-like mandibles of a scraper. Psilotreta did not reduce the numerical abundance of diatoms on either substratum, but did alter community structure by significantly reducing densities of stalked Gomphonema olivaceum and large species of Navicula and Nitzschia; densities of smaller diatoms (Achnanthes spp) increased. However, E. subvaria reduced densities of most algal species regardless of size on both substrata and also significantly altered community structure. SEMs of substrata grazed by mayflies showed reductions in fungal hyphae on all grazed leaf discs, decreases in filamentous algal forms on grazed tiles, and greatly shortened stalks of G. olivaceum (Paraleptophlebia only). Thus, periphyton communities are different on leaves versus tiles and grazers with different mouthpart morphologies have varying effects on both algal and heterotrophic microbial community structure.     

Predation pressure on the arboreal epiphytic herbivores of larch trees in southern England

Abstract. 1. Quantitative collections were made of arthropod predators from larch trees at Ranmore Common, Surrey, from August 1977 to September 1978.
2. Spiders were numerically the most important predators accounting for 64% of the total.
3. Antisera of the main larch dwelling, epiphyte herbivore orders, Collembola and Psocoptera, and the other common arboreal herbivore group at Ranmore, the Honioptera (principally Psyllidae) were prepared in rabbits.
4. Serological determinations of predator diets indicated that, with a few exceptions, the predators were taking approximately equal proportions of the three prey groups.
5. This apparent non-specificity of the predators is analysed in relation to both the numerical abundance and biomass of the three prey types during the study period.
6. These arboreal arthropod predators are selecting for large but rare prey items in preference to small and common ones.
7. These findings are discussed in relation to the population dynamics of arboreal epiphytic herbivores.     

Tank bromeliad transplants as an enrichment strategy in southern Costa Rica

Epiphytes represent up to 50% of all vascular plant species in neotropical forests but they are among the slowest plants to recolonize regenerating ecosystems. This discrepancy underlines the need for restoration ecologists to learn how to assist the colonization of organisms in this key functional group. Transplanting tank bromeliads (i.e. bromeliads featuring overlapping leaves that form a water impounding rosette) could be a good approach in the neotropics, where abundant, fallen bromeliads can be sustainably collected from the forest floor. Moreover, tank bromeliads could accelerate restoration processes by providing relatively stable microenvironments for invertebrates, thus helping them resist severe drought and high temperatures, such as predicted in light of many climate change models. We transplanted 60 individuals of the tank bromeliad Werauhia gladioliflora onto trunks and branches of comparable size and orientation on three host tree species. The study took place in three long‐term restoration plantations located in a tropical premontane rainforest zone in southern Costa Rica. Transplant survivorship after 9 months varied among sites, from 65 to 95%. Transplants hosted twice as many arthropod orders as untreated control branches, and they buffered microclimates during the driest (+1.7 to 19.7% relative humidity) and warmest (?0.5 to 5.0°C) times of the day. Our results suggest that bromeliad transplantation is a cost‐effective (circa Epiphytes represent up to 50% of all vascular plant species in neotropical forests but they are among the slowest plants to recolonize regenerating ecosystems. This discrepancy underlines the need for restoration ecologists to learn how to assist the colonization of organisms in this key functional group. Transplanting tank bromeliads (i.e. bromeliads featuring overlapping leaves that form a water impounding rosette) could be a good approach in the neotropics, where abundant, fallen bromeliads can be sustainably collected from the forest floor. Moreover, tank bromeliads could accelerate restoration processes by providing relatively stable microenvironments for invertebrates, thus helping them resist severe drought and high temperatures, such as predicted in light of many climate change models. We transplanted 60 individuals of the tank bromeliad Werauhia gladioliflora onto trunks and branches of comparable size and orientation on three host tree species. The study took place in three long‐term restoration plantations located in a tropical premontane rainforest zone in southern Costa Rica. Transplant survivorship after 9 months varied among sites, from 65 to 95%. Transplants hosted twice as many arthropod orders as untreated control branches, and they buffered microclimates during the driest (+1.7 to 19.7% relative humidity) and warmest (?0.5 to 5.0°C) times of the day. Our results suggest that bromeliad transplantation is a cost‐effective (circa $0.5 USD/successful transplant) strategy to assist the recovery of epiphyte diversity in forest restoration sites with minimal impact on source populations. Longer‐term studies are needed to test this strategy for other epiphyte families or for mixed‐taxa assemblages found on fallen branches.