Leaf photosynthetic traits scale with hydraulic conductivity and wood density in Panamanian forest canopy trees

We investigated how water transport capacity, wood density and wood anatomy were related to leaf photosynthetic traits in two lowland forests in Panama. Leaf-specific hydraulic conductivity (k_L) of upper branches was positively correlated with maximum rates of net CO₂ assimilation per unit leaf area (A_area) and stomatal conductance (g_s) across 20 species of canopy trees. Maximum k_L showed stronger correlation with A_area than initial k_L suggesting that allocation to photosynthetic potential is proportional to maximum water transport capacity. Terminal branch k_L was negatively correlated with A_area/g_s and positively correlated with photosynthesis per unit N, indicating a trade-off of efficient use of water against efficient use of N in photosynthesis as water transport efficiency varied. Specific hydraulic conductivity calculated from xylem anatomical characteristics (k_theoretical) was positively related to A_area and k_L, consistent with relationships among physiological measurements. Branch wood density was negatively correlated with wood water storage at saturation, k_L, A_area, net CO₂ assimilation per unit leaf mass (A_mass), and minimum leaf water potential measured on covered leaves, suggesting that wood density constrains physiological function to specific operating ranges. Kinetic and static indices of branch water transport capacity thus exhibit considerable co-ordination with allocation to potential carbon gain. Our results indicate that understanding tree hydraulic architecture provides added insights to comparisons of leaf level measurements among species, and links photosynthetic allocation patterns with branch hydraulic processes.     

Grazing catfish,fishing birds,and attached algae in a Panamanian stream

Synopsis In streams where algivorous fishes abound, striking variation of attached algae often develops along depth gradients, with bands of high standing crops in shallow water (<20 cm) and sparse standing crops on deeper substrates. Experimental results from a stream in central Panama support the hypothesis that vertical variation in algal standing crops arises when grazing fishes avoid predators in shallow water by forgoing food resources that accumulate there. When 38 rocks bearing algae in a stream in central Panama were transferred from shallow (<20 cm) to deeper (>20 cm) water, algae were rapidly consumed by grazing catfish. Catfish were removed from three stream pools and left in place in three control pools. Ten days after catfish removal, algal standing crops in deep and shallow areas of removal pools were similar, while algal standing crops were higher in shallow than in deep areas of control pools. Catfish were exposed to fishing birds in open-topped enclosures. In one of three series of these pens, most catfish in shallow pens (10 and 20 cm) disappeared after 14 days, while catfish in deeper pens (30 and 50 cm) did not. Other groups of catfish which were caged 8 days showed differences in behavior depending on whether they had been fed or starved. After their release into their home pool, starved catfish spent more time feeding than did fed catfish. Despite their apparently increased hunger levels, starved catfish did not venture into shallow water to obtain algae. These results support the view that predator induced avoidance by grazers of certain areas can produce spatial pattern in the flora of flowing water communities.     

Modification and Application of a Leaf Blower-vac for Field Sampling of Arthropods

Rice fields host a large diversity of arthropods, but investigating their population dynamics and interactions is challenging. Here we describe the modification and application of a leaf blower-vac for suction sampling of arthropod populations in rice. When used in combination with an enclosure, application of this sampling device provides absolute estimates of the populations of arthropods as numbers per standardized sampling area. The sampling efficiency depends critically on the sampling duration. In a mature rice crop, a two-minute sampling in an enclosure of 0.13 m² yields more than 90% of the arthropod population. The device also allows sampling of arthropods dwelling on the water surface or the soil in rice paddies, but it is not suitable for sampling fast flying insects, such as predatory Odonata or larger hymenopterous parasitoids. The modified blower-vac is simple to construct, and cheaper and easier to handle than traditional suction sampling devices, such as D-vac. The low cost makes the modified blower-vac also accessible to researchers in developing countries.     

Temporal patterns of water flux in trees and lianas in a Panamanian moist forest

Using constant heat sap flow sensors, xylem water fluxes in ten tree species and two liana species were monitored for 5–10 days during the beginning of the wet season in May, 1993. For a subset of the trees, a branch was also monitored at the top of the crown for 5 days. Xylem flux (J _S) was related diurnally in all plants to vapor pressure deficit (D) measured within the upper-third of the canopy, and to incoming shortwave radiation R _S above the canopy. Cross-correlation analysis was used to estimate time lags between diurnal patterns of J _S and D or R _S, and between J _S in stems and branches. The maximum correlation coefficient from cross-correlation of J _S with R _S (range=0.57–0.92) was often higher than the maximum of J _S with D (range=0.43–0.89), indicating that diurnal J _S was more dependent on R _S than D. Time lags (lag corresponding to maximum correlation) of J _S at stem-base with D was shorter (0–45 min) than with radiation (5–115 min), highly variable within a species, and uncorrelated to the height or exposure of tree crowns or liana in the canopy. On a stand level, not accounting for the diel lag between stem sap flux and canopy flux resulted in errors in estimated canopy transpiration of up to 30%. Received: 19 October 1998 / Accepted: 8 June 1999     

Soil nutrient dynamics in response to irrigation of a Panamanian tropical moist forest

We measured concentrations of soil nutrients (0–15 and 30–35 cm depths) before and after the dry season in control and dry-season irrigated plots of mature tropical moist forest on Barro Colorado Island (BCI) in central Panama to determine how soil moisture affects availability of plant nutrients. Dry-season irrigation (January through April in 1986, 1987, and 1988) enhanced gravimetric soil water contents to wet-season levels (ca. 400 g kg^–1 but did not cause leaching beyond 0.8 m depth in the soil. Irrigation increased concentrations of exchangeable base cations (Ca²⁺, Mg²⁺, K⁺, Na⁺), but it had little effect on concentrations of inorganic N (NH₄^+C, NO₃^– and S (SO₄^2–). These BCI soils had particularly low concentrations of extractable P especially at the end of the dry season in April, and concentrations increased in response to irrigation and the onset of the rainy season. We also measured the response of soil processes (nitrification and S mineralization) to irrigation and found that they responded positively to increased soil moisture in laboratory incubations, but irrigation had little effect on rates in the field. Other processes (plant uptake, soil organic matter dynamics) must compensate in the field and keep soil nutrient concentrations at relatively low levels.     

Leaf anatomy in a Venezuelan montane forest

A survey has been made of the leaf anatomy of 43 woody species in three montane vegetation types of Isla Margarita, Venezuela, differing in levels of cloud cover (and hence wetness) and exposure to wind. The only character that varies significantly with increasing probability of periodic drought is specific leaf area, which can be related to the higher proportion of deciduous species in the drier habitat. Leaves become significantly smaller and thicker with increased exposure to wind, and have thicker outer epidermal walls and cuticles. Most characters, including stomatal density, guard cell length, palisade: non-palisade ratios, and the incidence of such features as hypodermis, sclerenchyma, crystals and secretory structures are highly variable and show few or no trends according to habitat. Some of these characters are apparently more constrained by phylogeny than by immediate ecological circumstances. As a contribution to the continuing debate on the significance of xeromorphy in tropical montane forest leaves, it is suggested that low light levels due to cloud cover can be discounted as an important determining factor; in the case of the Isla Margarita vegetation, the need to avoid excessive leaf temperatures may be of greater significance.     

Species-specific foraging behavior of birds in a riparian forest

In riparian forest, insectivorous birds are largely dependent on aquatic preys. However, the contribution made by aquatic preys to bird diets varies considerably among bird species. In the present study, bird foraging behaviors were observed in order to examine the relationship between bird foraging method and the variation in the contribution of aquatic preys. The great tit, the black-faced bunting, and the wren are largely dependent on aquatic preys by capturing them on the ground. Sallyers, the brown flycatcher, the pale-legged willow warbler, and the narcissus flycatcher are also largely dependent on aquatic preys and capture them mostly during flight. The narcissus flycatcher frequently utilises aquatic invertebrates dwelling on the ground. The leaf and branch gleaner, the crowned willow warbler, does not depend on aquatic prey as much. Although both the pygmy woodpecker and nuthatch are branch and trunk gleaners, the nuthatch utilises aquatic preys frequently by capturing them on the ground as well as during flight, but the pygmy woodpecker does not depend on aquatic preys. The marsh tit also does not depend on aquatic preys but carefully searches terrestrial prey that hide in the vegetation. The differential dependence on aquatic preys among species can lead to the heterogeneous distribution of birds within a riparian forest, suggesting that the indirect effect of aquatic preys on a forest ecosystem via birds can vary within a forest–stream ecotone.     

Gas exchange and hydraulic properties in the crowns of two tree species in a Panamanian moist forest

Hydraulic properties and gas exchange were measured in branches of two tropical tree species (Simarouba amara Aubl. and Tapirira guianensis Aubl.) in a moist lowland forest in Panama. Branch-level sapflow, leaf-level stomatal conductance, and water potential measurements, along with measurements of specific hydraulic conductivity of stems in crown tops, were used to relate hydraulic parameters to leaf conductance in two individuals of each species. Branches of the taller trees for each species (28 m, 31 m) showed much higher leaf-specific hydraulic conductance and leaf vapor-phase conductance than those of the smaller trees (18m, 23m). This was probably related to the leaf-to-sapwood area ratio in branches of taller trees, which was less than half that in branches of smaller trees. Dye staining showed evidence of massive cavitation in all trees, indicating that stomata do not control leaf water potential to prevent xylem cavitation in these species. Stomatal conductance of intact leaves also appeared to be insensitive to leaf area removal treatment of nearby foliage. Nevertheless, a simple mass-balance model of water flux combining hydraulic and vapor transport was in close agreement with observed maximal vapor-phase conductance in the four trees (r²=0.98, P=0.006). Our results suggest that the major organismal control over water flux in these species is by structural (leaf area) rather than physiological (stomatal) means.     

Annual cycles in Jamaican forest birds

The annual cycles of forest birds in Jamaica were found to be very similar to those at higher latitudes. Most species bred between March and September, though a few possibly breed throughout the year, especially in cultivated areas. Primary moult followed immediately after breeding, and in some species was apparently arrested to allow a further breeding attempt. Several species were fatter outside the breeding season than during it, and this is interpreted as "winter fattening" comparable to that found in many birds at higher latitudes. Weights varied little but individuals retrapped were usually heavier outside the breeding season. In some species the first complete moult took place at the end of the first year, implying that the birds do not breed until at least two years old.     

An overview of surface specializations in the digenetic trematodes

The outer covering of digenetic trematodes undergoes a number of changes in the course of the life cycle in adaptation to parasitism and to the host-seeking requirements of the various stages. Using Schistosoma mansoni as a model, I review morphological, physiological, and cytochemical features of the coverings of miracidia, sporocysts, cercariae, and adults. Such features should be used with caution in evaluating phylogenetic relationships of any flatworms since the environment is itself a powerful determinant.     

Temporal patterns of soil nutrients in a Panamanian moist forest revealed by ion-exchange resin and experimental irrigation

The effect of seasonal water availability on soil nutrients and soil N transformations was investigated by irrigating two large plots of mature tropical forest on Barro Colorado Island (BCI), Panama, during the dry season for five consecutive years. Methods included (i) nutrient accumulation by ion-exchange resins placed on the surface of the mineral soil for contiguous 21-day periods, (ii) monthly mineral soil (0–10 cm) extractions and incubations for inorganic N and P concentrations, and (iii) leaching loss of nutrients from leaf litter samples. Rates of nutrient accumulation by the resins showed a great deal of variation between sampling dates and among years in control plots; albeit, seasonal patterns were slight, except for the highest Ca values near the end of the wet season and inorganic P (P_i) and SO₄ values that peaked during the dry season. Irrigation had remarkably little effect on nutrient accumulation rates by resins, except for an increase in Mg and Na values, but did affect the timing in the temporal variation in K, Na, N_i and P_i values. In contrast, inorganic N (N_i) and P_i pools and N transformation rates in the mineral soil hardly varied among sampling dates and did not show any response to irrigation. We hypothesize that the timing of leaf litterfall and nutrient leaching from forest floor litter can set up temporal patterns in the levels of soil nutrient at the surface of the mineral soil, but the temporal patterns essentially disappear with depth in the mineral soil.     

Lekking birds in a tropical forest forego sex for migration

Facultative, partially migratory animals provide a contemporary window into the evolution of migration, offering rare opportunities to examine the life-history trade-offs associated with migration. For the first time, to our knowledge, we describe the nature of these trade-offs, using a lek-breeding tropical bird, the white-ruffed manakin (Corapipo altera). Previous evidence indicated that weather drives post-breeding migration to lower elevations bringing condition-related benefits. Using elevation-sensitive stable isotope measurements and more than 1200 h of behavioural observations, we show that male manakins which migrate incur costs of diminished social status and matings with females the following breeding season. Because migratory tendency depends on inter-annual variation in weather, physical costs of displays and breeding prospects the following year, migratory decisions are subject to both natural and sexual selection, with the outcome of such decisions linked to changing climatic regimes.     

Neither host-specific nor random: vascular epiphytes on three tree species in a Panamanian lowland forest

BACKGROUND AND AIMS: A possible role of host tree identity in the structuring of vascular epiphyte communities has attracted scientific attention for decades. Specifically, it has been suggested that each host tree species has a specific subset of the local species pool according to its own set of properties, e.g. physicochemical characteristics of the bark, tree architecture, or leaf phenology patterns. METHODS: A novel, quantitative approach to this question is presented, taking advantage of a complete census of the vascular epiphyte community in 0.4 ha of undisturbed lowland forest in Panama. For three locally common host-tree species (Socratea exorrhiza, Marila laxiflora, Perebea xanthochyma) null models were created of the expected epiphyte assemblages assuming that epiphyte colonization reflected random distribution of epiphytes in the forest. KEY RESULTS: In all three tree species, abundances of the majority of epiphyte species (69-81 %) were indistinguishable from random, while the remaining species were about equally over- or under-represented compared with their occurrence in the entire forest plot. Permutations based on the number of colonized trees (reflecting observed spatial patchiness) yielded similar results. Finally, a third analysis (canonical correspondence analysis) also confirmed host-specific differences in epiphyte assemblages. In spite of pronounced preferences of some epiphytes for particular host trees, no epiphyte species was restricted to a single host. CONCLUSIONS: The epiphytes on a given tree species are not simply a random sample of the local species pool, but there are no indications of host specificity either.     

Landscape-dependent distribution of northern forest birds in winter

We evaluated the effects of landscape structure, along a broad gradient of deforestation (forest cover 8–88%, 500-m radius), on the spatial distribution of forest birds exposed to winter climatic conditions, in Quebec, Canada. Concurrently, we conducted an experiment to determine if these effects would persist if an unlimited source of energy, provided by food-supplementation, became available. We analyzed these effects at the population level, using count data of black-capped chickadees Poecile atricapillus , but also at the community level, referring to species richness. In one of the two years of the study, before food-supplementation began (November), forest integrity (a composite of forest cover and edge density) was positively associated with chickadee abundance and species richness. Each year, forest integrity was also positively associated with chickadee abundance and species richness in landscapes that were supplemented (December–February). However, in control landscapes, during the food-supplementation period, chickadee abundance and species richness tended to decrease with an increase in forest integrity. We argue that the more forested control landscapes facilitated winter emigration of juveniles and transient birds. Conversely, our results further suggest that, in the highly deforested and fragmented control landscapes, birds became gap-locked when rigorous winter climatic conditions exacerbated already existing movement constraints.     

Conservation of tropical forest birds in countryside habitats

The pressing need to increase agricultural production often seems at odds with conserving biodiversity. We find that if managed properly, the tropical countryside may provide a substantial opportunity for tropical bird conservation. We detected 144 bird species from 29 families in agricultural areas outside of extensive native forest in southern Costa Rica. The majority of the species detected were observed foraging, often kilometres from extensive native forest. We estimate that 46% of those native to this region (excluding nocturnal species and waterfowl) are utilizing the countryside in some manner. Forecasts of biodiversity change under various land-use scenarios indicate that policies that affect habitat composition could greatly impact the persistence of these species in the countryside. In particular, if tall trees and edge habitats were removed from this landscape, we predict that bird richness in the countryside would decline by approximately 40%.     

Characterization of poxviruses from forest birds in Hawaii

Two strains of avian pox viruses were isolated from cutaneous lesions in Hawaiian crows (Corvus hawaiiensis) examined in 1994 and a third from a biopsy obtained in 1992 from an infected bird of the Apapane species (Himatione sanguinea) by inoculation of the chorioallantoic membranes (CAM) of developing chicken embryos. The resulting proliferative CAM lesions contained eosinophilic cytoplasmic inclusion bodies characteristic of pox virus infection. The pathogenicity of these three viruses in domestic chickens was mild as evidenced by the development of relatively minor lesions of short duration at the sites of inoculation. Their virulence in this host was similar to that of a fowlpox virus (FPV) vaccine strain and contrasted greatly with the ability of two field strains of FPV to produce extensive proliferative lesions. One of the Hawaiian crow pox virus isolates as well as the one originating from the Apapane species could be propagated in two secondary avian cell lines, QT-35 and LMH. A comparison of the restriction fragment length polymorphisms (RFLP) of the genomes of the two cell line-adapted viruses, generated by EcoRI digestion, revealed a limited degree of similarity. Moreover, neither profile was comparable to those of the two field isolates of FPV, which were almost indistinguishable from each other. Thus, based on the genetic distinctness of the two Hawaiian bird viruses, they appear to represent different strains of avipoxvirus.     

Explosive increase in ectoparasites in Hawaiian forest birds

Ectoparasites, particularly chewing lice in the Phthiraptera (Insecta), affect the ecology of numerous host species. Most lice are highly host-specific, and there are no documented cases of major increases of chewing lice, within populations, over years. During continuous study from 1987-2005 at upper elevation forests on the island of Hawaii, chewing lice were exceedingly rare and, until 2003, were found in just 2 of 12 species of native and introduced birds. From 2003-2005, there was an explosive increase in the prevalence of chewing lice in all host species. There was no change in humidity, or in behavior of hosts, that could have caused an ecological release of existing lice. Based on reduced fat levels and increases in broken wing and tail feathers for most host species, there was apparently a food limitation that preceded the increase. The increase coincided temporally with detection of a nonnative bird that had recently been found in elevations below the study sites. Although there were isolated sightings of this bird on the study sites, seasonal movements and behavior of some species of native birds could also have allowed greater transmission to study sites. Both prevalence and intensity of infection, indexed by number of body regions parasitized, were lower in native species with greater bill overlap, a character that could help birds control lice. Seasonality of prevalence indicated that low prevalence preceded molt and high prevalence occurred after molting of hosts. The number of major fault bars in wing and tail feathers, a sign of nutritive stress, was correlated with intensity of infection, indicating an indirect cost to the hosts of being parasitized. In addition, birds with lice were less likely to be recaptured than birds without lice.     

Above and below ground impacts of terrestrial mammals and birds in a tropical forest

Understanding the impact of losing trophic diversity has global significance for managing ecosystems as well as important theoretical implications for community and ecosystem ecology. In several tropical forest ecosystems, habitat fragmentation has resulted in declines and local extinctions of mammalian and avian terrestrial insectivores. To assess the ability of a tropical rainforest community in Ivory Coast to resist perturbation from such loss of trophic diversity, I traced feedbacks in above and below ground communities and measured changes in nutrient levels and herbivory rates in response to an experimental exclosure of avian and mammalian terrestrial insectivores. I present evidence that loss of this functional group may result in increased tree seedling herbivory and altered nutrient regimes through changes in the abundance and guild structure of invertebrates. Exclusion of top predators of the forest floor resulted in increased seedling herbivory rates and macro-invertebrate (>5 mm) densities with strongest effects on herbivorous taxa, spiders and earthworms. Densities of microbivores including Collembola, Acarina and Sciaridae showed the opposite trend as did levels of inorganic phosphorus in the soil. Results were evaluated using path analysis which supported the presence of a top down trophic cascade in the detrital web which ultimately affected turnover of phosphorus, a limiting nutrient in tropical soils. Results illustrate the potential importance of vertebrate predators in both above and belowground food webs despite the biotic diversity and structural heterogeneity of the rainforest floor.