Rainforest Understory Research Articles

Some measurements of the microclimate within a Sri Lankan tropical rainforest

This study characterizes the microclimate and soilwater conditions of three gaps within mixed-dipterocarp rainforest of southwest Sri Lanka. The gaps were each of a different size and within a particular part of the forest topography. The largest gap was in a valley and the smallest on a ridge. Soil water, light quantity, and temperature were recorded within demarcated zones across each gap. Although trends between gaps and within gaps are relatively easily explained, the numerical values reported give a good impression of differences and amplitudes concerned as well as of the actual values in the rainforest understory. Surface soilwater measurements revealed that the ridge gap was more prone to water stress than the valley and midslope gaps. Gap centers of all three disturbances appeared to have lower amounts of surface soil water than adjacent forest understories. Light quantities on a sunny day in the forest understory were between 0.26 and 0.92 mols, of which sunflecks contributed between 24 and 62%. These understory light levels are approximately 1% of full sun exposure. The central zone of the valley gap had the longest duration of full sun (17–54% of the daily photosynthetic photon flux density (PPFD) of full sun). The center of the midslope gap had between 12 and 22% and the ridge had between 7 and 16% of daily PPFD of full sun. Diurnal temperature ranges in gap centers are wider than in adjacent forest understories. Sunny days had the greatest ranges of difference with a maximum of 34°C on the soil surface of the valley gap center, while surface temperatures in the understory did not exceed 25°C.

LEAF DEMOGRAPHY OF A NEOTROPICAL RAIN FOREST CYCAD, ZAMIA SKINNERI (ZAMIACEAE)

Tropical rain forest understories are extremely light‐limited environments. Little is known about the demography of leaves in these habitats. To determine leaf longevity and causes of mortality for a species in the understory of a neotropical rain forest, we studied 634 leaves on 173 individuals of the cycad shrub Zamia skinneri for 10 yr at the La Selva Biological Station, Costa Rica. Five different seasonal cohorts of leaves were followed. No significant differences among cohorts were found in median leaf longevity, but the cohorts did differ in the proportion of leaves dying in annual intervals from <1 to ≥6 yr. Median minimum time alive for all leaves was 4.6 yr. Epiphyll coverage increased from 0% in the first year of life to 75%‐100% by year 4. Leaf mortality was well described by two separate exponential functions, approximately 10% mortality/yr until year 4, then abruptly increasing to ca. 51%/yr. Of 625 leaf deaths, 77.0% were due to unexplained causes, 16.5% to physical damage by falling litter, 4.8% to attacks by larvae of the lycaenid butterfly Eumaeus minyas, and 1.8% to other causes. Leaf longevity was positively correlated with leaf size. Individuals of Z. skinneri frequently survive more than a century in very dark understory conditions. We suggest that leaf longevity in these circumstances is determined by the inevitable deterioration in leaf carbon balance due to leaf aging and increasing epiphyll coverage, and by loss due to physical damage and attacks by the specialist herbivore.

Leaf Demography of a Neotropical Rain Forest Cycad, Zamia skinneri (Zamiaceae)

Tropical rain forest understories are extremely light-limited environments. Little is known about the demography of leaves in these habitats. To determine leaf longevity and causes of mortality for a species in the understory of a neotropical rain forest, we studied 634 leaves on 173 individuals of the cycad shrub Zamia skinneri for 10 yr at the La Selva Biological Station, Costa Rica. Five different seasonal cohorts of leaves were followed. No significant differences among cohorts were found in median leaf longevity, but the cohorts did differ in the proportion of leaves dying in annual intervals from <1 to ≥6 yr. Median minimum time alive for all leaves was 4.6 yr. Epiphyll coverage increased from 0% in the first year of life to 75%-100% by year 4. Leaf mortality was well described by two separate exponential functions, approximately 10% mortality/yr until year 4, then abruptly increasing to ca. 51%/yr. Of 625 leaf deaths, 77.0% were due to unexplained causes, 16.5% to physical damage by falling litter, 4.8% to attacks by larvae of the lycaenid butterfly Eumaeus minyas, and 1.8% to other causes. Leaf longevity was positively correlated with leaf size. Individuals of Z. skinneri frequently survive more than a century in very dark understory conditions. We suggest that leaf longevity in these circumstances is determined by the inevitable deterioration in leaf carbon balance due to leaf aging and increasing epiphyll coverage, and by loss due to physical damage and attacks by the specialist herbivore.

Chamaedorea: Diverse species in diverse habitats

Abstract Remarkably diverse species occurring in equally diverse habitats characterize Chamaedorea, a genus of about 90 dioecious species restricted to the understory of neotropical rain and cloud forests from Mexico to Bolivia and Ecuador. A vast array of habits, stems, leaves, inflorescences, flowers, and fruits reflect the diversity of species. Although the genus is most species-rich in dense, moist or wet, diverse forests from 800-1,500 meters elevation, a few exceptional species occur in open and/or seasonally dry forests, on harsh substrates, or in other unusual habitats.

Relations Between Sunfleck Sequences and Photoinhibition of Photosynthesis in a Tropical Rain Forest Understory Herb

Relations Between Sunfleck Sequences and Photoinhibition of Photosynthesis in a Tropical Rain Forest Understory Herb

RELATIONS BETWEEN SUNFLECK SEQUENCES AND PHOTOINHIBITION OF PHOTOSYNTHESIS IN A TROPICAL RAIN FOREST UNDERSTORY HERB

We report the photosynthetic characteristics of a C3 shade plant native to the tropical rain forest understory. It was shown that Elatostema repens Lour. (Hall) f. (Urticaceae) presents a large light adjustment capacity. The effects of several lightfleck sequences on photoinhibition of photosynthesis and carbon gain are analyzed. Photoinhibition is measured both as a decrease in leaf net CO2 uptake in limiting light (shown to be linearly correlated to quantum yield of O2 evolution measured at saturating CO2) and as a decrease of the ratio of variable fluorescence (Fv) to maximum fluorescence (Fmax) measured in liquid nitrogen. It is shown that lightflecks (from 10 to 30 min in duration) of 700 μmol m–2 s–1 (high light) induce photoinhibition, and that the effects of those successive high light periods are additive; there is apparently no recovery from photoinhibition during the low light periods (from 10 to 45 min in duration). In contrast, the Fv/Fmax ratio, though decreasing similarly to quantum yield of net CO2 uptake on leaves submitted to a continuous illumination of 700 μmol m–2 s–1, is only decreased a little on leaves submitted to lightfleck sequences of the same photon flux density. Lightflecks of 250 μmol m–2 s–1 are not photoinhibitory. Compared to the control maintained under light growth condition (40 μmol m–2 s–1) carbon gain is increased on leaves submitted to lightflecks; this gain remains high throughout the light cycles on leaves submitted to nonphotoinhibitory lightflecks and to the photoinhibitory lightflecks followed by the shortest low light period. In the other cases, carbon gain, higher than that of the control at the beginning of the treatments, decreases and becomes lower than the control carbon gain. Finally, the relevance of photoinhibition in the tropical rain forest understory environment is discussed.

Canopy gaps and light penetration into a wind-exposed tropical lower montane rain forest

Penetration of light into the understory of tropical lower montane rain forests at Monteverde, Costa Rica, depends largely on the nature of canopy disruption by limb fall and tree fall. Midday photosynthetic photon flux densities (PPFD) above the forest canopy are low because of cloud cover (modal values are 400–500 μmol•m−2•s−1; daily integrations are 19 mol•m−2). The proportion of above-canopy PPFD that penetrates to the understory is greater in the elfin forests on wind-exposed ridge crests than in taller cloud forests in protected ravines (39 and 54% of understory locations receive &lt;2% PPFD, respectively). PPFD is higher, and its vertical gradients steeper, in elfin forest gaps than in gaps of the same area in the taller forest. Gaps may or may not influence PPFD in adjacent forest understory, depending on vegetation structure on the gap periphery. In the taller cloud forest, understory PPFD is not correlated with distance to the nearest gap, but in the elfin forest it is (r = −0.467). This variation in the light environment is an integral part of disturbance and regrowth in these forests and seems to have played a major role in the evolution of tree growth strategies.

GENOTYPIC VARIATION IN LEAF DAMAGE IN PIPER ARIEIANUM (PIPERACEAE) BY A MULTISPECIES ASSEMBLAGE OF HERBIVORES.

The shrub Piper arieianum (Piperaceae) has a diverse herbivore fauna (95 species total) in Costa Rican rain forest. The effect of plant genotype on leaf damage by individual herbivore species and total leaf area removed was studied in P. arieianum through a cloning experiment. Damage patterns were measured over 3.5 years for two plots, four genotypes per plot, in the understory of lowland rain forest. In both plots, there were significant differences among genotypes in total leaf area missing throughout the study period. Rankings of genotypes based on overall damage remained constant over time in plot 1 but changed in plot 2. Certain individual herbivore species caused significantly higher damage in some genotypes than in others; the change in genotype rankings in plot 2 was associated with increased damage to particular genotypes by specific herbivore groups. The genotype most heavily damaged by a given insect species varied depending on the herbivore species; thus, resistance to one herbivore species did not necessarily confer resistance against all species. Those herbivore species causing the greatest proportion of damage for a given plant changed over time. Because total damage resulted from the summation of losses to individual herbivore species, whether an individual plant lost more leaf area than its neighbors depended on the relative abundance of the herbivore species at any one time. Finally, for a portion of the study period in each plot, more heavily damaged clones grew less than lesser damaged clones. Together with previous reports that naturally growing plants differ significantly in damage and that these differences are sufficient to cause fitness differences, the results presented here suggest that the herbivores of P. arieianum represent a selective force for changes in resistance but that this selective force changes both in intensity and quality over time.

Breakage and Regrowth of Piper Species in Rain Forest Understory

Species of Piper in the lowland tropical forest of La Selva, Costa Rica, appear to have many adaptations to avoid death resulting from branches being broken or pinned to the ground. Analysis of breakage scars along the branches shows that the majority of the individuals have at least one broken branch, and some have many more

A Comparison of Mist Nets and Two Designs of Harp Traps for Capturing Bats

Two widely used methods for capturing bats away from their roosts are mist nets and harp traps (Kunz and Kurta, 1988; Tuttle, 1974). In Australia, Tidemann and Woodside (1978) found that harp traps were about 10 times more effective than mist nets at catching small to medium-sized vespertilionids and rhinolophids. In Costa Rica, Laval and Fitch (1977) found that traps were about 1.7 times as effective as nets. However, there is some evidence of variation among species in their susceptibility to traps. Laval and Fitch (1977) reported that large phyllostomids frequently were caught in mist nets and rarely were trapped, whereas the reverse was true for several small vespertilionids. Such interspecific variation in susceptibility can affect the choice of capture methods for a particular study, and must be considered in community studies if the number of captures is used as an index of relative abundance of species (Fleming, 1986). In this paper, I compare the effectiveness and selectivity of mist nets and harp traps in the understory of Malaysian rain forest. I also compare the relative success of two different designs of harp traps. The original harp traps had a single bank (row) of wires and were effective for capturing fast-flying bats such as molossids, but not for more maneuverable species (Constantine, 1958). Subsequently, traps with two banks of lines were used widely (Tidemann and Woodside, 1978; Tuttle, 1974), but even these may capture less than half the bats encountering them (Kunz and Anthony, 1977). I developed new designs of harp traps with additional banks of lines, and present here data on the trapping efficiency of four-bank traps compared with two-bank traps. The study was conducted between 1981 and 1987 in two areas of lowland rain forest in Malaysia: Pasoh

Biosystematics of two sympatric species ofEucharis (Amaryllidaceae)

Eucharis candida andE. formosa are two often sympatric species of bulbous geophytes restricted to neotropical rain forest understory. The species are most common in eastern Ecuador, and are the only two east Ecuadorean species of the genus found north of the Pastaza valley. Data from phenetic, karyotypic, and preliminary isozyme electrophoretic analyses of both species are represented. The species are distinguishable phenetically and karyologically, but isozyme-based relationships are more complex. Phenetic resolution of the isozyme phenotypes supports recognition of two species in Ecuador. A Peruvian isolate ofE. formosa, though not morphologically distinct, shows both allozyme and chromosomal divergence from Ecuadorean populations. Cladistic relationships based on overall allozyme data do not support species distinction, but a novel electrophoretic phenotype for glutathione reductase is shared only by individuals ofE. candida. An apparent geographic component within the monophyletic groups resolved in the cladogram suggests that some degree of gene flow between these two species has been maintained without the complete loss of morpholgoical species identity. This may have been mediated either by artificial population structures due to a probable long history of cultivation, or via Pleistocene refugia effects. Both species may have originated in eastern Ecuador from a common ancestral population which has since radiated outward, perhaps several times.

Patterns of Ant and Herbivore Activity on Five Understory Euphorbiaceous Saplings in Submontane Papua New Guinea

We present the results of a study of the ant and herbivore faunas found on the leaves of five species of euphorbiaceous saplings (Macaranga aleuritoides, M. punctata, M. quadriglandulosa, Mallotus philippensis, and Homalanthus novo-guineensis) during the wet season in a submontane tropical rain forest in Papua New Guinea. All of these species have foliar extrafloral nectaries. The abundance, density, and composition of the ant and herbivore communities differed among the tree species; ant densities per unit leaf area varied by a factor of ca 20; and herbivore densities varied by a factor of ca 3. The leaves of these species were visited by 6 to 13 species of ants. Ants were experimentally excluded from branches or small saplings of three tree species: Macaranga aleuritoides, M. punctata, and Homolanthus novo-guineensis. Numbers of herbivores and levels of leaf damage were found to be significantly greater on ant exclusion saplings than on ant access saplings for M. aleuritoides, but not for the other two species. Macaranga aleuritoides was visited by more ant species than the other four tree species and had the highest number of ants per leaf. The antplant associations we examined are relatively unspecialized compared with most of the ant associations with euphorbs that have been studied previously in the Indo-Malesian tropics. MANY PLANTS POSSESS EXTRAFLORAL NECTARIES (EFNs) that are visited by ants, and the role of ants in defending such plants from herbivory has been examined in a number of experimental studies (e.g., Koptur 1979, 1984; O'Dowd 1979; Schemske 1982; Stephenson 1982; Horvitz & Schemske 1984). Several of these studies have shown that ants can indeed protect plants from herbivores, but in others no such effect has been discerned (O'Dowd & Catchpole 1983, Tempel 1983, Boecklen 1984). This apparent variation in the efficacy of EFNs as defenses against herbivory has led some authors to conclude that the likelihood of a strong mutualistic interaction evolving between ants and plants is affected both by features of the plant (such as growth form and characteristics of the nectaries and nectar) and by temporal and spatial variation in the density and composition of the ant and herbivore fauna visiting the plant (e.g., Boecklen 1984, Horvitz & Schemske 1984, Barton 1986). However, very few data are available concerning this variation, particularly from ant-plant systems in the Indo-Malesian tropics. We present the results of a short-term investigation of the ant and herbivore fauna found on the leaves of five species of euphorbiaceous saplings [Macaranga aleuritoides F. Muell., M. punctata Schum., M. quadriglandulosa Warb., Mallotus philippensis (Lam.) Muell. Arg., and Homolanthus novo-guineensis (Warb.) Schum.} in the understory of a submontane tropical rain forest in Papua New Guinea. All five species have glands on the leaves near the petiole insertion, and our study was prompted by our observations of ants foraging at these glands on all species. In the Indo-Malesian tropics, species in the euphorbiaceous genera we examined show a range of associations with ants. Most previous work on these associations has been done on such species as Macaranga triloba that provide specialized food bodies and/or nest sites for one or a few species of attendant ants (e.g., Ridley 1910, Rickson 1980, Maschwitz et al. 1984); very little experimental work has been done on species that have EFNs but do not show highly specialized relationships with one or a few ant species. We attempted to determine the differences, if any, between the ant and herbivore communities on the leaves of the five tree species, and examine the consequences, in terms of herbivore numbers and herbivory, of excluding ants from leaves with active

Comparative Foraging Success of Insectivorous Birds in Tropical and Temperate Forests: Ecological Implications

Foraging success is the first indicator of food availability which in turn is a major determinant of clutch size, timing and output of reproduction as well as habitat selection, density and survival. Yet it has been rarely investigated among insectivorous birds and never along a latitudinal gradient. To allow a meaningful comparison between a tropical and a temperate guild of small foliage gleaners, the data were recorded in similar conditions and restricted to adult birds actively foraging, in the understory of primary rain forest (French Guiana) and old stands of broad leaved forest (France) during the breeding season. The mean attack rate of tropical species is 4 to 6 times lower than that of their temperate equivalents. The differences are highly significant, consistent over every species, and cannot be attributed to the birds' body size or prey size. The temperate species are more specialized on caterpillars and their foraging success has a lower variance than that of tropical species, suggesting that food resources may be on average higher, richer and more evenly distributed in temperate forests. Foraging speed is significantly higher in temperate than in tropical species and is correlated with attack rate and body size only among the latter. There are no consistent relationships between attack rate and foraging height, capture techniques or substrates used. Tropical species make a greater use of dead leaves and epiphytes than do temperate ones, as well as of the most expensive capture modes (probing, striking, sallying). They also take more prey under the leaf surface and have a larger foraging niche breadth than their temperate counterparts. Contrary to the widely assumed superabundance and stability of food resources in the tropics, it is suggested that a reduced prey availability (density, detectability, distribution, seasonality) may be a major cause of the lower foraging success of rain forest birds. Accordingly this constraint is likely to limit their reproductive rate and would in part explain their very low clutch size.

Geographical Variation in Fertility, Phenology, and Composition of the Understory of Neotropical Forests

We compare levels of flowering and fruiting in 55 samples of Neotropical forest understory from 13 sites in 6 countries. Each sample consists of a census of fertile understory plants along a transect. Changes in species richness and density of fertile understory plants are correlated with rainfall and soil fertility. Areas with weak (or no) dry seasons and intermediate to rich soils average 64 fertile plant species and 174 individuals per sample, whereas areas with poor soil and a strong dry season average only 5 fertile species and 8 fertile individuals. Areas with either strong dry seasons and good soils or weak dry seasons and very poor soils have intermediate values. Taxonomic composition of the understory also changes predictably with rainfall and soil fertility. In increasingly stressed forests changes are found in understory structure, with sequential loss of terrestrial herbs, epiphytes, understory shrubs, and lianas. The understory of the poorest soil site consists almost entirely of young trees. The effects of seasonal differences at a given site are small compared with between-site differences. We suggest that the level of understory fertility may provide a simple indicator of overall ecosystem productivity. THE UNDERSTORY OF A TROPICAL RAIN FOREST iS composed of a different set of species than is the canopy. The understory, though often neglected, is an integral and important part of the plant community. For example, understory species constitute 25 percent of the species and 24 percent of the individuals sampled in 1000 m2 at a wet-forest site in western Ecuador (Gentry and Dodson 1987). In a coastal Ecuador moist forest, 2 1 percent of the species and 44 percent of the individuals in a similar sample are restricted to the understory. The importance of understory plants is evident from local species lists: 46 percent of the Rio Palenque species (Dodson and Gentry 1978), 43 percent of the Barro Colorado Island, Panama species (Croat 1978), and 47 percent of the Jauneche (Dodson et al. 1985) species are herbs, shrubs, and small trees. These figures are overestimates, since a number of the species included in such lists are weedy and found rarely, if at all, inside the closed forest, but the overall importance of the understory to rainforest plant diversity is clear. The understory supports a different fauna than does the canopy. Many species of insects, birds, and mammals are restricted to the understory, and understory plants are major food sources for this biota. For example, hermit hummingbirds, major pollinators for many plant taxa, are exclusively understory birds. Such frugivores as birds of the family Pipridae and bats of the genus Carollia are largely dependent on understory fruits (e.g., Snow 1965, Stiles 1981, Fleming 1985). Temperate zone studies have shown that response of the forest understory to environmental gradients can differ from that of the canopy (summary in Whittaker 1977). For the tropics, we now have data that show predictable patterns of change in plant community composition across such environmental gradients as annual precipitation (Gentry 1982a, 1987, in press). Surprisingly little attention has been focused on the easily accessible understory, and few data are available for comparing the structure or floristic composition of the rainforest understory in differ-

Nitrogen Fixation by Epiphylls in a Tropical Rainforest

Nitrogen Fixation by Epiphylls in a Tropical Rainforest

Biosystematics of Tetraploid Eucharis (Amaryllidaceae)

Eucharis is a genus of 16 species of petiolate-leaved, neotropical Amaryllidaceae restricted to rainforest understory from Guatemala to Bolivia. The two northernmost species, E. bonplandil and E. bouchei, are the only two tetraploid (2n = 92) species so far known in the genus. Eucharis bonplandil is known from only a few localities in central Colombia. Eucharis bouchei, restricted to Central America, is particularly polymorphic and three varieties are recognized largely on the basis of staminal cup morphology. Data from phenetic, chromosomal, and preliminary electrophoretic analyses are presented for both tetraploid species. On the basis of 17 floral characters, the three varieties of E. bouchei do not resolve into discrete phenetic groups. The tetraploid representatives of Eucharis exhibit a wide degree of karyotypic heteromorphism. Eucharis bouchei var. dressleri is an unstable tetraploid. Electrophoretic banding patterns for aspartate-amino-transferase exhibit additive effects of polyploidy in some individuals. Isozyme phenotypes of Eucharis bouchei var. bouchei are quite variable and cladistic analysis of the isozyme data suggests that this variety may be polyphyletic. Eucharis bonplandii and E. bouchei may be monophyletic sister taxa and the remnants of a once more widespread tetraploid complex. The entry of Eucharis into Central America was probably a geologically recent event. It is hypothesized that E. bouchei has been steadily migrating away from the Colombian border. Eucharis bouchei is a semi-species complex of geographically isolated populations in the process of morphological diversification. The evolution of E. bouchei var. dressleri may be a sympatric speciation event. Founder effects, rapid chromosomal change, and geographical isolation are considered the most important factors in the evolution of the E. bouchei complex. Tetraploidy and attendant increased levels of heterozygosity may have been important in facilitating the migration of Eucharis across the Isthmus of Panama. The genus Eucharis Planchon & Linden [Amaryllidaceae infrafamily Pancratioidinae sensu Traub (1957, 1963)] consists of 16 species of rare, petiolate-leaved, bulbous geophytes inhabiting the understory of primary rainforest from Guatemala to Bolivia (Meerow, 1986). Most of the species are found in the western Amazon basin and adjacent slopes of the eastern Andes. Eucharis bouchei Woodson & Allen is a highly polymorphic tetraploid (2n = 92) complex of Central America (Fig. 1). The species is concentrated in Panama (Fig. 2), but has also been recorded from Costa Rica and Guatemala. Eucharis bouchei is the northernmost species of Eucharis and the only one found north of the Dari6n Gap. It is also the most variable species in the genus, in characteristics that elsewhere justify specific delimitation. Patterns of variation in floral size and tube and limb habit form a complete mosaic throughout the range of E. bouchei, showing little or no geographic consistency. In my recent monograph of Eucharis (Meerow, 1986), three varieties are recognized chiefly on the basis of staminal cup morphology (Fig. 3): E. bouchei var. bouchei, var. darienensis Meerow, and var. dressleri Meerow. Variety bouchei, most common around El Valle de Ant6n in Cocl& Province (Fig. 2), is recognized by its largely edentate staminal cup in which the trapezoidal free filament is not markedly constricted distally into a narrow subulate portion (Figs. LC, 3A, B). It is the most variable of the three varieties, both in flower size and staminal cup morphology. The staminal cup of variety ' I thank Robert Dressler, Mark Elliot, Mark Whitten, and Huntington Botanical Garden for providing living material of tetraploid Eucharis species. Bijan Dehgan provided the supportive environment where this work was accomplished. Charles Guy gave freely of his time and laboratory materials for the electrophoretic analyses. Bart Schutzman aided with computer applications. Brent Mishler suggested the cladistic analysis. Peter Goldblatt, George K. Rogers, and two anonymous reviewers provided useful criticism of an earlier version of this paper. Part of this work was supported by NSF Doctoral Dissertation Grant BSR-8401208 and a Garden Club of America/World Wildlife Fund Fellowship in Tropical Botany. Portions of this paper represent part of a doctoral dissertation submitted to the Graduate School of the University of Florida in partial fulfillment of the requirements for the degree of Doctor of Philososphy. Florida Agricultural Experiment Station Journal Series No. 7537. 2 Horticultural Systematics Laboratory, Department of Ornamental Horticulture, University of Florida, Gainesville, Florida 3261 1, U.S.A. ANN. MISSOURI BOT. GARD. 74: 291-309. 1987. This content downloaded from 157.55.39.177 on Tue, 15 Nov 2016 03:59:12 UTC All use subject to http://about.jstor.org/terms 292 ANNALS OF THE MISSOURI BOTANICAL GARDEN [VOL. 74

The Costs of Leaf Support in Understory Palms: Economy Versus Safety

Biomass allocation to nonproductive leaf-support structures represents an inescapable compromise between economy and safety. In the bifid-leaved understory palm species Geonoma cuneata and Asterogyne martiana, the costs of leaf support increase disproportionately with leaf size and crown size. Greater leaf longevities of larger leaves are insufficient to compensate for these escalating costs. Increases in leaf size also entail increases in stresses acting on the petiole, resulting in lower factors of safety in the largest leaves. In Geonoma congesta, ontogenetic changes in leaf morphology from bifid to split leaves at larger leaf sizes result in reduced costs of leaf support relative to bifid juvenile leaves. Despite differences in maximum leaf size, the leaves of reproductive individuals in all three species have similar factors of safety, indicating that mechanical constraints may be limiting leaf size in these species. Although reproductive G. congesta leaves have the greatest leaf-support efficiency (leaf area per unit of support biomass), total leaf area per whole-plant biomass is lowest. In the reproductive stage, plants of G. cuneata, the smallest species, have the highest leaf-support efficiency at leaf and crown levels and the highest total leaf area per whole-plant biomass. These characteristics, largely a consequence of small leaf and plant size, enable G. cuneata to exploit the most heavily shaded microsites within the rain-forest understory.

Trophic Organization of Understory Birds in a Malaysian Dipterocarp Forest

Abstract Single-sample studies suggested that understory flowers and fruits and their avian consumers are scarce in the Malaysian rain forest as compared with African and Central American rain forests. Results from my longer-term studies at Pasoh Forest Reserve (Negeri Sembilan, Peninsular Malaysia) established that flowers and fruits were consistently rare as food for birds. A comparison of two forest types at Pasoh revealed the effect of lower food availability on avian trophic organization. Food resources (e.g. flowers, fruits, arthropods) were less abundant in the regenerating than in the virgin forest, and bird species richness and individual abundance were also lower in the regenerating forest understory. However, the two forests did not differ significantly in the relative importance of the various foraging guilds, suggesting that similar types of resources were present in similar proportions. None of the birds sampled in the Malaysian rain-forest understory was a specialized consumer of understory flowers or fruit, whereas birds feeding mainly on foliage-dwelling arthropods were abundant and were represented by many species. This trophic organization is contrary to that reported for rain forests in other tropical regions but may simply reflect an allocation of harvestable productivity that is different rather than lower.

LEAF DISPLAY, CANOPY STRUCTURE, AND LIGHT INTERCEPTION OF TWO UNDERSTORY PALM SPECIES

The implications of leaf size, leaf display, and crown size for whole‐plant light interception were investigated in Geonoma cuneata and Asterogyne martiana, two understory palm species native to Central American rain forests. Adults of A. martiana had longer leaves, more leaves per plant, and greater total leaf area than G. cuneata adults. Geometric measurements within whole crowns were used to calculate light interception efficiency, a leaf‐based measure of the proportion of total incident light that is intercepted by a crown. Light interception efficiency was higher in adult G. cuneata than in adult A. martiana; seedlings of the two species did not differ significantly in light interception efficiency. Decreased efficiency of adult A. martiana crowns was largely due to an increased proportion of pendent leaves. Compared to G. cuneata, adults of A. martiana had greater light interception capacity (the product of light interception efficiency and total leaf area), but they also had higher biomass costs of light interception. Lower biomass costs of light interception in adult G. cuneata enable this species to exploit successfully the most deeply shaded microsites in the rain forest understory.