Costa Rican Tropical Wet Forest Research Articles

Microhabitat associations of vascular epiphytes in a wet tropical forest canopy

Summary In tropical forests, vascular epiphyte diversity increases with tree size, which could result from an increase in area, time for colonization or an increase in microhabitat heterogeneity within‐tree crowns if vascular epiphyte species are specialized to particular microhabitats within the crown. The importance of microhabitats in structuring epiphyte communities has been hypothesized for more than 120 years but not yet confirmed. We tested the importance of microhabitats in structuring epiphyte communities by examining microhabitat heterogeneity and epiphyte communities within the crowns of different‐sized Virola koschnyi (Myristicaceae) emergent trees in a Costa Rican tropical wet forest. We tested the degree to which epiphyte species composition was associated with environmental conditions and resources (i.e. microhabitats) using multivariate analyses and a null model that compared the observed epiphyte assemblages amongst different‐sized trees and crown zones with assemblages generated randomly. This study is the first to rigorously examine the degree of microhabitat specialization in epiphyte communities. Microhabitat heterogeneity, epiphyte species richness and abundance increased with tree size. The largest trees had the highest microhabitat and epiphyte diversity and a unique inner crown microhabitat with canopy humus. The few epiphytes found on small trees were mostly bark ferns. Large trees had different epiphyte communities in different parts of the crown; the inner crown contained species not abundant in any other microhabitat (i.e. aroids, cyclanths and humus ferns), and the outer crown contained bark ferns and atmospheric bromeliads. Variation in species composition amongst tree size classes was significantly related to the mean daily maximum vapour pressure deficit and tree diameter, while variation within large tree crowns was significantly related to canopy humus cover. Microhabitat specialization of epiphyte species increased with tree size with 6% of species significantly associated with small trees and 57% significantly associated with large trees. Of the species present in large tree crowns, 23% were specialized to the unique inner crown microhabitat. Synthesis. The increase in microhabitat heterogeneity within tree crowns as trees grow contributes to changes in epiphyte community structure, which supports decades‐old hypotheses of the importance of microhabitat diversity and specialization in structuring tropical epiphyte communities.

Does inundation risk affect leaf-cutting ant distribution? A study along a topographic gradient of a Costa Rican tropical wet forest

Abstract:Successional state of forest and availability of pioneer plants are recognized factors affecting densities of leaf-cutting ants. However little is known about how abiotic factors can shape nest distributions. We investigated the effect of topography, soil, forest successional state and inundation risk on nest density and size of Atta cephalotes colonies along streams in a tropical wet forest in Costa Rica. In each forest type, we surveyed 12 sites, each site comprising five transects (10 × 100 m) varying in topography and proximity to streambeds. We found no difference regarding nest size or density between forest types or soil consociation. Nest density varied with topographic environment, with significantly higher nest density on slope tops (farther from streambeds) and without colonies in valley bottoms (closer to streambeds). Nests found in areas affected by the last great inundation before our study were scarcer and smaller than nests in non-flooded areas. We showed that inundation events favour an accumulation of Atta colonies towards higher sites, where they are also allowed to become larger and may survive longer. Inundation risk may be a strong force shaping the distribution of leaf-cutting ant nests in tropical floodplain forests, even concealing the relevance of successional state of forest.

Litter Biomass and Nutrient Determinants of Ant Density, Nest Size, and Growth in a Costa Rican Tropical Wet Forest

ABSTRACTThe nutritional demands of animals vary by taxon. Across landscapes, communities of animals experience variability in the stoichiometry of carbon and nutrients within their resource base. Thus, we expect stoichiometry to contribute to the spatial variance in the demographic parameters of animal communities. Here, we measure how the composition of a litter‐nesting tropical rainforest ant community is influenced by spatial variation in environmental stoichiometry relative to litter biomass, a known predictor of ant density. We found the density of ants and their nests were strongly related to litter biomass and carbon: phosphorus stoichiometry. The spatial variation in soil nutrients, which determines leaf litter stoichiometry, was an excellent predictor of nest size in the two most common genera of ants. We found a negative relationship between species' growth rate and local soil stocks of phosphorus. Overall, the density of litter‐dwelling ants varied greatly across this tropical forest landscape and environmental stoichiometry can account for limits on ant density independent of the biomass of the leaf litter resource base.

Substantial labile carbon stocks and microbial activity in deeply weathered soils below a tropical wet forest

AbstractContrary to large areas in Amazonia of tropical moist forests with a pronounced dry season, tropical wet forests in Costa Rica do not depend on deep roots to maintain an evergreen forest canopy through the year. At our Costa Rican tropical wet forest sites, we found a large carbon stock in the subsoil of deeply weathered Oxisols, even though only 0.04–0.2% of the measured root biomass (>2 mm diameter) to 3 m depth was below 2 m. In addition, we demonstrate that 20% or more of this deep soil carbon (depending on soil type) can be mobilized after forest clearing for pasture establishment. Microbial activity between 0.3 and 3 m depth contributed about 50% to the microbial activity in these soils, confirming the importance of the subsoil in C cycling. Depending on soil type, forest clearing for pasture establishment led from no change to a slight addition of carbon in the topsoil (0–0.3 m depth). However, this effect was countered by a substantial loss of C stocks in the subsoil (1–3 m depth). Our results show that large stocks of relatively labile carbon are not limited to areas with a prolonged dry season, but can also be found in deeply weathered soils below tropical wet forests. Forest clearing in such areas may produce unexpectedly high C losses from the subsoil.

Resource heterogeneity affects demography of the Costa Rican ant Aphaenogaster araneoides

How do animals respond to an unpredictably heterogeneous environment? Ants foraging in the leaf litter of tropical wet forests experience unpredictably fluctuating food resources. To study how an ant species responds to these changes, foragers were tracked to determine home ranges of 51 colonies of Aphaenogaster araneoides, in three sites in a Costa Rican tropical wet forest. Of these colonies 16 were excavated to measure colony size, colony growth, and reproductive investment. These demographic variables were compared with two measures of home range quality: leaf litter dry weight and mass of arthropods. Home range areas of colonies were highly correlated with colony size, and moderately correlated with resource abundance. Colony growth was independent of colony size, as is found in other ants in unpredictable environments. The growth of colonies was closely associated with resource abundance. Production of the male reproductive caste was closely tied to the size of a colony rather than growth, but male production in slow-growing colonies was limited. Colonies foraging within high-quality environments grew at a faster rate, but reproduction was mainly correlated with colony size. Furthermore, it was found that the frequency of foragers in long-term treatment plots with supplemental food and reduced leaf-litter quality was not significantly different from the frequency of foragers in control plots. This rain-forest ant does not modify its home range areas in response to poor environments, and as a result, small-scale environmental heterogeneity strongly determines growth and reproduction.

Distribution and abundance of colonies of selected meliponine species in a Costa Rican tropical wet forest

The goal of this study was to assess territoriality and foraging home ranges for stingless bee species; and to develop methods for censusing stingless bee colonies. Colonies of two meliponine bee species, Trigona corvina Cockerell and Trigona amalthea Vachal, were censused using carrion and honey baits. Trigona corvina and Trigona amalthea are dispersed in a Costa Rican tropical wet forest (La Selva) so that foraging ranges of the colonies are non-overlapping. The nest density was c. 0.25 ha−1 for T. amalthea and 1.0 ha−1 for T. corvina. T. corvina visited carrion baits exclusively, while T. amalthea visited both carrion and honey baits. The identities of T. amalthea colonies were confirmed using a nestmate recognition assay. Partamona peckolti Friese, Plebeia frontalis Friese, and T. fulviventris Guerin also appeared at the baits. The reliability of baiting to determine colony abundance and distribution is assessed.

Influence of Attractants on Behavior of Screw worms (Diptera: Calliphoridae) in a Tropical Wet Forest in Costa Rica

In a mark-release-recapture study, sheep wounds and rotted liver were used as attractants to study movements of the screwworm fly, Cochliomyia hominivorax (Coquerel), in a Costa Rican tropical wet forest in the wet season. When sites were monitored for less than 1 h, liver attracted between 3 and 12.2 times more flies of both sexes than did wounds, but proportionately fewer gravid and parous females. Only 24.6% of females marked at liver sites were recaptured; seldom (3.1%) did they visit sheep wounds. Females originally marked at sheep wounds remained at the study site longer, visited both sheep and liver sites, and greater than 50% were recaptured. Some females were observed for 16 d and one laid four clutches of eggs. Only 8% of the marked males were recaptured and none was recaptured less than 2 d later. Only a small proportion of the adult screwworms at a locality visited wounds on sentinel sheep; the majority of the adults were transient.

Distribution and effects on tree growth of lianas and woody hemiepiphytes in a Costa Rican tropical wet forest

ABSTRACTWe evaluated occurrence and abundance of lianas and woody hemiepiphytes on canopy and emergent tree species in primary tropical wet forest at the La Selva Biological Station, Costa Rica. Two pioneers, Cecropia obtusifolia and C. insignis, lacked both lianas and hemiepiphytes. The seven non-pioneer species differed significantly in their loads of lianas and hemiepiphytes. For all non-pioneer species, two measures of liana and hemiepiphyte loads (the percentage of the crown occupied and the combined basal area of descending hemiepiphyte and liana roots and stems) increased significantly with tree diameter. In all non-pioneer species, most trees ∵ 70 cm diameter (50–97%) were colonized. Lianas occupied more trees and had a smaller mean host diameter than did hemiepiphytes; however, basal area of descending roots or stems was equivalent for the two life forms in occupied trees. We used partial correlations controlling for tree diameter to evaluate the relationship between annual tree diameter growth and loads of hemiepiphytes and lianas for six non-pioneer species. Five of the six species showed a significant negative correlation between loads and diameter growth. Existing published data show that the high incidence of lianas and hemiepiphytes at La Selva is paralleled in most other Neotropical wet forests.

The positional behavior and substrate use of squirrel monkeys: ecological implications

Age, sex, seasonal and contextual patterns in positional behavior and substrate use are described in a wild troop of squirrel monkeys ( Saimin oerstedii) in a Costa Rican tropical wet forest. Quadrupedal locomotion is predominant; relatively little climbing or leaping occurs. Most food is harvested and ingested by the animals while sitting on top of a branch. The animals also hang by hindlimbs, sit in tripod stances and use vertical clinging postures during foraging. Although the frequencies of foraging behaviors changed seasonally, the types of positional behaviors used in foraging and non-foraging contexts and substrate use were almost constant. Few differences among individuals or between seasons were found in positional behaviors that are not explicable by individual differences in time allocated to foraging. Neither the types of leaps made nor their distances differ among age-sex classes, although larger animals use substrates of larger diameter. Thinner horizontal, but not vertical, substrates are used in foraging compared to non-foraging contexts. The discussion addresses: (1) substrate preferences and sexual dimorphism in body size as mechanisms permitting resource partitioning among group members; (2) ecological and morphological constraints on positional behavior; and (3) posture as an important factor in the diverse foraging behaviors of the small-bodied New World primates.

Radial Gradients in the Specific Gravity of Wood in Some Tropical and Temperate Trees

Abstract Pith to bark wood samples from 20 Costa Rican tropical wet forest and 17 United States temperate forest tree species showed wide variability in specific gravity trends among species. Specific gravity increased significantly from pith to bark in 16 of the tropical tree species. The increases were most dramatic (90-270%) in species that are typically colonizers of clearings and disturbed sites; species of later stages of succession showed moderate increases (20-70%) or, in some instances, no change in specific gravity from pith outward. Statistically significant linear pith to bark trends were found in only four temperate species—two of these increased specific gravity with diameter and two decreased. None of the temperate species showed a change greater than 40%, and no clear relationship between specific gravity pattern and shade tolerance was evident. In tropical wet forests, increase in wood specific gravity with diameter may be an adaptation by shade intolerant species to the highly competitive conditions on disturbed sites. For. Sci. 35(1):197-210.

Internal heterogeneity of gaps and species richness in Costa Rican tropical wet forest

ABSTRACTIn Costa Rican tropical wet forest the distribution of seedlings of most tree species colonizing 51 tree-fall gaps is clumped, probably because of differences in the proximity of gaps to fruiting trees. Different gap zones (root, bole and crown) are more similar to one another, in terms of their species composition, than they are to other zones in the same gap. This nonrandom pattern is established soon after gap formation, indicating that mortality of young seedlings is both high and related to gap zones. Some tree species are strongly positively associated with one of the gap zones, whereas others are associated with the age of the gap at the time the census was taken or the species of tree whose fall caused the gap. Root zones are dominated by fewer species than are bole or crown zones. Results are consistent with the assumption that an initial random distribution of seedlings is quickly changed to a strongly non-random pattern by selective mortality of seedlings of different tree species in the different gap zones. The internal heterogeneity of gaps is probably one of the factors helping to maintain the high tree species richness characteristic of tropical wet forests.

Habitat use by squirrel monkeys (Saimiri oerstedi) in Costa Rica.

This paper analyses movement patterns, habitat preferences, activity schedules, and dispersion of troop members in squirrel monkeys (Saimiri oerstedi) in relation to seasonal changes in food abundance in a Costa Rican tropical wet forest. Secondary forest was the preferred habitat and use of primary forest and late successional forest was limited primarily to seasons when food availability was low. Range area differed between seasons, varying from 79 to 110 ha, and totaling 176 ha over 11 months. The number of hectares used, hourly rate of group movement, and proportion of time spent foraging each season were all negatively related to relative food abundance. There was a tendency to spend less time in foraging activities in the middle of the day and to spend more time exclusively in travel at dawn and dusk. In all seasons dispersion was least when the troop was travelling and it was generally greatest during seasons of low food abundance. Measures of the allocation of time by the troop to food-related activities and the extent of troop dispersion each season were consistent with estimates based on behavior sampling of individuals.

Height and Stem Diameter Relationships for Dicotyledonous Trees and Arborescent Palms of Costa Rican Tropical Wet Forest

RICH, P. M. (Harvard University, Harvard Forest, Petersham, MA 01366), K. HELENURM (Dept. Biology, Washington University, St. Louis, MO 63130), D. KEARNS (Institute of Ecology, University of Georgia, Athens, GA 30602), S. R. MORSE (Dept. Botany, University of California, Berkeley, CA 94708), M. W. PALMER (Dept. Botany, Duke University, Durham, NC 27706), L. SHORT (Dept. Botany, University of Wisconsin, Madison, WI 53704). Height and stem diameter relationships for dicotyledonous trees and arborescent palms of Costa Rican tropical wet forest. Bull. Torrey Bot. Club 113:241-246. 1986.-Allometric relationships between stem diameter and height were studied for dicotyledonous trees and arborescent palms in a tropical wet forest of Costa Rica. In a mixed population of dicotyledonous trees, stem diameter varies with the 3/2 power of height. The climax forest tree Pentaclethra macroloba (Willd.) Kuntze. appears to have a greater margin of safety against mechanical failure than the faster growing tree Pourouma aspera Trecul. This is consistent with Pourouma's shorter life span and narrower crown. As the arborescent palms Welfia georgii Wendl. ex Burret and Socratea durissima (Derst.) Wendl. grow in height, the margin of safety against mechanical failure decreases and/or the stem tissue stiffness and strength increases. Welfia shows little capacity to increase stem diameter during height growth. Socratea shows major stem diameter increase during height growth, but not enough to maintain elastic or geometric similarity. The tallest individuals of Socratea exceed McMahon's (1973) theoretical buckling limit for dicotyledonous trees. This is consistent with the observation that tall palms have stronger, stiffer stem tissue and narrower crowns than dicotyledonous trees. Differences in allometry of height and stem diameter indicate differences in stem tissue mechanical properties, the margin of safety against mechanical failure, and/or crown weight; however, we generally can not distinguish the relative importance of these three possibilities on the basis of studies of height and stem diameters alone.