Costa Rican Rain Forest Research Articles

Successional dynamics of nitrogen fixation and forest growth in regenerating Costa Rican rainforests.

Regenerating tropical forests have an immense capacity to capture carbon and harbor biodiversity. The recuperation of the nitrogen cycle following disturbance can fuel biomass regeneration, but few studies have evaluated the successional dynamics of nitrogen and nitrogen inputs in tropical forests. We assessed symbiotic and asymbiotic nitrogen fixation, soil inorganic nitrogen concentrations, and tree growth in a well-studied series of five tropical forest plots ranging from 19yr in age to old-growth forests. Wet-season soil inorganic nitrogen concentrations were high in all plots, peaking in the 29-yr-old plot. Inputs from symbiotic nitrogen fixation declined through succession, while asymbiotic nitrogen fixation peaked in the 37-yr-old plot. Consequently, the dominant nitrogen fixation input switched from symbiotic fixation in the younger plots to asymbiotic fixation in the older plots. Tree growth was highest in the youngest plots and declined through succession. Interestingly, symbiotic nitrogen fixation was negatively correlated with the basal area of nitrogen-fixing trees across our study plots, highlighting the danger in using nitrogen-fixing trees as a proxy for rates of symbiotic nitrogen fixation. Our results demonstrate that the nitrogen cycle has largely recuperated by 19yr following disturbance, allowing for rapid biomass regeneration at our site. This work provides important insight into the sources and dynamics of nitrogen that support growth and carbon capture in regenerating Neotropical forests.

Behavioral repeatability and choice performance in wild free-flying nectarivorous bats (Glossophaga commissarisi)

Animal individuals show patterns of behavior that are stable within individuals but different among individuals. Such individual differences are potentially associated with differences in foraging efficiency and in fitness. Furthermore, behavioral responses may be correlated in specific suites of so-called behavioral syndromes that are consistent across different contexts and with time. Here, we present a field investigation on individual differences between wild, free-flying nectarivorous bats (Glossophaga commissarisi) in the foraging context. We further investigated how individual differences affect choice performance, and we examined their interdependence within hypothesized behavioral syndrome structures. Free-ranging bats were individually identified as they visited an array of 24 artificial flowers with nectar of high or low sugar concentration. We found that three behavioral measures of foraging behavior were individually stable over the two-month observation period. We investigated the link between individual behavioral measures and measures of choice performance using generalized linear mixed models. Individual measures of choice performance showed significant repeatability, and we found evidence that bats making more visits per bout tend to be slower in learning to avoid unprofitable flowers. We used a multi-response generalized linear mixed model to estimate between-individual correlations and compare hypothesized syndrome structures. There were no clear patterns of between-individual correlations among the behavioral measures in our study, despite the measures exhibiting significant repeatability. This may indicate that foraging behavior depends on multiple individual behavior dimensions that are not adequately described by simple models of behavioral syndromes. Nectar-feeding bats, like other animals including humans, have their own peculiar ways of consuming food that differ among individuals of the same species. We characterized the feeding habits of individuals in a population of wild, free-flying bats that were trained to gather nectar from computer-automated artificial flowers in a Costa Rican rainforest. Individual bats responded to the experimental conditions in different ways, but consistently over the two-month observation period. For example, some bats frequently returned to the same flowers, while others tended to meticulously probe most flowers they encountered before returning to a previously visited location. Interestingly, bats also consistently differed in how fast they learned to avoid flowers with dilute nectar and the faster learners were the bats that made only a few visits on each feeding trip. This suggests that individual foraging strategies might be associated with differences in foraging efficiency.

Convergence of ecophysiological traits drives floristic composition of early lineage vascular plants in a tropical forest floor.

Tropical understorey plant communities are highly diverse and characterized by variable resource availability, especially light. Plants in these competitive environments must carefully partition resources to ensure ecological and evolutionary success. One mechanism of effective resource partitioning is the optimization of functional traits to enhance competition in highly heterogeneous habitats. Here, we surveyed the ecophysiology of two early lineage vascular plant groups from a tropical forest understorey: Selaginella (a diverse lineage of lycophytes) and ferns. In a lowland rain forest in Costa Rica, we measured a suite of functional traits from seven species of Selaginella and six fern species. We evaluated species microclimate and habitat; several photosynthetic parameters; carbon, nitrogen and phosphorus content; chlorophyll concentration; leaf mass per area (LMA); and stomatal size and density. We then compare these two plant lineages and search for relationships between key functional parameters that already exist on a global scale for angiosperms. Convergence of trait function filtered Selaginella species into different habitats, with species in heavily shaded environments having higher chlorophyll concentrations and lower light compensation points compared with open habitats. Alternatively, lower foliar nitrogen and higher stomatal densities were detected in species occupying these open habitats. Selaginella species had denser and smaller stomata, lower LMA and lower foliar nutrient content than ferns, revealing how these plant groups optimize ecophysiological function differently in tropical forest floors. Our findings add key pieces of missing evidence to global explorations of trait patterns that define vascular plant form and function, which largely focus on seed plants. Broadly predictable functional trait relationships were detected across both Selaginella and ferns, similar to those of seed plants. However, evolutionary canalization of microphyll leaf development appears to have driven contrasting, yet successful, ecophysiological strategies for two coexisting lineages of extant homosporous vascular plants.

A Light in the Dark: The Discovery of Another Fluorescent Frog in the Costa Rican Rainforests

A Light in the Dark: The Discovery of Another Fluorescent Frog in the Costa Rican Rainforests

The influence of anthropogenic edge effects on primate populations and their habitat in a fragmented rainforest in Costa Rica.

When a forest is fragmented, this increases the amount of forest edge relative to the interior. Edge effects can lead to loss of animal and plant species and decreased plant biomass near forest edges. We examined the influence of an anthropogenic forest edge comprising cattle pasture, coconut plantations, and human settlement on the mantled howler (Alouatta palliata), white-faced capuchin (Cebus capucinus), Central American spider monkey (Ateles geoffroyi), and plant populations at La Suerte Biological Research Station (LSBRS), Costa Rica. We predicted that there would be lower monkey encounter rate, mean tree species richness, and diameter at breast height (DBH) in forest edge versus interior, and that monkeys would show species-specific responses to edge based on diet, body size, and canopy height preferences. Specifically, we predicted that howler monkeys would show positive or neutral edge effects due to their flexible folivorous diet, large body size, and preference for high canopy, capuchins would show positive edge effects due to their diverse diet, small body size, and preference for low to middle canopy, and spider monkeys would show negative edge effects due their reliance on ripe fruit, large body size, and preference for high upper canopy. We conducted population and vegetation surveys along edge and interior transects at LSBRS. Contrary to predictions, total monkey encounter rate did not vary between the forest edge and forest interior. Furthermore, all three species showed neutral edge effects with no significant differences in encounter rate between forest edge and interior. Interior transects had significantly higher mean tree species richness than edge transects, and interior trees had greater DBH than edge trees, although this difference was not significant. These results suggest that forest edges negatively impact plant populations at La Suerte but that the monkeys are able to withstand these differences in vegetation.

Mineral-Lick Use ByCholoepus hoffmanni(Pilosa: Megalonychidae) At Las Cruces Biological Station, Coto Brus, Costa Rica

To date, only one record of mineral licking exists for sloths (i.e., Choloepus didactylus) from Ecuador. Here we present the first record of mineral licking for C. hoffmanni from a tropical rainforest in Costa Rica. This behavior has been mainly associated with detoxification in herbivores, but no evidence exists for such needs in sloths.

Nitrogen-fixing trees inhibit growth of regenerating Costa Rican rainforests

More than half of the world's tropical forests are currently recovering from human land use, and this regenerating biomass now represents the largest carbon (C)-capturing potential on Earth. How quickly these forests regenerate is now a central concern for both conservation and global climate-modeling efforts. Symbiotic nitrogen-fixing trees are thought to provide much of the nitrogen (N) required to fuel tropical secondary regrowth and therefore to drive the rate of forest regeneration, yet we have a poor understanding of how these N fixers influence the trees around them. Do they promote forest growth, as expected if the new N they fix facilitates neighboring trees? Or do they suppress growth, as expected if competitive inhibition of their neighbors is strong? Using 17 consecutive years of data from tropical rainforest plots in Costa Rica that range from 10 y since abandonment to old-growth forest, we assessed how N fixers influenced the growth of forest stands and the demographic rates of neighboring trees. Surprisingly, we found no evidence that N fixers facilitate biomass regeneration in these forests. At the hectare scale, plots with more N-fixing trees grew slower. At the individual scale, N fixers inhibited their neighbors even more strongly than did nonfixing trees. These results provide strong evidence that N-fixing trees do not always serve the facilitative role to neighboring trees during tropical forest regeneration that is expected given their N inputs into these systems.

The effects of background coloration and dark spots on the risk of predation in poison frog models

Protective coloration is a well-known predator avoidance strategy in prey species. Aposematic species often display a contrasting color pattern consisting of dark spots of different shapes and sizes on a bright background coloration. Both elements, background color and spots are expected to serve different purposes. While the ecological function of the bright coloration has been addressed in many studies, the question of whether the interaction with differently sized spots influences predator behavior has received less attention by researchers. In a lowland rain forest in Costa Rica we used 2700 clay models that imitated the polytypic strawberry poison frog (Oophaga pumilio) as a proxy for an aposematic prey species. We manipulated the dorsal color pattern by using a local and a non-local aposematic and a non-local cryptic background color and combined them with black spots increasing in size (none, small, medium, large). The major objective was to test if spot size alters the survival rate of differently colored models. Background coloration and spot size were significant predictors of being attacked. However, the interaction between both effects was not. During five trials predators avoided the non-local aposematic color morph and did not discriminate between local aposematic and non-local cryptic models. Spot size and attack rate were negatively linear correlated which suggests that predator selection promotes the evolution of dark spots. We further conclude that spot size matters in a contrasting color pattern and plays an important role in predator avoidance.

Potential trajectories of old‐growth Neotropical forest functional composition under climate change

Quantifying relationships between plant functional traits and abiotic gradients is valuable for evaluating potential responses of forest communities to climate change. However, the trajectories of change expected to occur in tropical forest functional characteristics as a function of future climate variation are largely unknown. We modeled community level trait values of Costa Rican rain forests as a function of current and future climate, and quantified potential changes in functional composition. We calculated per‐plot community weighted mean (CWM) trait values for leaf area (LA), specific leaf area (SLA), leaf dry matter content (LDMC), leaf nitrogen (N) and phosphorus (P) content, and wood basic specific gravity (WSG), for tree and palm species in 127 0.25 ha plots. We modeled the response of CWM traits to current temperature and precipitation gradients using generalized additive modeling. We then predicted and mapped CWM traits values under current and future climate, and quantified potential changes under a global warming scenario (RCP8.5, year 2050). We calculated the area within the multi trait functional space occupied by forest plots under both current and future climate, and determined potential changes in functional space occupied by forest plots. Overall, precipitation predicted CWM traits better than temperature. Models indicated increases in CWM SLA, N and P, and a decrease in CWM LDMC under climate change. Lowland forest communities converged on a single direction of change towards more acquisitive CWM trait values, indicating a change in forest functional composition resulting from a changed climate. Functional space occupied by forest plots was reduced by 50% under the future climate. Functional composition changes may have further effects on forests ecosystem services. Assessing functional trait spatial‐gradients can help bridge the gap between species‐based biogeography and biogeochemical approaches to strengthen biodiversity and ecosystem services conservation efforts.

Cognition-mediated evolution of low-quality floral nectars.

Plants pollinated by hummingbirds or bats produce dilute nectars even though these animals prefer more concentrated sugar solutions. This mismatch is an unsolved evolutionary paradox. Here we show that lower quality, or more dilute, nectars evolve when the strength of preferring larger quantities or higher qualities of nectar diminishes as magnitudes of the physical stimuli increase. In a virtual evolution experiment conducted in the tropical rainforest, bats visited computer-automated flowers with simulated genomes that evolved relatively dilute nectars. Simulations replicated this evolution only when value functions, which relate the physical stimuli to subjective sensations, were nonlinear. Selection also depended on the supply/demand ratio; bats selected for more dilute nectar when competition for food was higher. We predict such a pattern to generally occur when decision-makers consider multiple value dimensions simultaneously, and increases of psychological value are not fully proportional to increases in physical magnitude.

Sweet, but not too sweet

Evolutionary Biology Nectar-feeding pollinators, when given a choice, tend to prefer nectar with high concentrations of sugar. Nectar-producing plants, however, tend to produce more dilute nectar. This mismatch between selective force and trait value has long been seen as an evolutionary paradox. Nachev et al. used a dynamic flower array that evolved in real time in a Costa Rican rainforest to show that the “paradox” is, in fact, driven by pollinator choices (see the Perspective by Farris). Bat pollinators based their choices on small, nonlinear differences in nectar sweetness, which led to selection for less sweet nectar overall. Science , this issue p. [75][1]; see also p. [25][2] [1]: /lookup/doi/10.1126/science.aah4219 [2]: /lookup/doi/10.1126/science.aal4193

Propagation of the Palm Flora in a Lowland Tropical Rainforest in Costa Rica: Fruit Collection and Germination Patterns

Palms are important structural components of tropical rain forests. Nonetheless, information on their propagation potential under nursery conditions to facilitate their use in forest restoration is...

Chamber measurements of high CO2 emissions from a rainforest stream receiving old C-rich regional groundwater

Carbon emissions from fluvial systems are a key component of local and regional carbon cycles. We used floating chambers to investigate the CO2 flux from stream water to air (\({\text{f}}_{{{\text{CO}}_{ 2} }}\)) in the Arboleda, a stream in the lowland rainforest of Costa Rica, fed partly by old regional groundwater high in dissolved inorganic carbon (DIC). Drifting and static chambers showed \({\text{f}}_{{{\text{CO}}_{ 2} }}\) averaging 35.5 and 72.7 μmol C m−2 s−1, respectively, bracketing the previously-published \({\text{f}}_{{{\text{CO}}_{ 2} }}\) value of 56 μmol C m−2 s−1 obtained using tracer methods in this stream. These values are much higher than most \({\text{f}}_{{{\text{CO}}_{ 2} }}\) data in the literature and reflect a large flux of deep crustal (non-biogenic) CO2 out of the Arboleda, a flux that does not represent a component of ecosystem respiration. Static chambers appeared to overestimate \({\text{f}}_{{{\text{CO}}_{ 2} }}\) by creating artificial turbulence, while drifting chambers may have underestimated \({\text{f}}_{{{\text{CO}}_{ 2} }}\) by under-sampling areas of potentially high gas exchange (e.g., riffles around coarse woody debris obstructions). Both static and drifting chambers revealed high spatial heterogeneity in \({\text{f}}_{{{\text{CO}}_{ 2} }}\) at the scale of 5–30 m reaches. Some observed temporal trends were localized, e.g., among three reaches with repeated measurements through the wet and dry seasons, (1) only the reach located between the other two showed significantly lower \({\text{f}}_{{{\text{CO}}_{ 2} }}\) during the dry season, and (2) the highest and lowest \({\text{f}}_{{{\text{CO}}_{ 2} }}\) were consistently observed in the reaches farthest upstream and downstream, respectively. Streams like the Arboleda receiving significant inputs of high-DIC regional groundwater merit additional study as hotspots for C emissions from terrestrial ecosystems.

A new genus of mites of the subfamily Platyseiinae associated with Azteca ant galleries in Cecropia trees in Costa Rica (Acari: Mesostigmata: Blattisociidae)

The genus Calyptoseius gen. nov. of the subfamily Platyseiinae Evans is described, based on adults and nymphs of one newly described species associated with ants of the genus Azteca occupying hollow stems of Cecropia in lowland rainforests of Costa Rica. Several unusual morphological attributes are noted, particularly the autapomorphic presence of four elongated setae on each of telotarsi II to IV. Some perspectives of these mites in the Cecropia-Azteca association are discussed, including a possible dispersal link via nematoceran flies to gain access to such an unusual habitat. The definition of the previously monobasic genus Cheiroseiulus is augmented, and also modified in view of an undescribed species at hand, and the subfamily definition is modified to account for morphological aspects of the new genus. A key to the genera of the Platyseiinae is given.

Higher survival drives the success of nitrogen-fixing trees through succession in Costa Rican rainforests.

Trees capable of symbiotic nitrogen (N) fixation ('N fixers') are abundant in many tropical forests. In temperate forests, it is well known that N fixers specialize in early-successional niches, but in tropical forests, successional trends of N-fixing species are poorly understood. We used a long-term census study (1997-2013) of regenerating lowland wet tropical forests in Costa Rica to document successional patterns of N fixers vs non-fixers, and used an individual-based model to determine the demographic drivers of these trends. N fixers increased in relative basal area during succession. In the youngest forests, N fixers grew 2.5 times faster, recruited at a similar rate and were 15 times less likely to die as non-fixers. As succession proceeded, the growth and survival disparities decreased, whereas N fixer recruitment decreased relative to non-fixers. According to our individual-based model, high survival was the dominant driver of the increase in basal area of N fixers. Our data suggest that N fixers are successful throughout secondary succession in tropical rainforests of north-east Costa Rica, and that attempts to understand this success should focus on tree survival.

Interactions between plant size and canopy openness influence vital rates and life-history tradeoffs in two neotropical understory herbs.

• For tropical forest understory plants, the ability to grow, survive, and reproduce is limited by the availability of light. The extent to which reproduction incurs a survival or growth cost may change with light availability, plant size, and adaptation to shade, and may vary among similar species.• We estimated size-specific rates of growth, survival, and reproduction (vital rates), for two neotropical understory herbs (order Zingiberales) in a premontane tropical rainforest in Costa Rica. During three annual censuses we monitored 1278 plants, measuring leaf area, number of inflorescences, and canopy openness. We fit regression models of all vital rates and evaluated them over a range of light levels. The best fitting models were selected using Akaike's Information Criterion.• All vital rates were significantly influenced by size in both species, but not always by light. Increasing light resulted in higher growth and a higher probability of reproduction in both species, but lower survival in one species. Both species grew at small sizes but shrank at larger sizes. The size at which shrinkage began differed among species and light environments. Vital rates of large individuals were more sensitive to changes in light than small individuals.• Increasing light does not always positively influence vital rates; the extent to which light affects vital rates depends on plant size. Differences among species in their abilities to thrive under different light conditions and thus occupy distinct niches may contribute to the maintenance of species diversity.

Carbohydrate regulation of photosynthesis and respiration from branch girdling in four species of wet tropical rain forest trees.

How trees sense source-sink carbon balance remains unclear. One potential mechanism is a feedback from non-structural carbohydrates regulating photosynthesis and removing excess as waste respiration when the balance of photosynthesis against growth and metabolic activity changes. We tested this carbohydrate regulation of photosynthesis and respiration using branch girdling in four tree species in a wet tropical rainforest in Costa Rica. Because girdling severs phloem to stop carbohydrate export while leaving xylem intact to allow photosynthesis, we expected carbohydrates to accumulate in leaves to simulate a carbon imbalance. We varied girdling intensity by removing phloem in increments of one-quarter of the circumference (zero, one--quarter, half, three-quarters, full) and surrounded a target branch with fully girdled ones to create a gradient in leaf carbohydrate content. Light saturated photosynthesis rate was measured in situ, and foliar respiration rate and leaf carbohydrate content were measured after destructive harvest at the end of the treatment. Girdling intensity created no consistent or strong responses in leaf carbohydrates. Glucose and fructose slightly increased in all species by 3.4% per one-quarter girdle, total carbon content and leaf mass per area increased only in one species by 5.4 and 5.5% per one-quarter girdle, and starch did not change. Only full girdling lowered photosynthesis in three of four species by 59-69%, but the decrease in photosynthesis was unrelated to the increase in glucose and fructose content. Girdling did not affect respiration. The results suggest that leaf carbohydrate content remains relatively constant under carbon imbalance, and any changes are unlikely to regulate photosynthesis or respiration. Because girdling also stops the export of hormones and reactive oxygen species, girdling may induce physiological changes unrelated to carbohydrate accumulation and may not be an effective method to study carbohydrate feedback in leaves. In three species, removal of three-quarters of phloem area did not cause leaf carbohydrates to accumulate nor did it change photosynthesis or respiration, suggesting that phloem transport is flexible and transport rate per unit phloem can rapidly increase under an increase in carbohydrate supply relative to phloem area. Leaf carbohydrate content thus may be decoupled from whole plant carbon balance by phloem transport in some species, and carbohydrate regulation of photosynthesis and respiration may not be as common in trees as previous girdling studies suggest. Further studies in carbohydrate regulation should avoid using girdling as girdling can decrease photosynthesis through unintended means without the tested mechanisms of accumulating leaf carbohydrates.

Tropical rain forest structure, tree growth and dynamics along a 2700-m elevational transect in Costa Rica.

Rapid biological changes are expected to occur on tropical elevational gradients as species migrate upslope or go extinct in the face of global warming. We established a series of 9 1-ha plots in old-growth tropical rainforest in Costa Rica along a 2700 m relief elevational gradient to carry out long-term monitoring of tropical rain forest structure, dynamics and tree growth. Within each plot we mapped, identified, and annually measured diameter for all woody individuals with stem diameters >10 cm for periods of 3-10 years. Wood species diversity peaked at 400-600 m and decreased substantially at higher elevations. Basal area and stem number varied by less than two-fold, with the exception of the 2800 m cloud forest summit, where basal area and stem number were approximately double that of lower sites. Canopy gaps extending to the forest floor accounted for <3% of microsites at all elevations. Height of highest crowns and the coefficient of variation of crown height both decreased with increasing elevation. Rates of turnover of individuals and of stand basal area decreased with elevation, but rates of diameter growth and stand basal area showed no simple relation to elevation. We discuss issues encountered in the design and implementation of this network of plots, including biased sampling, missing key meteorological and biomass data, and strategies for improving species-level research. Taking full advantage of the major research potential of tropical forest elevational transects will require sustaining and extending ground based studies, incorporation of new remotely-sensed data and data-acquisition platforms, and new funding models to support decadal research on these rapidly-changing systems.

Increased abundance, but reduced nest predation in the chestnut-backed antbird in costa rican rainforest fragments: surprising impacts of a pervasive snake species

Understory insectivorous birds often disappear from fragmented tropical rainforest landscapes before mechanisms such as increased rate of nest depredation can be evaluated. Here, we took advantage of chestnut-backed antbird (Myrmeciza exsul), a representative rainforest understory insectivore that persists in fragments (unlike many other understory species), to identify variables influencing nest predation rate and to test the hypothesis that nest predation underlies avian extirpation in tropical fragments. We compared nest predation rates, bird density, and predator identities in three habitats of lowland Caribbean Costa Rica: two fragments, a peninsular reserve (La Selva Biological Station), and unfragmented rainforest. Our results suggest an inversely density-dependent nest predation pattern: In fragments, chestnut-backed antbirds reached their highest density and—contrary to predictions—experienced their lowest nest predation rates; La Selva on the other hand experienced the lowest density and highest predation rate. Because nest predation decreased with fragmentation, it appears not to explain declines of understory insectivores from forest fragments generally. Nest survival models indicated that habitat best explained nest predation likelihood, whereas edge, annual, and nest age effects were unimportant. Video surveillance documented both bird-eating snake (Pseustes poecilonotus) causing 80% of nest loss overall (37 of 46 nests) and a larger variety of predators in fragments; thus, landscape factors influenced an understory bird’s nest predation. Given the large effect on our focal species, Pseustes likely affects other understory nesters, a topic warranting further study. Tropical reserve conservation plans should consider potential impacts of specialized nest predators on vulnerable understory birds.

Reproductive Biology and Herkogamy of &lt;i&gt;Psychotria elata&lt;/i&gt; (Rubiaceae), a Distylous Species of the Tropical Rain Forests of Costa Rica

The floral morphology, breeding system and pollinators of four natural populations of Psychotria elata (Rubiaceae), found in tropical rainforests of Costa Rica, were examined. Anisoplethy and distylous morphology were observed in all studied populations. The number of flowers per inflorescence was significantly higher in the most abundant morph (p 0.05). Reciprocal herkogamy was detected in only one of the morphs in one population. Breakdown of the heteromorphic incompatibility system was observed in the populations Tirimbina and Rara Avis. Pollen production was significantly lower in thrum morphs of Tirimbina. Seedless fruits were formed in two of the four populations. One-seeded fruits were formed in all populations and were predominant in thrum morphs of Tirimbina (9.15%) and Zurqui (75%). According to our visitation records, hummingbirds, butterflies and moths are the main pollinators of P. elata flowers.