Host-specificity of folivorous insects in a moist tropical forest

1. To assess the degree of herbivore host-specificity in the moist tropical forest on Barro Colourado Island, Panama, I conducted an extensive series of feeding trials on the common insect herbivores from 10 tree species.
2. The herbivores were offered leaves from both congeneric and confamilial plant species to their known host species, as well as leaves from the most abundant tree species in the forest.
3. The amount of damage caused by these herbivores to young, expanding leaves was also measured on nine of the tree species.
4. Of 46 herbivores species (seven Coleoptera, one Orthoptera, 38 Lepidoptera), 26% were specialized to a single plant species, 22% were limited to feeding on a single genus and 37% were able to feed on several genera within a single family. The remaining 15% were generalists, able to feed from several different plant families.
5. The causes of leaf damage varied extensively across the tree species. On average, specialist herbivores caused 58% of the damage to young leaves, generalists herbivores 8% and fungal pathogens 34%. For four of the tree species, pathogens were the most important cause of leaf damage.
6. In this forest, most chewing herbivores appear to have fairly narrow diets, and these specialists are responsible for most of the insect herbivory.     

Variation in stand structure,light and seedling abundance across a tropical moist forest chronosequence,Panama

Abstract. We asked whether forest structure and understory light environments across a tropical moist forest chronosequence followed predictions of a 4‐phase model of secondary succession (establishment, thinning, transition and steady‐state) and whether seedling density and diversity were functions of light availability as predicted by this model. Using aerial photographs, we identified eight second‐growth stands (two each aged ca. 20, 40, 70, and 100 yr) and two old‐growth stands within Barro Colorado Nature Monument, Panama. Trees and seedlings were sampled in nested, contiguous quadrats in 2 160‐m transects in each stand. Light was measured as percent transmittance of diffuse photosynthetically active radiation (TPAR) at each seedling quadrat and by estimation of percent total incident radiation during the growing season from hemispherical canopy photographs. Basal area, tree density, and canopy height followed predictions of the 4‐phase model. Percent total radiation, but not TPAR, declined with stand age as did seedling density. While seedlings were more likely to occur in quadrats at higher light levels, much variation in seedling density was not related to light availability. Seedling patch sizes were small irrespective of light patches, estimated as semivariance ranges. Seedling species richness was a function of seedling density; estimates of species diversity unbiased by density did not vary systematically as a function of stand age. Proximate seed sources, efficient dispersal mechanisms, and appropriate establishment conditions can promote establishment of species‐rich communities early in successions of heterogeneous tropical moist forest.     

Polypore fungal diversity and host density in a moist tropical forest

In a moist tropical forest in Panama, the wood-decay polyporefungi comprise many rare species (more than half found only once) andexhibit diversity that exceeds that of the supporting tree community.The most abundant fungal species were non-specialists, each found onseveral host species from multiple plant families. In diverse fungalcommunities, each of many species should infect a given host species ina density-dependent manner, so that the infected proportion of a hostpopulation should increase with host density. Applied across hostspecies, hosts with denser populations should support greater fungaldiversity. For 10 tree species, fungal incidence and diversity increasedwith abundance of the host in the community, consistent withacross-species density-dependent infection. Fungal diversity associatedwith individual trunks did not, however, vary with host-species density.Both host density and persistence of decaying logs may be important indetermining fungal diversity associated with tree species.     

Soil seed bank composition along a forest chronosequence in seasonally moist tropical forest,Panama

Abstract. We used a forest chronosequence at the Barro Colorado Nature Monument (BCNM) to examine changes in the abundance and species composition of seeds in the soil during forest succession. At each of eight sites varying from 20 yr to 100 yr since abandonment, and at two old-growth (> 500 yr) forest stands, we established two 160-m transects and sampled the surface 0–3 cm of soil in cores collected at each 5 m interval. Seed densities were estimated from the number of seedlings germinated from the soil over a six-week period. Contrary to expectation, neither the density of the soil seed bank, nor species richness or diversity were directly related to age since abandonment, but the density of the soil seed bank was correlated with the abundance of seed-bank-forming species in the standing vegetation. In marked contrast to published studies, herbaceous taxa were rare even in the youngest stands, and the common tree species, which accounted for most seeds in the soil, were present in all stands. The pioneer tree Miconia argentea (Melastomataceae) was the single most common species in the seed bank, accounting for 62% of seeds and present in 92% of soil samples. Rapid recovery of the vegetation of young regrowth stands on BCNM, when compared to sites elsewhere may be partly due to allochthonous seed rain from nearby mature forest stands and the lack of seed inputs of weeds and grasses from agricultural and pasture lands which may inhibit forest succession.     

Propagation of local interactions create global gap structure and dynamics in a tropical rainforest

Gap dynamics in tropical forests are of interest because an understanding of them can help to predict canopy structure and biodiversity. We present a simple cellular automaton model that is capable of capturing many of the trends seen in the canopy gap pattern of a complex tropical rainforest on the Barro Colorado Island (BCI) using a single set of model parameters. We fit the global and local densities, the cluster size distributions, and two correlation functions, for gaps, gap formations, and gap closures determined from a spatial map of the forest (1983-1984). To the best of our knowledge, this is the first report that the cluster size distributions of gap formations and closures in the BCI are both power laws. An important element in the model is that when a transition from gap to non-gap (closure), or vice versa (formation), occurs, this transition is allowed to expand into adjacent cells in order to make different cluster sizes of transitions. Model results are in excellent agreement with reported field data. The propagation of local interactions is necessary in order to obtain the complex dynamics of the gap pattern. We also establish a connection between the global and local densities via the neighborhood-dependent transition rates and the effective global transition rates.     

Solar irradiance as the proximate cue for flowering in a tropical moist forest

We compared flowering times predicted by six possible proximate cues involving seasonal changes in rainfall and irradiance and flowering times observed over 30 years of weekly censuses for 19 tree and liana species from Barro Colorado Island (BCI), Panama. Hypotheses concerning variation in the timing and intensity of rainfall failed to predict flowering times in any species. In contrast, 10 to 12 weeks of consistent high levels of irradiance predicted flowering times well for eight species, and six or seven weeks of rapidly increasing levels of irradiance predicted flowering times well for two species. None of the six possible proximate cues predicted flowering times adequately for the nine remaining species. We conclude that high and increasing levels of irradiance are the proximate cues for flowering in many BCI species. The seasonal movements of the Intertropical Convergence Zone cause strong seasonal changes in cloud cover, atmospheric transmissivity, and irradiance reaching the BCI forest. Inter‐annual variation in the timing of movements of the Intertropical Convergence Zone is the most likely cause of inter‐annual variation in the timing of flowering in these species. Much work remains to be done as the physiological mechanisms linking flowering and irradiance are unknown and the proximate cues for flowering remain to be identified for many other BCI species.     

Spatial scale and sampling resolution affect measures of gap disturbance in a lowland tropical forest: implications for understanding forest regeneration and carbon storage

Treefall gaps play an important role in tropical forest dynamics and in determining above-ground biomass (AGB). However, our understanding of gap disturbance regimes is largely based either on surveys of forest plots that are small relative to spatial variation in gap disturbance, or on satellite imagery, which cannot accurately detect small gaps. We used high-resolution light detection and ranging data from a 1500 ha forest in Panama to: (i) determine how gap disturbance parameters are influenced by study area size, and the criteria used to define gaps; and (ii) to evaluate how accurately previous ground-based canopy height sampling can determine the size and location of gaps. We found that plot-scale disturbance parameters frequently differed significantly from those measured at the landscape-level, and that canopy height thresholds used to define gaps strongly influenced the gap-size distribution, an important metric influencing AGB. Furthermore, simulated ground surveys of canopy height frequently misrepresented the true location of gaps, which may affect conclusions about how relatively small canopy gaps affect successional processes and contribute to the maintenance of diversity. Across site comparisons need to consider how gap definition, scale and spatial resolution affect characterizations of gap disturbance, and its inferred importance for carbon storage and community composition.     

Communities of fungal endophytes in tropical forest grasses: highly diverse host- and habitat generalists characterized by strong spatial structure

Plant-symbiotic fungi influence the structure and function of all terrestrial ecosystems, but factors shaping their distributions in time and space are rarely well understood. Grasses (Poaceae), which first arose and diversified in tropical forests, harbor diverse but little-studied endophytes in the lowland forests of Panama. We used sequence data for 402 isolates from two sampling years, 11 host species, and 55 microsites at Barro Colorado Island, Panama to investigate the influence of host and habitat (soil type, forest age) in shaping endophyte diversity and composition. In contrast to previous studies, we found no evidence for host- or habitat specificity. Instead, endophytes demonstrated strong spatial structure consistent with dispersal limitation, with community similarity decaying markedly over a scale of hundreds of meters. Spatial structure that is independent of host species and habitat reveals remarkable heterogeneity of endophyte–host associations at small geographic scales and adds an important spatial component to extrapolative estimates of fungal diversity.     

Tri-trophic interactions affect density dependence of seed fate in a tropical forest palm

Natural enemies, especially host-specific enemies, are hypothesised to facilitate the coexistence of plant species by disproportionately inflicting more damage at increasing host abundance. However, few studies have assessed such Janzen-Connell mechanisms on a scale relevant for coexistence and no study has evaluated potential top-down influences on the specialized pests. We quantified seed predation by specialist invertebrates and generalist vertebrates, as well as larval predation on these invertebrates, for the Neotropical palm Attalea butyracea across ten 4-ha plots spanning 20-fold variation in palm density. As palm density increased, seed attack by bruchid beetles increased, whereas seed predation by rodents held constant. But because rodent predation on bruchid larvae increased disproportionately with increasing palm density, bruchid emergence rates and total seed predation by rodents and bruchids combined were both density-independent. Our results demonstrate that top-down effects can limit the potential of host-specific insects to induce negative-density dependence in plant populations.     

The epiphyte vegetation of Annona glabra on Barro Colorado Island, Panama

Aim Information on the community composition, structure, and dynamics of epiphyte vegetation is scarce. A survey of the epiphytes occurring on all individuals of one particular host tree species in a well-studied neotropical research site allowed us a comparison of the epiphyte flora of this tree with the local epiphyte flora, the analysis of spatial distribution patterns and the use of these patterns as indications for changes in time. In the future, our results can be used as a baseline data-set for the direct observation of the long-term dynamics in epiphyte communities. Location The study was conducted on Barro Colorado Island (BCI), Panama. Methods We recorded all individuals of the vascular epiphytes growing on Annona glabra L., a flood-tolerant, multiple-stemmed tree, which is restricted to the shoreline of BCI. Data on tree biometrics, epiphyte species, and epiphyte abundances were collected for more than 1200 trees. Results In total, we encountered almost 15,000 epiphytic individuals in sixty-eight species, corresponding to more than one third of the entire epiphyte flora of Barro Colorado Island. The component species differed strongly in abundance: the four most important species accounted for >75% of all individuals. In most cases, the same four species were also the first to colonize a tree (=phorophyte). Colonization patterns indicated no replacement of early colonizers by late arrivals. Species richness and epiphyte abundances showed a positive correlation with the size and the density of the host trees. All species showed a highly clumped distribution and the physiognomy of epiphyte communities of individual trees was dominated either by one or several of the four most common species or by a set of frequently co-occurring tank bromeliads. Other species were dominant only in exceptional cases. Most species were always rare. A distance effect on community composition was mostly confined to a local scale with an increased similarity in the species assemblage of stems of a tree v. neighbouring trees. Main conclusions The epiphytes on a single small phorophyte species may encompass a surprisingly large proportion of the local epiphyte flora. The observations that most tree crowns are inhabited by a single or only very few species, and that all epiphyte species show highly clumped distributions suggest a predominance of very local dispersal within a tree crown, which is only infrequently interrupted by successful long-distance dispersal between crowns.     

A model-based method for estimating effective dispersal distance in tropical plant populations

Dispersal is a key mechanism to help populations propagate across space and thus is important in helping to understand spatial patterns. However, it is often difficult to quantify empirically as it requires intensive and detailed field study. Here we describe a method for estimating the effective dispersal distance of tropical plant populations. The method integrates a simple spatially explicit, individual-based dynamic model and spatial statistical analysis. The model is partly parameterized from spatial point pattern data as well as time series data from a 50 ha tropical forest plot in Barro Colorado Island (BCI) in Panama. Correlation between our estimated dispersal distances and those from inverse modeling based on field studies to date on BCI raises some questions about the match between our methods and those previously used. The method we propose can be generalized to any population for which spatial point pattern data are available. Additional field studies would be useful to further validate our method.     

Characterization of 14 microsatellite DNA markers for the tropical forest tree Virola surinamensis (Rol.) Warb. (Myristicaceae)

Fourteen microsatellite DNA markers were developed for studies of gene flow in the Neotropical rain forest tree Virola surinamensis. The loci were unlinked and polymorphic in a sample of 21 individuals, with two to 10 alleles per locus and observed heterozygosity ranging from 0.14 to 0.76. The overall exclusion probability (0.997) indicates high resolution for parentage-based analyses of gene flow.     

Stomatal behavior and photosynthetic performance under dynamic light regimes in a seasonally dry tropical rain forest

Rates of photosynthetic induction upon exposure to high light and rates of induction loss after darkening the leaf were measured in the field for four species of tropical shrubs in the family Rubiaceae. During wet season mornings, stomatal conductance (g _s) in the shade prior to induction was generally high enough so that the time course of induction was determined primarily by rates of activation of biochemical processes. During wet season afternoons, however, g _s values in the shade tended to be considerably lower and photosynthetic induction following a light increase exhibited a slower time course. In the afternoon, the time course of induction was determined by a combination of stomatal opening time and the rates of activation of light regulated enzymes. Stomatal behavior was also correlated with patterns of induction loss following a transfer from high light to darkness. In the afternoon, maximum g _s was lower for all species, and for a given time in the darkness, leaves showed a greater loss of induction in the afternoon than in the morning. During the dry season, maximum g _s and average values for g _s in the shade were reduced in all species. Along with these shifts in stomatal behavior, reduced rates of photosynthetic induction were observed. In the high-light species, the lower maximum g _s values observed during the dry season were also correlated with increased induction loss for a given time in the darkness. For all species, stomatal behavior was affected by season and time of day and, with the exception of wet season mornings, stomata appeared to exert significant control over rates of induction and patterns of induction loss. The results of simulation modeling suggest that the observed seasonal and diurnal changes in rates of induction and induction loss can have significant consequences on sunfleck carbon gain under a dynamic light regime. Received: 17 March 1999 / Accepted: 26 October 1999     

Impact of Research Trails on Seedling Dynamics in a Tropical Forest

We evaluated the impact of research access trails on adjacent seedling density, survival, and recruitment in a permanent tropical forest plot in Panama. Significant differences were detected up to 20 m from trails, indicating that data collected close to trails may be biased. However, observed effects were not substantial enough to affect plot-wide estimates of seedling dynamics, suggesting that research trail impacts are negligible when affected areas constitute only a small fraction of the total area sampled.     

Local neighborhood effects on long-term survival of individual trees in a neotropical forest

The survival of approximately 235 000 individual tropical trees and saplings in the 50 ha permanent plot on Barro Colorado Island (BCI), Panama was analyzed over a 13-year interval (1982–1995) as a function of four biotic neighborhood variables: (i) total stem density; (ii) conspecific density; (iii) relative plant size; and (iv) relative species richness. These neighborhood variables were measured in annular rings of width 2.5 m, extending 30 m from a given focal plant, and in one more distant annulus at 47.5–50 m. Because survival was spatially autocorrelated, a Gibbs sampler and a Monte Carlo Markov chain method were used for fitting an autologistic regression model to obtain unbiased estimates of parameter variances for hypothesis testing. After pooling all species at the community level, results showed that all four variables had significant and often strong effects on focal plant survival. Three of the four variables had negative effects on focal plant survival; relative plant size was the only variable with a positive effect (18% increase in the survival odds ratio). The variables with a negative effect on the survival odds ratio, in order of their effect strength in the nearest annulus, were: stem density (a 70% reduction in the survival odds ratio), conspecific density (50% reduction) and species richness (13% reduction). A guild-level analysis revealed considerable heterogeneity among guilds in their responses to these variables. For example, survival of gap species showed a much larger positive response to relative plant size than did survival of shade-tolerant species. Survival of shrub species was positively affected by conspecific density, but canopy tree survival was negatively affected. Conspecific density negatively affected survival of rare species much more strongly than survival of common species. The neighborhood effects of conspecific density disappear within approximately 12–15 m of the focal plant. Although locally strong, the rapid spatial decay of these effects raises unanswered questions about their quantitative contribution to the maintenance of tree diversity on landscape scales in the BCI forest.     

Diapause in a large aggregation of a tropical beetle

Abstract. 1_˜ Diapause is described for adults of Stenotarsus rotundus Arrow (Endomychidae, Coleoptera) in the tropical lowland forest on Barro Colorado Island, Panama.
2 . Diapause lasted up to 10 months each year, covering some 6 months of the wet season plus 4 months of the dry season. When in diapause the beetles clustered in an aggregation of up to 70 ,000 individuals on the stem of a palm tree, the same individual palm tree in at least four successive years.
3. The habitat and habits of the beetle when not in diapause are unknown, but one or two generations may be produced during this period. Beetles returned to the diapause site from June until late August.
4. Soon after arrival at the diapause site the flight muscles degenerated and did not develop again until February/March. The ovaries were undeveloped when the beetles arrived at the site and remained in that condition until the onset of yolk deposition in March. At the beginning of the rainy season in April the beetles mated and dispersed.
5. Lean dry weight of the beetles did not decrease noticeably during diapause, but fat content did. It is suggested that the beetles do not feed, or feed very little, while in diapause. During diapause, the metabolic rate is surprisingly low considering a mean ambient temperature of 25° C.
6. Juvenile hormone analogue (ZR515) stimulated flight muscle and ovary development. Daylength, when different from LD 12: 12 , initiated a partial flight muscle development, but there was no effect on the ovaries. Field observations suggest that neither temperature nor humidity had an effect on the termination of diapause.
7. Although the development of both flight muscles and ovaries were stimulated by juvenile hormone analogue and both developed around March of each year, it is suggested that the mechanisms causing this development are different.