Habitat destruction threatens biodiversity by reducing the amount of available resources and connectivity among geographic areas. For organisms living in fragmented habitats, population persistence may depend on dispersal, which maintains gene flow among fragments and can prevent inbreeding within them. It is centrally important to understand patterns of dispersal for bees living in fragmented areas given the importance of pollination systems and recently documented declines in bee populations. We used population and landscape genetic techniques to characterize patterns of dispersal over a large fragmented area in southern Costa Rica for the orchid bee species Euglossa championi. First, we estimated levels of genetic differentiation among forest fragments as φpt, an analog to the traditional summary statistic F st, as well as two statistics that may more adequately represent levels of differentiation, G'st and Dest . Second, we used a Bayesian approach to determine the number and composition of genetic groups in our sample. Third we investigated how genetic differentiation changes with distance. Fourth, we determined the extent to which deforested areas restrict dispersal. Finally, we estimated the extent to which there were temporal differences in allele frequencies within the same forest fragments. Within years we found low levels of differentiation even over 80 km, and no effect of land use type on level of genetic differentiation. However, we found significant genetic differentiation between years. Taken together our results suggest that there are high levels of gene flow over this geographic area, and that individuals show low site fidelity over time.
Most plant species in tropical forests are pollinated by animals, and yet the diversity and specificity of pollinator assemblages are poorly documented. Here, we investigated pollinator relationships for 11 species of understory herbs in the genus Costus , with the goal of documenting visitation rates and pollinator assemblages among a variety of habitats. For a subset of species, we documented pollinator visitation for multiple years and/or multiple sites to examine temporal and spatial variation in pollinator relationships. Furthermore, we examined the extent to which specialization in pollination systems can contribute to reproductive isolation for sympatric species. Each species was primarily pollinated by either euglossine bees or hummingbirds. Total visitation rates were generally low, averaging 3.2 visits per flower per hour for bee–pollinated species and 0.5 visits per flower per hour for hummingbird–pollinated species. All of the higher elevation species studied were hummingbird–pollinated, while low elevation species were pollinated either by euglossine bees or hummingbirds. Spatial and temporal variation in visitation rates and pollinator identities was minimal. Pollinator specificity was found to contribute strongly to reproductive isolation for the 11 pairwise combinations of sympatric species differing in pollination syndrome, in some cases functioning as a complete barrier to potential pollen flow.
La mayoría de las especies vegetales de los bosques tropicales son polinizadas por animates, pero la diversidad y las especies de los gremios de polinizadores son poco conocidas. En este trabajo investigamos las interacciones de los polinizadores de 11 especies de hierbas del sotobosque pertenecientes al género Costus , con el objetivo de documentar la frecuencia de visitas y los gremios de polinizadores en distintos hábitats. Para un subconjunto de las especies, documentamos las visitas de los polinizadores en varios años y/o sitios para examinar la variación espacial y temporal de las interacciones de los polinizadores. Además, examinamos como la especialización de los sistemas de polinización puede contribuir al aislamiento reproductive de especies simpátricas. Cada especie fue polinizada principalmente ya sea por abejas euglosinas o colibríes. Las frecuencia de visitas fue baja en general, con un promedio de 3.2 visitas/flor/ hora en las especies polinizadas por abejas y 0.5 visitas/flor/hora en las especies polinizadas por colibríes. Las especies localizadas a mayor altitud fueron polinizadas por colibríes, mientras que las de bajas elevaciones fueron polinizadas por ambos. La diferencia espacial y temporal en frecuencia de visitas e identidad de polinizadores fue mínima. Se encontró que la especificidad de los polinizadores contribuye al aislamiento reproductivo, en las 11 combinaciones reciprocas de especies simpátricas se observaron síndromes de polinización diferentes, que en algunos caso sirvieron como una barrera efectiva al flujo potencial de polen.
We studied the decomposition of Cyrilla racemiflora logs over a 13-yr period in tropical dry and wet forests in Puerto Rico. The mean mass loss, ratio of soft to hard wood, nutrient concentrations, and the diversity of wood-inhabiting organisms were greater in logs decomposing in the dry forest than in the wet forest. Termites were also more abundant in the logs collected from the tropical dry forest than the tropical wet forest. High moisture content and a low animal diversity on the logs in the wet forest seem to retard wood decay in this habitat. Wood decay rates in the tropical dry forest can be related to the high diversity of species and functional groups of wood-inhabiting organisms.RESUMENEfectuamos un estudio de la descomposición de troncos de Cyrilla racemiflora por un período de 13 años en bosques tropicales secos y húmedos en Puerto Rico. El promedio de pérdida de masa, la proporción de madera blanda a dura, la concentración de nutrientes y la diversidad de organismos en la madera fueron mayor en los troncos descompuestos en el bosque tropical seco que en los troncos del bosque tropical húmedo. El numero de termitas también fue más abundante en los troncos colectados en el bosque tropical seco que en el bosque tropical húmedo. Un alto contenido de humedad y una baja diversidad de organismos en los troncos en el bosque tropical húmedo parecen retardar la descomposición de madera en esta región. La rápida descomposición de madera en el bosque tropical seco puede estar relacionada a la alta diversidad de organismos y de grupos funcionales de los organismos que habitan en su madera.
Figs are a critical resource for many tropical frugivores, yet they often are referred to as low quality fruits. To determine their nutritional value, both as a group and for individual species, we analyzed 14 fig species from Barro Colorado Island (BCI), Panama, for fiber, tannins, lipids, protein, carbohydrates, amino acids, and minerals. Seeds and pulp were analyzed separately. Fig fruit pulp consisted of about one-third digestible components, mostly carbohydrates with some lipids and proteins. Tannin, lignin, and water-soluble carbohydrates showed considerable variation among species, as did fruit size. Figs contained high amounts of amino acids, such as leucine, lysine, valine, and arginine, and minerals, such as potassium, calcium, magnesium, sodium, and phosphorus. One species, Ficus insipida, contained the highest concentrations of almost all amino acids, many minerals, and protein. Small figs had as much nutritional value per gram as large figs. Free-standing figs had higher percentages of protein, complex carbohydrates, and ash than strangler figs, which had higher percentages of water-soluble carbohydrates, tannins, and hemicellulose. The guild of fruit-eating bats on BCI included ten common species with diets dominated by figs. Fecal analyses and captures at ripe fig trees showed a consistent pattern of resource partitioning. Small bats preferentially ate small-fruited and strangler figs while large bats consumed mostly large-fruited and free-standing figs. Small bats most often ate F. bullenei, which has high levels of lipid and carbohydrates, and F. yoponensis, which has high levels of protein. Medium and large bats most often ate F. insipida, a nutritionally superior species; their second most eaten species was F. obtusifolia, in which the large size may make it efficient to eat. Each bat ate a variety of fig species, supporting the idea that although no single species of fig may be sufficient to sustain frugivores, a mix of fig species can provide a complete set of nutrients.
In an old-growth tropical wet forest at La Selva, Costa Rica, we combined radiocarbon (C-14) dating and tree-ring analysis to estimate the ages of large trees of canopy and emergent species spanning a broad range of wood densities and growth rates. We collected samples from the trunks, of 29 fallen, dead individuals. We found that all eight sampled species formed visible growth rings, which varied considerably in distinctiveness. For five of the six species for which we combined wood anatomical studies with C-14-dates (ring ages), the analyses demonstrated that growth rings were of annual formation. The oldest tree we found by direct ring counting was a Hymenolobium mesoamericanum Lima (Papilionaceae) specimen, with an age of ca. 530 years at the time of death. All other sampled individuals, including very large trees of slow-growing species, had died at ages between 200 and 300 years. These results show that, even in an everwet tropical rain forest, tree growth of many species can be rhythmic, with an annual periodicity. This study thus raises the possibility of extending tree-ring analyses throughout the tropical forest types lacking a strong dry season or annual flooding. Our findings and similar measurements from other tropical forests indicate that the maximum ages of tropical emergent trees are unlikely to be much greater than 600 years, and that these trees often die earlier from various natural causes.
Degraded grasslands resulting from intensive land use appear to be highly resistant to tree invasion due to interactions between land use, climate, grazing and fire. We describe long-term patterns of tropical montane forest regeneration into degraded grasslands and analyze their relationships with historical changes in rainfall, grazing and fire in Los Toldos valley (Northwest Argentina), cloud forest life zone (1600 m asl). We used dendrochronological techniques to reconstruct spatial and temporal patterns of Podocarpus parlatorei establishment (the dominant tree species in secondary forests) and grassland fires for the last 150 yr. We assessed current livestock spatial distribution along the valley through feces sampling. Inferred tree establishment patterns (i.e., considering age structure and mortality) were analyzed in relation to temporal and spatial patterns of grazing and fire derived from our own analyses and from government statistics, and to rainfall patterns derived from previous dendrochronological reconstructions. Current grazing intensity was higher close to the local township. Fire occurrence increased with periods of above-average rainfall (higher fuel productivity), and tended to increase with distance to township (less grazing). Tree establishment in grasslands was spatially associated with high grazing intensity and low fire frequency, and temporally associated with periods of high grazing intensity and below-average rainfall. Despite climatic and land-use changes leading to conditions potentially favorable for trees (i.e., more rainfall, less grazing), grasslands persist in this study area, likely due to the direct (saplings burning) and indirect (soil degradation and desiccation) effects of recurrent fires, enhanced by decreasing grazing and increasing rainfall.
Harengula jaguana is a dominant species in the southern Gulf of Mexico. It is a demersal, estuarine-dependent species, distributed throughout the Campeche Sound area (12-54 m). It is found in areas influenced by coastal discharges. The size at first maturity of females is 117 mm total length. Reproduction occurs from February to October in depths 18-36 m. The juveniles are found on the shelf (<20 m) and inside the Terminos Lagoon. Recruitment is in two distinct periods, March–June and August–November, when adults and juveniles migrate to the outer shelf. Greatest abundance is observed in depths of 12-36 m. The biology and ecology of H. jaguana was analyzed in the Campeche Sound to better understand the structure and function of the demersal fish communities. It is a very important species in the trophic dynamics of the ecosystem because it transports matter and energy. A model of the biological pattern for this species in the area is proposed.
Se analizan diversos aspectos de la biología y ccología de Harengula jaguana, especie dominante en el sur del Golfo de México. Esta especie demersal dependiente estuarina sc distribuye en la plataforma continental de la Sonda de Campeche, entre los 12 y 54 m. Las hembras tienen una talla de primera madurez de 117 mm de longitud total (LT). Se reproduce de febrero a octubrc a profundidades de 18-36 m. Los juveniles se encuentran en la plataforma a profundidades menores de 20 m y dentro de la Laguna de Términos. El reclutamiento se presenta en dos periodos: marzo-junio y agosto-noviembre, migrando hacia aguas de mayor profundidad. Las abundancias máximas en biomasa y densidad se presentan alrededor de las isóbatas de 12 y 36 m. For su dependencia estuarina y su dominancia, es una especie de gran importancia en la dinámica trófica del ecosistema al actuar como transportador de materia y energyía entre la plataforma continental de la Sonda de Campeche y la Laguna de Términos.
Deforestation rates were estimated in 270 km2 of lowland moist forest on the eastern slope of the Colombian Andes (300–800 m). Deforestation was quantified by determining areas of pasture, secondary growth, and forest in aerial photographs (1:19,000–1:60,000) taken in the 1930s, 1950s, 1960s, 1970s, and 1980s. Mean annual deforestation rate for the 1938–1988 period was 1.5 percent, and ranged from <0.1 percent (1938–1955) to 4.4 percent (1970s–1980s). Between the 1930s and the 1980s, areas covered by pasture and young secondary forest increased from 26–53 percent and from 2–14 percent, respectively. This suggests that, although there are areas with clear signs of forest regeneration, the overall trend is the conversion of mature tropical forests to pastures for cattle ranching.
En un área de 270 km2 de bosque húmedo tropical de tierras bajas localizados en el piedemonte oriental de los Andes colombianos (300-800 m), fué cuantificada la deforestación mediante la determinación de áreas de pastizales, rastrojos y bosques en fotografías aéreas (1:19.000-1:60.000) tomadas en las décadas de los años 1930, 1950, 1960, 1970 y 1980. La tasa media anual de deforestación fue de 1.5 por ciento para el periodo comprendido entre 1938 y 1988, y varió entre menos de 0.1 por ciento (1938-1955) y 4.4 por ciento (1979–1988). Entre 1938 y 1988 las áreas cubiertas por pastizales y rastrojos aumentaron de 26 a 53 por ciento y de 2 a 14 por ciento, respectivamente, lo cual sugiere que a pesar de que hay áreas en proceso de regeneración, la tendencia general es hacia la conversión de bosques para pastizales ganaderos.
Quantifying forest change in the tropics is important because of the role these forests play in the conservation of biodiversity and the global carbon cycle. One of the world's largest remaining areas of tropical forest is located in Papua New Guinea. Here we show that change in its extent and condition has occurred to a greater extent than previously recorded. We assessed deforestation and forest degradation in Papua New Guinea by comparing a land-cover map from 1972 with a land-cover map created from nationwide high-resolution satellite imagery recorded since 2002. In 2002 there were 28,251,967 ha of tropical rain forest. Between 1972 and 2002, a net 15 percent of Papua New Guinea's tropical forests were cleared and 8.8 percent were degraded through logging. The drivers of forest change have been concentrated within the accessible forest estate where a net 36 percent were degraded or deforested through both forestry and nonforestry processes. Since 1972, 13 percent of upper montane forests have also been lost. We estimate that over the period 1990–2002, overall rates of change generally increased and varied between 0.8 and 1.8 percent/yr, while rates in commercially accessible forest have been far higher—having varied between 1.1 and 3.4 percent/yr. These rates are far higher than those reported by the FAO over the same period. We conclude that rapid and substantial forest change has occurred in Papua New Guinea, with the major drivers being logging in the lowland forests and subsistence agriculture throughout the country with comparatively minor contributions from forest fires, plantation establishment, and mining.
Forest closure on the three original Krakatau Islands (Panjang, Rakata, and Sertung) took place ca 1930, about 50 yr after the apparent sterilization of the islands due to volcanic eruptions. Two permanent forest plots were established on each of these islands in 1989. A full enumeration of these plots, of two additional Rakata plots, and of two “mainland”plots from the Ujung Kulon National Park, West Java, was undertaken in 1992. These data provide the first estimates of aboveground biomass from Krakatau. The values reported for Krakatau are below the local West Javan figures, with considerable variation occurring within the islands. In 1992, the fourth Krakatau island, Anak Krakatau, began an eruption sequence which continued through the study period, depositing ash on the study sites of Panjang and Sertung. A further partial survey of these plots in 1994–1995 revealed a significant increase in mortality since the volcanic activity recommenced, with an increase in deaths of larger stems. Although Rakata has not been influenced directly by volcanism, three sites surveyed on Rakata in 1994–1995 have experienced increased tree mortality, in two cases as a consequence of storm damage and in particular, of lightning strikes. Stand dynamics on Krakatau thus appear to be strongly influenced by episodic environmental disturbance with varying degrees of dependence on the volcanic activity of Anak Krakatau.
Tropical ecosystems support a diversity of species and ecological processes that are unparalleled anywhere else on Earth. Despite their tremendous social and scientific importance, tropical ecosystems are rapidly disappearing. To help tropical ecosystems and the human communities dependent upon them better face the challenges of the 21st century, tropical biologists must provide critical knowledge in three areas: (1) the structure and functioning of tropical ecosystems; (2) the nature and magnitude of anthropogenic effects on tropical ecosystems; and (3) the socio‐economic drivers of these anthropogenic effects. To develop effective strategies for conservation, restoration, and sustainable management of tropical ecosystems, scientific perspectives must be integrated with social necessities. Three principles for guiding tropical biological research are suggested: (1) broadening the set of concerns; (2) integration of biological knowledge with the social sciences and traditional knowledge; and (3) linking science to policy and action. Four broad recommendations are proposed for immediate action in tropical biology and conservation that are fundamental to all biological and social disciplines in the tropics: (1) assemble and disseminate information on life's diversity in the tropics; (2) enhance tropical field stations and build a worldwide network to link them with tropical field biologists at their field sites; (3) bring the field of tropical biology to the tropics by strengthening institutions in tropical countries through novel partnerships between tropical and temperate zone institutions and scientists; and (4) create concrete mechanisms to increase interactions between tropical biologists, social scientists, and policy makers.
Los ecosistemas tropicales mantienen una diversidad de especies y procesos ecológicos como ningún otro en la tierra. A pesar de su gran importancia a nivel social y cientifico, los ecosistemas tropicales están desapareciendo rápidamente. Para ayudar a estos y a las comunidades humanas que dependen de ellos a encarar mejor los desafios del siglo 21, los biólogos tropicales deben proveer informacion critica en tres áreas: (1) la estructura y funcionamiento de los ecosistemas tropicales; (2) la naturaleza y magnitud de los efectos antropogénicos sobre los ecosistemas tropicales; y (3) las fuerzas socio‐económicas de esos efectos antropogénicos. Para desarrollar estrategias efectivas para la conservación, restauración y manejo sostenible de los ecosistemas tropicales, las perspectivas cientificas deben ser integradas a las necesidades sociales. Tres principios para orientar la investigación en biologia tropical son sugeridos: (1) ampliación del grupo de interés; (2) integración del conocimiento boilógico con las ciencias sociales y el conocimiento traditional; y (3) enlazar la ciencia con las politicas y la acción. Para una acción inmediata en biologia tropical y la conservación, se proponen cuatro recomendaciones amplias que son fundamentales a todas la disciplinas boilógicas y sociales en los trópicos: (1) recopilar y diseminar información sobre diversidad boilógica en los trópicos; (2) mejorar las facilidades para la investigación en los trópicos (estaciones biológicas) y construir una red mundial que las una con los biólogos tropicales; (3) apoyar el campo de la biologia tropical fortaleciendo las instituciones de 10s paises tropicales a travts de la colaboraci6n con instituciones y cienrificos de zonas ternpladas; y (4) crear rnecanismos concretos para incrementar la interacci6n entre bi6logos rropicales, cientificos sociales y tomadores de decisiones.
Os ecossistemas tropicais abrigam uma diversidade de espécies e de processes ecológicos sem paralelo em qualquer outro lugar da Terra. Apesar de sua tremenda importa̧ncia social e cientifica, os ecossistemas tropicais esta̧o desaparecendo rapidamente. Para ajudar os ecossistemas tropicais e as populaço̧es humanas que dependem deles a melhor enfrentar os desafios do século 21, os biólogos tropicais precisam gerar conhecimentos cruciais em trȩs áreas: (1) a estrutura e o funcionamento dos ecossistemas tropicais; (2) a natureza e a magnitude dos efeitos antrópicos sobre os ecossistemas tropicais; e (3) as diretrizes sócio‐econo̧micas destes efeitos antrópicos. Para se desenvolver estratégias efetivas para a conservação, restauracao e manejo sustentável dos ecossistemas tropicais, as perspectivas cientificas tem que ser integradas às necessidades sociais. Trȩs principios norteadores da pesquisa em biologia tropical são indicados: (1) ampliar o conjunto de interesses; (2) integrar o conhecimento boilógico com o conhecimento tradicional e as ciȩncias sociais; e (3) unir ciȩncia com politica e tomada de decisão. Quatro amplas recomendaço̧es são propostas para açã imediata na biologia tropical e conservação e que são fundamentals para todas as disciplinas boilógicas e sociais nos trópicos: (1) organizar e disseminar informação sobre a diversidade boilógica nos trópicos; (2) consolidar as estaço̧es de pesquisa de campo nos trópicos e construir uma rede mundial para ligar estaço̧es e os biólogos tropicais atuando nelas; (3) levar o campo da biologia tropical para os trópicos pelo fortalecimento das instituiço̧es nos paises tropicais e através de novas parecerias entre estes pafses e seus cientistas com os cientistas e as instituiço̧es de paises da região temperada; e (4) criar mecanismos concretos para aumentar as interaço̧es entre os biólogos tropicais, os cientistas sociais e os politicos tomadores de decisão.
Mangroves are prone to bearing frequently the full brunt of hurricanes and tropical storms. The extent of destruction and early regeneration are widely studied. The purpose of this study was to add a long-term view of mangrove regeneration and assess the potential effects on mangrove horizontal zonation patterns of catastrophic destruction. Hattie, a category five hurricane, hit the Belizean coast in 1961. It passed directly over the Turneffe Atoll where our study area, Calabash Cay, is located. At four sites on this island, we analyzed mangrove forest structure along transects parallel to the shoreline within zones delineated by species dominance and tree height. We propose an index based on the Simpson index of diversity to express changes in the heterogeneity of the species dominance. Physical–chemical parameters and nutrient availability were also measured. The destruction levels were estimated by analysis of the distribution of diameter at breast heights of the bigger trees in the inland zones. Variations in species dominance among sites and zones could be explained by interactions of various factors. Further, different levels of destruction between the two sides of the island had a significant effect on current patterns of species and structural zonation at Calabash. We conclude that disturbance regime in general should be considered as a factor potentially influencing mangrove horizontal zonation patterns.
We compared vegetation structure and species richness across a 56-yr chronosequence of six replicated age classes of dry tropical forest on the island of Providencia, Colombia, in the Southwest Caribbean. Stand age classes were determined using sequential, orthorectified panchromatic aerial photos acquired between 1944 and 1996 and Landsat 7 ETM + satellite imagery from 2000. Along the chronosequence we established 59 plots of 2 × 50 m (0.01 ha) to document changes in species richness, basal area, tree height, stem density, and sprouting ability. All woody trees and shrubs >2.5 cm diameter at breast height (DBH) were censused and measured. Although woody species density reached a peak in stands from 32 to 56 yr old, rarefaction analysis showed that species richness increased linearly with stand age and was highest in stands 56 yr old or greater. Nonparametric, abundance-based estimators of species richness also showed positive and linear associations with age. Basal area and mean tree height were positively associated with age since abandonment, while sprouting ability showed a negative relationship. Our results indicate rapid recovery of woody species richness and structural characteristics along this tropical dry forest chronosequence.
We describe patterns of tree community change along a 700-km transect through terra firme forests of western Amazonia, running from the base of the Andes in Ecuador to the Peru-Brazil border. Our primary question is whether floristic variation at large scales arises from many gradual changes or a few abrupt ones. Data from 54 1-ha tree plots along the transect support the latter model, showing two sharp discontinuities in community structure at the genus level. One is located near the Ecuador-Peru border, where the suite of species that dominates large areas of Ecuadorean forest declines abruptly in importance to the east. This discontinuity is underlain by a subterranean paleoarch and congruent with a change in soil texture. A second discontinuity is associated with the shift from clay to white sand soils near Iquitos. We hypothesize that the first discontinuity is part of an edaphic boundary that runs along the Andean piedmont and causes a transition from tree communities preferring richer, younger soils near the base of the Andes to those preferring poorer, older soils farther east. Because the floristic changes observed at this discontinuity are conserved for large distances to the east and west of it, the discontinuity is potentially key for understanding floristic variation in western Amazonia. The significant floristic turnover at the Ecuador-Peru border suggests that the only large protected area in the region - Ecuador's Yasuní National Park - is not adequate protection for the very diverse tree communities that cover vast areas of northern Peru.
To study the structure and composition of old-growth forest in the Saracá-Taquera National Forest near Porto Trombetas, Brazil, we established 36 0.25 ha plots and described the vegetation. We collected charcoal from the A2 soil horizon of each plot for radiocarbon dating. Although fires have been very rare in this forest during historic times, the presence of charcoal in these soils indicates fire at some earlier period. The ages (conventional radiocarbon age adjusted to 1997) of the charcoal ranged from 177 to 1547 years. These ages, however, did not correlate significantly with any of several measures of biodiversity or stand characteristics. The relative uniformity of the current old-growth forest indicates that either the prehistoric fires were of such low intensity that they had little long-term effect on the vegetation or that the present stands have progressed to near steady state.
Vegetation, seed rain, seed germination, microclimate, and soil physical and chemical parameters were measured in a recently abandoned pasture and adjacent primary rain forest in southern Costa Rica. The goal of this study was to assess the importance of these factors in limiting forest regeneration in abandoned pastures. Seed rain of animal dispersed species decreased dramatically in the pasture >5 m from the forest/pasture edge; fewer wind dispersed seeds fell in the pasture than in the forest, but the difference was much less than for animal dispersed seeds. Percent seed germination of most species studied was similar in the forest and in pasture with grasses; seed germination was lower during the dry season in areas of pasture cleared of grasses. Air temperature, vapor pressure deficit (VPD), and photon flux density (PFD) were much higher in the pasture than in the forest at 1 m above the ground. VPD and PFD at ground level and soil temperature were similar in the pasture and the forest, indicating that pasture grasses strongly modify microclimatic conditions near the soil surface. The lowest gravimetric water content recorded in the pasture during the dry season was 0.5 and leaf relative water contents of the two species measured in the forest and pasture were identical, suggesting that plants in the pasture were not water stressed. Levels of most soil nutrients were lower in the pasture as compared to the forest; however, aboveground and root biomass for seedlings grown in pasture and forest soils did not differ significantly. Although a number of factors impede forest recovery in abandoned pastures, these results suggest that the most important limitation is lack of seed dispersal.
We characterized plant regeneration in four old logging roads (700–1000 m long), 12–17 yr after abandonment, in selectively logged forests in lowland Costa Rica. Sets of 4‐m ² plots were laid out at 20‐m intervals in three distinct microhabitats: road track (topsoil eliminated), road edge (where removed topsoil accumulates on the sides after road construction), and adjacent logged forest. Density of stems ≥ 1 m tall and ≤ 5 cm DBH (included canopy tree, midstory tree, liana, palm, shrub, and tree fern species) was highest in the road edge plots than either the track or logged forest plots. This “edge effect” is presumably due to buried seed germination of light‐demanding trees and shrubs after moderate soil disturbance, less compaction, and higher substrate fertility than in road tracks. Species richness was the lowest, but relative dominance the highest, in the track plots of all roads: 6–9 species comprised alone 50 percent of the Importance Value Index (IVI), in contrast to 11–15 and 16–22 species required to reach 50 percent IVI in edge and forest plots, respectively. We found evidence of soil compaction in tracks of three out of four roads which, in addition to low substrate fertility, and initial lack of on‐site plant propagules, could explain slower recovery of stem density and species richness compared to edge and logged forest plots. For stems >5 cm and ≤ 20 cm DBH, density and basal area in the track plots averaged about one‐fourth of edge and logged forest plot values. We estimated recovery of basal area in road tracks to take at least 80 yr to reach the status found in logged forest, and species richness over an even longer period. We suggest that abandoned logging roads serve as long corridors of relatively uniform and long‐lasting floristic and structural characteristics that may confer particular ecological roles in selectively logged forests.
Caracterizamos la regeneración vegetal en cuatm caminos de arrastre de troncos (700–1000 m de largo) que fueron abandonandos entre 12–17 años luego del madereo selectivo en bosqucs de bajura en Costa Rica. Parcelas de 4 m ² se colocaron a lo largo de los caminos en tres micrositios contrastantes: centm del camino (desprovisto de suelo orgánico), borde (en donde se acumula el suelo orgánico remaido luego de construir el camino), y en bosque perturbado adyacente. En general, la densidad de tallos ≥1 m alto y ≤5 cm DAP (incluídas especies de árboles del dosel y subdosel, lianas, palmas, arbustos y helechos arborescentes) fue más alta en las parcelas del borde que en las del centm del camino o bosque perturbado adyacente. Este “efecto de borde” se debe probablemente a la germinación de semillas de árboles y arbustos demandantes de luz luego de la perturbación moderada del suelo, menor compactación y mayor fertilidad del substrato con respecto al centro del camino. En todos los caminos, la riqueza de especies fue más baja y la dominancia relativa más alta en las parcelas colocadas en el centro, en las cuáles sólo 6–9 especies acumularon el 50 por ciento del Indice de Valor de Importancia (IVI), en contraste con 11–15 y 16–22 espies requeridas para acumular 50 por ciento IVI en parceles del borde del camino y bosque perturbado, respectivamente. Encontramos evidencia de compactación en el centro de tres de los cuatro caminos estudiados, lo cual sumado a una posible baja fertilidad del suelo y escasze de propágulos autóctonos podría explicar la baja densidad de tallos y riqueza de cspecies en las parcelas del centro del camino respecto a parcelas en el borde y bosque perturbado. La densidad y el área basal de individuos >5 cm y ≤ 20 cm DAP en parcelas del centro de los caminos fue en promedio una cuarta parte de los valores en el borde y bosque perturbado. Estimamos que la recuperación de área basal en el centro de los caminos respecto al bosque perturbado podría tomar un mínimo de 80 años y mucho más tiempo para la riqueza de especies. Proponemos que los caminos de arrastre abandonados sirven como corredores de una relativa uniformidad florística y estructural, los cuales pueden jugar un papel ecológico particular en bosques perturbados por el madereo seletivo.
We compared early plant succession in four abandoned pastures of differing age since abandonment and a nearby secondary forest site in northwestern Ecuador. Two 'Open' pastures had no tree canopy covering, and two 'Guava' pastures had a well-developed canopy cover of Psidium guajava. No site had been seeded with pasture grasses. All pastures were compared in a chronological sequence; two were monitored for 18 months. Species richness was consistently higher in Guava sites than in Open sites and it continued to increase over time, whereas it remained static in Open sites. Species richness was highest in secondary forest. Recruitment of tree saplings in Guava sites was lower than in secondary forest; however, it was nearly absent in Open sites. The seed bank contained predominantly herbaceous species at all sites, and was highly dissimilar to aboveground vegetation. Dominance-diversity curves for Guava sites showed a more equitable distribution of species that increased over time. In contrast, dominance-diversity curves for Open sites were static and indicated dominance by a few aggressive species. Soil characteristics among sites were variable; however, a principal components analysis on soils isolated the older Open site from all others. The older Open site had the lowest species richness and was dominated by Baccharis trinervis, an aggressive shrub species. The site appears to be in a state of arrested succession and some form of restorative intervention may be necessary to initiate succession toward a forested condition. Succession in Guava sites appears headed toward secondary forest, whereas it does not in Open sites.
The indirect impacts of Shorea siamensis-logging on the reproductive ecology of Dipterocarpus obtusifolius, a self-incompatible butterfly-, moth-, and bird-pollinated tree, were studied in tropical dry forest in Thailand. Pollinator activity at D. obtusifolius trees and subsequent seed production were recorded in three forest areas subject to differing intensities of S. siamensis extraction. The pollinator and plant understory communities in these areas were also noted. Forest areas subject to high S. siamensis extraction intensities had very high understory flowering plant cover, dominated by the exotic invader Chromolaena odorata. Activity of butterfly pollinators at D. obtusifolius trees decreased in these disturbed areas, although their abundance remained comparable to other forest areas subject to only moderate or no extraction. For sphingid moth pollinators, there was no difference across differentially disturbed forest areas in either abundance or in the proportion bearing pollen. Pollinator activity by birds increased at highly disturbed locations but was not sufficient to offset a decline in overall pollinator activity at D. obtusifolius canopies in areas of heavy 5. siamensis extraction. Thus, extraction of S. siamensis indirectly affected the pollination of D. obtusifolius, primarily by causing changes in the foraging behavior of butterfly pollinators rather than their abundance. A shift in the relative abundance of floral nectar resources from the canopy to the understory, a consequence of 5. siamensis extraction and invasion by C. odorata, led to a parallel shift in foraging location of the principal diurnal pollinators, the butterflies, toward the understory. Despite reduced pollination at disturbed sites, behavioral changes did not translate into a D. obtusifolius seed set effect, possibly because pollination by birds (or moths) at the disturbed site compensated for reduced butterfly pollination.
We evaluated temporal patterns of seedling survival of eight Neotropical tree species generated under multiple abiotic and biotic hazards (vertebrates, disease, litterfall) in the forest understory on Barro Colorado Island, Panama. Seedlings were transplanted at first leaf expansion in low densities along a 6-km transect and damage and mortality were recorded for 1 yr. We also planted and monitored small and large artificial seedlings to estimate physical disturbance regimes. During 0–2 mo after transplant, vertebrate consumers of reserve cotyledons caused high mortality of real seedlings, but little damage to artificial seedlings. On real seedlings after 2 mo, disease became an important agent of mortality, despite a decrease in overall mortality rates. Damage by litterfall remained relatively low during the 1-yr study period. Survival ranks among species showed ontogenetic shifts over time, as species changed susceptibility to the mortality agents. Survival after 2 mo was positively correlated with stem toughness, not because species with tough stems were less likely to receive mechanical damage, but because they survived better after receiving mechanical damage. Within each transplant station, artificial seedlings were not good predictors of litterfall damage experienced by real seedlings. Forest-wide litterfall damage level, however, was similar for both real and artificial seedlings (ca 10%/yr), a moderate level compared to other tropical forests. In conclusion, species traits including biomechanical traits interact to create complex temporal patterns of first year seedling survival, resulting in ontogenetic shifts that largely reflect changes in the relative importance of vertebrate consumers relative to other hazards.
The control of vegetative phenology in tropical trees is not well understood. In dry forest trees, leaf abscission may be enhanced by advanced leaf age, increasing water stress, or declining photoperiod. Normally, it is impossible to dissect the effects of each of these variables because most leaves are shed during the early dry season when day length is near its minimum and leaves are relatively old. The 1997 El-Niño Southern Oscillation caused a ten-week long, severe abnormal drought from June to August in the semi-deciduous forests of Guanacaste, Costa Rica. We monitored the effect of this drought on phenology and water status of trees with young leaves and compared modifications of phenology in trees of different functional types with the pattern observed during the regular dry season. Although deciduous trees at dry sites were severely water stressed (Ψstem < -7MPa) and their mesic leaves remained wilted for more than two months, these and all other trees retained all leaves during the abnormal drought. Many trees exchanged leaves three to four months earlier than normal during the wet period after the abnormal drought and shed leaves again during the regular dry season. Irrigation and an exceptional 70 mm rainfall during the mid-dry season 1998/1999 caused bud break and flushing in all leafless trees except dormant stem succulents. The complex interactions between leaf age and water stress, the principal determinants of leaf abscission, were found to vary widely among trees of different functional types.