Osvaldo Calderón

Pervasive density-dependent recruitment enhances seedling diversity in a tropical forest.

Negative density-dependent recruitment of seedlings, that is, seeds of a given species are less likely to become established seedlings if the density of that species is high, has been proposed to be an important mechanism contributing to the extraordinary diversity of tropical tree communities because it can potentially prevent any particular species from usurping all available space, either in close proximity to seed sources or at relatively larger spatial scales. However, density-dependent recruitment does not necessarily enhance community diversity. Furthermore, although density-dependent effects have been found at some life stages in some species, no study has shown that density-dependent recruitment affects community diversity. Here we report the results of observations in a lowland, moist forest in the Republic of Panamá in which the species identities of 386,027 seeds that arrived at 200 seed traps were compared with the species identities of 13,068 seedlings that recruited into adjacent plots over a 4-year period. Across the 200 sites, recruit seedling diversity was significantly higher than seed diversity. Part of this difference was explained by interspecies differences in average recruitment success. Even after accounting for these differences, however, negative density-dependent recruitment contributes significantly to the increase in diversity from seeds to seedling recruits.

THE EL NIÑO SOUTHERN OSCILLATION, VARIABLE FRUIT PRODUCTION, AND FAMINE IN A TROPICAL FOREST

We tested the hypothesis that the El Nino Southern Oscillation influences forest-wide fruit production, which, in turn, limits frugivorous and granivorous mammals on Barro Colorado Island (BCI), Panama. Observations of BCI mammals have been compiled for 49 years. Frugivorous mammals experienced famine between September and January in 1931–1932, 1958–1959, 1970–1971, and 1993–1994. The most recent famine is evident from an 11-yr record of natural deaths of mammals and a 2-yr record of population densities. Famine occurred every time a mild dry season followed an El Nino event in the 49-yr record. This coincidence is statistically improbable, as demonstrated by a randomization test. A 2-yr cycle of high, then low community-level fruit production has been observed twice for BCI when a mild dry season followed an El Nino event. We used 260 litter traps to monitor community- and species-level fruit production from 1 January 1987 through 30 June 1996. Community-level fruit production was greatest during the 199...

Are Lianas Increasing in Importance in Tropical Forests? A 17-Year Record from Panama

The relative importance of large lianas (woody vines) increased by 100% for stem enumerations conducted during the 1980s and 1990s in widely scattered Neotropical forests. We use three independent types of data to evaluate the hypothesis that lianas have increased in importance in old growth forests on Barro Colorado Island, Panama. Liana leaf litter production and the proportion of forest-wide leaf litter composed of lianas increased between 1986 and 2002. In contrast, liana seed production and liana seedling densities were much more variable through time with particularly high levels during and immediately after El Nino years. Longer time series will be required to detect shifts in life-form composition for highly dynamic seed and seedling communities. The Barro Colorado Island leaf production data are, however, consistent with the hypothesis that lianas are increasing in importance in Neotropical forests.

ANNUAL AND SPATIAL VARIATION IN SEEDFALL AND SEEDLING RECRUITMENT IN A NEOTROPICAL FOREST

2 National Center for Ecological Analysis and Synthesis, Santa Barbara, California 93101-5504 USA Abstract. An economy of scale may lead to selection to increase interannual variation in seed production when the per seed probability of seedling establishment increases with seed production. Variable annual seedfall will, however, reduce this probability when post- dispersal seed fate is negatively density dependent on the local density of seeds, and seed dispersal and density dependence act identically across years. Intuitively, more variable annual seedfall causes the representative seed to experience a greater density of conspecific seeds and suffer greater density-dependent effects. This handicap must be overcome for the per seed probability of recruitment to be greater in years with greater seed production. We quantified spatial and annual variation in seedfall and seedling recruitment, evaluated density dependence and economies of scale during the seed-to-seedling transition, and investigated the synergistic consequences of density dependence and variable annual seed- fall for seedling recruitment on Barro Colorado Island (BCI), Panama. Weekly censuses of 200 0.5-m 2 seed traps documented seedfall for 15 years and 108 plant species. Annual censuses of 600 1-m 2 seedling plots documented recruitment for nine years and 32 species. The density of seedling recruits tended to increase with the density of seeds; however, the per seed probability of recruitment invariably decreased with seedfall density. Negative density dependence characterized the seed-to-seedling transition. Observed levels of spatial and interannual variation in seedfall density would reduce long-term recruitment by up to 28% if negatively density-dependent survival acted identically across years; however, the strength of negative density dependence varied significantly among years for 12 of 32 species. Negative density dependence occurred in all years for these species but was sig- nificantly weaker during the one or two years of greatest seedfall than during the remaining years of lower seedfall. The per seed probability of recruitment increased significantly with annual seedfall for eight of these species. These eight species realized postdispersal econ- omies of scale despite the reduction in long-term recruitment expected from the synergism between variable annual seed production and negatively density-dependent seed fate.

Flowering and fruiting phenologies of seasonal and aseasonal neotropical forests : the role of annual changes in irradiance

The seasonality of both rainfall and solar irradiance might influence the evolution of flowering and fruiting in tropical forests. In seasonally dry forests, to the degree that soil moisture limits plant productivity, community-wide peaks in reproduction are expected during the rainy season, with seedfall and germination timed to allow seedlings to become well established while soil moisture is available. Where soil moisture is never seasonally limiting, seasonal changes in light availability caused by periods of cloudiness or seasonally low zenithal sun angles should favour reproduction during seasons when irradiance levels are high. To evaluate these predictions, we documented the timing of flower and fruit fall for 10 and 15 y at El Verde, Puerto Rico, and Barro Colorado Island (BCI), Panama. At El Verde, rainfall is abundant year-round and solar declination largely determines seasonal variation in irradiance. At BCI, rainfall is abundant throughout the 8-mo wet season while drought develops and average solar irradiance increases by 40-50% over the 4-mo dry season. Seasonal variation in the number of species flowering and fruiting at both sites was generally consistent with the hypothesis that seasonal variation in irradiance limits the evolution of reproductive phenologies. Community-level metrics provided no evidence for a similar role for moisture availability at BCI. Seasonal variation in irradiance also strongly influenced seed development times at both sites. Thus, community-wide phenologies reveal a strong signature of seasonal changes in irradiance, even in those forests that exhibit some degree of seasonality in rainfall.

Interspecific synchrony and asynchrony in the fruiting phenologies of congeneric bird-dispersed plants in Panama

Plants potentially compete for seed dispersal. Selection may favour temporally segregated fruiting phenologies to minimize this competition and also to maintain resident populations of dispersal agents. Alternatively, selection may favour temporally aggregated fruiting phenologies when the effectiveness of seed dispersal agents varies seasonally or when large, synchronous fruit displays enhance dispersal. These evolutionary scenarios assume that plants share seed dispersal agents. This assumption and temporal overlap in fruiting phenologies were evaluated for the Miconia and Psychotria of central Panama. These two genera accounted for 18 and 27%, respectively, of 1096 fleshy fruits found in regurgitation or faecal samples taken from 2054 birds of 103 species netted in the forest under- storey. Two species of manakins accounted for 62% (123/200) of all Miconia fruit taken. Three species of manakins and three species of migratory thrushes accounted for 97% (282/292) of all Psychotria fruits taken. There is a high potential for intrageneric competition for seed dispersal for both plant genera. Null model analyses showed that the fruiting phenologies of Miconia (14 species) are segreg- ated in time, while fruiting of Psychotria (21 species) is highly aggregated. The Miconia were found in up to 24% of the diet samples for the two manakin species, suggesting that Miconia may be a critical resource for both species. The Psychotria fruited when the diversity of understorey fruits was greatest, suggesting a high potential for both intra- and extrageneric competition. The abundance and nomad- ism of the six bird species that consumed most Psychotria fruit peaked when the Psychotria fruited, supporting the enhancement hypothesis.

Seed arrival in tropical forest tree fall gaps

Tree deaths open gaps in closed-canopy forests, which allow light to reach the forest floor and promote seed germination and seedling establishment. Gap dependence of regeneration is an important axis of life history variation among forest plant species, and many studies have evaluated how plant species differ in seedling and sapling performance in gaps. However, relatively little is known about how seed arrival in gaps compares with seed arrival in the understory, even though seed dispersal by wind and animals is expected to be altered in gaps. We documented seed arrival for the first seven years after gap formation in the moist tropical forests of Barro Colorado Island (BCI), Panama, and evaluated how the amount and functional composition of arriving seeds compared with understory sites. On average, in the first three years after gap formation, 72% fewer seeds arrived in gaps than in the understory (207 vs. 740 seeds x m(-2) x yr(-1)). The reduction in number of arriving seeds fell disproportionately on animal-dispersed species, which suffered an 86% reduction in total seed number, while wind-dispersed species experienced only a 47% reduction, and explosively dispersed species showed increased seed numbers arriving. The increase in explosively dispersed seeds consisted entirely of the seeds of several shrub species, a result consistent with greater in situ seed production by explosively dispersed shrubs that survived gap formation or recruited immediately thereafter. Lianas did relatively better in seed arrival into gaps than did trees, suffering less of a reduction in seed arrival compared with understory sites. This result could in large part be explained by the greater predominance of wind dispersal among lianas: there were no significant differences between lianas and trees when controlling for dispersal syndromes. Our results show that seed arrival in gaps is very different from seed arrival in the understory in both total seeds arriving and functional composition. Differential seed arrival in gaps will help to maintain wind-dispersed, explosively dispersed, and possibly other understory species in the community of plants that regenerate in gaps.

Resource acquisition and reproductive strategies of tropical forest in response to the El Niño–Southern Oscillation

The El Niño–Southern Oscillation (ENSO) is the largest source of interannual climate variability in much of the tropics. We hypothesize that tropical plants exhibit interannual variation in reproduction and resource acquisition strategies driven by ENSO that mirrors their seasonal responses. We analyze the relationship of leaf and seed fall to climate variation over 30 years in a seasonally dry tropical forest in Panama where El Niño brings warm, dry, and sunny conditions. Elevated leaf fall precedes the onset of El Niño, and elevated seed production follows, paralleling associations with dry seasons. Our results provide evidence of a shift in allocation from leafing to fruiting in response to a warming phase of ENSO. This shift may enable plants to take advantage of higher light availability, while coping with higher atmospheric water demand and lower water supply. These findings might be an indicator of adaptive strategies to optimize reproduction and resource acquisition.It has been suggested that tree phenology may be regulated by climatic oscillations. Here, Detto et al. present a 30 year tropical forest dataset that suggests leaf and fruit production is coordinated with ENSO cycles, with greater leaf fall observed prior to El Niño followed by greater seed production.

Long-term increases in tropical flowering activity across growth forms in response to rising CO2 and climate change

Mounting evidence suggests that anthropogenic global change is altering plant species composition in tropical forests. Fewer studies, however, have focused on long-term trends in reproductive activity, in part because of the lack of data from tropical sites. Here, we analyze a 28-year record of tropical flower phenology in response to anthropogenic climate and atmospheric change. We show that a multidecadal increase in flower activity is most strongly associated with rising atmospheric CO2 concentrations using yearly aggregated data. Compared to significant climatic factors, CO2 had on average an approximately three-, four-, or fivefold stronger effect than rainfall, solar radiation, and the Multivariate ENSO Index, respectively. Peaks in flower activity were associated with greater solar radiation and lower rainfall during El Niño years. The effect of atmospheric CO2 on flowering has diminished over the most recent decade for lianas and canopy trees, whereas flowering of midstory trees and shrub species continued to increase with rising CO2 . Increases in flowering were accompanied by a lengthening of flowering duration for canopy and midstory trees. Understory treelets did not show increases in flowering but did show increases in duration. Given that atmospheric CO2 will likely continue to climb over the next century, a long-term increase in flowering activity may persist in some growth forms until checked by nutrient limitation or by climate change through rising temperatures, increasing drought frequency and/or increasing cloudiness and reduced insolation.