This study tested the hypotheses that seed dispersal enhances seedling survival by (1) escape of distance—dependent or density—dependent mortality or both, and (2) colonization of light—gaps. Spatial patterns of seedling survival for 1 yr in shade and in light—gaps and causes of seedling mortality for the first 2 mo were determined for one tree of each of nine species that use wind dispersal on Barro Colorado Island, Panama. Dispersal was advantageous to all nine trees, but for different reasons. The colonization hypothesis was supported by all nine trees. At some time interval during the 1st yr, the escape hypothesis was supported by eight of the nine trees; dispersal away from the parent to shaded conditions lowered the probability of seedling mortality. However, the trees differed greatly in (1) when the dispersal advantage became apparent, (2) whether the advantages persisted through time, (3) the distance at which survival was most enhanced, and (4) the strength of the advantage. By 1 yr only four trees showed a moderate to strong advantage of dispersal to shaded areas; two trees showed a very weak advantage, and three trees showed no advantage. Pathogens caused the largest proportion of deaths among shaded seedlings in six of the nine trees. For the two trees with the strongest support for the escape hypothesis, pathogens caused distance— and/or density—dependent mortality; those trees had high seedling densities near the parent. Four other trees had moderate to high mortality by pathogens, irrespective of dispersal distance; those trees had low seedling densities at all distances from the parent. For these six trees in which pathogens were responsible for the largest proportion of deaths in the shade, this mortality was significantly reduced in light—gaps, even though seedlings occurred in high densities. The study indicates that the location where seedlings survive is greatly influenced by pathogen activity. Only the four trees with moderate to strong support for the escape hypothesis had large numbers of seedlings survive to 1 yr in the shade. Seedling recruitment is expected to occur away from the parent tree, in shaded conditions for trees with distance— and/or density—dependent mortality, and in light—gaps for all trees studied here.

We present results of two experiments designed to identify the relative importance of dispersal distance, seedling density, and light conditions on pathogen-caused mortality of tropical tree seedlings. The field experiment on Barro Colorado Island, Panama, demonstrated that both an increase in dispersal distance and a decrease in seedling density reduce levels of damping-off disease among seedlings of Platypodium elegans, and that there is an interaction between the two factors. The results indicated significant variation among sites in pathogen activity and suggested that seedlings are more vulnerable to disease when establishing around their parent tree than around other conspecific trees.The second experiment in a screened enclosure used potted seedlings of 18 wind-dispersed tree species exposed to two levels of sunlight and seedling density. The results indicated that environmental conditions similar to those in light-gaps significantly reduce pathogen activity. They also confirmed that high seedling density increases disease levels, especially under shaded conditions.Seedlings of 16 of the 18 species experienced pathogencaused mortality, but in widely varying amounts. Seed weight was not a good predictor of a species' vulnerability to pathogens. Adult wood density, an indicator of growth rate and successional status, was inversely correlated with a species' vulnerability to pathogens. Fast-growing, colonizing species, whose seedlings require light-gaps, lacked strong resistance to seedling pathogens, relative to slow-growing species able to tolerate shade and escape seedling pathogens. We discuss these results in the context of seed dispersal as a means of escaping from seedling pathogens.

This study compares reproductive success of individual plants flowering and fruiting in and out of synchrony with the population. I test the hypotheses that reproductive synchrony enhances a plant's ability to: (I) attract pollinators, and (2) avoid seed predators. The prediction is that an individual in synchrony with its population has higher fruit and ovule set after pollination and has more seeds at dispersal time than an individual out of synchrony.The test species is Hybanthus prunifolius (Schult.) Schulze (Violaceae), a shrub on Barro Colorado Island, Panama. It normally flowers in response to a heavy rain that interrupts a drought in the dry season. An individual produces most of its flowers on a single day. The flowers are self-compatible, but require animal vectors to effect any pollination. To test hypotheses (I) and (2) I used water sprinklers to induce individual plants to Hower prior to the normal triggering rain, thereby creating a highly asynchronous population which flowered before the natural population. Peak flowering day for individuals in the experimental asynchronous population spanned 35 d; the natural population spanned 4 d. Measurements included pollination success (ovule and fruit set) due primarily to the social bee Melipona interrupta, occupation of fruits by microlepidopteran larvae (Cosmoptericidae) and dipteran larvae (Lonchaeidae, Silba sp.), and final reproductive output (number of mature fruits and seeds).Individuals in all size categories in the natural synchronous population matured a greater number of seeds than individuals in the asynchronous population. Mean number of mature seeds per individual was 658 in the synchronous population; it was 62 mature seeds per individual in the asynchronous population.The large difference in seed output occurred primarily because individuals in the synchronous population had greater pollination success than individuals in the asynchronous population (86% vs. 58% fruit set; 78% vs. 40% ovule set). In contrast, fruit infestation by microlepidopteran larvae was greater among individuals in the asynchronous population than in the synchronous population (II% vs. 5%). The combined effects of pollinators and seed predators were thus additive and produced intense selection against temporally isolated individuals.Similarly low seed output was occasionally observed for the few nonexperimental plants that flowered in response to a light rain during the dry season. Such individuals had lower levels of pollination and mature fruit production than plants that flowered when all others in the entire forest were in Hower. The effect of space was similar to that of time. During the natural synchronous flowering period, individuals in sites of low spatial density attracted fewer pollinators and incurred more predation than individuals in sites with high density.An evolutionary interpretation of these results is that stabilizing selection by both pollinators and seed predators maintains the present low variance in the timing of flowering and fruiting within Hyb...

Climate change, with both warmer spring temperatures and greater temperature fluctuations, has altered phenologies, possibly leading to greater risk of spring frost damage to temperate deciduous woody plants. Phenological observations of 20 woody species from 1993 to 2012 in Trelease Woods, Champaign County, Illinois, USA, were used to identify years with frost damage to vegetative and reproductive phases. Local temperature records were used in combination with the phenological observations to determine what combinations of the two were associated with damage. Finally, a long-term temperature record (1889-1992) was evaluated to determine if the frequency of frost damage has risen in recent decades. Frost < or = -1.7 degrees C occurred after bud-break in 14 of the 20 years of observation. Frost damage occurred in five years in the interior and in three additional years at only the forest edge. The degree of damage varied with species, life stage, tissue (vegetative or reproductive), and phenological phase. Common features associated with the occurrence of damage to interior plants were (1) a period of unusual warm temperatures in March, followed by (2) a frost event in April with a minimum temperature < or = -6.1 degrees C with (3) a period of 16-33 days between the extremes. In the long-term record, 10 of 124 years met these conditions, but the yearly probability of frost damage increased significantly, from 0.03 during 1889-1979 to 0.21 during 1980-2012. When the criteria were "softened" to < or = -1.7 degrees C in April and an interval of 16-37 days, 31 of 124 years met the conditions, and the yearly damage probability increased significantly to 0.19 for 1889-1979 and 0.42 for 1980-2012. In this forest, the combination of warming trends and temperature variability (extremes) associated with climate change is having ecologically important effects, making previously rare frost damage events more common.

The presence of leaf litter of different depths within a tropical forest creates many different microsites for plant establishment. The amount and distribution of leaf litter within a forest can influence patterns of plant establishment. In this study, we determined the spatial variability in leaf litter in the forest understory, and investigated how different litter depths (bare, 1, 6, and 12 cm) affected the establishment of several tropical tree species in both growth house (sun and shade) and field (gap and understory) experiments in the semideciduous tropical forest of Barro Colorado Island, Panama. The tree species used in this study (Aspidospermum cruenta, Ceiba pentandra, Cordia alliodora, Gustavia Superba, Luehea semmannii, Ochroma pyrimidale) were chosen to represent a range of seed masses and a gradient in the light requirement for establishment of the species. The spatial distribution of leaf litter was not correlated between adjacent sampling points within the forest understory, suggesting that the establishment environment for seedlings, with respect to litter, is highly variable at scales of 1—20 m. The presence of litter affected five of the six species, but the nature and the magnitude of the effect were species specific. The smaller seeded shade—intolerant species had fewer seedlings establishing under leaf litter than on bare ground. The species ranged from strongly nagatively affected (Luehae) to moderately negatively affected (Cordia, Ochroma) to affected only by extreme amounts of litter (Ceiba). The presence of litter influenced Gustavia, one of the larger seeded shade—tolerant species, but did not affect Aspidospermum, the other large seeded species. The effect of litter on Gustavia depended on the light environment. Gustavia had more seedlings establishing under litter in the sun, but the presence of litter had no effect in the shade. Differences among the smaller seeded shade—intolerant species in the amount they were negatively influenced by litter were not correlated with seed mass. Data from our field study were consistent with our growth house results for the shade—intolerant species. Additional data from the field study indicated that these species with similar habitat requirements differed in the developmental stage at which they were affected by the presence of litter. Luehea had fewer seeds germinating under litter while the other two species, Ochroma and Cordia, were affected only after germination. Interspecific comparisons done for each light level and litter depth indicated that the presence of litter caused reversals in the relative ranking of species success. For example, Gustavia preferentially established under relatively deep litter depths in the sun where Luehea could not establish. In conclusion, the presence of litter can potentially increase seedling diversity within the forest by creating heterogeneity in the establishment environment and by causing reversals in species' rankings.