Cogongrass is a highly invasive perennial grass that threatens agriculture, forestry, and native plant assemblages in many regions of the world. Cogongrass is a prolific seed producer, but the ecological importance of seeds as a vector of invasion has not been adequately addressed. Propagule pressure plays a key role in many successful invasions and the relative importance of propagule pressure across environmental gradients is an area of research that is quickly becoming important in understanding invasion success. The present study was conducted to test the effects of varying propagule density upon the ability of cogongrass seedlings to grow and establish across experimentally manipulated light and soil gradients. Seedlings growing in high-nutrient soil performed the best regardless of the amount of available light, but overall biomass was always greatest in high-light environments. Cogongrass showed a very strong trend in biomass allocation, with seedlings in reduced light environments partitioning more biomass to aboveground tissue. The data provided no evidence that initial propagule density affected the growth and establishment of cogongrass seedlings, but growth and biomass production were positively correlated with final seedling density.

Roads and streams are important conduits for the spread of invasive species, and croftonweed is an invasive plant in southwest China. We established six transects along one road and stream in Huili County, Sichuan Province to determine the pattern of invasion and identify major factors potentially influencing it. Results demonstrated that the invasion (cover, abundance, and number of clusters) of croftonweed declined significantly with distance from the road and stream. However, after compensation for environmental factors, only distance from the road was significantly negatively related to number of clusters of this plant. Instead, native plant species cover and richness were significantly negatively associated with croftonweed invasion along the road; elevation and native species cover were negatively correlated with the invasion along the stream. These findings indicated that roads and streams were main conduits for the spread of croftonweed in southwest China, and reduced native species cover could facilitate invasion by croftonweed in habitats along roads and streams.

The relative effects of irradiance and spectral composition on the colonization and development of macroalgal communities were examined in four streams exposed to full sunlight in southern Brazil. A set of solar filters surrounded the artificial substrata to provide various quantitative and qualitative light conditions. In addition to the control, the conditions were full sunlight attenuated by 50, 70 and 90%, blue light (filter transmitting 430–500 nm), green light (500–600 nm) and red light (>600 nm). Macroalgal percent cover was determined three times during the colonization period (15, 39 and 70 days). Significant differences occurred in the mean values for macroalgal community percent cover between treatments at 15 and 39 days, suggesting that early and intermediate stages of the colonization were directly influenced by irradiance. There was an almost complete absence of macroalgae with 90% attentuation, but progressive increases with 70 and 50% attenuation. However, the mean values for macroalgal community percent cover did not differ significantly between color treatments, suggesting that the effects of irradiance on macroalgal colonization were stronger than those of spectral composition at similar intensity. The quantitative responses differed among the taxa.

Vegetation and environment change mutually during secondary succession, yet the idiosyncrasies of the vegetation effect on the understorey environment are poorly understood. To test whether the successional understorey environment changes predictably and is shaped by the structure and seasonality of tropical dry forests, we estimated basal area and vegetation cover, and measured understorey temperature, light and moisture conditions, in 17 plots forming a 60-y chronosequence and a mature forest. Light and air and soil temperature decreased with time (75−15% of open-sky radiation, 31.7−29.3 °C, and +2.5 °C to −0.5 °C relative to ambient, respectively), whereas relative humidity increased (67−74%). Soil water availability increased with early-successional development (−45 to −1 kPa) but decreased afterwards (to −18 kPa). The first axis of a PCA of the rainy-season environment explained 60% of the variation and was strongly related to air temperature and relative humidity. During tropical dry-forest succession, such factors may be more important than light, the reduction in which is not extreme compared with taller and more vertically stratified wet forests. Seasonality significantly affected the successional environmental gradients, which were marked mainly during the wet season. Environmental heterogeneity was higher in the wet than in the dry season, and larger for resources (light and water) than for conditions (temperature and humidity). The wet-season increase in environmental heterogeneity potentially creates differential growing scenarios; the environmental harshness of the dry season would mostly challenge seedling survival.

Based on findings in vascular plants showing that the capacity to provide enough carbon skeletons for rapid ammonium assimilation into amino acids is a prerequisite for tolerance to eutrophication, we tested the hypothesis that lichens from shady habitats are particularly sensitive to nitrogen pollution. We tested this hypothesis using published ecological indicator values (estimates of eutrophication tolerance and light preferences on an ordinal scale) for more than 500 central European lichen species. Our results show that shade-adapted lichens are indeed at the same time intolerant to eutrophication. However, not all eutrophication-sensitive lichens inhabit shady environments, suggesting the existence of several independent mechanisms causing intolerance of high nitrogen levels in lichens. The correlation of shade adaptation with nitrogen intolerance is limited to epiphytic and saxicolous species, since terricolous lichens are out-competed by vascular plants in dense vegetation. Our results suggest that lichen communities of shady bark, wood and rock are particularly sensitive to eutrophication.

The effect of canopy species dominance on understorey light availability in secondary forests was examined for Pentaclethra macroloba and Goethalsia meiantha, two common tree species with contrasting canopy architecture at La Selva Biological Station in the Atlantic lowlands of Costa Rica. In each of six 12- to 16-year-old successional stands dominated by one of these species, relative abundance and basal area of the focal species were determined within a 20 m × 50 m plot. Light availability at 1 m height was measured within each plot using a Licor-2000 Plant Canopy Analyzer at 27 locations and by analysing hemispherical photographs taken at 10 locations. Across stands, mean LAI values were significantly negatively correlated with canopy openness. Understorey microsites beneath Pentaclethra had significantly higher light availability than microsites beneath Goethalsia. Across stands, however, light availability was not correlated with either total basal area or stem density of trees ≥10 cm DBH. These results indicate that basal area and stem density of trees in the upper levels of the forest canopy are poor predictors of light penetration to the understorey. One hypothesis suggested here is that dense subcanopy and understorey vegetation in tropical secondary forests can strongly influence understorey light levels. Alternatively, light transmittance characteristics of the upper forest canopy may be influenced by species-specific differences in canopy architecture or foliage density that vary independently from tree diameter and density.

Quantitative analyses of the factors driving the distribution of bryophytes in the phyllosphere of tropical rain forests are rare. In this study we sampled epiphyllous bryophytes on two leaves of each of 57 individuals belonging to four phorophyte species (Asplundia pittieri, Carludovica drudei, Costus laevis and Dieffenbachia concinna) at three adjacent, but microclimatically varied sites in the Esquinas forest, Costa Rica. Microclimatic parameters and phorophyte identities were correlated to differences in cover, diversity and species composition of bryophyte assemblages by means of Kruskal–Wallis tests, non-metric multidimensional scaling and indicator species analysis. High relative humidity and in particular daily fluctuations in relative humidity proved to be the most important factors for epiphyll distribution suggesting severe constraints of epiphyll colonisation by pronounced humidity fluctuations. Differences in air temperature and light availability as well as phorophyte identity were only weakly correlated with bryophyte cover and diversity. However, species composition of epiphyllous assemblages was related to all microclimatic variables as well as to phorophyte identity. The strong response of epiphyllous bryophytes to even subtle microclimatic variations suggests that undisturbed forest canopies and their control on microclimate may be essential for the development of epiphyll communities.

We examined effects of experimental manipulations of vegetation cover on recruitment, mortality and density of seedlings (20–100 cm tall) and saplings (≥100 cm tall) of woody growth forms over a 2.5-y period. We created four treatments in each of three 15–20-y-old tropical forest stands in Costa Rica: a large canopy gap (270–350 m2), a small canopy gap (50–100 m2), understorey vegetation removal, and an unmanipulated control treatment. Creation of canopy gaps, especially large ones, increased first-year recruitment and density, as well as overall mortality of seedlings. Saplings experienced lower mortality and more prolonged gap-enhanced recruitment and density than seedlings. Removal of understorey vegetation had little or no effect on tree seedling and sapling dynamics. Recruitment and density of lianas responded only to large gaps, whereas understorey species responded to both gap treatments and to spatial heterogeneity within gaps. Tree species exhibited diverse regeneration requirements, whereas liana and understorey species were more specialized to the high and low ends of the light availability gradient, respectively. Canopy gaps provide a critical mechanism for regeneration of lianas, and canopy tree species that dominate during the early stages of secondary forest succession. The choice of management system for these secondary forests can determine the direction and rate of succession.

The dependence of tree seedling survival and growth on light availability was evaluated over 15 mo in three second-growth, wet forest stands (15–18 y old) in north-eastern Costa Rica. Seedlings of four canopy tree species (Dipteryx panamensis, Hyeronima alchorneoides, Virola koschnyi and Vochysia guatemalensis) were planted into four canopy treatments in three replicated stands and in a pasture site, spanning a nearly complete gradient of light availability. Survival and growth of all species increased in response to increasing light availability, but species differed in survival in shaded microsites (6–20% light transmittance) and in growth increments per light increase. Hyeronima showed the highest mortality at low light levels and the highest relative height growth increase per light increase. In contrast, Virola showed high survival at low and moderate light, but showed the lowest leaf area and above-ground biomass per light increase among all species. Dipteryx and Vochysia maintained relatively high rates of survival and growth across the entire light gradient. Hyeronima and Virola showed trade-offs between growth and survival responses to light, unlike Dipteryx and Vochysia. Differences among species in seedling survival at low light may be determined by a variety of physiological and morphological traits that may or may not be mechanistically linked to growth responses.

Plants often modify microsite conditions important for seedling establishment. In tropical moist forest, advance regeneration in the form of shade-suppressed seedlings is a major component of regrowth in new gaps. Tree seedlings may be filtered by the composition and structure of the forest understorey. In a lowland forest in Central Panama, we examined light availability, litter accumulation and the seedling community (abundance, diversity and composition) under and away from the canopies of a common subcanopy, clonal palm, Oenocarpus mapora, and tested whether seedling abundance varies as a function of changes in understorey light conditions and leaf litter depth. We found evidence that leaf litter was five times deeper and light availability 27% lower under O. mapora canopy than where it was absent. The probability of finding a seedling in plots under O. mapora canopies was 33% lower than in plots without the palm. Plots under O. mapora canopies also had 49% fewer species of seedlings and those seedlings came from significantly larger seeds than seedlings in plots not under the palm. Overall, seedling density was positively correlated with light availability and negatively correlated with leaf litter depth. A transplant experiment confirmed that for at least one large-seeded species of dicot, seedlings are negatively affected by the reduction in light availability and increase in leaf litter. Under O. mapora, seedlings of Gustavia superba (Lecythidaceae) had lower survivorship and biomass accumulation than away from the palm. Thus, site conditions produced by large, clonal, multi-layered palms such as O. mapora may reduce establishment of small-seeded and shade-intolerant species, potentially affecting species composition in the understorey seedling pool.

Variation in herbivory is often associated with plant density and light environment. The effect of these variables was studied on leaf production and herbivory of Manilkara bidentata in Puerto Rico. The major herbivore of M. bidentata is the leaf miner Acrocercops sp. To determine the effect of plant density, twenty-four 20-m × 20-m plots were established and the densities of saplings, juveniles and adults were determined. Leaf production, herbivory and growth were measured on saplings. Plant density was determined in eight 20-m × 20-m plots surrounding the focal plots. The effect of light was determined by comparing leaf phenology, leaf quality and herbivory in vertical and horizontal profiles. Sapling density in 60-m × 60-m plots was associated with herbivory. In the vertical profile, leaf production was continuous only for the canopy: herbivory increased from the canopy (1.3%) towards the understorey (35.6%). In the horizontal profile leaf production was related to light. Saplings in low light environments had narrow peaks in leaf production compared with saplings in high light environments. Differences in leaf phenology did not result in differences in herbivory possibly because of variation in herbivory among leaves. Although many saplings lost more than 80% of new leaf area, there was no detectable effect on growth. High levels of intra-specific variation in herbivory suggest that to better understand plant/herbivore interactions it is necessary to consider all size classes in a population. It is also important to document these pattern across different spatial scales.

New leaf production in seasonal tropical forests may result from changes in water or light availability. In this study, the relationship between leaf flushing, photosynthetically active radiation (PAR) and rainfall was examined for understorey saplings in a moist tropical forest over one year. During the wet season, weeks with greater PAR were correlated with a greater proportion of saplings flushing new leaves during subsequent weeks in nine out of ten species. Rainfall was negatively correlated with subsequent leafing during the wet season for six of ten species. However, during the dry season, rainfall was positively correlated with leafing during the following weeks for six species, but the relationship was much weaker. PAR in the dry season was negatively correlated with flushing in eight species. These results support the hypothesis that under well-watered conditions, light limits leaf production, and peaks in insolation result in greater leaf production.

The effect of natural canopy gaps on the performance of naturally occurring tree seedling populations was studied in a tropical rain forest in French Guiana. This was done at two levels of scale. Firstly, on a 20 m × 250 m forest transect intersecting four canopy gaps, it was investigated how patterns of recruitment, growth and survival of seedlings of Cecropia obtusa, Dicorynia guianensis and Pourouma bicolor differed between canopy gaps and closed forest. Secondly, for one large natural canopy gap, performance of seedling cohorts established before (‘pre-gap’ cohorts) and after (‘post-gap’ cohorts) gap formation was studied in relation to environmental heterogeneity. The direct site factor (DSF) was used as an indicator of light availability in the gap zone. Cecropia specialised in large gaps, and also in specific sites within the large gap: seedling performance was increased by light, and by dead wood. Pourouma mainly germinated under closed forest conditions, where it can survive for a long period until a canopy gap is formed nearby. Pourouma seedlings adjusted well to the new gap environment but they did not specialise in specific places within the gap. Seedling location of Pourouma seedlings was mainly determined by accidental positioning before gap formation. Dicorynia was able to germinate both in gaps and under closed forest conditions. Although the number of gaps studied in this study was low, the results show that seedling establishment, early growth and survival of the three tree species were affected by canopy gaps and by the environmental heterogeneity within a large gap.