The responses to shade of seedlings of very small-seeded tree and shrub species from tropical rain forest in Singapore

1. Newly germinated seedlings of six tree and shrub species with very small seeds (31–460 μg dry mass), one light-demanding and five shade-tolerant at the stage of establishment in the wild, were grown for 5 months in neutral shade houses with 0·5, 1, 3·5 and 7·5% daylight.
2. The ratio of yield in 7·5% to that in 1% was 8:1 for the light-demanding Melastoma malabathricum but only 2:1 for the confamilial shade-tolerator Pternandra echinata. The Pternandra, Urophyllum hirsutum, Ficus chartacea, Ficus grossularioides and Pellacalyx saccardianus showed a graded series of responses to irradiance, generally consistent with their apparent demands for light in the wild. In contrast, survival of very deep shade was not clearly related to light demand in nature.
3. The results support the conclusion drawn from observational studies that large seed size is not primarily adaptive in resisting shade but in resisting the associated risks of burial by litter, desiccation during dry spells, uprooting by birds and other kinds of damage by animals or falling debris.     

The effect of seed mass and gap size on seed fate of tropical rain forest tree species in guyana

Abstract: For eleven tree species, differing in seed mass, germination success (emergence success for two small-seeded species) and the causes of failure to germinate were studied in the forest understorey and in logging gaps in the tropical rain forests of Guyana. In the forest understorey, germination success increased with seed mass. However, as gap size increased the difference between smaller and larger seeded species diminished because germination success of smaller-seeded species increased slightly, while that of larger-seeded species decreased dramatically. The negative effect of gap size on germination success of larger-seeded species was caused by an increased risk of desiccation with gap size, which was a far more important seed mortality agent for larger than for smaller-seeded species. Generally, seeds of smaller-seeded species suffered more from insect predation and were removed at higher rates than larger-seeded species. On the other hand, larger-seeded species were eaten more by mammals than smaller-seeded species. It is concluded that logging can result in shifts in the species composition in the tropical rain forests of Guyana which are dominated by species with large seeds, since germination success of larger-seeded species is dramatically reduced in large logging gaps.     

Ecology of seed germination of eight non-pioneer tree species from a tropical seasonal rain forest in southwest China

We compared various aspects of the seed biology of eight non-pioneer tree species from a tropical seasonal rain forest in Xishuangbanna, SW China, that differ in time of dispersal, size and fresh seed moisture content (MC). Seeds were tested for germination under laboratory conditions after dehydration to different moisture levels and under 3.5, 10 and 30% solar irradiances in neutral-shade houses. For six species, germination was also compared in forest understory (3.5% light) and center of a forest gap (32.5% light). Under continuous dehydration over activated silica gel, 100% of seeds of four species had lost the ability to germinate after 48 h, and those of all species except Castanopsis hystrix (decreased from >90 to 30% germination) had lost the ability to germinate after 120 h. Four species did not differ in final germination percentages at the three irradiances (i.e. uniform germination). However, final germination percentages of Horsfieldia pandurifolia and Litsea pierrei var. szemaois were significantly lower in 30% than in 10 or 3.5% light, and seeds of Antiaris toxicaria and C. hystrix germinated to higher percentages in 30 and 10% than in 3.5% light. Mean time to germination (MTG) of the eight species (forest and shade house data combined) ranged from 5–5 days for Pometia tomentosa to 72–207days for L. pierrei; MTG for four species was ≤21 days. There was no obvious relationship between relative desiccation resistance and either time of dispersal, MTG or uniformity of germination at the three light levels, or between seed size and MC or MTG. However, the relationship between seed MC at maturity (25–60% fresh mass basis) and MC at 50% loss of seed viability (12.4–42.5%) was significant. Seven of the species fit Garwood’s (Ecol Monogr 53:159–181, 1983) rapid-rainy germination syndrome and one, L. pierrei, either her delayed-rainy or intermediate-dry germination syndrome. However, fresh, non-dehydrated seeds of all eight species germinated in ≤30 days at constant 30°C in light.     

Why tropical forest lizards are vulnerable to climate warming

Biological impacts of climate warming are predicted to increase with latitude, paralleling increases in warming. However, the magnitude of impacts depends not only on the degree of warming but also on the number of species at risk, their physiological sensitivity to warming and their options for behavioural and physiological compensation. Lizards are useful for evaluating risks of warming because their thermal biology is well studied. We conducted macrophysiological analyses of diurnal lizards from diverse latitudes plus focal species analyses of Puerto Rican Anolis and Sphaerodactyus. Although tropical lowland lizards live in environments that are warm all year, macrophysiological analyses indicate that some tropical lineages (thermoconformers that live in forests) are active at low body temperature and are intolerant of warm temperatures. Focal species analyses show that some tropical forest lizards were already experiencing stressful body temperatures in summer when studied several decades ago. Simulations suggest that warming will not only further depress their physiological performance in summer, but will also enable warm-adapted, open-habitat competitors and predators to invade forests. Forest lizards are key components of tropical ecosystems, but appear vulnerable to the cascading physiological and ecological effects of climate warming, even though rates of tropical warming may be relatively low.     

Changes in tree phenology along natural regeneration in a seasonally dry tropical forest

The applicability of succession models from temperate and tropical wet forests to threatened seasonally dry tropical forests (SDTFs) is questioned. Plant phenology affects ecosystem functions and changes along forest regeneration gradient. To investigate the recovery of ecological functions after disturbances in a SDTF, we recorded the vegetative and reproductive phenologies for trees (DBH >5 cm) for 17 months in southeast Brazil in three successional stages: early (10–15 years after clearing), intermediate (25–30) and late (>50). The vegetative phenology of the 523 individuals was strongly seasonal, with 3% of individuals presenting green leaves in a deciduous dry season. Besides structural and floristic differences, phenological trends were similar between the later stages. Reproduction occurred with higher intensities in the early stage and in the advanced stages only in the dry season, providing key resources to local fauna. The studied SDTF is resilient to ecological functions, rapidly recovering functional processes. The integration of structural and functional knowledge of succession of STDFs may lead to better management of its secondary remnants. Our study suggests that classical forest succession theory developed for other ecosystems may not fully reflect the pattern of SDTF succession, an ecosystem that originally covered 42% of the earth's tropical and subtropical landmass.     

西双版纳热带季节雨林四种珍稀濒危树种种子萌发对脱水和光的响应

绒毛番龙眼(Pometia tomentosa)、琴叶风吹楠(Horsfieldia pandurifolia)、箭毒木(Antiaris toxicaria)和思茅木姜子(Litseapierrei var. szemois)是西双版纳热带季节雨林乔木层的优势树种,也属于珍稀濒危物种。实验研究了4个树种的种子在经过不同脱水水平、人工遮荫条件下的不同光照水平以及热带季节雨林之林下和林窗的萌发特性。结果显示:脱水对于种子萌发特征具有显著影响,绒毛番龙眼和琴叶风吹楠经48h的连续脱水后丧失了萌发能力;箭毒木和思茅木姜子具较强的脱水耐性,经96h的连续脱水后仍具有较高的种子含水量,且种子萌发率分别为15%和33%。干旱季散布的种子比雨季散布的种子具有更强的脱水耐性。遮荫及森林种子萌发试验中,除绒毛番龙眼外,其它3个树种的种子萌发率以及平均萌发周期在不同光水平或生境下差异显著。琴叶风吹楠和思茅木姜子在30%的光照以及林窗中心的萌发率要显著低于低光处理(10%和3.5%光照)和林下,而箭毒木在30%和10%光水平以及林窗中心获得较高萌发率。雨季传播的具有干燥敏感性的种子,可以减少种子由于干燥而引起的死亡。试验结果可以为4种珍稀濒危植物的保护和人工繁育提供一定理论依据。     

Life-history traits are correlated with geographical distribution patterns of western European forest herb species

Aim To investigate whether six plant life‐history traits that have been related to colonization ability at local scales are also related to the geographical range characteristics of 273 forest plant species. Location Continental western Europe, five countries in particular: France, Luxemburg, Belgium, the Netherlands and Germany. The region is situated between 42° and 55°N and 5°W and 15°E and has a summed total area of 971,404 km². Methods Distribution data were compiled from five national data bases and converted to a 10′ grid. Life‐history traits were taken from existing compilations of autecological information of European species. The spatial arrangement of occupied grid cells was investigated using Ripley's K. Cross‐species correlations and phylogenetically independent contrasts were used to investigate the relationships between plant life‐history traits and three range characteristics: area of occupancy, latitudinal extent and centroid latitude. Results For herbaceous species, seed dispersal mode, seed production and seed bank longevity exhibited significant associations with geographical range characteristics, including area of occupancy. Woody plant species exhibited fewer significant associations, although maximum height was positively associated with range centroid latitude within the study area. Furthermore, the ranges of species with limited dispersal ability were found to be more clustered than the ranges of species with morphological adaptations for long‐distance seed dispersal. Main conclusions For western European forest plant species, life‐history traits that are related to colonization ability at local scales are associated with variation in large‐scale geographical range characteristics. This finding implies that the distributions of some forest plant species in the study area may be limited by seed dispersal and colonization capacity rather than climate or other environmental factors.     

Postdispersal survivorship of north Queensland rainforest seeds and fruits: Effects of forest,habitat and species

Abstract Diaspore (seed and/or fruit) survivorship of 12 species of North Queensland rainforest trees was monitored over a 4 week period in two rainforests on the Atherton Tableland, north Queensland, Australia, with replicated sites in forest interiors, canopy gaps, forest edges and adjacent pastures. General linear modelling was used to estimate the relative importance of forest site, habitat, species and diaspore type on survival. The probability of survival varied significantly among species, between habitat, forest and diaspore types. Survival averaged 41.2% at day 28, was greater in one forest (Curtain Fig, 56.1%) than the other (Lamins Hill, 25.3%) and was apparently inversely related to the abundance of rodents. In both forests, survival declined in the order: pasture ≥ forest edge ≥ forest interior = canopy gap. A lack of significant difference between the forest interior and canopy gap is consistent with findings in other rainforest sites within Australia, but contrasts with most results from other continents. The study also indicated that diaspore weight was an important intrinsic variable affecting survival, the predators (mostly rodents) taking small to moderately large diaspores rather than the very large ones.     

Light environment,sapling architecture,and leaf display in six rain forest tree species

Architecture and leaf display were compared in saplings of six rain forest tree species differing in shade tolerance. Saplings were selected along the whole light range encountered in a forest environment. Species differed largely in realized height and crown expansion per unit support biomass, but this could not be related to differences in shade tolerance. The results demonstrate that there exist various solutions to an effective expansion of plant height and crown area. It is argued that choice of the study species and the ontogenetic trajectory regarded determine to a large extent the outcome of interspecific comparisons. No evidence was found that pioneers were characterized by a multilayered and shade tolerants by a monolayered leaf distribution. Yet, sun plants had a similar crown area, a deeper crown, and a higher leaf area index compared to shade plants and their leaves were more evenly distributed along the stem. This suggests that differences in leaf layering are found between plants growing in different light environments, rather than between species differing in shade tolerance.     

Conflicting selection pressures on seed size: evolutionary ecology of fruit size in a bird-dispersed tree, Olea europaea

Recent evidence indicates that fruit size has evolved according to dispersers' size. This is hypothesized to result from a balance between factors favouring large seeds and dispersers setting the maximum fruit size. This hypothesis assumes that (1) the size of fruits that can be consumed by dispersers is limited, (2) fruit and seed size are positively correlated, and (3) the result of multiple selection pressures on seed size is positive. Our studies on the seed dispersal mutualism of Olea europaea have supported the first and second assumptions, but valid tests of the third assumption are still lacking. Here we confirm the third assumption. Using multiplicative fitness components, we show that conflicting selection pressures on seed size during and after dispersal reverse the negative pattern of selection exerted by dispersers.     

Photosynthetic acclimation to light in juveniles of two cloud forest tree species

The photosynthetic response of juveniles of Decussocarpus rospigliosii, an emergent primary forest species and shade tolerant in its juvenile stages and Alchornea triplinervia, a gap-colonizing species of tropical cloud forest in Venezuela was studied. Daily courses of microenvironmental variables and gas exchange under contrasting light conditions (gap and understory) were carried out in their natural environment and transplanted to different light regimes (shade and sun) in the field. The photosynthetic response and some anatomical characteristics of plants from different treatments were analyzed in the laboratory. Photosynthetic rates were low for both species, and were negative during some diurnal periods, related to the low photosynthetically active radiation levels obtained at both gap (6% of total radiation) and understory (2%). A. triplinervia shows higher rates (1.5–3.0 molm^{-2 -1}) than D. rospigliosii (0.7–1.1 molm^-2s^-1). Both species showed increased photosynthetic rates when grown in gaps. A. triplinervia did not adjust its maximum photosynthetic rates to the prevailing light conditions. In contrast, D. rospigliosii responded to increased light levels. Both species showed low light compensation points when grown under total shade. There was a partial stomatal closure generally during midday in D. rospigliosii. A. triplinervia presented lower leaf conductances, transpiration rates and lesser stomatal control. Some leaf anatomical characteristics, in both species, were affected by variations in the light regime (i.e. increased leaf thickness, leaf specific weight and stomatal density). These results suggest that both species have the ability to respond to variations in their natural light environments, therefore maintaining a favorable carbon balance during the day.     

Linking frugivore activity to early recruitment of a bird dispersed tree,Eugenia umbelliflora (Myrtaceae) in the Atlantic rainforest

Seed dispersal by animals is a complex process involving several distinct stages: fruit removal by frugivores, seed delivery in different microhabitats, seed germination, seedling establishment, and adult recruitment. Nevertheless, studies conducted until now have provided scarce information concerning the sequence of stages in a plant's life cycle in its entirety. The main objective of this study was to evaluate the immediate consequences of frugivore activity for Eugenia umbelliflora (Myrtaceae) early recruitment by measuring the relative importance of each fruit‐eating bird species on the establishment of new seedlings in scrub and low restinga vegetation in the Atlantic rainforest, Brazil. We conducted focal tree observations on E. umbelliflora trees recording birds' feeding behaviour and post‐feeding movements. We also recorded the fate of dispersed seeds in scrub and low restinga vegetation. We recorded 17 bird species interacting with fruits in 55 h of observation. Only 30% of the handled fruits were successfully removed. From 108 post flight movements of exit from the fruiting trees, 30.6% were to scrub and 69.4% to low restinga forest. Proportion of seed germination was higher in low restinga than in the scrub vegetation. Incorporating the probabilities of seeds' removal, deposition, and germination in both sites, we found that the relative importance of each frugivorous bird as seed dispersers varies largely among species. Turdus amaurochalinus and Turdus rufiventris were the best dispersers, together representing almost 12% probability of seed germination following removal. Our results show the importance of assessing the overall consequence of seed dispersal within the framework of disperser effectiveness, providing a more comprehensive and realistic evaluation of the relative importance of different seed dispersers on plant population dynamics.     

Loss of animal seed dispersal increases extinction risk in a tropical tree species due to pervasive negative density dependence across life stages

Overhunting in tropical forests reduces populations of vertebrate seed dispersers. If reduced seed dispersal has a negative impact on tree population viability, overhunting could lead to altered forest structure and dynamics, including decreased biodiversity. However, empirical data showing decreased animal-dispersed tree abundance in overhunted forests contradict demographic models which predict minimal sensitivity of tree population growth rate to early life stages. One resolution to this discrepancy is that seed dispersal determines spatial aggregation, which could have demographic consequences for all life stages. We tested the impact of dispersal loss on population viability of a tropical tree species, Miliusa horsfieldii, currently dispersed by an intact community of large mammals in a Thai forest. We evaluated the effect of spatial aggregation for all tree life stages, from seeds to adult trees, and constructed simulation models to compare population viability with and without animal-mediated seed dispersal. In simulated populations, disperser loss increased spatial aggregation by fourfold, leading to increased negative density dependence across the life cycle and a 10-fold increase in the probability of extinction. Given that the majority of tree species in tropical forests are animal-dispersed, overhunting will potentially result in forests that are fundamentally different from those existing now.