Abstract Mangroves are one of the most important ecosystems in the world being found in the tropical–subtropical belt. Despite their significance, they have been highly disturbed due to many ...anthropogenic and natural causes. A significant effort has been made to restore mangroves around the world. However, a lack of information on the seed biology of mangrove species has impeded restoration. Thus, this study aimed to produce a seed dormancy profile for selected plant species of mangroves in Sri Lanka. This profile would allow restoration ecologists to better understand what kinds of dormancy are present, how to alleviate dormancy and how to best stimulate germination to generate seedlings for nursery stock or out-planting. Mature fruits/seeds were collected from coastal zone mangroves in Sri Lanka. Germination and imbibition of non-scarified and manually scarified seeds and embryo:seed length (E:S) ratio of fresh and radicle-emerged seeds were evaluated to assess the class of dormancy. Of the 30 species, seeds from 12 (40%) were non-dormant and 18 (60%) were dormant. Three dormancy classes physiological (PD), physical (PY) and morphophysiological (MPD) and presence of epicotyl dormancy were identified. Among species producing dormant seeds, most of them showed PD (44%). PY, MPD and presence of epicotyl dormancy were represented by 28, 17 and 11% of the species, respectively. These findings aid practitioners to craft strategies to effectively break dormancy and germinate seeds for conservation and restoration activities of mangroves.
► Native tree species can be used to accelerate succession on degraded grasslands. ► We examined the mechanisms limiting tree seedling growth and survival. ► Root competition exerted by grass ...inhibited seedling performance. ►
Symplocos cochinchinensis and
Macaranga indica can be used to accelerate succession.
The density of colonising tree seedlings is often very low in degraded human-induced tropical grasslands. To investigate the mechanisms that constrain seedling establishment in grasslands on former tea plantations in Sri Lanka we planted seedlings of the native tree species
Dimocarpus longan Lour.,
Macaranga indica Wight,
Symplocos cochinchinensis (Lour.) S. Moore and
Syzygium spathulatum Thw. and examined effects of vertebrate herbivory, and above- and below-ground competition exerted by the grass sward on seedling growth and survival over 28 months. Seedlings of the same species were also planted in remnant patches of lower montane rain forest to determine the effects of habitat on seedling growth and survival. Less than 40% of seedlings survived to 28 months post-transplantation. The highest survival was recorded for
Symplocos cochinchinensis in both grassland and forest, while Macaranga indica seedlings had the highest relative growth rate of height (RGRh) in both habitats. Root competition reduced survival of
Macaranga indica and the RGR
h of
Macaranga indica,
Symplocos cochinchinensis and
Syzygium spathulatum in the grassland, while above-ground competition and exclusion of vertebrate herbivores had no effects on seedling establishment. These results suggest that
Symplocos cochinchinensis would be suitable for re-establishing forest cover on degraded grasslands, although
Macaranga indica would be more likely to catalyse succession because it possesses animal-dispersed fruit. Measures that overcome or restrict the effects of root competition from grasses would enhance tree seedling growth and survival more than manipulation of the light environment or protection from vertebrate herbivores.
•A long term growth study across an elevation gradient.•MDF structure is potentially linked to landform stability.•MDF tree family dominance increases with time.•Canopy trees showed greater site ...overlap than understory.
Little work has examined the spatial and temporal changes of a tropical rain forest for long time periods. Here, we present an analysis of long-term plot data from a Sri Lankan Mixed-Dipterocarp forest (MDF). The plots were established in 1978 at three different elevations (low - 335m, medium - 560 m, and high - 915 m). At each site we measured all trees ≥ 10 cm DBH in 16 or 18 plots. The total area sampled amounted to 12.5 ha. We recorded 166 tree species, with each site comprising between 95 and 121 species. Stem densities and basal areas were different among sites, and declined over the period from 642.8 stems ha−1 and 41.8 m2 ha−1 for the 1978 census, to 496.2 stems ha−1 and 35.2 m2 ha−1 for the 2018 census. The lowest elevation exhibited the highest recruitment, mortality and turnover among sites but patterns changed and became less marked over time. Forest wide tree recruitment was about 1% y−1, while mortality was twice as much averaging about 2% y−1. Four families made up between 44% (1978 census) and 54% (2018 census) of the trees sampled. Two families – Clusiaceae, Dipterocarpaceae contributed up to 33.4% and 37.3% respectively, depending upon site and year of census. The five most common species represent between 31 and 54% of the basal area and between 25 and 66% of the stem density depending upon site. Percentage dominance by the most common species increased over the 40-year time period. Though many trees showed some degree of spatial differentiation, canopy trees showed greater site overlap in their distribution than understory and sub-canopy species. Our results provide an improved picture of variations in MDF structure and composition across the ever-wet realms of equatorial south and southeast Asia. We suggest that dominance in dipterocarps and the degree of closed canopy structure, as measured by basal area, is influenced by time, elevation, the degree of topographic variability within elevation, and topographic interactions with variabilities in climate (drought and windstorms). We propose higher standing basal areas and super dominance of dipterocarps in MDF are linked to site, succession, and landform stability. Dipterocarps increase in dominance with succession (time), with topography (ridges greater than valleys) and with elevation (lowland < hill < lower montane).
Mangroves are highly adapted to extreme environmental conditions that occur at the interface of salt and fresh water. Adaptations to the saline environment during germination are a key to mangrove ...survival, and thereby, its distribution. The main objective of this research was to study the effect of salinity on seed germination of selected mangrove species and the application of a hydrotime model to explain the relationship between water potential of the medium and rate of seed germination. Germination of seeds was examined at 15, 25 and 35°C in light/dark over a NaCl gradient. Germination time courses were prepared, and germination data were used to investigate whether these species behave according to the principles of the hydrotime model. The model was fitted for the germination of Acanthus ilicifolius seeds at 25°C. Final germination percentage was significantly influenced by species, osmotic potential and their interaction at 25°C. Moreover, temperature had a clear effect on seed germination (Sonneratia caseolaris and Pemphis acidula) which interacted with osmotic potential. Only A. ilicifolius seeds behaved according to the hydrotime principles and thus its threshold water potential was –1.8 MPa. Optimum germination rates for seeds of the other species occurred at osmotic potentials other than 0 MPa. The descending order of salinity tolerance of the tested species was Aegiceras corniculatum > Sonneratia caseolaris > Acanthus ilicifolius > Pemphis acidula > Allophylus cobbe, suggesting that the viviparous species (A. corniculatum) is highly salt tolerant compared with the non-viviparous species. The results revealed that seeds of the study species exhibited facultative halophytic behaviour in which they can germinate over a broad range of saline environments. Use of a hydrotime model for mangroves was limited as germination of their seeds did not meet model criteria.
The Western Ghats of India and Sri Lanka biodiversity hotspot is often regarded as one unit because of shared biogeographical history. However, recent studies suggest that certain faunal components, ...particularly in the wet zones are distinct. This article looks at the existing information on species richness and endemicity of taxa in both regions. Data are available on some taxa but not on others, and many taxa have not received equal attention in both regions. The natural ecosystems of this hotspot are under threat and urgent conservation action is needed, especially in augmenting the protected area network. There is also the need for increasing the level of scientific collaboration in biodiversity studies between the two countries.
The future trajectory of global forests is closely intertwined with tree demography, and a major fundamental goal in ecology is to understand the key mechanisms governing spatio‐temporal patterns in ...tree population dynamics. While previous research has made substantial progress in identifying the mechanisms individually, their relative importance among forests remains unclear mainly due to practical limitations. One approach to overcome these limitations is to group mechanisms according to their shared effects on the variability of tree vital rates and quantify patterns therein. We developed a conceptual and statistical framework (variance partitioning of Bayesian multilevel models) that attributes the variability in tree growth, mortality, and recruitment to variation in species, space, and time, and their interactions – categories we refer to as organising principles (OPs). We applied the framework to data from 21 forest plots covering more than 2.9 million trees of approximately 6500 species. We found that differences among species, the species OP, proved a major source of variability in tree vital rates, explaining 28–33% of demographic variance alone, and 14–17% in interaction with space, totalling 40–43%. Our results support the hypothesis that the range of vital rates is similar across global forests. However, the average variability among species declined with species richness, indicating that diverse forests featured smaller interspecific differences in vital rates. Moreover, decomposing the variance in vital rates into the proposed OPs showed the importance of unexplained variability, which includes individual variation, in tree demography. A focus on how demographic variance is organized in forests can facilitate the construction of more targeted models with clearer expectations of which covariates might drive a vital rate. This study therefore highlights the most promising avenues for future research, both in terms of understanding the relative contributions of groups of mechanisms to forest demography and diversity, and for improving projections of forest ecosystems.
1. One of the primary goals in community ecology is to determine the relative importance of processes and mechanisms that control biodiversity. Here, we examined habitat-driven species assemblages ...and species distribution patterns as well as their temporal variations for three life stages of two censuses of a 25-ha mixed dipterocarp forest at Sinharaja (Sri Lanka). 2. Our general objective was to find out whether the species assemblages and associated habitat types changed with life stage, spatial scale and species attributes. We also analyse whether the habitat types were related to certain indicator species. Habitat types were determined with multivariate regression tree analyses driven by topographic variables. 3. We found species assemblages associated with five distinct habitat types that appeared consistently for all life stages of the two censuses. These habitats were related to ridge-valley gradients and a pronounced contrast in south-west versus north-east aspect. Habitat-driven structuring was weak at the recruit stage but strong in the juvenile and adult stages. The species assemblage variance explained by topographic variables for different life stages ranged between 10% for recruits and 23% for juveniles. 4. The species assemblages determined for different spatial scales (10, 20, 50 m) showed similar habitat partitioning, but the variance explained by the topographic variables increased in all life stages with spatial scale. This could be due to the homogenizing effect of topographic variables at the larger scales and unaccounted environmental variation at the smaller scales. The number of indicator species identified in the two censuses was higher in the juvenile stage than in the adult stage, and nearly all indicator species in the adult stage were also indicator species in the juvenile stage. 5. Synthesis. Our study showed that approximately 75% of the variance in local species composition is unexplained. This may be due to spatially structured processes such as dispersal limitation, unaccounted biotic and abiotic environmental variables, and stochastic effects, but only 25% were due to topographic habitat association. Although the pronounced ridge-valley gradient and contrast of south-west versus north-east aspect created consistent habitats, our results suggest that local species assemblages at Sinharaja forest are jointly shaped by neutral and niche processes.
Understanding how plant species coexist in tropical rainforests is one of the biggest challenges in community ecology. One prominent hypothesis suggests that rare species are at an advantage because ...trees have lower survival in areas of high conspecific density due to increased attack by natural enemies, a process known as negative density dependence (NDD). A consensus is emerging that NDD is important for plant-species coexistence in tropical forests. Most evidence comes from short-term studies, but testing the prediction that NDD decreases the spatial aggregation of tree populations provides a long-term perspective. While spatial distributions have provided only weak evidence for NDD so far, the opposing effects of environmental heterogeneity might have confounded previous analyses. Here we use a novel statistical technique to control for environmental heterogeneity while testing whether spatial aggregation decreases with tree size in four tropical forests. We provide evidence for NDD in 22% of the 139 tree species analyzed and show that environmental heterogeneity can obscure the spatial signal of NDD. Environmental heterogeneity contributed to aggregation in 84% of species. We conclude that both biotic interactions and environmental heterogeneity play crucial roles in shaping tree dynamics in tropical forests.
1. In the moist tropics, studies have demonstrated poor seedling establishment of latesuccessional trees on lands cleared of forest. Our study examined the potential for establishing ...late-successional tree species that dominate the canopy of rainforest by planting within and adjacent to experimental openings that were created within a Pinus caribaea plantation. 2. We tested five canopy tree species (Dipterocarpus zeylanicus, Mesua ferrea, Shorea disticha, S. megistophylla and S. trapezifolia) of tropical forest in south-western Sri Lanka. Seedlings were monitored for 2 years within treatments that removed either three rows or one row of Pinus canopy, a canopy edge treatment and a control that left the canopy intact. 3. The greatest growth and dry mass for all species were in the canopy removal treatments. In particular, S. trapezifolia and S. disticha exhibited the greatest height growth in these treatments. In the three-row canopy removal treatment, M. ferrea had a significantly lower dry mass than the other species. 4. Differences were shown in the number and area of leaves among species. Shorea trapezifolia and, to a lesser degree, S. disticha increased area by increasing leaf production. Dipterocarpus zeylanicus and, to a lesser degree, M. ferrea increased area by increasing the size of individual leaves. 5. Guidelines based on results from this study recommend that species grow best when seedlings are planted within openings created by the removal of three rows of Pinus canopy. Where planting without canopy removal is required, S. disticha or S. megistophylla should be selected because of greater shade and drought tolerance. 6. This experiment demonstrated that Pinus can be used as a nurse for facilitating the establishment of site-sensitive tropical forest tree species that are late-successional. In particular, results have application for similar mixed dipterocarp forest types in southeast Asia.
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