► 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.
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.
The pollination biology of two sympatric species, Polyalthia coffeoides and Polyalthia korinti (Annonaceae), is described in detail. An Endaeus species (Coleoptera: Curculionidae) is shown to be the ...major pollinator of both species, with Carpophilus plagiatipennis (Coleoptera: Nitidulidae) as the secondary pollinator of P. coffeoides. Both Polyalthia species show intrafloral dichogamy (protogyny) with a reproductively inactive phase between the pistillate and staminate phases, although there is no evidence of interfloral dichogamy. A pollination chamber is formed by the inner petals throughout the reproductively active phases. Thermogenesis occurs in P. korinti, with internal floral temperatures up to 6°C above ambient levels. The heat is presumably an energy reward for the beetles. Although most pollination systems are regarded as diversified and opportunistic, specialized pollination systems are typical of the Annonaceae. Although P. coffeoides and P. korinti have overlapping distributions, habitats, and flowering seasons and share the same pollinators, the extent of competition for pollinators is likely to be lessened due to the abundance and nonspecificity of the beetles. PUBLICATION ABSTRACT
•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 breeding systems of two sympatric species, Polyalthia coffeoides and Polyalthia korinti (Annonaceae), are assessed using a range of approaches, including controlled pollination experiments and ...analysis of intersimple sequence repeat markers within and between populations. Natural (open) pollination resulted in similar levels of fruit formation as artificial cross-pollination, suggesting that pollinator availability is not a limiting factor in reproduction. Both species possess facultatively xenogamous breeding systems, with 33%-36% fruit formation in artificially crossed experiments and 17%-19% fruit formation following geitonogamy. Reduced fruit set following geitonogamy suggests partial self-incompatibility; this is supported by index of self-incompatibility values of ca. 0.5 in both species. Analysis of population genetic structure supports the hypothesis of a mixed but largely xenogamous mating system. Genetic diversity within populations was estimated to be ca. 80% and 66% of total genetic diversity for P. coffeoides and P. korinti, respectively. The levels of gene flow between populations were moderate (Nm = 2.033 for P. coffeoides and 0.970 for P. korinti), and genetic identity (I) values between populations within species were high. This possibly reflects the fragmentation of a previously more extensive population, correlated with the historical deforestation associated with crop cultivation and irrigation in Sri Lanka. PUBLICATION ABSTRACT
1 Responses to the addition of P and Mg are described for eight species of Shorea section Doona (Dipterocarpaceae) which vary in their adult distribution across a topographic/soils gradient at ...Sinharaja Forest Reserve, Sri Lanka. 2 All combinations of the two nutrients resulted in increased dry mass yield, seedling height and leaf number after 24 months for seedlings of these species growing in pots of soil taken from a nearby Pinus plantation compared to a control which did not receive nutrients. In the presence of P, dry mass yield, leaf number and lateral root ratio declined in response to increasing Mg. In the presence of Mg, seedling height and stem mass ratio increased in response to increasing P. 3 After 24 months maximum dry mass yield in response to nutrient addition was greater for the four species which occur on more nutrient-rich soils in the field. Maximum percentage increase in dry mass in response to nutrient addition was negatively correlated with mean dry mass of unfertilized seedlings, but the relationship did not discriminate between species according to adult distribution across the catena. 4 There was no relation between seedling root mass ratio (RMR) or phenotypic plasticity in seedling RMR and adult distribution in relation to nutrient supply. 5 In Shorea section Doona, trade-offs between seedling `responsiveness' and `tolerance' to nutrient supply, in terms of either growth or dry mass allocation, are not powerful determinants of differences in adult distribution in relation to nutrient supply. Differential dry mass yield in response to nutrient addition was determined by differences in seed size and seedling relative growth rates under the experimental conditions, as well as the degree of response to nutrient addition. Response to nutrient addition may be constrained by seedling characteristics relating to variation in irradiance and water availability.
Data on the reproductive biology of the Annonaceae are rather fragmentary, particularly for paleotropical species. The pollination ecology and breeding system of the Sri Lankan endemic Xylopia ...championii (Annonaceae) are described in detail. The pollination ecology was investigated using a diverse range of approaches, including (1) observations of flower-level and population-level phenology, (2) assessments of floral visitors and effective pollinators, (3) monitoring of floral temperature in situ using a digital data logger, and (4) analysis of scent chemistry using solid-phase microextraction sampling and gas chromatography-mass spectrometry identification of volatiles. The breeding system was evaluated using pollen/ovule ratios and field-based controlled-pollination experiments. Intrafloral dichogamy (protogyny) occurs over a 2-d period, with a reproductively inactive phase between the pistillate and staminate phases, although there is no evidence of interfloral dichogamy. The inner petals close to form a pollination chamber during the reproductively active phases. The flowers are pollinated by a species of Endaeus weevil (Coleoptera: Curculionidae). Floral chamber temperatures are elevated to 8°C above ambient levels. The floral scent contains a combination of volatiles that have previously been observed in fruits and other flowers and that possibly mimic insect pheromones. Xylopia championii has an essentially xenogamous breeding system, promoted by protogyny. Although X. championii possesses numerous clear adaptations for cantharophily, there is no evidence for a species-specific interaction. The beetles are attracted to the flowers by strong scents; rewards offered to the beetles include heat energy and protection from predators. Low levels of fruit set in natural conditions suggest that pollinator availability may be a limiting factor.
The angiosperm flora of Sri Lanka, which has been comparatively well studied by many eminent botanists for over two centuries, has a rich endemic component and also exhibits strong affinities to that ...of peninsular India. Some of the 15 floristic regions recognized in the island have exceptionally rich, highly localized floras. But increased forest disturbances over the years have taken a heavy toll. A plea is made to conserve at least one sufficiently large forest reserve in each floristic region. A meaningful conservation program has been launched in the Sinharaja MAB Reserve; similar programs are urgently needed for other protected areas of the island.
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