I. M. Turner

Species loss in fragments of tropical rain forest : a review of the evidence

A review of the literature shows that in nearly all cases tropical rain forest fragmentation has led to a local loss of species. Isolated fragments suffer reductions in species richness with time after excision from continuous forest, and small fragments often have fewer species recorded for the same effort of observation than large fragments or areas of continuous forest. Birds have been the most frequently studied taxonomic group with respect to the effects of tropical forest fragmentation. The mechanisms of fragmentation-related extinction include the deleterious effects of human disturbance during and after deforestation, the reduction of population sizes, the reduction of immigration rates, forest edge effects, changes in community structure (second- and higher-order effects) and the immigration of exotic species. The relative importance of these mechanisms remains obscure. Animals that are large, sparsely or patchily distributed, or very specialized and intolerant of the vegetation surrounding fragments, are particularly prone to local extinction. The large number of indigenous species that are very sparsely distributed and intolerant of conditions outside the forest make evergreen tropical rain forest particularly susceptible to species loss through fragmentation. Much more research is needed to study what is probably the major threat to global biodiversity.

Responses to nutrient addition among shade-tolerant tree seedlings of lowland tropical rain forest in Singapore

1 Two bioassays of growth limitation were carried out for seedlings of four shadetolerant tree species (Antidesma cuspidatum, Calophyllum tetrapterum, Dipterocarpus kunstleri and Garcinia scortechinii) growing in P-deficient soil taken from lowland dipterocarp forest in Singapore, as a test of the hypothesis that growth would be limited by the availability of phosphorus. 2 Seedlings of only one species, Antidesma cuspidatum, showed increased growth in response to increased nutrient supply and in that case the limiting nutrient was not P. A majority of seedlings of Antidesma, Calophyllum and Garcinia in this experiment possessed VA mycorrhizas. 3 For seedlings of Antidesma, addition of magnesium led to an increase in the concentration of Mg in all fractions and a positive relation between Mg concentrations and dry mass yield. Addition of potassium and calcium resulted in reductions in concentrations of these elements in the leaves of Antidesma. 4 Seedlings of Antidesma, Calophyllum and Dipterocarpus responded to P by altering distribution of dry mass between different plant parts; the pattern of response varied between species. Phosphorus taken up in excess of requirements for vegetative growth was transferred to plant stems rather than leaves. 5 The outcome of pot bioassays may be dependent on factors such as pot size, irradiance and soil moisture conditions; therefore conclusions drawn here need to be tested by field fertilization experiments.

Responses to Simulated Drought and Elevated Nutrient Supply among Shade-Tolerant Tree Seedlings of Lowland Tropical Forest in Singapore1

Seedlings of three tree species of lowland tropical rain forest in Singapore grown in pots of forest soil at low irradiance were subjected to simulated drought and nutrient addition treatments. The gravimetric soil water contents applied during drought simulations were within the range found during an eight-month survey of soil moisture content at 10 understory sites in primary forest in Singapore. Aspects of plant growth and dry mass allocation were influenced strongly by simulated drought for all three species. Morphological responses to the low-frequency watering regime were similar to those described for herbaceous crop plants subjected to water shortage, and suggest that the tree seedlings used are capable of withstanding substantial periods of low soil water availability. In addition, analysis of covariance showed that height and leaf area growth of seedlings of Antidesma cuspidatum were limited by Mg and height growth of seedlings of Vatica maingayi by one or more macro-nutrients. These results suggest that tree seedlings of tropical lowland rain forests may be limited by the availability of soil resources, as well as irradiance, when growing in deep shade.

Tree species richness in primary and old secondary tropical forest in Singapore

If secondary succession can accumulate species rapidly, then tropical secondary forests may have an important role to play in the conservation of biodiversity. Data on the floristic composition of forest stands in the Central Catchment Nature Reserve, Singapore, have been analysed to investigate the diversity of approximately 100-year-old tropical secondary forest. Classification using TWINSPAN indicated that three floristic communities could be recognized from 59 0.2 ha plots enumerated for trees >30 cm gbh. These were two types of secondary forest, both dominated by Rhodamnia cinerea (Myrtaceae), and dryland primary forest. The secondary forest was developed on land abandoned after use for agriculture at the end of the 19th century. The 16 primary forest plots contained a total of 340 species, more than the 281 recorded from the 43 plots of the two secondary forest types combined. The mean species number per plot in the more diverse of the two secondary forests was only about 60% of the primary forest. Thus the secondary forest, despite a century or so for colonization by species and the presence of contiguous primary forest, was still significantly less diverse than primary forest areas. It is concluded that secondary forest cannot be assumed to accrete biodiversity rapidly in the tropics, and may not be of direct value in conservation. However, other indirect roles, such as providing resources for native animals, and buffering and protecting primary forest fragments may make the protection of secondary forest worthwhile.

A quantitative analysis of leaf form in woody plants from the world's major broadleaved forest types

The results of a compilation and analysis of pub- lished data on average leaf characteristics from multi-species samples of the worlds major broadleaved woody vegetation types are presented. Highly significant linear regressions indi- cate several important trends across the major forest types. As lamina thickness increases, outer epidermal wall and cuticle thickness increases and specific leaf area (SLA) decreases. Thick leaves tend to have a lower thickness ratio of palisade to non-palisade mesophyll. Foliar concentrations of nitrogen (N) and phosphorus (P) are closely correlated with SLA. Crude fibre concentration and sclerophylly index (crude fibre/ crude protein) exhibit similar non-linear trends with leaf P concentration, the latter being governed by a negative expo- nential relationship where the degree of sclerophylly increases rapidly below 0.1% P. Together these represent a trend of increasing sclerophylly, a convergence point in leaf design shared by diverse vegetation types including Mediterranean shrublands, tropical heaths and tropical upper montane forests. The analysis shows that there is considerable variation both within and between vegetation types in leaf form. It does not satisfactorily distinguish any major groups within the sclero- phyllous communities. Further studies of community leaf form are required before we can have a quantitative understanding of these biogeographically important variables.

Mineral nutrient status of coastal hill dipterocarp forest and adinandra belukar in Singapore: analysis of soil, leaves and litter

The soil of remnant primary rainforest on granite in Singapore is very acidic (pH mostly 3.5-4.2 at 0-10 cm) and has unusually low total concentrations of nitrogen and phosphorus, but the mean concentrations of N and P in the living leaves and freshly fallen leaves of the more shade-tolerant species are within the ranges found for other lowland forests on infertile oxisols and ultisols. The concentration of Ca in freshly fallen leaves is very low. The soil under secondary forest (belukar) on sandstone dominated by Adinandra dumosa (Theaceae) was degraded during use for agriculture. It has the same pH range but even lower values of total N and P. The mean concentrations of N, P, K, Ca and Mg in the living leaves are slightly higher than in leaves of the more shade-tolerant trees of primary forest, but lower than in the leaves of the species which require canopy gaps for establishment or early onward growth. The few species sampled in both primary forest and belukar show no consistent trend in foliar concentrations. The degraded soil has selected species with inherently lower foliar concentrations. Vitousek & Sanford (1986) showed that the nutrient status of a tropical rainfor- est measured in terms of the nutrient concentrations in the plant parts, and in terms of the amounts of nutrients cycling each year, could be related to soil type. They contrasted three groups of lowland forests: those on moderately fertile soils, those on infertile oxisols and ultisols, and those on extremely infer- tile spodosols and psamments. They emphasized the fact that many of the most detailed studies on nutrient relations of tropical rainforests have been carried out on extreme types, e.g. lowland forests on spodosols and montane forests, and called for more work on middle of the range forests. Our paper is concerned with coastal hill dipterocarp forest, a forest type which is developed predomin- antly on ultisols over granite and which might be expected to fall in the middle of the range of soil fertility for lowland rainforests. Our study has been made

The ecology of very small-seeded shade-tolerant trees and shrubs in lowland rain forest in Singapore

A study was made in Bukit Timah Nature Reserve of niche differentiation among 11 woody species that have very small seeds (22–460 µg) and establish in deep shade, i.e., where the indirect site factor (isf) under cloudy conditions is ≤2%, and one taxonomically related light-demander (seed mass 33 µg). Comparative observations were made on changing light requirement with age. The species varied from shrubs and treelets (Urophyllum) via small to medium-height trees (Ficus, Pternandra) to tall trees (Gynotroches and Pellacalyx). In general, the shorter species fruited in deep shade while the taller needed direct irradiance. All produced ripe fruit at least once a year.In 1–3% isf seedling mortality over 11 months was 24% for Urophyllum hirsutum and 57% for Pternandra echinata (two strongly shade-tolerant species); survivors produced appreciable new stem and leaf tissue. For two more light-demanding species (Ficus grossularioides and F. fistulosa) mean lamina area was much larger in small gaps (10% isf) than in the understorey (<3% isf), while for Urophyllum hirsutum it was much smaller. These three species showed the greatest difference in SLA. There were few significant intraspecific differences between gaps and understorey in concentrations of N, P or K. In a valley where there was no significant difference in isf between a path and adjacent understorey, seedlings of species with seeds <1 mg mean dry mass were five times as dense on transects along steep pathside banks as in the understorey 5 m upslope. The mean mid-bank slope was 66° (cf. 20° upslope). On mid-bank transects litter had a cover of 36% (cf. 95% upslope), and was thinner (0.8 vs 3.2 cm). The soil at 0–7 cm depth into the mid-bank had much lower concentrations of organic matter, total N and P (but not total K) and fine roots than that of upslope transects. All species were found to have vesicular-arbuscular mycorrhiza when 1–2 m tall in deep shade, several having very dense infections. Seedling form and responses are discussed in relation to potential adaptive value and possible evolutionary origin.

Soil seed bank from lowland rain forest in Singapore: canopy-gap and litter-gap demanders

The vast array of species found in the humid lowland tropical forests of the world are frequently divided into two general groups, light-demanders and shade-tolerators (Denslow 1980, Swaine & Whitmore 1988). In this dichotomy the characteristics of the gap-demanders are often given to include small seed size, dormancy mechanisms, potentially rapid growth, and requirement of direct sunlight for germination and establishment. Conversely, shade-tolerators are considered to have large, short-lived seeds, the potential to germinate in canopy shade and an ability to grow slowly in deep shade (Swaine & Whitmore 1988). Failing to conform to either general pattern, small-seeded shade-tolerant species have been documented from both the Central American and South-east Asian tropics (Ellison at al. , Metcalfe & Grubb 1995). Some of these small-seeded species have been shown to be able to germinate in filtered light approximating to canopy shade (Metcalfe 1996), and seedlings of other small-seeded species have been recorded growing under a closed canopy (Grubb 1996, Kiew 1988, Raich & Gong 1990). Several taxa shown by Metcalfe (1996) and Metcalfe & Grubb (1997) to be small-seeded and shade-tolerant were found in considerable numbers in the soil seed bank in Sabah by Kennedy (1991), and one ( Urophyllum glabrum ) by Putz & Appanah (1987) in Peninsular Malaysia. Species with small seeds (less than about 1-10 mg) do not have the resources to emerge from underneath a covering of leaf litter, and require litter-free sites for successful establishment (Guzman-Grajales & Walker 1991, Molofsky & Augspurger 1992, Putz 1983).

Artificial nest and seed predation experiments in tropical lowland rainforest remnants of Singapore

Abstract Tropical lowland rainforests of Southeast Asia are continually being fragmented and lost at an alarming rate. Little is known about the consequences of this large-scale habitat modification. Using artificial nest and seed experiments, we determined predation rates in forest remnants (2·5–1100 ha in area) of Singapore and Pulau Ubin. Singapore (641 km 2 ) is highly urbanised with only 5% of the native forest cover remaining, whereas Pulau Ubin (11 km 2 ) is relatively less developed with about 60% of the native forest cover remaining. We specifically determined if predation rates varied in relation to: (i) distance from edge, (ii) forest types, (iii) forest areas, (iv) isolation, (v) edge to area ratio, (vi) canopy closure, and/or (vii) area (Singapore vs Pulau Ubin). Overall, 80·5% ( n = 328) of artificial ground nests were depredated, of which 55·3% were predated by small mammals. For seeds, 98·2% ( n = 219) experimental stations were predated. Generally, the predation rate did not vary significantly in relation to the distance from the edge, though a primary forest remnant had lower artificial nest and seed predation rates than the other remnants. Pulau Ubin remnants were found to have lower predation rates than remnants on Singapore island. No significant correlations were found between predation rates and remnant area, isolation from other remnants, or the edge/area ratio, or canopy density. Relatively high predation rates in some tropical secondary and primary forest remnants may greatly influence plant regeneration and bird community structure.

Mineral nutrient status of coastal hill dipterocarp forest and adinandra belukar in Singapore: bioassays of nutrient limitation

Bioassays of nutrient limitation were carried out for Melastoma malabathricum growing on soil from primary lowland dipterocarp rainforest in Singapore, and for Dillenia suffruticosa on soil from adinandra belukar, a nutrient-poor secondary forest type dominated by Adinandra dumosa . Three questions were addressed. 1. What is the nutrient most limiting to growth in primary forest? 2. What is the nature of nutrient limitation under conditions of adequate P supply? 3. Is there a qualitative difference in the nature of nutrient limitation under primary forest and adinandra belukar? Results showed that there was a strong limitation by P availability in both primary forest and adinandra belukar under the experimental conditions used. Once plants had an adequate P supply, all other nutrients became limiting to growth in primary forest soil. These findings are interpreted as support for the hypothesis that P availability would limit the productivity of moist tropical forests in general in the absence of mycorrhizas; tentative conclusions are drawn on the assumption that most woody tropical plants are mycorrhizal. It is argued that limitation by major cations may be common on old, highly leached tropical rainforest soils.