Early stages of secondary succession on abandoned cropland in north-east Borneo Island

A study on community structure and floristic change in the early stages of the tropical old-field succession in an intensive shifting cultivation area, Sabah, north-east Borneo Island, was conducted in March and September 1997. Dense therophytic communities were established soon after abandonment, and rapidly changed: namely, from Galinsoga parviflora (in approximately 1–2 months) to Crassocephalum crepidioides (in approximately 3–5 months) and further to Conyza sumatrensis (in approximately 1 year). These therophytic communities were replaced by perennial grass and shrub species, such as Imperata cylindrica and Eupatorium odoratum, within 3 years after abandonment. At this stage, a few pioneer tree species such as Trema orientalis made a sparse canopy layer approximately 3–4 m high. The three pioneer annuals have wind-dispersed small seeds. The plant densities of the three annuals did not appear to change between areas fallow for 1 month and 4 months. Almost all of the seedlings of the three annuals might simultaneously have invaded the fallowed field soon after abandonment. The three pioneer annuals take a different amount of time to reach their respective mature sizes. This resulted in the rapid successional changes in the tropical old-field succession. The seed-bank annuals, whose seeds germinate after the winter pre-chilling, occurred in the temperate old-fields as the first-year pioneers under seasonal condition. Intermittent shifting cultivation and tropical conditions without seasonality favour the occurrence of wind-dispersed annuals with a spatially fugitive strategy.     

Responses of an avian community to rain forest degradation

Cumulative impacts of logging and road building in a previously undisturbed tract of tropical rain forest in French Guiana were assessed by random sampling of the bird community and 1-km² plot counts of diurnal raptors. Similar surveys were carried out during road construction and 15 years later within 5km of the road and were also compared with the undisturbed bird community of a nearby primary forest. The main disturbance was the change in forest structure brought about by logging and secondarily the road opening, the roadside second growth and the depletion of large vertebrates by hunting pressure. The responses of different bird guilds were highly divergent according to their natural habitat requirements, their diet and their vulnerability to hunting. They resulted in a moderate decrease in overall species richness and equitability. All species pooled, the abundance of 118 species was lowered, that of 45 species did not change appreciably and 89 were favored or even appeared. The guilds most affected were those with large body sizes, from terrestrial foragers to canopy frugivores, mostly by hunting, and the open understorey specialists, notably large insectivores and mixed flock members, because of changes in forest structure after logging. The guilds favored by logging and road opening were many of the hummingbirds, upper canopy frugivores and omnivores, and gap, edge or low secondary growth specialists. Recommendations to minimize the negative consequences of human exploitation in rain forests include reducing the width of deforested roadsides to no more than 10–15m on either side, implementing much more careful logging practices and strongly limiting hunting pressure in newly opened areas.     

Genetic variation of Calycophyllum spruceanum in the Peruvian Amazon Basin, revealed by amplified fragment length polymorphism (AFLP) analysis

An understanding of the level, structure and origin of genetic variation within and among populations of tropical trees is essential for devising optimum management strategies for their sustainable utilization and conservation. Here, amplified fragment length polymorphism (AFLP) analysis was used to partition genetic variation within and among nine populations of the predominantly riverine tree, Calycophyllum spruceanum , sampled across a wide geographical range along river tributaries of the Peruvian Amazon Basin. Analysis of molecular variance ( AMOVA ) employed 65 AFLP markers and revealed most variation among individuals within populations (91%), although variation among populations was highly significant ( P < 0.001). Calculation of genetic distances and nested AMOVA indicated a degree of structuring among populations based on geographical proximity, although clustering did not depend on geographical distance alone. No firm evidence was obtained for unidirectional seed dispersal by water playing an important role in determining genetic structure over the geographical range sampled. Implications of data for optimising genetic management of the species are discussed and areas for further study identified.     

云南石林喀斯特地区半湿润常绿阔叶林附生植物的初步研究

对云南石林喀斯特山地原生植被——半湿润常绿阔叶林中附生植物的组成、生物量以及附生植物持水量等进行研究,结果表明该区森林林木附生植物种类相对较少,只有8科9种附生维管束植物和5种苔藓植物,而林内岩石表面则有石附生维管束植物16科17种,苔藓植物3种,两种不同生境附生植物的相似性系数为0.5。相同取样面积(400cm2)条件下,岩石表面附生植物平均生物量(11.7g)比林木表面(4.9g)的高。附生植物一次持水量为自身干重的5.5~9.7倍。     

Phytoplankton and Epiphyte Development and Their Shading Effect on Submerged Macrophytes in Lakes of Different Nutrient Status

The annual mean light intensity at the depth limit of the Littorella vegetation was 24–33% of the subsurface light intensity, despite large variations in each attenuation component (lake water, phytoplankton, and epiphytes). In oligotrophic, silicate-poor lakes, the light attenuation above the submerged vegetation was dominated by the water itself, which accounted for 65–72% of the total attenuation. Phytoplankton and epiphytes were equal in importance to each other. In oligotrophic, silicate-rich lakes and lakes receiving a nitrogen supply above background level, the epiphytes were more abundant, accounting for about 50% of the light attenuation. In one lake with a high nutrient supply, the epiphytes were responsible for 86% of the light attenuation. A new method of measuring the effect of shading by the epiphytic community on submerged macrophytes is presented. The light attenuation caused by the phytoplankton and the epiphytes was investigated and related to the depth distribution of the submerged angiosperm, Littorella uniflora. It is shown that the biomass of the epiphytes increased more than the biomass of the phytoplankton in response to an external or internal nutrient loading. Shading by epiphytes is of decisive importance for the depth distribution of Littorella at increasing nutrient supply.     

The Citreoline Trogon as an Ecosystem Engineer1

The western Mexican endemic Citreoline Trogon (Trogon citreolus) builds its nest cavities in arboreal termitaria. We studied the reuse of these holes by other organisms after the trogons abandoned them in the tropical dry forest of western Mexico. Of the 24 cavities examined, 19 were occupied (5 by mammals and 14 by arthropods). We found two western Mexico endemic mammals (Marmosa canescens and Xenomys nelsoni) using these spaces. Our results suggest that the T. citreolus provides a critical role in creating necessary cavities for many organisms in tropical dry forests.     

Impacts of Riparian Forest Removal on Palauan Streams

The 2006 completion of the circum‐island Compact Road on the island of Babeldaob in the Republic of Palau resulted in several deforested stream reaches with modified stream channels. To determine the impacts of deforestation and road construction, various ecosystem parameters were compared between road‐impacted reaches, reforested savanna reaches, and forested reaches. Compared to adjacent forested reaches, road‐impacted reaches received significantly more light (0.4 ± 0.1 vs. 87.8 ± 4.1 % light transmittance, respectively), were significantly warmer (25.7 ± 0.1 vs. 26.1 ± 0.1°C, respectively), and received higher nutrient and sediment loads, all of which were attributed to the removal of riparian vegetation and increased surface runoff from the road. These differences were believed to have shifted the benthic algal community in road‐impacted reaches from diatoms to filamentous algae with significantly greater chl a biomass (10×) and benthic algal ash free dry mass AFDM (3×) compared to adjacent forested reaches. Savanna‐impacted and forested reaches had similar chl a, algal AFDM, and received similar amounts of light. Nutrient and sediment concentrations varied between the two reach types. Results from this study emphasize the need for the maintenance of riparian forests especially with predicted increases in population, development, and deforestation. Future studies are needed to determine effective riparian widths and riparian forest community structure to help resource managers and land owners protect and preserve the many ecosystem services that Palauan streams and watersheds provide.     

Tropical Dry Forests of Venezuela: Characterization and Current Conservation Status1

Tropical dry forests are located predominantly in the northern portion of Venezuela, above 6°N. Although their potential extent covers ca 400,000 km² (44% of the land), they currently occupy about 10 percent of this area. The diversity and complexity of Venezuelan dry forests increases from north to south along a gradient of decreasing severity of the dry season. A typical dry forest in Venezuela presents ca 110–170 species of plants from ca 40 to 50 families within an area of approximately 10 ha. Species composition and forest structure, however, are dependent on local landscape conditions (e.g., soil type, topography), and nearby forest types can be very different. Our analysis of five dry forest variants showed a maximum family similarity of 67 percent, although most values fell in the 50–60 percent interval. They are currently considered as one of Venezuela's most threatened ecosystems, but only 5 percent of extant dry forests are included in protected areas; this represents 0.5 percent of their potential extent. It is fundamental to promote the creation of at least 3 or 4 more large protected areas (ca 5000 ha), with different climatic and orographic characteristics, in combination with the recovery of threatened species, the restoration of degraded systems, and the implementation of sustainable development projects. Their apparent high resilience suggests that with the proper management we can restore and maintain the integrity of Venezuelan dry forests.