J. W. Ferry Slik

Drought–mortality relationships for tropical forests

*The rich ecology of tropical forests is intimately tied to their moisture status. Multi-site syntheses can provide a macro-scale view of these linkages and their susceptibility to changing climates. Here, we report pan-tropical and regional-scale analyses of tree vulnerability to drought. *We assembled available data on tropical forest tree stem mortality before, during, and after recent drought events, from 119 monitoring plots in 10 countries concentrated in Amazonia and Borneo. *In most sites, larger trees are disproportionately at risk. At least within Amazonia, low wood density trees are also at greater risk of drought-associated mortality, independent of size. For comparable drought intensities, trees in Borneo are more vulnerable than trees in the Amazon. There is some evidence for lagged impacts of drought, with mortality rates remaining elevated 2 yr after the meteorological event is over. *These findings indicate that repeated droughts would shift the functional composition of tropical forests toward smaller, denser-wooded trees. At very high drought intensities, the linear relationship between tree mortality and moisture stress apparently breaks down, suggesting the existence of moisture stress thresholds beyond which some tropical forests would suffer catastrophic tree mortality.

Effects of fire and selective logging on the tree species composition of lowland dipterocarp forest in East Kalimantan, Indonesia

Tree species composition (diameter at breast height (dbh) ≥ 10 cm) was studied in primary, selectively logged and heavily burnt forests in East Kalimantan, Indonesia. The number of trees, tree species, and the Fisherss-α diversity index were determined for the first 15 years (burnt forest) and 25 years (selectively logged forest) after disturbance. Additionally the population structure of six common and typical Macaranga pioneer tree species was compared through time between selectively logged, burnt and primary forest. Both selectively logged and burnt forest showed a significant reduction in number of trees and tree species per surface area directly after disturbance. Fire especially affected dominant tree species, while for selective logging the opposite was observed. In selectively logged forest the number of trees, tree species and the Fisherss-α index reached pre-disturbance levels within c. 15 years. For burnt forest, only the number of trees recovered to pre-disturbance levels. The number of tree species stayed constant after disturbance, while the Fisherss-α index decreased. The six studied Macaranga pioneer tree species seedlings were present in all forest types. Their density seems to be unrelated to light levels in the forest understorey but strongly related to the number of mature parent trees. Their sapling densities were strongly related to light levels in the forest understorey. The studied Macaranga species formed an important part of both under- and over-storey in burnt forest 15 years after disturbance, while they were almost absent in the understorey and only moderately common in the overstorey of selectively logged forest.

An estimate of the number of tropical tree species

Significance People are fascinated by the amazing diversity of tropical forests and will be surprised to learn that robust estimates of the number of tropical tree species are lacking. We show that there are at least 40,000, but possibly more than 53,000, tree species in the tropics, in contrast to only 124 across temperate Europe. Almost all tropical tree species are restricted to their respective continents, and the Indo-Pacific region appears to be as species-rich as tropical America, with each of these two regions being almost five times as rich in tree species as African tropical forests. Our study shows that most tree species are extremely rare, meaning that they may be under serious risk of extinction at current deforestation rates. The high species richness of tropical forests has long been recognized, yet there remains substantial uncertainty regarding the actual number of tropical tree species. Using a pantropical tree inventory database from closed canopy forests, consisting of 657,630 trees belonging to 11,371 species, we use a fitted value of Fisher’s alpha and an approximate pantropical stem total to estimate the minimum number of tropical forest tree species to fall between ∼40,000 and ∼53,000, i.e., at the high end of previous estimates. Contrary to common assumption, the Indo-Pacific region was found to be as species-rich as the Neotropics, with both regions having a minimum of ∼19,000–25,000 tree species. Continental Africa is relatively depauperate with a minimum of ∼4,500–6,000 tree species. Very few species are shared among the African, American, and the Indo-Pacific regions. We provide a methodological framework for estimating species richness in trees that may help refine species richness estimates of tree-dependent taxa.

Functional and phylogenetic assembly in a Chinese tropical tree community across size classes, spatial scales and habitats

Summary 1. Increasingly, ecologists are using functional and phylogenetic approaches to quantify the relative importance of stochastic, abiotic filtering and biotic filtering processes shaping the pattern of species co-occurrence. A remaining challenge in functional and phylogenetic analyses of tropical tree communities is to successfully integrate the functional and phylogenetic structure of tree communities across spatial and size scales and habitats in a single analysis. 2. We analysed the functional and phylogenetic structure of tree assemblages in a 20-ha tropical forest dynamics plot in south-west China. Because the influence of biotic interactions may become more apparent as cohorts age, on local scales, and in resource-rich environments, we perform our analyses across three size classes, six spatial scales and six distinct habitat types, using 10 plant functional traits and a molecular phylogeny for the >400 tree taxa found in the plot. 3. All traits, except leaf area and stem-specific resistance, had significant, albeit weak phylogenetic signal. For canopy species, phylogenetic clustering in small and medium size classes turned to phylogenetic overdispersion in the largest size class and this change in dispersion with size was found in each habitat type and across all spatial scales. On fine spatial scales, functional dispersion changed from clustering to overdispersion with increasing size classes. However, on larger spatial scales assemblages were functionally clustered for all size classes and habitats. 4. Phylogenetic and functional structure across spatial and size scales and habitats gave strong support for a deterministic model of species co-occurrence rather than for a neutral model. The results also support the hypothesis that abiotic determinism is more important at larger scales, while biotic determinism is more important on smaller scales within habitats.

Biodiversity Conservation in the REDD

Deforestation and forest degradation in the tropics is a major source of global greenhouse gas (GHG) emissions. The tropics also harbour more than half the worlds threatened species, raising the possibility that reducing GHG emissions by curtailing tropical deforestation could provide substantial co-benefits for biodiversity conservation. Here we explore the potential for such co-benefits in Indonesia, a leading source of GHG emissions from land cover and land use change, and among the most species-rich countries in the world. We show that focal ecosystems for interventions to reduce emissions from deforestation and forest degradation in Indonesia do not coincide with areas supporting the most species-rich communities or highest concentration of threatened species. We argue that inherent trade-offs among ecosystems in emission reduction potential, opportunity cost of foregone development and biodiversity values will require a regulatory framework to balance emission reduction interventions with biodiversity co-benefit targets. We discuss how such a regulatory framework might function, and caution that pursuing emission reduction strategies without such a framework may undermine, not enhance, long-term prospects for biodiversity conservation in the tropics.

Macaranga and Mallotus species (Euphorbiaceae) as indicators for disturbance in the mixed lowland dipterocarp forest of East Kalimantan (Indonesia)

The indicator value (IV) of Macarangaand Mallotus species (Euphorbiaceae) for different types of disturbance in lowland dipterocarp forest was assessed by counting and identifying all individuals of species of these genera taller than 30 cm in 45 (10 m × 300 m) plots at nine locations. Twelve Macarangaand nine Mallotus species were found. The main forest disturbance types (primary forest, secondary forest, selectively logged forest, forest burned once, and repeatedly burned forest used for shifting-cultivation) each had their own set of indicator species. The level of disturbance in the forest types was assessed by measuring nine forest structural parameters. The occurrence of Macarangaand Mallotusspecies was closely related to the level of disturbance in a forest. Most Macarangaspecies were characteristic of high disturbance levels, while most Mallotusspecies preferred intermediate to low levels of disturbance. However, both genera had species at both disturbance extremes. Using multiple regression analysis, combinations of Macarangaand Mallotus species were formed and used to predict the separate forest structural parameters and the general level of disturbance of a forest. The Macaranga and Mallotus species could be grouped into (1) primary forest ‘remnant’ species; (2) generalist pioneer species; and (3) high disturbance pioneer species.

Soils on exposed Sunda Shelf shaped biogeographic patterns in the equatorial forests of Southeast Asia

The marked biogeographic difference between western (Malay Peninsula and Sumatra) and eastern (Borneo) Sundaland is surprising given the long time that these areas have formed a single landmass. A dispersal barrier in the form of a dry savanna corridor during glacial maxima has been proposed to explain this disparity. However, the short duration of these dry savanna conditions make it an unlikely sole cause for the biogeographic pattern. An additional explanation might be related to the coarse sandy soils of central Sundaland. To test these two nonexclusive hypotheses, we performed a floristic cluster analysis based on 111 tree inventories from Peninsular Malaysia, Sumatra, and Borneo. We then identified the indicator genera for clusters that crossed the central Sundaland biogeographic boundary and those that did not cross and tested whether drought and coarse-soil tolerance of the indicator genera differed between them. We found 11 terminal floristic clusters, 10 occurring in Borneo, 5 in Sumatra, and 3 in Peninsular Malaysia. Indicator taxa of clusters that occurred across Sundaland had significantly higher coarse-soil tolerance than did those from clusters that occurred east or west of central Sundaland. For drought tolerance, no such pattern was detected. These results strongly suggest that exposed sandy sea-bed soils acted as a dispersal barrier in central Sundaland. However, we could not confirm the presence of a savanna corridor. This finding makes it clear that proposed biogeographic explanations for plant and animal distributions within Sundaland, including possible migration routes for early humans, need to be reevaluated.

Wood Density as a Conservation Tool: Quantification of Disturbance and Identification of Conservation-Priority Areas in Tropical Forests

Inventories of tree species are often conducted to guide conservation efforts in tropical forests. Such surveys are time consuming, demanding of expertise, and expensive to perform and interpret. Approaches to make survey efforts simpler or more effective would be valuable. In particular, it would be good to be able to easily identify areas of old-growth forest. The average density of the wood of a tree species is closely linked to its successional status. We used tree inventory data from eastern Borneo to determine whether wood density can be used to quantify forest disturbance and conservation importance. The average density of wood in a plot was significantly and negatively related to disturbance levels, with plots with higher wood densities occurring almost exclusively in old-growth forests. Average wood density was unimodally related to the diversity of tree species, indicating that the average wood density in a plot might be a better indicator of old-growth forest than species diversity. In addition, Borneo endemics had significantly heavier wood than species that are common throughout the Malesian region, and they were more common in plots with higher average wood density. We concluded that wood density at the plot level could be a powerful tool for identifying areas of conservation priority in the tropical rain forests of Southeast Asia.

Biodiversity inventory and informatics in Southeast Asia

Rapidly changing land use in Southeast Asia threatens plant diversity, and reduces the time we have left to document it. Despite over 200 years of scientific plant exploration, many plant species have yet to be discovered. Moreover, we still have a very poor understanding of the distribution of known taxa in this biogeographically complex region. We review the current state of biodiversity exploration, using plants in Indonesia as an example. Traditional methods of collecting and describing species have provided a solid foundation for our understanding of plant biodiversity, but are insufficient for the pragmatic task of rapidly discovering and documenting today’s biodiversity before it is gone, because general collecting expeditions tend to be infrequent, and documentation of most new species must await taxonomic revisions many years in the future. Solutions to this exploration and documentation crisis (i) could use the abundant resource of enthusiastic, networked, national biology students, (ii) should employ biodiversity informatics tools to efficiently engage both specialists and parataxonomists, and (iii) might require adoption of new types of α-taxonomy, utilizing increasingly low-cost molecular methods and high resolution photographs. We describe emerging technologies that will facilitate this taxonomic development. We believe that a new golden age of biodiversity exploration may be dawning, just as biodiversity itself is most threatened, and are hopeful that increasing knowledge of biodiversity will be a positive force to slow its loss.

Variation in forest soil fungal diversity along a latitudinal gradient

In forest ecosystems, plant communities shape soil fungal communities through the provisioning of carbon. Although the variation in forest composition with latitude is well established, little is known about how soil fungal communities vary with latitude. We collected soil samples from 17 forests, along a latitudinal transect in western China. Forest types covered included boreal, temperate, subtropical and tropical forests. We used 454 pyrosequencing techniques to analyze the soil communities. These data were correlated with abiotic and biotic variables to determine which factors most strongly influenced fungal community composition. Our results indicated that temperature, latitude, and plant diversity most strongly influence soil fungal community composition. Fungal diversity patterns were unimodal, with temperate forests (mid latitude) exhibiting the greatest diversity. Furthermore, these diversity patterns indicate that fungal diversity was highest in the forest systems with the lowest tree diversity (temperate forests). Different forest systems were dominated by different fungal subgroups, ectomycorrhizal fungi dominated in boreal and temperate forests; endomycorrhizal fungi dominated in the tropical rainforests, and non-mycorrhizal fungi were best represented in subtropical forests. Our results suggest that soil fungal communities are strongly dependent on vegetation type, with fungal diversity displaying an inverse relationship to plant diversity.