Tree species composition and diversity gradients in white-water forests across the Amazon Basin

Aim Attention has increasingly been focused on the floristic variation within forests of the Amazon Basin. Variations in species composition and diversity are poorly understood, especially in Amazonian floodplain forests. We investigated tree species composition, richness and α diversity in the Amazonian white‐water (várzea) forest, looking particularly at: (1) the flood‐level gradient, (2) the successional stage (stand age), and (3) the geographical location of the forests. Location Eastern Amazonia, central Amazonia, equatorial western Amazonia and the southern part of western Amazonia. Methods The data originate from 16 permanent várzea forest plots in the central and western Brazilian Amazon and in the northern Bolivian Amazon. In addition, revised species lists of 28 várzea forest inventories from across the Amazon Basin were used. Most important families and species were determined using importance values. Floristic similarity between plots was calculated to detect similarity variations between forest types and over geographical distances. To check for spatial diversity gradients, α diversity (Fisher) of the plots was correlated with stand age, longitudinal and latitudinal plot location, and flood‐level gradient. Results More than 900 flood‐tolerant tree species were recorded, which indicates that Amazonian várzea forests are the most species‐rich floodplain forests worldwide. The most important plant families recorded also dominate most Neotropical upland forests, and c. 31% of the tree species listed also occur in the uplands. Species distribution and diversity varied: (1) on the flood‐level gradient, with a distinct separation between low‐várzea forests and high‐várzea forests, (2) in relation to natural forest succession, with species‐poor forests in early stages of succession and species‐rich forests in later stages, and (3) as a function of geographical distance between sites, indicating an increasing α diversity from eastern to western Amazonia, and simultaneously from the southern part of western Amazonia to equatorial western Amazonia. Main conclusions The east‐to‐west gradient of increasing species diversity in várzea forests reflects the diversity patterns also described for Amazonian terra firme. Despite the fine‐scale geomorphological heterogeneity of the floodplains, and despite high disturbance of the different forest types by sedimentation and erosion, várzea forests are dominated by a high proportion of generalistic, widely distributed tree species. In contrast to high‐várzea forests, where floristic dissimilarity increases significantly with increasing distance between the sites, low‐várzea forests can exhibit high floristic similarity over large geographical distances. The high várzea may be an important transitional zone for lateral immigration of terra firme species to the floodplains, thus contributing to comparatively high species richness. However, long‐distance dispersal of many low‐várzea trees contributes to comparatively low species richness in highly flooded low várzea.     

Mapping gradual landscape-scale floristic changes in Amazonian primary rain forests by combining ordination and remote sensing

Aim We present a new method to economically map gradual changes in plant species composition in lowland rain forests using field data and satellite images. Such a method will be a useful tool in planning the sustainable use and conservation of Amazonian rain forests. Location The study covered an area of c. 700 km² of primary rain forest in Amazonian Ecuador. Methods We field inventoried the species composition of pteridophytes and Melastomataceae in 340 inventory plots (5 m × 50 m), described the prevailing topography and analysed soil cation concentration and texture. We used non‐metric multidimensional scaling (NMDS) to summarize the floristic variation among the inventory plots in three ordination dimensions. The scores of the three ordination axes were predicted to non‐visited places using a Landsat TM (thematic mapper) satellite image and the k nearest neighbours (knn) estimation method. To avoid extrapolation, we excluded from the analysis those pixel windows whose spectral values were not represented in the areas covered by field sampling. The accuracy of the predictions was evaluated by cross‐validation and by comparing the predictions based on spectrally nearest neighbours to the predictions based on random neighbours. Results The floristic gradients presented by NMDS ordination were interpretable in terms of topography, drainage and soil cation content. Thirteen percent of the cloud‐free pixels were excluded from the knn analysis to avoid extrapolation. The estimates of the floristic ordination scores based on spectrally nearest neighbours were always more accurate than estimates based on random neighbours. Main conclusions The presented method needs a relatively small input of work and resources, is mechanistic and produces maps that give relevant information on floristic variation over forest areas that are traditionally considered essentially homogeneous. Therefore, the method appears to have a great potential for use in mapping large areas of Amazonian rain forests.     

Spatial distribution and floristic composition of trees and lianas in different forest types of an Amazonian rainforest

A quantitative inventory of trees and lianas was conducted (1) to compare floristic composition, diversity and stem density variation between three different forest types (tierra firme, floodplain and swamp), and (2) to analyse the relationships between floristic similarity and forest structure in two regions ~60 km apart in Yasuní National Park, Amazonian Ecuador. A total of 1,087 species with a diameter at breast height ≥ 2.5 cm were recorded in 25 0.1-ha plots. Tierra firme was the habitat with the highest number of species and stem density for trees and lianas, followed by floodplain and swamp in both regions. Two hypotheses that have been independently proposed to describe plant distribution in tropical rain forests, together explain species spatial distribution in this study. The fact that the 30 most important species per forest type (totalling 119 species) accounted for 48.2% of total individuals supports the oligarchy hypothesis. Likewise, 28 out of these 119 species are reported as restricted to a single forest type, which supports the environmental-determinism hypothesis. In general, both canopy and understorey trees and lianas showed rather similar floristic patterns across different forest types and regions.     

Elevational gradient and vegetation‐environmental relationships in the central Hyrcanian forests of northern Iran

Tertiary‐relict Hyrcanian (Caspian) forest along the shores of the southern Caspian Sea is a center of biodiversity. Still, there is little information on plant diversity patterns in this area. This study evaluated plant diversity, variation in life forms, and geographical distribution of the zonal vegetation types and their relationships with environmental variables, in the educational and experimental forest of Kheyrudkenar, an important protected area in the central Hyrcanian forest of northern Iran. For this purpose, 226 vegetation plots of 400 m² were laid out along two altitudinal transects from the lowlands (100 m a.s.l.) to the timberline (2000 m a.s.l.). Four vegetation types were identified using modified TWINSPAN, indirect and direct gradient analyses. Species‐related (species diversity indices, life form and phytogeographical elements) and environmental variables (climate, topographic and soil variables) were calculated and subjected to one‐way ANOVA among the vegetation types. Both constrained (CCA) and unconstrained (DCA) ordination analyses showed an almost identical variation of the floristic composition along their axes and demonstrated that there are two main gradients in the Hyrcanian forest. Elevation together with annual precipitation and mean annual temperature were the most important factors controlling the floristic composition in the area. Topographic features such as slope inclination and heat index were found to be important within an elevation zone/vegetation type. Soil physical and chemical properties were of secondary importance for the separation of the vegetation types. This knowledge will be useful for forest management and conservation practices in the Hyrcanian area with its distinct and unique flora and vegetation.     

A transitional xerophytic tropical plant community of the Cape Region,Baja California

Abstract. Our main objective was to use comparative floristic and structural criteria in order to understand the differences in assemblages brought about by the meeting of arid and dry‐tropical environments. We sampled 33 vegetation plots and recorded all perennial species. The data were subjected to multivariate analysis, including Principal Component Analysis (PCA). The floristic variation detected by the PCA was strongly related to altitude and rainfall. The gradient detected by the ordination procedure had Tropical deciduous forest (TDF) and Sonoran Desert (SD) at opposite extremes and Cape sarcocaulescent shrubland (CSCS) in an intermediate position. The numerical classification algorithm detected six distinct groups of species with clearly identifiable field distributions. Vegetation structure also differed significantly between the regions. SD had the lowest species‐richness (α‐diversity) and inter‐site similarity in the CSCS was highest, indicating that this environment is relatively uniform and has low species turnover (β‐diversity). It is concluded that the CSCS is distinctly different from the sarcocaulescent shrublands of the more northern plant communities of the peninsula, where Sonoran Desert floristic affinities prevail. The floristic composition of the CSCS is rich in dry‐tropical affinities, its species richness is higher, it is homogeneous in its species content and turnover and is more dense with a taller canopy than the northern desert scrubs. It is proposed that the boundary line separating the Cape sediments of granitic origin from the basalt‐derived sediments of the northern‐lying Sierra de la Giganta should be used as an easily identifiable landscape trait to delimitate this unique community.     

Floristic patterns of epiphytes in the Brazilian Atlantic Forest,a biodiversity hotspot

To assess the main factors driving epiphytic angiosperm distribution throughout the Brazilian Atlantic Forest, we compiled 57 floristic surveys and analysed species composition under the influence of environmental variables, space and vegetation type using canonical correspondence analysis (CCA), similarity (Sorensen) and Mantel's tests. The indicator value index (IndVal) was used to find indicator species of each Brazilian Atlantic Forest vegetation type. Group sharpness analysis was performed in order to determine the appropriate group partition level. CCA showed a separation of the epiphytic flora reflecting temperature and rainfall gradients. Mantel's test showed that environment and space were highly correlated with floristic similarity. Cluster analysis, indicating floristic similarity, resolved five groups, mainly grouped by region. Clear differentiation of the Brazilian Atlantic Forest epiphytic flora on a north–south axis with a strong correlation with temperature and rainfall gradients was found. The role of space and environment on species composition varied according to distinct epiphytic species groups. In particular, for Bromeliaceae and Orchidaceae, the main factor associated with floristic similarity was space. Indicator species were found for all vegetation types apart from the Seasonal Semideciduous Forest that seemed to represent a subset of a more humid forest type. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 179 , 587–601.     

Ethnobotanical ground‐truthing: indigenous knowledge,floristic inventories and satellite imagery in the upper Rio Negro,Brazil

Aim To assess the utility of indigenous habitat knowledge in studies of habitat diversity in Amazonia. Location Baniwa indigenous communities in Rio Içana, upper Rio Negro, Brazil. Methods Six campinarana vegetation types, recognized and named by a consensus of Baniwa indigenous informants according to salient indicator species, were studied in 15 widely distributed plots. Floristic composition (using Baniwa plant nomenclature only, after frustrated attempts to obtain botanical collection permits), quantitative measures of forest structure and GPS waypoints of the 4‐ha composite plot contours were registered, permitting their location on Landsat satellite images. Non‐metric multidimensional scaling (NMDS) ordination was carried out using pc‐ord software. Results The NMDS ordinations of the plot data revealed a clear gradient of floristic composition that was highly correlated with three quantitative measures of forest structure: basal area, canopy height and satellite reflectance. Main conclusions Baniwa‐defined forest types are excellent predictors of habitat diversity along the structural gradient comprising distinctive white‐sand campinarana vegetation types. Indigenous ecological knowledge, as revealed by satellite imagery and floristic analyses, proves to be a powerful and efficient shortcut to assessing habitat diversity, promoting dialogue between scientific and indigenous worldviews, and promoting joint study and conservation of biodiversity.     

Environmental drivers of tree community turnover in western Amazonian forests

A more comprehensive understanding of the factors governing tropical tree community turnover at different spatial scales is needed to support land‐management and biodiversity conservation. We used new forest inventory data from 263 permanent plots in the Carnegie Biodiversity‐Biomass Forest Plot Network spanning the eastern Andes to the western Amazonian lowlands of Peru to examine environmental factors driving genus‐level canopy tree compositional variation at regional and landscape scales. Across the full plot network, constrained ordination analysis indicated that all environmental variables together explained 23.8% of the variation in community composition, while soil, topographic, and climatic variables each explained 15.2, 10.9, and 17.0%, respectively. A satellite‐derived metric of cloudiness was the single strongest predictor of community turnover, and constrained ordination revealed a primary gradient of environmentally‐driven community turnover spanning from cloudy, high elevation sites to warm, wet, lowland sites. For three focal landscapes within the region, local environmental variation explained 13.4–30.8% of compositional variation. Community turnover at the landscape scale was strongly driven by topo‐edaphic factors in the two lowland landscapes examined and strongly driven by potential insolation and topography in the montane landscape. At the regional scale, we found that the portion of compositional variation that was uniquely explained by spatial variation was relatively small (2.7%), and was effectively zero within the three focal landscapes. Overall, our results show strong canopy tree compositional turnover in response to environmental gradients at both regional and landscape scales, though the most important environmental drivers differed between scales and among landscapes. Our results also highlight the usefulness of key satellite‐derived environmental covariates that should be considered when conducting biodiversity analyses in tropical forests.     

Avian Composition Co‐varies with Floristic Composition and Soil Nutrient Concentration in Amazonian Upland Forests

Spatial heterogeneity in the plant species composition of tropical forests is expected to influence animal species abundance and composition because vegetation constitutes the primary habitat feature for forest animals. Floristic variation is tied to variation in soils, so edaphic properties should ultimately influence animal species composition as well. The study of covariation in floristic and faunistic turnover has been hindered by the difficulty of completing coordinated surveys in hyperdiverse tropical communities, but this can be overcome with the use of a few plant taxa that function as surrogates for general floristic turnover. We used avian and plant transect surveys and soil sampling in a western Amazonian upland (terra firme) forest landscape to test whether spatial variation in bird community composition is associated with floristic turnover and corresponding edaphic gradients. Partial Mantel tests and Non‐metric Multidimensional Scaling showed floristic distinctiveness between two forest types closely associated with differences in soil cation concentrations, and differences in both floristic composition and cation concentrations were further linked to compositional differences in avian species, independent of geographic distances among sites. Ten percent of bird species included in Indicator Species Analyses showed significant associations with one of the two forest types. The upland forest types that we sampled, each corresponding to a different geological formation, are intermediate relative to edaphically extreme environments in the region. Models of avian diversification should take into account this environmental heterogeneity, as should conservation planning approaches that aim to represent faunal diversity. Abstract in Spanish is available in the online version of this article.     

Understorey vegetation in boreal Picea mariana and Populus tremuloides stands in British Columbia

Abstract. We compared the species composition and species density of vascular plants in the understorey vegetation of boreal forest between Picea mariana (Black spruce) and Populus tremuloides (Trembling aspen) stands in British Columbia, Canada, and related differences in species composition and species density between the two forest types to dominant canopy tree species as well as a wide variety of environmental factors. We analysed 231 stands, distributed in three different climatic regions representing drier, wetter, and milder variations of montane boreal climate. Of these stands 118 were dominated by P. mariana and 113 by P. tremuloides. P. tremuloides stands had higher species density than P. mariana stands in all climatic regions, but species density of each dominance type varied among climatic regions. The floristic composition of the understorey vegetation was markedly different for P. mariana and P. tremuloides dominated stands. A detailed study on the effect of canopy dominance and local environmental factors on the understorey vegetation of the boreal forest was conducted using 88 stands from one of the three climatic regions. Using a combination of ordination and variation partitioning by constrained ordination we demonstrated a small but unique effect of canopy dominance type on the understorey vegetation, while a larger amount of compositional variation was shared with other factors. Our results accord with a scenario in which differences in primary environmental factors and humus form properties, the latter accentuated by the canopy dominants themselves, are the most important causes of higher species density in P. tremuloides stands than in P. mariana stands, as well as differences in species composition among the two canopy dominance types. Processes and time scales involved in the small but significant direct and indirect effects of the canopy dominant on understo‐ rey species composition are discussed.     

Effects of age and distance on the composition of mixed deciduous forest fragments in an agricultural landscape

Abstract. Forest patches in central Belgium were inventoried twice for the presence or absence of forest plant species to study the effects of age and distance on species composition. All forests in the study area were subdivided based on their land use history. To avoid effects of autocorrelated environmental characteristics on species composition, habitat homogeneity was indirectly investigated using a TWINSPAN classification of the vegetation data. Two major habitats (alluvial and non‐alluvial forests) were distinguished and analysed separately. Patterns of species composition were investigated at the landscape level using Mantel tests. Species composition similarity measures were calculated between all pairs of fragments based on the floristic data. A highly significant correlation was found between species composition similarity and inter‐patch distance. Difference in age, which we used as a measure for habitat quality, was less important in explaining species composition patterns. The effects of spatial configuration became significant when difference in age was accounted for, and a partial correlation was calculated between inter‐patch distance and species composition similarity. Different results were found for alluvial and non‐alluvial forest types. Alluvial forests were more influenced by the spatial configuration than the non‐alluvial. For the non‐alluvial forest type effects measured with the difference in age between forest fragments obscured the effects of inter‐patch distance. Based on our findings we suggest that species composition is not only internally generated, but external processes such as differential colonization caused by varying degrees of isolation may be of overriding importance.     

Indicators of restoration success in riparian tropical forests using multiple reference ecosystems

Forest restoration by planting trees often accelerates succession, but the trajectories toward reference ecosystems have rarely been evaluated. Using a chronosequence (4–53 years) of 26 riparian forest undergoing restoration in the Brazilian Atlantic Forest, we modeled how the variables representing forest structure, tree species richness and composition, and the proportion of plant functional guilds change through time. We also estimated the time required for these variables to reach different types of reference ecosystems: old‐growth forest (OGF), degraded forest, and secondary forest. Among the attributes which follow a predictable trajectory over time are: the basal area, canopy cover, density and tree species richness, as well as proportions of shade tolerant and slow growing species or individuals. Most of the variation in density of pteridophythes, lianas, shrubs and phorophythes, proportion of animal‐dispersed individuals, rarefied richness and floristic similarity with reference ecosystems remain unexplained. Estimated time to reach the reference ecosystems is, in general, shorter for structural attributes than for species composition or proportion of functional guilds. The length of this time varies among the three types of reference ecosystems for most attributes. For instance, tree species richness and proportion of shade tolerant or slow growing individuals become similar to secondary forests in about 40 years, but is estimated to take 70 years or more to reach the OGF. Of all the variables considered, canopy cover, basal area, density, and richness of the understory—by their ecological relevance and predictability—are recommended as ecological indicators for monitoring tropical forest restoration success.     

Regional-Scale Drivers of Forest Structure and Function in Northwestern Amazonia

Field studies in Amazonia have found a relationship at continental scales between soil fertility and broad trends in forest structure and function. Little is known at regional scales, however, about how discrete patterns in forest structure or functional attributes map onto underlying edaphic or geological patterns. We collected airborne LiDAR (Light Detection and Ranging) data and VSWIR (Visible to Shortwave Infrared) imaging spectroscopy measurements over 600 km² of northwestern Amazonian lowland forests. We also established 83 inventories of plant species composition and soil properties, distributed between two widespread geological formations. Using these data, we mapped forest structure and canopy reflectance, and compared them to patterns in plant species composition, soils, and underlying geology. We found that variations in soils and species composition explained up to 70% of variation in canopy height, and corresponded to profound changes in forest vertical profiles. We further found that soils and plant species composition explained more than 90% of the variation in canopy reflectance as measured by imaging spectroscopy, indicating edaphic and compositional control of canopy chemical properties. We last found that soils explained between 30% and 70% of the variation in gap frequency in these forests, depending on the height threshold used to define gaps. Our findings indicate that a relatively small number of edaphic and compositional variables, corresponding to underlying geology, may be responsible for variations in canopy structure and chemistry over large expanses of Amazonian forest.     

Peatland forests are the least diverse tree communities documented in Amazonia,but contribute to high regional beta‐diversity

Western Amazonia is known to harbour some of Earth's most diverse forests, but previous floristic analyses have excluded peatland forests which are extensive in northern Peru and are among the most environmentally extreme ecosystems in the lowland tropics. Understanding patterns of tree species diversity in these ecosystems is important both for quantifying beta‐diversity in this region, and for understanding determinants of diversity more generally in tropical forests. Here we explore patterns of tree diversity and composition in two peatland forest types – palm swamps and peatland pole forests – using 26 forest plots distributed over a large area of northern Peru. We place our results in a regional context by making comparisons with three other major forest types: terra firme forests (29 plots), white‐sand forests (23 plots) and seasonally‐flooded forests (11 plots). Peatland forests had extremely low (within‐plot) alpha‐diversity compared with the other forest types that were sampled. In particular, peatland pole forests had the lowest levels of tree diversity yet recorded in Amazonia (20 species per 500 stems, Fisher's alpha 4.57). However, peatland pole forests and palm swamps were compositionally different from each other as well as from other forest types in the region. Few species appeared to be peatland endemics. Instead, peatland forests were largely characterised by a distinctive combination of generalist species and species previously thought to be specialists of other habitats, especially white‐sand forests. We suggest that the transient nature and extreme environmental conditions of Amazonian peatland ecosystems have shaped their current patterns of tree composition and diversity. Despite their low alpha‐diversity, the unique combination of species found in tree communities in Amazonian peatlands augment regional beta‐diversity. This contribution, alongside their extremely high carbon storage capacity and lack of protection at national level, strengthens their status as a conservation priority.     

Riqueza y Distribución Ecológica de Especies de Pteridofitas en la Zona del Río Yavarí-Mirín, Amazonía Peruana

We studied the ecological distribution of pteridophytes (ferns and fern allies) along eight 8-km transects covering 12.7 ha in Peruvian Amazonia. Subunits of 200 m² of the transects have previously been classified into four different forest types, and here we document and quantify the floristic differences among these forest types. Pteridophytes have been suggested as an indicator group to classify rain forest habitats, but this requires that the ecological preferences of the species are well documented and consistent across geographic regions. Here we analyzed in detail the distribution and diversity patterns of 130 species across the four rain forest types. Relative species abundance and species diversity were similar among some of the forest types and differed among others, but the species composition differed markedly. Our results largely confirmed the earlier interpretation of the edaphic preferences of the pteridophyte species in western Amazonia. This supports the proposition that deterministic processes have an important role in influencing the floristic composition of Amazonian forests.     

Diversity and biogeography of vascular epiphytes in Western Amazonia, Yasuní, Ecuador

Aim Although vascular epiphytes are important components of species richness and complexity of Neotropical forests, vascular epiphytes are under‐represented in large scale biogeographical analyses. We studied the diversity, biogeography and floristic relationships of the epiphytic flora of the Yasuní region (Western Amazonia) in a Neotropical context, with special emphasis on the influence of the Andean flora on floristic composition and diversity of surrounding lowland forests. Location Western Amazonian lowland rainforest, Tiputini Biodiversity Station (0°38′ S 76°09′ W, 230 m a.s.l., 650 ha), Yasuní National Park, Ecuador. Methods We compared the vascular epiphyte flora of Yasuní with 16 published Neotropical epiphyte inventories. Secondly, based on a floristic database with records of more than 70,000 specimens of vascular epiphytes from the Neotropics the elevational composition of eight selected inventories was analysed in detail. Results The vascular epiphyte flora of Yasuní is characterized by a very high species richness (313 spp.). A moderate portion of species is endemic to the Upper Napo region (c. 10%). However, this figure is much higher than previous analyses primarily based on woody species suggested. Geographical ranges of these species match with a proposed Pleistocene forest refuge. Compared with Northern and Central Amazonian sites, Western Amazonian epiphyte communities are characterized by a higher portion of montane and submontane species. Species richness of vascular epiphytes at the sites was correlated with the amount of rainfall, which is negatively correlated with the number of dry months. Main conclusion Recent and historic patterns of rainfall are the driving forces behind diversity and floristic composition of vascular epiphytes in Western Amazonia: high annual rainfall in combination with low seasonality provides suitable conditions to harbour high species richness. The proximity to the Andes, the most important centre of speciation for most Neotropical epiphytic taxa, in combination with the climatic setting has allowed a continuous supply of species richness to the region. At least for epiphytes, the borderline between the Andean and Amazonian flora is much hazier than previously thought. Moreover, the comparatively moist climate in Western Amazonia during the Pleistocene has probably led to fewer extinctions and/or more speciation than in more affected surrounding lowlands.     

Pollen–vegetation relationships along steep climatic gradients in western Amazonia

Question: How accurately do Amazonian montane forest pollen spectra reflect the vegetation? Can compositional changes observed in the vegetation along environmental gradients be identified in the pollen spectra? How well do herbarium collection data and bioclimatic envelopes represent abundance changes along elevation gradients? Location: Amazonian montane forests, Peru. Methods: Moss polsters collected along five altitudinal transects spanning over 3000 m a.s.l. were used to characterize pollen spectra. Vegetation plot data from a network of 15 1‐ha permanent plots were used to correlate pollen spectra with present‐day vegetation. Probability density functions (PDFs) fitted to pollen and plot data allowed comparisons using Spearman correlation coefficients. Ordination analyses were used to summarize changes in pollen spectra. Correlations between pollen‐based PDFs and previously‐published herbarium collection PDFs were also evaluated. Results: Pollen spectra closely reflected changes in species composition along elevation gradients. A mid‐elevation shift in pollen spectra was identified using ordination analyses. Pollen spectra from the driest forest in our data set were statistically different from those of wet forests. Pollen abundance PDFs along the altitudinal gradient were significantly correlated (P<0.01) with PDFs fitted to plot abundance, basal area and herbarium collection data for ten out of 11 taxa analysed. Conclusions: Pollen spectra closely reflected the vegetation composition of Amazonian montane forests. The differentiation of pollen spectra from dry localities showed the potential of genus‐level pollen data to reflect precipitation gradients. Pollen spectra also reflected mid‐elevation compositional changes well along the lower elevation limit of ground cloud formation. Despite collection biases, herbarium‐based bioclimatic envelope PDFs also represented well forest compositional changes along elevation gradients.     

Modern pollen rain in mangroves from San Andres Island, Colombian Caribbean

The precise characterisation of present-day mangrove ecosystems from modern pollen rain facilitates the accurate use of fossil pollen data for late Quaternary sea level and environmental reconstructions. Here, we investigate whether the analysis of pollen rain data corroborates existing floristic and structural characterisation of different mangrove types at the Caribbean island of San Andrés, Colombia. At 82 plots along 20 transects of four distinct mangrove types, samples were obtained of (i) surface sediments for pollen analysis, and (ii) a range of environmental parameters (including inundation levels, salinity and pH). This information was compared to previously sampled mangrove composition and tree basal area. In surface sediment samples 82 pollen taxa were found, from which 19 were present in the vegetation plots. However, because pollen may be transported by wind and/or watercourses, the overall floristic composition of the different forest types may not necessarily be reflected by the pollen spectra. Local vegetation (i.e. mangroves and beach) represented > 90% of the pollen spectra, while the regional one (i.e. hinterland forests) represented < 5% of it. Unlike the four mangrove types that were previously described in the vegetation, the analysis of pollen samples suggested only three distinct types of forest.The groups were characterised based on (i) the dominance of at least one of the true mangrove species from pollen data ordination and the presence of associated species, and (ii) their relationship with environmental parameters. Rhizophora was present in all plot samples, but did not contribute to forest type separation. In fact, just three true mangrove species proved reliable indicators of (i) high salinity and fringe mangroves (i.e. Avicennia), (ii) high pH levels and landward mangroves (i.e. Conocarpus), and (iii) natural or anthropogenic caused disturbance of forest stands (Laguncularia and associated Acrostichum fern). Hence our study confirms that mangrove pollen spectra can be accurately used to describe different mangrove environments for fossil based palaeoecological reconstructions.     

Forest structure and tree species composition of the understory of two central Amazonian várzea forests of contrasting flood heights

We investigated species composition, distribution, and forest structure of understory trees (≥1 m height, <10 cm diameter at breast height) in two late-successional várzea forests subject to contrasting levels of inundation within the Mamirauá Sustainable Development Reserve, western Brazilian Amazon, and compared it with the overstory flora at the same study sites. In total, 1486 individuals and 116 woody species were recorded on an area totaling 3140 m². Individual densities and tree species richness were considerably higher in the high várzea than in the low várzea, which suggests that the heights and durations of the annual inundations are the main factor limiting species regeneration. In addition, approximately one third of the recorded species with densities ≥8 individuals showed regular or random spatial distribution patterns, which suggests that floodwaters act on dispersal strategies and species establishment.Independent of the forest type, floristic similarity between the understory and the overstory amounted to approximately 35%, and to approximately 10% when compared to other understory inventories in Amazonian várzea. Although the inventoried area of the understory amounted to only 16% of that of the overstory, species richness accounted for approximately 52-56% of that of the overstory. The results indicate that the understory flora of várzea forests is distinct and that it significantly increases local tree species richness. The understory flora of várzea forests therefore should be addressed in floristic inventories that provide the basis for regional and/or basin-wide estimations of tree diversity.     

Plant composition associated with environmental gradients in tropical montane forests (Cueva de Los Guacharos National Park,Huila, Colombia)

Niche differentiation among tropical forest plants can generate species turnover along gradients of soil, topography, climate, and land use history. In this study we explore the relative importance of these variables as drivers of floristic composition in Cueva de Los Guacharos National Park. We established twenty 0.1‐ha plots, within which trees, lianas, and shrubs (diameter ≥ 2.5 cm) were censused. We selected plot locations in primary and disturbed forests, and we measured topography and soil variables. Despite their structural similarity, primary and disturbed forests differed floristically, and also differed in environmental variables measured. A NMDS ordination showed that variation in the floristic composition across plots is highly correlated to the exchangeable acidity, elevation, temperature, and magnesium availability. Variance partitioning analysis shows that together spatial and environmental variables explain 24.2 percent of the variation in species composition. ‘Pure environmental’ variables were more important in explaining compositional variability than ‘pure spatial’ processes (9.8% and 1.4%, respectively). Residual variance may be attributed to stochastic process or non‐measured biotic effects.