Soil factors determining the change in forests between dry and wet Chacos

This work consists of an extensive study of soil properties and floristic composition of the Paraguayan Chacoan forests. The role of the soil factors was determined conditioning the classical landscape differentiation between the dry western Chaco and the wet eastern Chaco. Soil horizon A best explains the previous classification of the forests. It highlights the soil differences between the eastern and western Chaco. CCA ordination including all forest, soil and geographical factors primarily arranged the plant associations along a W-E gradient. A significant relationship was found between the composition of the northern Paraguayan Chaco forest and geographic longitude, drainage and altitude. CCA ordination focusing on alluvial forests and soil features identified clay content as the primary soil factor discriminating forest types; it also underlined the importance of cation exchange and the content of exchange bases in the soil for differentiating dry and wet Chacoan forests.     

Geographical zonation in the Neotropics of tree species characteristic of the Paraguay-Paraná Basin

Aim This paper uses data from the literature (monographs and taxonomic reviews) to investigate the geographical zonation in South America of 32 common tree species encountered in Paraguay. The actual spatial distribution of the species is then used to provide clues on the plant communities present in the past, and especially during the Wisconsinan age (Last major ice age: 80,000–10,000 bp ). The floristic relatedness between the Paraguayan flora and the neighbouring floras is also investigated. Methods The main vegetation patterns were highlighted using a Discriminant Analysis of the Eigenvectors of Neighbourhood Operator to emphasize the geographical zonation in South America of 32 tree species predominant in the Paraguay‐Paraná Basin. Results Three main vegetation patterns were emphasized: the Chaco, the Paraná‐São Francisco gradient and the Peri‐Amazonian gradient. The Chaco is well defined. The Peri‐Amazonian gradient is characterized by a continuous change in species from the Colombian pole towards the São Francisco pole. The São Francisco gradient shows a continuous change from the São Francisco pole towards the Paraná pole. Some of the species are monocentric (related to only one dispersal centre), whereas others are polycentric (distributed in several poles). Main conclusions The Chaquean xeromorphic forests are considered as a climax, with a stable composition resulting of the saline soil conditions. The Chaco acts as an edaphic barrier to many species, although the migration of some non‐Chaquean species is made possible by the net of gallery‐forests and low montane forests between the Río Paraguay and the Andean Piedmont. The Cerrado and the Chaco can be considered as a barrier limiting the expansion of the Paranean forest. This latter formation is a semi‐deciduous forest made of a mix of monocentric and polycentric elements. The numerous species found both in Colombia and in Paraná indicate that bridges exist, or have existed, between these two poles. The persistence in the Paraguay‐Paraná Basin of a moist forest made of the Paranean assemblages highlighted in this study confirms the hypothesis of moister and cooler phases than today.     

Native woody vegetation in central Argentina: Classification of Chaco and Espinal forests

Question

What are the composition and spatial patterns of native woody plant communities in the southern Great Chaco and Espinal?

Location

Córdoba Province, central Argentina, an area of ca. 161,000 km².

Methods

We collected 351 geo‐referenced relevés representative of the geographic, topographic and ecological variation of the Chaco and Espinal woody vegetation in central Argentina. The relevés were classified into vegetation types using the hierarchical ISOPAM method. Forest and shrubland types were described on the basis of diagnostic species occurrences and their distribution in relation to environmental factors. A map of the actual vegetation derived from remote‐sensed images (Landsat) and field data was used to describe the current distribution and abundance of the different vegetation types.

Results

The classification of the 351 plots × 837 species matrix revealed two major clusters comprising seven woody vegetation types corresponding to Chaco lowland and mountain forests and shrublands, Espinal forests and edaphic vegetation. The most important gradients in woody vegetation types are related to elevation, temperature and rainfall variables.

Conclusions

Subtropical seasonally dry woody plant communities from the southern extreme of the Great Chaco and Espinal forests were described for the first time based on complete floristic data. Our results show that lowland Chaco native forests, as well as replacement communities, are still present in its southern distribution range and are well distinguishable from other vegetation types such as the Espinal and mountain forests. Overall, extensive Espinal forests have almost disappeared while Chaco vegetation is highly fragmented and degraded.

     

Vegetation in Danish beech forests: the importance of soil,microclimate and management factors,evaluated by variation partitioning

The importance of management regime on floristic variation (mosses and vascular plants) in four Danish beech forests was investigated. Sixty-four blocks were sampled, representing beech stands of different age and management types. Nineteen potential explanatory variables were recorded and tested with Monte-Carlo tests and Canonical Correspondence Analysis. In addition results were evaluated by use of Detrended Correspondence Analysis. Explanatory variables were divided into three groups; soil, microclimatic and management parameters. The amount of variation explained by each group of variables was calculated by use of variation partitioning. The group consisting of management variables explained most variation, on local as well as regional scale. Management related variables explained more variation in vegetation than any other variables together. This indicates the importance of management as determining species composition in Danish beech forests. Management related variables explained most variation on local scale. On a regional scale, soil parameters explained the major part of the variation. The results suggest that thirty years without management are sufficient to change species composition significantly, as compared to managed forests.     

Is there environmental differentiation in the <Emphasis Type="Italic">Quercus</Emphasis>-dominated forests of west-central Mexico?

Detailed information on 38 species and 26 environmental variables was recorded from a network of 86 permanent plots across a geographical range of 10 km, in order to determine the patterns of floristic composition in Quercus-dominated forests; to elucidate environmental differentiation in such forests; and to determine whether species are partitioning their environment. To examine likely patterns of floristic composition, a data matrix expressed as relative volume + relative density was used to run non-metric multidimensional scaling. Canonical correspondence analysis extracted the environmental variation that best correlates with the observed patterns of floristic composition. Our results indicate that congeneric Quercus individuals represent the largest proportion of the species pool in the study plots. They coexist with other species having similar ecological requirements in at least three distinct floristic groups. Examination of the two largest groups and their species compositions reveals that one floristic gradient runs across the most xeric zone of the study area, and the second major floristic gradient runs across a mesic zone. The most important environmental variable explaining the observed patterns of floristic composition is altitude, although partial canonical correspondence analysis suggests that micro-habitat heterogeneity (catena position and canopy maturity) was most significant.     

The role of dispersal ability, climate and spatial separation in shaping biogeographical patterns of phylogenetically distant plant groups in seasonally dry Andean forests of Bolivia

Aim To assess biogeographical patterns of Acanthaceae, Bromeliaceae, Cactaceae and Pteridophyta in Bolivian Andean seasonally dry forest islands and to explain current floristic differences between these islands by means of extrinsic (precipitation, elevation) and intrinsic (dispersal ability) factors. Location Ten isolated and disjunct seasonally dry forest areas in the Bolivian Andes and the adjacent seasonal forest areas of the Chiquitanía and Chaco regions. Methods We collated species data from recently updated and revised taxonomic treatments and herbarium collections for Acanthaceae, Bromeliaceae, Cactaceae and Pteridophyta, constructed floristic distance matrices to estimate beta diversity at the study sites and subjected them to Mantel correlation analyses. Multiple regressions on distance matrices allowed us to test the influence of geographical distance and environmental (elevation and precipitation) differences on floristic differentiation. Results Acanthaceae and Bromeliaceae, and to a lesser extent Cactaceae, showed coincident biogeographical patterns and suggested the presence of two seasonally dry forest groups in Andean Bolivia: one including all small isolated northern dry valleys and another including all southern valleys with connections to the lowland seasonal forests of the Chiquitanía and the Chaco. Most of the variation in the floristic distance matrices in these plant groups, with seed dispersal typically restricted to short distances, was explained by spatial separation between habitat islands. In contrast, pteridophytes showed a different biogeographical pattern. Their floristic differences between sites were determined by the environmental variables. The anemochorous and spore‐based dispersal system of this plant group seems to be a highly effective mechanism allowing pteridophytes to easily reach even the isolated dry valleys in inter‐Andean Bolivia. Main conclusions Current biogeographical patterns in dry Andean habitat islands can provide insights into the factors that control the processes of community assembly. We show that differences in community composition of phylogenetically distant plant groups in the understorey of seasonally dry forest islands can be explained by a combination of the habitat characteristics where the group is present (either precipitation, elevation or both) and, more interestingly, by group specific dispersal limitation (as inferred by geographical distance between island habitats).     

Patterns and Determinants of Floristic Variation across Lowland Forests of Bolivia

Floristic variation is high in the Neotropics, but little is known about the factors shaping this variation at the mesoscale. We examined floristic composition and its relationship with environmental factors across 220 1‐ha permanent plots in tropical lowland Bolivia. For each plot, abundance of 100 species (93 tree and 7 palm species ≥10 cm diam) was obtained. Climatic data, related to rainfall seasonality and temperature, were interpolated from all available weather stations in the region, and soil properties, related to texture and fertility, were obtained for each plot. Floristic variation was strongly associated with differences in water availability and temperature, and therefore the climatic gradient shaped floristic variation more strongly than the edaphic gradient. Detrended correspondence analysis ordination divided lowland Bolivia primarily into two major groups (Southern Chiquitano region vs. the Amazon region) and a multiple response permutation procedure distinguished five floristic regions. Overall, the tested environmental variables differed significantly among the five regions. Using indicator species analysis, we distinguished 82 strong indicator species, which had significant environmental preferences for one floristic region. These species can be used as indicators of environmental conditions or to determine which floristic region a certain forest belongs. Given the predicted decreases in rainfall and increases in temperature for tropical lowland forests, our gradient approach suggests that species composition may shift drastically with climate change. Abstract in Spanish is available at http://www.blackwell‐synergy.com/loi/btp .     

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.     

Classification,species richness,and environmental relations of monsoon rain forest in northern Australia

A floristic classification for monsoon rain forest vegetation in the Northern Territory, Australia, is derived based on comprehensive floristic inventory and environmental data. Allied aims include relating the floristic classification to Australia-wide structural and floristic schema, documenting species richness, and exploring site-environmental relations. TWINSPAN classification and complementary DCA analysis of a data set comprising 1219 sites x 55 9 rain forest taxa yielded 16 floristic assemblages. A diagnostic floristic key to these groups is presented. Eight groups describe rain forests associated with sites of perennial moisture; eight groups are associated with seasonally dry landforms. The structural typology of Australian rain forests is found wanting when applied to relatively simple monsoon rain forest communities. Rain forest patches are mostly less than 5 ha in size; maximum species richness is ca. 135 species per patch. Two major environmental gradients are identified through indirect gradient analysis: a primary latitudinal-moisture gradient and a subsidiary topographic-drainage gradient. Given the demonstrated tolerance of monsoon rain forest to a broad range of environmental conditions, the question remains: why is this vegetation type so restricted in occurrence in northern Australia?     

Distance Decay of Tree Species Similarity in Protected Areas on Terra Firme Forests in Colombian Amazonia

In this study, we investigated the pattern of floristic similarity as a function of geographical distances and environmental variability in well-drained uplands (terra firme) in Colombian Amazonia. The study site comprised three National Natural Parks, Tinigua, Chiribiquete, and Amacayacu, located in different geological units that represent a soil fertility gradient linked to parental materials. Differences in species richness between sites were compared using rarefaction analysis. A clear floristic transition appeared in the east–west direction following a soil fertility gradient along the first PCoA axis. In multiple regression analyses based on distance matrices, both geographical distances and geology explained 64 percent of the total floristic variation. Geographical distances alone accounted for 12 percent of variation in floristic similarities among plots, while geology alone accounted for 1 percent, and the joint effect of both explained 51 percent of the floristic variation. The species richness trend supports the existence of a latitudinal corridor southward of the geographical Equator in the Amazon basin, where tree diversity reaches the maximum expected values. A coupled effect of stochastic dispersal limitation and habitat specialization would certainly appear to be an appropriate explanation for tree species turnover in terra firme forests in Colombian Amazonia, strongly emphasizing that competition and neutrality must be supplementary rather than mutually exclusive processes. This result pinpoints the effect of dispersal on floral mixing as an ongoing active process for structuring tree communities in NW Amazonia, and the size of the reserves as a relevant issue to protect rare species from extinction by chance.
     

Spatial Scale or Amplitude of Predictors as Determinants of the Relative Importance of Environmental Factors to Plant Community Structure

The literature on tropical rain forest plant‐community relationships with environmental factors usually does not recognize that the relative importance of environmental factors recorded in each study might be due to their amplitude of variation within sites. Geographic scale, however, is recognized as an important modulator of this relative importance. To disentangle the effects of scale and environmental amplitude, ferns and trees in two landscapes of the same size (each 25 km²) with different soil‐fertility amplitudes but similar soil‐texture range were sampled in central Amazonia. We found that major determinants of community structure were the same for ferns and trees. Texture was the main predictor of community structure in the site with homogeneous soil fertility, while availability of exchangeable cations was the main predictor in the site with a wider fertility range. When both sites were analyzed together, soil fertility was the main predictor of community structure and soil texture segregated floristic subgroups within certain ranges of the soil‐fertility gradient. We conclude that: (1) floristic patterns for trees and ferns are congruent; (2) floristic variation depends on the amplitude of the studied gradients, more than on geographical scale; (3) limiting factors are not necessarily the most important predictors of compositional patterns; and (4) communities are structured hierarchically. Therefore, landscape structure (meaning which combinations of environmental factors, their amplitude of variation and which part of the gradient is found within the landscape) affect our perception of the relative importance that environmental factors will have as predictors of species composition.     

Some synchorological aspects of basiphilous pine forests in Fennoscandia

Basiphilous pine forests and related birch forests are herb-and grass-rich forests on calcareous substrate. These forests are complex communities with floristic/ecological elements from different vegetation types occurring in a subtle micromosaic. These elements are e.g. species from acidophilous conifer forests, thermophilous forest-rim communities, calcareous shallow-soil and steppe communities, eutrophic wet meadows and fens, and in northern Fennoscandia also species from alpine Dryas heaths. Four associations are recognized in Fennoscandia: Convallario-Pinetum, Melico-Piceetum pinetosum, Peucedano-Pinetum and Epipacto atrorubentis-Betuletum. The main association is the Convallario-Pinetum, a widespread community in Fennoscandia and Estonia with a considerable floristic variation between the different regions. Examples of the floristic variation along west-east profiles and south-north profiles in Fennoscandia are presented. The basiphilous pine forest complex can be divided into a number of ecological types along the moisture and nutritional gradients. A further subdivision into geographical types (races) is presented.Nomenclature follows Lid (1974) for vascular plants, Nyholm (1954–1969) for musci and Dahl & Krog (1973) for lichens.     

The inselberg flora of Atlantic Central Africa. I. Determinants of species assemblages

Aims To identify the relative contributions of environmental determinism, dispersal limitation and historical factors in the spatial structure of the floristic data of inselbergs at the local and regional scales, and to test if the extent of species spatial aggregation is related to dispersal abilities. Location Rain forest inselbergs of Equatorial Guinea, northern Gabon and southern Cameroon (western central Africa). Methods We use phytosociological relevés and herbarium collections obtained from 27 inselbergs using a stratified sampling scheme considering six plant formations. Data analysis focused on Rubiaceae, Orchidaceae, Melastomataceae, Poaceae, Commelinaceae, Acanthaceae, Begoniaceae and Pteridophytes. Data were investigated using ordination methods (detrended correspondence analysis, DCA; canonical correspondence analysis, CCA), Sørensen's coefficient of similarity and spatial autocorrelation statistics. Comparisons were made at the local and regional scales using ordinations of life‐form spectra and ordinations of species data. Results At the local scale, the forest‐inselberg ecotone is the main gradient structuring the floristic data. At the regional scale, this is still the main gradient in the ordination of life‐form spectra, but other factors become predominant in analyses of species assemblages. CCA identified three environmental variables explaining a significant part of the variation in floristic data. Spatial autocorrelation analyses showed that both the flora and the environmental factors are spatially autocorrelated: the similarity of species compositions within plant formations decreasing approximately linearly with the logarithm of the spatial distance. The extent of species distribution was correlated with their a priori dispersal abilities as assessed by their diaspore types. Main conclusions At a local scale, species composition is best explained by a continuous cline of edaphic conditions along the forest‐inselberg ecotone, generating a wide array of ecological niches. At a regional scale, these ecological niches are occupied by different species depending on the available local species pool. These subregional species pools probably result from varying environmental conditions, dispersal limitation and the history of past vegetation changes due to climatic fluctuations.     

Soil and Geographic Distance as Determinants of Floristic Composition in the Azuero Peninsula (Panama)

Many studies analyzing the relative contribution of soil properties versus distance‐related processes on plant species composition have focused on lowland tropical forests. Very few have investigated two forest types simultaneously, to contrast ecological processes that assemble the communities. This study analyses—at the landscape scale—the relative contribution of soil and distance on lowland and submontane tropical forests, which co‐occur in two reserves of the Azuero peninsula (Panama). Floristic inventories and soil sampling were conducted in 81 0.1‐ha plots clustered in 27 sites, and data were analyzed using Mantel tests, variance partitioning and non‐metric multidimensional scaling. The largest differences in floristic composition occurred between reserves in both forest types. Soil variation and geographic distance were important determinants of floristic composition, but their effects were highly correlated; together they explained 7–25 percent and 46–50 percent of the variation in lowland and submontane forests, respectively. Soil variables that had the best correlations with floristic composition were iron, zinc, and silt content in lowland, and calcium, copper, iron, potassium, magnesium, phosphorus, zinc, and sand content in submontane forests. The studied forests showed a high beta diversity that seems to be related primarily with soils and, secondarily, with dispersal limitation and stochastic events. The results reveal a response of tree assemblages to environmental gradients, which are particularly conspicuous in Panama. The effects of limited dispersal seem to be more important in submontane than in lowland forests, probably as a result of higher isolation.     

Convergence in growth responses of tropical trees to climate driven by water stress

Widely documented for temperate and cold forests in both hemispheres, variations in tree growth responses to climate along environmental gradients have rarely been investigated in the tropics. Seven tree‐ring chronologies of Centrolobium microchaete (Fabaceae) in the Cerrado tropical forests of Bolivia are used to determine the growth responses to climate along a precipitation gradient. Chronologies are distributed from the humid Guarayos forests (annual precipitation > 1600 mm) in the transition to the Amazonia to the dry‐mesic Chiquitos forests (annual precipitation < 1200 mm) in the proximity to the dry Chaco. On a large spatial scale, radial growth is positively influenced by rainfall and negatively by temperature at the end of the dry season. However, this regional pattern in climate‐tree growth relationship shows differences along the precipitation gradient. Relationships with climate are highly significant and extend over longer periods of the year in sites with low rainfall and extremely severe dry seasons. At wet sites, larger water soil capacity and endogenous forest dynamics partially mask the direct influence of climate on tree growth. Stronger similarities in tree‐growth responses to climate occur between sites in the dry Central Chiquitos and in the transition to the Guarayos forests. In contrast, the relationships show fewer similarities between sites in the humid Guarayos. We conclude that growth responses to climate in the tropics are more similar between sites with limited rainfall and severe and prolonged dry seasons. Our study points to a convergence in the patterns of growth responses of tropical trees to climate, modulated by scarce rainfall and marked seasonality. The negative impact of water deficits on tree physiological processes induces not only the documented reduction in forest species richness, but also a convergence in tree‐growth responses to climate in dry tropical forests.     

Litterfall and organic matter decomposition in a seasonal forest of the eastern Chaco (Argentina).

Amazonia and the Gran Chaco are the largest phytogeographic units of the Neotropical Region. The Forest Wedge of the Santa Fe province of Argentina is the southernmost part of the Eastern Chaco, and there are three main types of forest, 1) the mixed dense forests, 2) the Schinopsis balansae "quebrachal" and 3) the Prosopis nigra var. ragonesei forests, distributed along an environmental gradient correlated with topographic elevation. In this paper litterfall and organic matter decomposition of four species were studied in two different microsites in a Schinopsis balansae forest during three years. Litter fall varied along the year following seasonal environmental variation but litterfall variations between years were not significant. Litterfall was not uniform over the whole surface, under close canopy monthly average litterfall amounts 32.67 g/m2, in open gaps 4.47 g/m2 and 15.07 g/m2 under medium density canopy. Organic matter decomposition, like in other forests, was a negative exponential function of time, and the decomposition rate is independent from the microsites where it falls, 0.15 and 0.12 in sunny and shadow microsites respectively for Maytenus vitis-idaea, 0.10 and 0.11 in sunny and shadow microsites respectively for Achatocarpus praecox, 0.04 and 0.03 for Acacia praecox, 0.04 and 0.06 for Schinopsis balansae in sunny and shadow microsites respectively. Shurbs litter was decomposed faster than the tree litter, and the shurbs litter nutrientes level was also higher. Therefore the rate of organic matter decomposition is more correlated with leaf characteristics than with environmental variables.     

The concept of stability in connection with the gallery forests of the Chaco region

This paper discusses the concept of stability in a concrete natural situation: the gallery forests in the Chaco region. On the basis of Pickett's definition of minimal structure, stability is considered as the response which enables a system to keep its (minimal and configurational) structure once an alteration has taken place (Pickett et al. 1989). Resilience is, instead, another kind of response which enables the system to keep its minimal structure intact regardless of what might happen to its configurational structure.The system to be analysed is that of the gallery forests in the Lower Bermejo Basin, in Argentina, which is subject to intense river bed migrations and floods caused by this allochthonous watercourse. The effect of this alteration is analysed on different space-time scales.Locally floods behave as a completely unpredictable phenomena, starting a complex floristic changes mechanism that would eventually enable the system to recover its structure once the alteration is over. These forests appear to be resilient but little adaptable, with low resistence and low elasticity. The latter, when combined with the alternation's return interval, which is under the ecosystem's generation time, upset the theoretical stability conditions, thus making the system seem apparently unstable.Regionally, instead, migrations act as regular and permanent phenomena (highly predictable) always affecting some part of the area. The autochthonous system river side forests have become adapted through evolution to this new situation, so much so that this phenomenon has become a part of the system's internal dynamics. Chaco physiognomy, were it not due to the allochthonous phenomena affecting it today, would certainly be very different from the present one.     

Relationship between vegetation structure and microenvironment in Fagus grandifolia subsp. mexicana forest relicts in Mexico

Aims Changes in the structure and composition of forests, whether caused by natural or anthropic events, alter the microenvironment, sometimes irreversibly. Since the local environment has a direct impact on basic ecological processes, this has become a key component of research. Mexican beech forests (Fagus grandifolia subsp. mexicana) in the Sierra Madre Oriental are restricted to sites with specific climate, soils and topography, making them an ideal natural system for ecological research. The objectives of this study were to identify the relationship between the microenvironment and the tree and shrub structure and composition of Mexican beech forests in the state of Hidalgo, and to compare the floristic similarity of these forests on the country scale using data from seven localities.Methods Specimens were collected for a period of one year at all localities in the state of Hidalgo where beech forests are located. At each locality, five 400 m 2 plots were established, and structural attributes (basal area, coverage, density and species richness) and six environmental variables were measured in the plots. The relationship between structure and microenvironment was estimated by simple correlation and canonic correspondence analysis (CCA). In addition, floristic similarity between different beech forest localities in the Sierra Madre Oriental was estimated by correspondence analysis (CA).Important findings Twenty tree species and eight shrub species were identified; at all localities studied F. grandifolia subsp. mexicana dominated the canopy. The multivariate analysis indicated that (i) in the four localities in the state of Hidalgo, all microenvironmental variables except pH are related to the variation observed in species composition and structure; (ii) the El Gosco locality had both tree and shrub species and microenvironmental factors different from those observed in the Fagus forests at the other localities in the study and (iii) the localities studied in order to draw country-scale comparisons could be divided into three groups by floristic similarity. The first group consisted of the Hidalgo localities, the second of the Veracruz localities, and the third, more different from the others, of the Tamaulipas locality. The results of this study provide the first reference for the relationship between the range of microenvironments and species structure in Mexican beech forests. Microenvironmental conditions in the larger beech forests could be used as a model for designing management and conservation programs for this plant association. Because of its particular ecological and historical characteristics, this association could serve as an example of biodiversity conservation in Mexico.     

Local Hydrological Conditions Explain Floristic Composition in Lowland Amazonian Forests

Amazonian forests harbor a large variety of understory herbs adapted to areas with different hydrological conditions, ranging from well‐drained to seasonally flooded forests. The presence versus absence of flooding forms the extremes of a hydrological gradient, with various intermediate conditions, such as seasonal soil waterlogged areas, in between. We investigated the relationship between understory herbs and hydrological conditions in Central Amazonian forests using eighty‐eight 250 × 2 m plots distributed along a 600‐km transect. Hydrological conditions were determined regionally by precipitation and locally by topographic conditions based on drainage potential, flooding height and soil permeability (sand content). Soil cation concentration was used as a proxy for soil fertility. The floristic dissimilarities among plots were visualized by Nonmetric Multidimensional Scaling, and simple and multiple regressions were used to identify the best predictor of herb species composition. Local drainage potential was more important in determining herb species composition than soil fertility or precipitation at non‐flooded and flooded sites. Flooded sites comprised a very distinctive herb species composition even when the flood height was low (0.3 m). We conclude that hydrological conditions are the primary constraint of herb distribution within this flat regional landscape with moderate amounts of soil fertility variation (0.09–2.280 cmol(+)/kg). Hydrological models that consider local water conditions explained the largest part of herb species composition. Therefore, predictions of species distribution based on large‐scale climatic variables may underestimate the favorable area for understory herbs if the variation on local hydrological conditions is not considered.     

Relating variation in the understorey of beech forests to ecological factors

A data set of beech (Fagus sylvatica) forests in the Basque Country (Northern Spain), including biotic (floristic relevés) and environmental data (pH, organic matter, slope, altitude, rock cover), is analyzed in order to describe the relationships between understorey composition and environmental parameters. An ordination technique (DCA) is used to display the floristic gradients, and the interpretation of the floristic space in environmental terms is carried out by a spatial analysis technique (kriging). The composition of the beech forest understorey responds to a complex environmental gradient in the study area. Forests growing on acid substrata differ markedly from those growing on soils with higher pH, as the sharp transition between the pH 5 and 6 isolines reflects. When pH values exceed 6, organic matter content becomes the most important factor explaining the vegetation pattern and the rate of change is greater as organic matter content tends to change at a smaller scale than sample size and, as a consequence many transitional forms are found.