Altitudinal zonation of Andean cryptogam communities

To test whether cryptogamic plant communities in tropical Andean rain forests are distributed in floristically discrete communities corresponding to altitudinal belts, I subjected the elevational distribution of pteridophytes along two elevational gradients in Bolivia, and of bryophytes and lichens along two transects in Peru and Colombia (data from Gradstein & Frahm, 1987 ; Wolf, 1993 ) to an analysis of deviance. All well‐defined elevational boundaries in floristic composition were related to marked ecological changes: the transition from the steep mountains to the hilly lowland zone coupled with a change in geological substrate at 400 m along the Bolivian Carrasco transect, a strong humidity gradient at 1000 m at the Bolivian Masicurí transect and at 1250–1980 m along the Colombian transect, and the transition from mixed cloud forests to forests dominated by Polylepis or Podocarpus at 3400–3600 m in Carrasco, at 1650–1800 m in Masicurí, and at 3670 m in Colombia. Consequently, floristic elevational belts appear to be well‐defined at strong environmental boundaries and in fairly species‐poor forest communities where the presence or absence of one or a few tree species influences the whole ecosystem while they are ill‐defined in species‐rich communities such as tropical forests at low to mid‐elevations.     

Evolutionary heritage shapes tree distributions along an Amazon‐to‐Andes elevation gradient

Understanding how evolutionary constraints shape the elevational distributions of tree lineages provides valuable insight into the future of tropical montane forests under global change. With narrow elevational ranges, high taxonomic turnover, frequent habitat specialization, and exceptional levels of endemism, tropical montane forests and trees are predicted to be highly sensitive to environmental change. Using plot census data from a gradient traversing > 3,000 m in elevation on the Amazonian flank of the Peruvian Andes, we employ phylogenetic approaches to assess the influence of evolutionary heritage on distribution trends of trees at the genus‐level. We find that closely related lineages tend to occur at similar mean elevations, with sister genera pairs occurring a mean 254 m in elevation closer to each other than the mean elevational difference between non‐sister genera pairs. We also demonstrate phylogenetic clustering both above and below 1,750 m a.s.l, corresponding roughly to the cloud‐base ecotone. Belying these general trends, some lineages occur across many different elevations. However, these highly plastic lineages are not phylogenetically clustered. Overall, our findings suggest that tropical montane forests are home to unique tree lineage diversity, constrained by their evolutionary heritage and vulnerable to substantial losses under environmental changes, such as rising temperatures or an upward shift of the cloud‐base.     

Richness,species composition and functional groups in Agaricomycetes communities along a vegetation and elevational gradient in the Andean Yungas of Argentina

The Neotropics are among the least explored regions from a mycological perspective. A few recent molecular studies in South America have shown high fungal diversity as well as numerous groups of mostly undescribed taxa. Through soil metabarcoding analysis we compared richness and species composition among macrofungal communities, belonging to Agaricales, Russulales, Boletales and Phallomycetidae groups, in three elevational forests types in the subtropical Yungas of Northwestern Argentina (Piedmont forest; Montane forest, Montane cloud forest). The aims of this study were to assess richness of taxonomic and functional groups along the elevation gradient and to assess the relationships between environmental variables and species composition in the studied fungal communities. The results have shown rich Agaricomycetes communities, diversely structured among forests habitats. The elevation gradient differentially affected the richness and distribution of Agaricales, Russulales, Boletales and Phallomycetidae. Based on fungal trophic modes and guilds, the gradient also affected the ectomycorrhizal taxa distribution. When considering the basidiomata growth forms (agaricoid, boletoid, gasteroid, etc.), only the secotioid type showed significant elevational differences. Additional analyses indicated that saprotrophic nutritional mode was dominant along the entire gradient, being partially replaced by biotrophic modes at higher elevations. Fungal communities in the Montane cloud forests are most dissimilar when compared with communities at the Piedmont forest and Montane forest, which is consistent with the different biogeographic origins of these forests. DNA metabarcoding sequence analysis provided detailed information on the diversity and taxonomic and functional composition of macrofungal communities.     

Elevational distribution and zonation of tropical pteridophyte assemblages in Costa Rica

The use of species turnover rates to identify discontinuities in vegetation assemblages along environmental gradients is hampered by the difficulty of the application of significance tests. We used a simulation procedure to detect abrupt transitions in fern species assemblages along a tropical elevational gradient to detect elevational zonations between 100 and 3400 m in a species-rich tropical forest in Costa Rica. Significant species turnover values were only observed at 300, 1000, and 3200 m. These boundaries in vegetation assemblages were caused by different lifeforms within the pteridophyte species set: epiphytes attained high turnover values at low, terrestrials at high elevations. Accordingly, the mean elevational ranges of epiphytes and terrestrials differed clearly at high elevations, where those of terrestrial species showed a sharp decline above ca. 2500 m, but those of epiphytes kept more or less constant. The boundaries of significant range limit accumulations coincide with shifts in the environmental settings, namely humidity conditions and forest structure. The location of the observed boundaries in turn influences the elevational amplitudes of species restricted to the resulting elevational zones, determining the elevational distribution of range amplitudes. Between 1000 and 3200 m no significant boundaries were observed, although it has been argued that the lower cloud condensation layer and the frost line are the prime drivers of elevational zonations.     

What Factors Affect Diversity and Species Composition of Endangered Tumbesian Dry Forests in Southern Ecuador?

This paper reports a study on species richness and composition of Tumbesian dry forest communities. We tested two alternative hypotheses about species assemblage processes in tropical dry forests: (1) species assemblage is determined by the filtering effect of environmental conditions and (2) species assemblage is determined by facilitative processes along the gradient of water availability, and thus, species richness and evenness increase as water becomes limited. In addition, we also explored the effect of climate and soil conditions on species composition in tropical dry forests. Species composition was sampled in 109 plots in terms of cover and tree diameter at breast height. Climatic, edaphic, topographic and anthropogenic degradation variables were obtained for each plot. We used generalized linear models and canonical correspondence analyses to evaluate the effect of environmental variables on species composition, richness and evenness. Water availability negatively affected richness and significantly determined the species assemblage. Species richness increased from ridges to valleys and evenness increased at higher altitudes. Soil characteristics showed no effect on richness and evenness but soil moisture, nitrogen concentration and soil temperature explained significant fractions of species composition. Although timber extraction and livestock in our study area were of low intensity, it negatively affected richness but had only a minor effect on species composition. Our results suggest that species composition in these endangered tropical dry forests may be at least partially explained by the stress‐gradient hypothesis, with higher species richness at drier conditions probably induced by facilitation processes.     

Climatic correlates of phylogenetic relatedness of woody angiosperms in forest communities along a tropical elevational gradient in South America

Aims This study assesses the relationship between phylogenetic relatedness of angiosperm tree species and climatic variables in local forests distributed along a tropical elevational gradient in South America. In particular, this paper addresses two questions: Is phylogenetic relatedness of plant species in communities related to temperature variables more strongly than to water variables for tropical elevational gradients? Is phylogenetic relatedness of plant species in communities driven by extreme climatic conditions (e.g. minimum temperature (MT) and water deficit) more strongly than by climatic seasonal variability (e.g. temperature seasonality and precipitation seasonality)?Methods I used a set of 34 angiosperm woody plant assemblages along an elevational gradient in the Andes within less than 5 degrees of the equator. Phylogenetic relatedness was quantified as net relatedness index (NRI) and nearest taxon index (NTI) and was related to major climatic variables. Correlation analysis and structure equation modeling approach were used to assess the relationships between phylogenetic relatedness and climatic variables.Important findings Phylogenetic relatedness of angiosperm woody species in the local forest communities is more strongly associated with temperature-related variables than with water-related variables, is positively correlated with mean annual temperature (MAT) and MT, and is related with extreme cold temperature more strongly than with seasonal temperature variability. NTI was related with elevation, MAT and MT more strongly than was NRI. Niche convergence, rather than niche conservatism, has played a primary role in driving community assembly in local forests along the tropical elevational gradient examined. Negative correlations of phylogenetic relatedness with elevation and higher correlations of phylogenetic relatedness with elevation and temperature for NTI than for NRI indicate that evolution of cold tolerance at high elevations in tropical regions primarily occurred at recent (terminal) phylogenetic nodes widely distributed among major clades.     

Distribution,species diversity and life-form spectra of plant communities along an altitudinal gradient in the northern slopes of Qilianshan Mountains,Gansu, China

We studied the distribution pattern, species diversity and life-formspectra of plant communities along an altitudinal gradient in the mid-sectionofthe northern slopes of Qilianshan Mountains by means of multivariate analyses.Two data sets (167 species × 75 plots, 10 environmental variables ×75 plots), originated from the fieldworks in 1998–1999, were subjected toTWINSPAN and DCCA, resulting in 8 major plant communities: 1)Asterothamnus centraliasiaticus–Halogetonarachnoideus desert grassland on azonal substrates from 1450 to 1600m and 2) zonal Reaumuria soogorica desertgrassland on gravels from 1470 to 1900 m; 3) Stipaprzewalskii–Stipa purpurea montane grassland from 2200 to 2900m; 4) Polygonum viviparum alpine grasslandfrom 2900 to 3700 m; 5) Caraganastenophylla–Ajaniafruticulosa dry-warm shrubland from 2350 to 2800 m; 6)Sabina przewalskii mid-wet warm forest from 2700 to 3300m; 7) Picea crassifolia cold coniferousforestfrom 2450 to 3200 m; 8) Caragana jubatawet-cold alpine shrubland from 3100 to 3700 m. Species diversityand species richness of both grasslands and forests peaked at the intermediateportion of the elevational gradient. Evenness might be strongly influenced byeither biotic or abiotic factors at a local scale, while seems quiteindependentof an elevational gradient at landscape scales. Beta-diversity decreased from1500 to 3700 m, indicating that species turnover declined withincreased elevation. Both richness of life-form and total species richness in agiven altitudinal belt (gamma-diversity) peaked at intermediate elevations,while relative species richness of different life-form varied differently alongthe altitudinal gradient.     

Assessing the effects of selective logging on birds in Neotropical piedmont and cloud montane forests

In tropical and subtropical forests there is limited information about how to integrate sustainable timber management with the conservation of biodiversity. We examined the effect of selective logging on the bird community to help develop management guidelines to assure the conservation of biodiversity in forests managed for timber production. The study design consisted of control and harvested plots in piedmont and cloud forests of the subtropical montane forests of the Andes in northwestern Argentina. We conducted bird point-count surveys combined with distance estimation. Breeding season bird community composition was more similar between control and logged forest in both the cloud forest and piedmont, than between the two elevations, probably because Neotropical bird communities change dramatically along elevational gradients. Within each elevation zone, community composition changed significantly between harvested and control forests. Both between and within each elevation zone no significant differences in bird density were detected. Similarly, when we analyzed bird density according to diet guilds no general pattern could be extracted. However, we found a significantly greater density of cavity nesters and lower of non-cavity nesters in control plots, probably because most trees that can develop suitable cavities were extracted in logged plots and these plots had a greater structural diversity enabling more nesting resources. Grouping species according to their nesting habitat requirements has rarely been used in the neotropics and other tropical and subtropical forests, but focusing management attention on cavity nesters might address the most sensitive portion of the avian community as well as other species dependent on trees likely to hold cavities.     

The influence of historical dispersal on the phylogenetic structure of tree communities in the tropical Andes

We test for evidence of the Tropical Niche Conservatism or the Out of The Tropics hypotheses in structuring patterns of tree community composition along a 2000 + meter elevational gradient in the northern tropical Andes. By collecting and integrating data on the presence–absence of tree species within plots with phylogenetic information, we analyzed the following: (a) patterns of phylogenetic dispersion and species diversity along the elevational gradient based on indexes of net relatedness, nearest taxon relatedness, and species richness (α‐diversity); and (b) the replacement of lineages along the gradient using the PhyloSorensen metric (β‐diversity). More specifically, we established 20 0.25‐ha permanent tree inventory plots between 750 and 2,802 m asl where all individuals with diameter at breast height (DBH) ≥ 10 cm were measured and identified. We then used a series of linear models to test for changes in α and β diversity between plots in relation to elevation. Neither the net relatedness index nor the nearest taxon index showed a significant relationship with elevation. However, there was greater phylogenetic overdispersion at intermediate elevations; this likely reflects the mixing of species with contrasting origins from tropical and temperate lineages. β‐diversity between plots was negatively related to the corresponding difference in elevation, indicating that closely related lineages occupy similar ranges of elevation and temperature. We conclude that the immigration of lineages from extra‐tropical regions has significant effects in determining the phylogenetic structure of tree communities in tropical Andean forests. Abstract in Spanish is available with online material.     

Ant community structure during forest succession in a subtropical forest in South-East China

Understanding how communities respond to environmental gradients is critical to predict responses of species to changing habitat conditions such as in regenerating secondary habitats after human land use. In this study, ground-living ants were sampled with pitfall traps in 27 plots in a heterogeneous and diverse subtropical forest to test if and how a broad set of environmental variables including elevation, successional age, and tree species richness influence ant diversity and community composition. In total, 13,441 ant individuals belonging to 71 species were found. Ant abundance was unrelated to all environmental variables. Rarefied ant species richness was negatively related to elevation, and Shannon diversity decreased with shrub cover. There was considerable variation in ant species amongst plots, associated with elevation, successional age, and variables related to succession such as shrub cover. It is shown that younger secondary forests may support a species-rich and diverse community of ants in subtropical forests even though the species composition between younger and older forests is markedly different. These findings confirm the conservation value of secondary subtropical forests, which is critical because subtropical forests have been heavily exploited by human activities globally. However, the findings also confirm that old-growth forest should have priority in conservation as it supports a distinct ant community. Our study identifies a set of ant species which are associated with successional age and may thus potentially assist local conservation planning.     

Tropical Rain Forest Structure,Tree Growth and Dynamics along a 2700-m Elevational Transect in Costa Rica

Rapid biological changes are expected to occur on tropical elevational gradients as species migrate upslope or go extinct in the face of global warming. We established a series of 9 1-ha plots in old-growth tropical rainforest in Costa Rica along a 2700 m relief elevational gradient to carry out long-term monitoring of tropical rain forest structure, dynamics and tree growth. Within each plot we mapped, identified, and annually measured diameter for all woody individuals with stem diameters >10 cm for periods of 3-10 years. Wood species diversity peaked at 400-600 m and decreased substantially at higher elevations. Basal area and stem number varied by less than two-fold, with the exception of the 2800 m cloud forest summit, where basal area and stem number were approximately double that of lower sites. Canopy gaps extending to the forest floor accounted for <3% of microsites at all elevations. Height of highest crowns and the coefficient of variation of crown height both decreased with increasing elevation. Rates of turnover of individuals and of stand basal area decreased with elevation, but rates of diameter growth and stand basal area showed no simple relation to elevation. We discuss issues encountered in the design and implementation of this network of plots, including biased sampling, missing key meteorological and biomass data, and strategies for improving species-level research. Taking full advantage of the major research potential of tropical forest elevational transects will require sustaining and extending ground based studies, incorporation of new remotely-sensed data and data-acquisition platforms, and new funding models to support decadal research on these rapidly-changing systems.     

The recovery of ground ant diversity in secondary Lacandon tropical forests

Ants in tropical forests participate in a wide variety of processes, but human activity can disturb their communities and cause the loss of key species. In this study, we evaluated the effects of human disturbance on the alpha and beta diversity of ant assemblages in a successional gradient of secondary forests growing in abandoned maize fields of the Lacandon region, previously covered by tropical rain forest. Our results show that the alpha diversity of ant species and genera increases with the age of the secondary forests; however, deforestation could cause the loss of certain genera. The turnover of ant species is low along the successional gradient and it is regulated by ecological filters. Secondary forests serve as a refuge for certain ant species and as their successional age increases, they tend to recover the species richness of the mature forest.     

Deterministic tropical tree community turnover: evidence from patterns of functional beta diversity along an elevational gradient

Explaining the mechanisms that produce the enormous diversity within and between tropical tree communities is a pressing challenge for plant community ecologists. Mechanistic hypotheses range from niche-based deterministic to dispersal-based stochastic models. Strong tests of these hypotheses require detailed information regarding the functional strategies of species. A few tropical studies to date have examined trait dispersion within individual forest plots using species trait means in order to ask whether coexisting species tend to be more or less functionally similar than expected given a null model. The present work takes an alternative approach by: (i) explicitly incorporating population-level trait variability; and (ii) quantifying the functional beta diversity in a series of 15 tropical forest plots arrayed along an elevational gradient. The results show a strong pattern of decay in community functional similarity with elevation. These observed patterns of functional beta diversity are shown to be highly non-random and support a deterministic model of tropical tree community assembly and turnover.     

Changes in Species Richness,Abundance, and Composition of Arboreal Twig‐nesting Ants Along an Elevational Gradient in Coffee Landscapes

The distribution, diversity, and assembly of tropical insects have long intrigued ecologists, and for tropical ants, can be affected by competitive interactions, microhabitat requirements, dispersal, and availability and diversity of nesting sites. Arboreal twig‐nesting ants are limited by the number of hollow twigs available, especially in intensive agricultural systems. Ant diversity and abundance may shift along elevation gradients, but no studies have examined if the proportion of occupied twigs or richness of arboreal twig‐nesting ants vary with elevation. In coffee agroecosystems, there are over 40 species of arboreal twig‐nesting ants. We examined communities of twig‐nesting ants in coffee plants along an elevational gradient to answer the following questions: (1) Do species richness and colony abundance decline with elevation or show a mid‐elevation peak? (2) Does community composition change with elevation? (3) Is elevation an important predictor of change in ant abundance, richness, and relative abundance of common species? We surveyed 42 10 × 10 m plots in 2013 from 450 to1550 m elevation across a coffee landscape in Chiapas, Mexico. We sampled a total of 2211 hollow coffee twigs, 77.1 percent of which were occupied by one of 28 species of ants. Pseudomyrmex simplex was more abundant in lower elevations, whereas Pseudomyrmex ejectus dominated in high elevations. Species richness and the percent of occupied hollow twigs both peaked at mid‐elevations (800–1050 m). In sum, we found that species richness, abundance, and composition of arboreal twig‐nesting ants shift with elevation. These findings may provide important insights for understanding ant communities in coffee agroecosystems.     

Changes in Patterns of Species Co‐occurrence across Two Tropical Cloud Forests Differing in Soil Nutrients and Air Temperature

Patterns of co‐occurrence of species are increasingly used to examine the contribution of biotic interactions to community assembly. We assessed patterns of co‐occurrence at four scales, in two types of tropical cloud forests in Hainan Island, China (tropical montane evergreen forests, TMEF and tropical dwarf forests, TDF) that varied significantly in soil nutrients and temperature. We tested if the patterns of co‐occurrence changed when we sorted species into classes by abundance and diameter at breast height (dbh). Co‐occurrence differed by forest type and with plot size, with significant species aggregation observed across larger plots in TDF and patterns of species segregation observed in smaller plots in TMEF. Analyses of differential abundance and dbh classes also showed that smaller plots in TMEF tend to have negative co‐occurrence patterns, but larger plots in TDF tend to show patterns of aggregation, suggesting competitive and facilitative interactions. This underscores the scale‐dependence of the processes contributing to community assembly. Furthermore, it is consistent with predictions of the stress gradient hypothesis that facilitation will be most important in biological systems subject to abiotic stress, while competition will be more important in less abiotically stressful habitats. Our results clearly demonstrate that these two types of tropical cloud forest exhibit different co‐occurrence patterns, and that these patterns are scale‐dependent, though independent of plant abundance and size class.     

Competition and habitat filtering jointly explain phylogenetic structure of soil bacterial communities across elevational gradients

The importance of assembly processes in shaping biological communities is poorly understood, especially for microbes. Here, we report on the forces that structure soil bacterial communities along a 2000 m elevational gradient. We characterized the relative importance of habitat filtering and competition on phylogenetic structure and turnover in bacterial communities. Bacterial communities exhibited a phylogenetically clustered pattern and were more clustered with increasing elevation. Biotic factors (i.e., relative abundance of dominant bacterial lineages) appeared to be most important to the degree of clustering, evidencing the role of the competitive ability of entire clades in shaping the communities. Phylogenetic turnover showed the greatest correlation to elevation. After controlling the elevation, biotic factors showed greater correlation to phylogenetic turnover than all the habitat variables (i.e., climate, soil and vegetation). Structural equation modelling also identified that elevation and soil organic matter exerted indirect effects on phylogenetic diversity and turnover by determining the dominance of microbial competitors. Our results suggest that competition among bacterial taxa induced by soil carbon contributes to the phylogenetic pattern across elevational gradient in the Tibetan Plateau. This highlights the importance of considering not only abiotic filtering but also biotic interactions in soil bacterial communities across stressful elevational gradients.     

Microclimatic factors associated with elevational changes in army ant density in tropical montane forest

Abstract.  1. The density (rate of encountering foraging raids) and species richness of army ants (Formicidae: Ecitoninae, and behaviourally convergent Ponerinae) was measured in montane tropical forest. Above-ground and subterranean army ant raids were sampled using standard protocols at four sites across an elevational gradient (1200–1650 m above mean sea level) in and near cloud forest in the area of Monteverde, Costa Rica.
2. Mean ambient temperature differed among sites, and decreased with elevation. For the above-ground foraging army ant species, raid rates also declined with elevation. Surface army ant raid rates, however, were not affected by day to day weather variation within sites (temperature, cloud cover, or precipitation).
3. For the underground foraging army ant species, raid rates did not vary directionally with elevation, and subterranean raid rates were not affected by day to day weather variation within sites.
4. Army ant species richness was not directionally related to elevation, and species sharing among sites was generally high.
5. Army ant community structure changes with elevation in Neotropical montane forest, and the results suggest that the strongest effects are of temperature regimes on the density of raids. These findings provide a baseline against which to detect changes in army ant communities that may accompany directional climate change in tropical cloud forests.     

Elevational changes in the avian community of a Mesoamerican cloud forest park

Harboring many range‐restricted and specialized species, high elevation tropical cloud forests are diverse habitats represented in many protected areas. Despite this, many such areas receive little practical protection from deforestation and land conversion. Moreover, montane species may be more sensitive to climate change owing to various factors affecting community assembly across elevational gradients. Few studies have used annual monitoring to assess how biological communities in cloud forests may be shifting in response to habitat or climate change or assessed the efficacy of protected areas in buffering these effects. We analyzed avifaunal community trends in a 10‐yr dataset of constant‐effort bird point‐count data in a cloud forest national park in Honduras, Central America. We found that species richness and diversity increased at higher elevations, but decreased at lower elevations. Abundances of most dietary and forest‐dependency groups exhibited similar trends, and many key cloud forest species shifted upslope and/or increased in abundance. Taken together, our results suggest that the avian community is moving upslope and species composition is changing. Results for species richness and diversity were similar when only nondegraded transects were considered, suggesting the role of climate change as an important driver. At lower elevations, however, many species may be negatively affected by increased habitat degradation, favoring species with low forest dependency. Continued habitat conversion and climate change could push the cloud forest bird community further upslope, potentially resulting in increased competition, mortality, and even extirpation of some species. Increased protection is unlikely to mitigate the effects of climate change.     

The inability of tropical cloud forest species to invade grasslands above treeline during climate change: potential explanations and consequences

The upper elevational range edges of most tropical cloud forest tree species and hence the ‘treeline’ are thought to be determined primarily by temperatures. For this reason, the treeline ecotone between cloud forests and the overlying grasslands is generally predicted to shift upslope as species migrate to higher elevations in response to global warming. Here, we propose that other factors are preventing tropical trees from shifting or expanding their ranges to include high elevation areas currently under grassland, resulting in stationary treelines despite rising mean temperatures. The inability of cloud forest species to invade the grasslands, a phenomenon which we refer to as the ‘grass ceiling’ effect, poses a major threat to tropical biodiversity as it will greatly increase risk of extinctions and biotic attrition in diverse tropical cloud forests. In this review, we discuss some of the natural factors, as well as anthropogenic influences, that may prevent cloud forest tree species from expanding their ranges to higher elevations. In the absence of human disturbances, tropical treelines have historically shifted up‐ and down‐slope with changes in temperature. Over time, increased human activity has limited forests to lower elevations (i.e. has depressed treelines), and often broken the equilibrium between species range limits and climate. Yet even in areas where anthropogenic influences are halted, cloud forests have not expanded to higher elevations. Despite the critical importance of understanding the distributional responses of tropical species to climate change, few studies have addressed the factors that influence treeline location and dynamics, severely hindering our ability to predict the fate of these diverse and important ecosystems.