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Questions: To gain insights into the role of species‐by‐species replacements in cloud forest community structuring, we asked: (1) What are the effects of the spatial distribution of standing individuals on the seed rain, soil seed bank, and sapling density and survival in this cloud forest?; and (2) What is the prevalence of conspecific vs heterospecific replacements in the regeneration of this forest? Location: Santo Tomás Teipan, Oaxaca State, southern Mexico. Methods: In a 1‐ha cloud forest plot, we assessed seed rain, seed bank, and sapling density and survival of four canopy tree species (Chiranthodendron pentadactylon, Cornus disciflora, Quercus laurina and Oreopanax xalapensis). All standing individuals of these and other tree species (diameter at breast height [DBH] ≥ 2.5 cm) were mapped. We used neighbourhood models to examine the spatial patterns of the three life cycle stages relative to the spatial distribution of adults. The neighbourhood effect was assessed through the Neighbourhood Index, which integrates information on size (DBH) and distance to adults. Data analysis was based on maximum likelihood and model selection procedures. Results: We found large between‐species differences regarding the spatial patterns of seeds and saplings. Three species showed evidence for the Janzen–Connell effect operating at the seed (Chiranthodendron pentadactylon and Quercus laurina) or sapling (Oreopanax xalapensis) stage. We also found support for a critical role of specific microsite factors (i.e. niche differentiation) in the regeneration of two species (Chiranthodendron pentadactylon and Cornus disciflora). Conclusions: Seed and sapling distribution patterns suggest the prevalence of heterospecific replacements and that both Janzen–Connell and niche differentiation effects contribute to this pattern. Our results largely support the notion that the prevalence of heterospecific replacements among canopy species promotes species co‐existence in cloud forests. [ABSTRACT FROM AUTHOR]

We examined general and family-specific patterns of vascular plant richness along a large elevational gradient (0-3,670 m a.s.l.), assessed the continuity of these patterns and analysed their potential underlying causes in a high diversity region of the Sierra Madre del Sur, Oaxaca, Mexico. We used a vascular plant database constructed previously. The gradient was divided into 18 200-m elevation belts. To examine elevational patterns of richness, we used both observed and estimated (interpolated) species richness, as well as genus and family observed richness, for each belt. A generalised linear model (GLM) was used to assess the effect of altitude on area-corrected species richness (standard area = 100 km), and a numerical classification of the elevational belts based on species richness was performed. Overall, richness at the three taxonomic levels decreased with elevation, but some individual families departed from this pattern. A sharp drop in species richness was observed at 1,800 m, and the dendrogram separated two elevational floristic groups at this elevation. The GLM revealed a significant negative effect of elevation on species richness. Despite this overall decreasing pattern for vascular plants along this extensive gradient, an examination of some family-specific patterns revealed the existence of other elevation-diversity relationships, indicating taxon-specific responses to elevation. The most noticeable discontinuity in species richness, at ca. 1,800 m, is likely related to a critical temperature isocline. [ABSTRACT FROM AUTHOR]

We updated the floristic checklist of the Nizanda region, Isthmus of Tehuantepec (southern Mexico), characterized the occurring plant communities based on dominant species, and described the region’s flora according to life form, growth form, growth type, and growth habit spectra. Ten years of botanical exploration, along with surveys in 188 100-m2 samples from different vegetation types, provided the baseline floristic information. Ordination and classification analyses were performed to examine the degree of differentiation between communities. Geographical ranges of all species were used to assess biogeographical relationships of this flora. The inventory includes 920 species (553 genera, 124 families). More than one-third of the families were represented by a single species, whereas the 10 richest families had 43% of the species richness. Dendrograms showing plot classification at three taxonomic levels (species, genus and family) revealed savannah as the most strongly differentiated community amid seven vegetation types. Regarding growth forms, forbs and trees prevailed. Phanerophytes were the most common life form category, whereas herbs and woody plants were the dominant growth types. The largest richness for all taxonomic levels was recorded in the tropical dry forest. The expanded floristic knowledge gained for the Nizanda region provided better criteria to revise the classification scheme of its vegetation. Our preliminary biogeographical analysis illustrates the role of the Isthmus of Tehuantepec as a corridor for thermophilous floras between two oceanic watersheds, and as a natural distributional limit for several Mesoamerican plant species. [ABSTRACT FROM AUTHOR]

Topography strongly affects the distribution of insolation in the terrain. Patterns of incoming solar radiation affect energy and water balances within a landscape, resulting in changes in vegetation attributes. Unlike other regions, in seasonally dry tropical forest areas the potential contribution of topography-related environmental heterogeneity to β-diversity is unclear. In Mt. Cerro Verde (Oaxaca), S. Mexico, we: (1) modelled potential energy income for N- and S-facing slopes based on a digital elevation model, (2) examined the response of vegetation structure to slope aspect and altitude and (3) related variations in plant diversity to topographyrelated heterogeneity. Vegetation survey and modelling of potential energy income (SOLEI-32 model) were based on 30 plots equally distributed among three altitudinal belts defined for each slope of the mountain; combining the three altitudinal belts and the two slopes produced six environmental groups, represented by five vegetation plots each. Potential energy income was about 20% larger on the S than on the N slope (9,735 versus 8,138 MJ/m2), but it did not vary with altitude. In addition, the temporal behaviour of potential energy income throughout the year differed greatly between slopes. Vegetation structure did not show significant changes linked to the environmental gradients analysed, but altitude and aspect did affect β-diversity. We argue that the classic model of slope aspect effect on vegetation needs reconsideration for tropical landscapes. [ABSTRACT FROM AUTHOR]

Current knowledge about savanna vegetation in Mexico is still incomplete, and it is dominated by an ongoing debate over their natural versus anthropogenic origin. In this paper we provide a detailed structural and floristic description of the savannas of the Nizanda region (Oaxaca, S Mexico). Vegetation was sampled in 20 sites of 15 m² each (totaling 300 m²). A total of 135 morphospecies were encountered, 86 of which could be determined to species level; they were distributed in 29 families and 68 genera. Leguminosae, poaceae and asteraceae were the families with the largest taxonomic richness. Total community cover was 200.7 m², of which 69% was accounted for by grasses and sedges. Despite a clear dominance of Trachypogon spicatus (berger-parker index = 0.82), this community had a relatively large diversity (log[sub 2] shannon = 3.64). Life form spectrum showed a dominance of chamaephytes and hemicryptophytes, indicating a harsh environment. Notwithstanding the widespread occurrence of secondary vegetation and ruderal species in the region, the virtual lack of exotic species in these savannas, together with the presence of some endemics and the identity of the dominant species, strongly support the primary nature of this plant community. [ABSTRACT FROM AUTHOR]