Floristic diversity of Mexican seasonally dry tropical forests

Studies of the variation in tropical plant species diversity and itsrelationship with environmental factors are largely based on research intropical moist/wet forests. Seasonally dry tropical forests (SDTFs), incontrast, have been poorly investigated. In this paper we present data from 20Mexican SDTF sites sampled to describe the magnitude of floristic diversity inthese forests and to address the following questions: (i) to what extent isspecies diversity related to rainfall? (ii) Are there other climatic variablesthat explain variation in species diversity in SDTFs? (iii) How does speciesidentity vary spatially (species turnover) within the country? We found thatspecies diversity was consistently greater (a ca. twofold difference) than wouldbe expected according to the sites' precipitation. Rainfall did notsignificantly explain the variation in species diversity. Likewise, the numberof dry and wet months per year was unrelated to species diversity. In contrast,a simple measure of potential evapotranspiration (Thornthwaite's index)significantly explained the variation in species diversity. In addition to thegreat diversity of species per site (local diversity), species turnover wasconsiderable: of a total of 917 sampled species, 72% were present only in asingle site and the average similarity (Sorensen's index) among sites wasonly 9%. These aspects of floristic diversity and the high deforestation ratesof these forests in Mexico indicate that conservation efforts should be directedto tropical forests growing in locations of low and seasonal rainfall.     

High genetic diversity and contrasting fine-scale spatial genetic structure in four seasonally dry tropical forest tree species

In this study we compared population structure, genetic diversity and fine-scale spatial genetic structure (SGS) in four Bignoniaceae tree species, Handroanthus chrysotrichus, H. impetiginosus, Tabebuia roseoalba and H. serratifolius in a remnant of seasonally dry tropical forest in Central-West Brazil, based on polymorphisms at six microsatellite loci. All species, except T. roseoalba, presented the inverted ‘J’ population structure indicating recruitment of juveniles. Juveniles presented a clumped distribution suggesting limitation in dispersal or patchy distribution of suitable microhabitat for recruitment. All species showed high levels of polymorphism and genetic diversity but without a clear pattern of distribution among life stages. The SGS was significant for all species, except T. roseoalba, but the pattern and strength of the spatial genetic structure differed among species. Handroanthus serratifolius had stronger SGS with significant kinship until 77 m. For H. impetiginosus and H. chrysotrichus, kinship was significant just until 23 and 6 m, respectively. Despite the high genetic diversity, all species showed low number of adults and high fixation indices suggesting that habitat fragmentation and disturbance have been affecting these populations in Central-West Brazil.     

Fire alters diversity,composition, and structure of dry tropical forests in the Eastern Ghats

Fire is known to have dramatic consequences on forest ecosystems around the world and on the livelihoods of forest‐dependent people. While the Eastern Ghats of India have high abundances of fire‐prone dry tropical forests, little is known about how fire influences the diversity, composition, and structure of these communities. Our study aimed to fill this knowledge gap by examining the effects of the presence and the absence of recent fire on tropical dry forest communities within the Kadiri watershed, Eastern Ghats. We sampled plots with and without evidence of recent fire in the Eswaramala Reserve Forest in 2008 and 2018. Our results indicate that even though stem density increases in the recently burned areas, species richness is lower because communities become dominated by a few species with fire resistance and tolerance traits, such as thick bark and clonal sprouting. Further, in the presence of fire, the size structure of these fire‐tolerant species shifts toward smaller‐sized, resprouting individuals. Our results demonstrate that conservation actions are needed to prevent further degradation of forests in this region and the ecosystem services they provide.     

Dynamics of viable nitrifier community, N-mineralization and nitrification in seasonally dry tropical forests and savanna

The study was conducted in Vindhyan region, to assess the N-mineralization, nitrification and size of viable community of ammonium- and nitrite-oxidizing bacteria as affected by different sites and seasons. Six different ecosystems (four forests and two savannas), which differ in terms of topography, vegetation and moisture status, were selected for the present study. The soils of the study sites differ significantly in its physico-chemical properties. The savanna site had significantly higher pH (7.2), bulk density (1.37 g cm(-3)) and silt content (67.80%) but lower water holding capacity (1.37%), total-C (16,356 microg g(-1) dry soil), N (1090 microg g(-1) dry soil) and P (213 microg g(-1) dry soil) than forest sites. The soil moisture content, N-mineralization, nitrification rates and numbers of ammonium- and nitrite-oxidizing bacteria were highest in the wet season and lowest in dry season, while the size of mineral-N (NH4(+)-N and NO3(-)-N) showed a reverse trend at the sites. The N-mineralization, nitrification and nitrifier population size differ significantly across the site and season. The numbers of free-living cells of ammonium- and nitrite-oxidizing bacteria were significantly related to each other and to N-mineralization, nitrification, soil moisture and mineral-N components. The N-mineralization, nitrification and the viable number of nitrifying cells were consistently higher for forest soils compared to savanna sites. It was concluded that soil microbial process (N-mineralization and nitrification) and nitrifier population size were dependent on site topography, vegetation cover and soil moisture status.     

Woody plant diversity in relation to environmental factors in a seasonally dry tropical forest landscape

Questions

Water availability is known to be a first‐order driver of plant diversity; yet water also affects fire regimes and soil fertility, which, in turn, affect plant diversity. We examined how precipitation, fire and soil properties jointly determine woody plant diversity. Specifically, we asked how woody plant diversity varies along a sharp precipitation gradient (about 600–1,800 mm mean annual precipitation [MAP ]within a ~45‐km distance) exhibiting considerable variation in long‐term fire burn frequency and soil fertility, in a southern Indian seasonally dry tropical forest (SDTF ) landscape.

Location

Mudumalai, Western Ghats, India.

Methods

Woody plants ≥1‐cm DBH were enumerated in 19 1‐ha permanent plots spanning a range of tropical vegetation types from dry thorn forest, through dry and moist deciduous forest to semi‐evergreen forest. Burn frequencies were derived from annual fire maps. Six measures of surface soil properties – total exchangeable bases (Ca + Mg + K), organic carbon (OC ), total N, pH , plant available P and micronutrients (Fe + Cu + Zn + Mn) were used in the analyses. Five measures of diversity – species richness, Shannon diversity, the rarefied/extrapolated versions of these two measures, and Fisher's α – were modelled as functions of MAP , annual fire burn frequency and the principal components of soil properties.

Results

Most soil nutrients and OC increased with MAP , except in the wettest sites. Woody productivity increased with MAP , while fire frequency was highest at intermediate values of MAP . Woody plant diversity increased with MAP but decreased with increasing fire frequency, resulting in two local diversity maxima along the MAP gradient – in the semi‐evergreen and dry thorn forest – separated by a low‐diversity central region in dry deciduous forest where fire frequency was highest. Soil variables were, on the whole, less strongly correlated with diversity than MAP .

Conclusions

Although woody plant diversity in this landscape, representative of regional SDTF s, is primarily limited by water availability, our study emphasizes the role of fire as a potentially important second‐order driver that acts to reduce diversity in this landscape.

     

Neotropical seasonally dry forests and Quaternary vegetation changes

Seasonally dry tropical forests have been largely ignored in discussions of vegetation changes during the Quaternary. We distinguish dry forests, which are essentially tree‐dominated ecosystems, from open savannas that have a xeromorphic fire‐tolerant, grass layer and grow on dystrophic, acid soils. Seasonally dry tropical forests grow on fertile soils, usually have a closed canopy, have woody floras dominated by the Leguminosae and Bignoniaceae and a sparse ground flora with few grasses. They occur in disjunct areas throughout the Neotropics. The Chaco forests of central South America experience regular annual frosts, and are considered a subtropical extension of temperate vegetation formations. At least 104 plant species from a wide range of families are each found in two or more of the isolated areas of seasonally dry tropical forest scattered across the Neotropics, and these repeated patterns of distribution suggest a more widespread expanse of this vegetation, presumably in drier and cooler periods of the Pleistocene. We propose a new vegetation model for some areas of the Ice‐Age Amazon: a type of seasonally dry tropical forest, with rain forest and montane taxa largely confined to gallery forest. This model is consistent with the distributions of contemporary seasonally dry tropical forest species in Amazonia and existing palynological data. The hypothesis of vicariance of a wider historical area of seasonally dry tropical forests could be tested using a cladistic biogeographic approach focusing on plant genera that have species showing high levels of endemicity in the different areas of these forests.     

Phenological diversity in tropical forests

One of the most intriguing and complex characteristics of reproductive phenology in tropical forests is high diversity within and among forests. To understand such diversity, Newstrom et al. provided a systematic framework for the classification of tropical flowering phenology. They adopted frequency and regularity as criteria with priority, and classified plants in La Selva, Costa Rica, where most plants reproduced more than once a year irregularly. Many other studies have demonstrated annual cycles corresponding to rainfall patterns at the community level in Neotropical forests, including La Selva. On the other hand, supraannual flowering synchronized among various plant species, called general flowering, is known from aseasonal lowland dipterocarp forests in Southeast Asia. Within both forests, a wide spectrum of flowering patterns is found. This range of patterns suggests the great potential of tropical phenological studies to explore the selective pressures on phenology. Various abiotic and biotic factors can be selective agents. The shared pollinators hypothesis suggests that plant species sharing pollinators segregate flowering temporarily to minimize interspecific overlap in flowering times and thus minimize ineffective pollination or competition for pollinators, indicating strong phylogenetic constraints in timing and variation of flowering. Comparison of phenology within and among forests may help our understanding of phenological diversity. Attempts are now being made to develop a common language to communicate concepts and render interpretations of data more compatible among investigators and to create a network to promote comparative studies. Received: September 8, 2000 / Accepted: January 30, 2001     

Vegetation patterns and environmental gradients in tropical dry forests of the northern Yucatan Peninsula

Patterns of plant species composition and their relationships to soil and topographic variables were investigated in tropical dry forests across the north central Yucatan, Mexico. Seven sites were studied in the oldest accessible forests along a 200–km transect oriented northwest to southeast; an eighth site was located in a little‐disturbed area located 75 km northeast of the transect. Two of the sites were on Mayan ruins. All sites were sampled using 9–24, 10m × 20m plots (<n= 132) for woody stems ≥ 3.0 cm diameter breast height. The important natural forest species were Bursera simaruba, Caesalpinia gaumeri, Gymnopodium floribundum, Piscidia piscipula, and Thouinia paucidentata. The two most important woody species in ruin woodlands were Brosimum alicastrum and Croton lundellii. Forest plots (n=108) had 17 species on average, ruin plots (n= 24) nine species. Mean basal area of stems at the forest plots (20.7 m².ha^‐1) was lower than in ruin plots (28.4 m².ha^‐1). Detrended Correspondence Analysis generally placed plots by site along the geographic transect. Natural forest plots and sites were separated from the plots on ruin sites. The five soil and topographic variables (slope, soil depth, percent surface rock, soil pH, total soil organic matter) differed significantly among sites. Plot values were correlated with DCA axe scores. Intersite floristic variation reflects an overall west to east environmental gradient affected by climate.     

Ecology and diversity of leaf litter fungi during early-stage decomposition in a seasonally dry tropical forest

Leaf litter samples of 12 dicotyledonous tree species (belonging to eight families) growing in a dry tropical forest and in early stages of decomposition were studied for the presence of litter fungi. Equal-sized segments of the leaves incubated in moist chambers were observed every day for 30 d for the presence of fungi. Invariably, the fungal assemblage on the litter of each tree species was dominated by a given fungal species. The diversity of fungi present in the litter varied with the tree species although many species of fungi occurred in the litter of all 12 species. A Pestalotiopsis species dominated the litter fungal assemblage of five trees and was common in the litter of all tree species. The present study and earlier studies from our lab indicate that fungi have evolved traits such as thermotolerant spores, ability to utilize toxic furaldehydes, ability to produce cell wall destructuring enzymes and an endophyte-litter fungus life style to survive and establish themselves in fire-prone forests such as the one studied here. This study shows that in the dry tropical forest, the leaf litter fungal assemblage is governed more by the environment than by the plant species.     

Functional diversity of macromycete communities along an environmental gradient in a Mexican seasonally dry tropical forest

Macromycetes are important for ecosystem functioning due to their role in the nutrient cycling, and their function as pathogens and mutualists. Diversity metrics based on functional traits are robust predictors of ecosystem functionality since they incorporate an evolutionary and ecologic background. We examined diversity patterns of macrofungi using functional trait-based metrics of diversity along an altitudinal gradient in a seasonally dry tropical forest in southern Mexico. Our findings show that: (1) functional diversity varies with elevation, relating more to climatic variables than to vegetation structure; (2) functional diversity indexes exhibited contrasting patterns, so measures reflecting heterogeneity on trait abundance and niche complementarity tend to increase with elevation, whereas the measure of trait evenness decreases; and (3) functional diversity patterns depend on the type of functional trait considered and how they respond to environmental conditions. Our results indicate that functional diversity analyses help understanding of how macrofungal communities respond to environmental variation.     

Fine-scale habitat structure complexity determines insectivorous bird diversity in a tropical forest

Habitat complexity in reforested stands has been acknowledged as a key factor that influences habitat use by birds, being especially critical for habitat disturbance-sensitive species such as tropical understory insectivorous birds. Most studies regarding the relationship between forest structure and species diversity were conducted at the landscape scale, but different diversity patterns may emerge at a finer scale (i.e., within a habitat patch). We examined a tropical reforested area (State of Caldas, Colombia), hypothesizing that insectivorous bird richness, abundance, and foraging guild abundance would increase as intra-habitat complexity increases. We established 40 monitoring plots within a reforested area, measured their structural features, and determined their relationships with species richness, total abundance, and foraging guild abundance, using Generalized Additive Models. We found that the increasing variation in basal area, stem diameter, and number of stems was positively correlated with species richness, total abundance, and foraging guild abundance. Relationships between richness or abundance and structural features were not lineal, but showing curvilinear responses and thresholds. Our results show that heterogeneity on basal area, stem diameter, and the number of stems was more correlated to insectivorous bird richness and abundance than the average of those structural features. Promoting structural variation on reforested areas by planting species with different growth rates may contribute to increase the richness and abundance of a tropical vulnerable group of species such as the understory insectivorous birds.     

Holocene southward expansion in seasonally dry tropical forests in South America: phylogeography of Ficus bonijesulapensis (Moraceae)

We conducted a phylogeographical and niche modelling study of the tree Ficus bonijesulapensis, endemic to Brazilian seasonally dry tropical forests (SDTFs), in order to evaluate the effects of Quaternary climatic fluctuations on population dynamics. The trnQ–5′rps16 region of plastid DNA was sequenced from 15 populations. Three phylogeographical groups were identified by the median‐joining algorithm network and spatial analysis of molecular variance (SAMOVA) (F_CT = 0.591): a central‐west, a central‐east and a scattered group. The central groups had higher total haplotype and nucleotide diversities than the scattered group. Ecological niche modelling suggested that, since the Last Interglacial (130 kyr bp ), the central and north regions have been relatively stable, whereas the southern region of the species distribution has been less stable. The phylogeographical groups showed concordance with the floristic units described for SDTFs. The low genetic diversity, unimodal mismatch distribution and unfavourable climatic conditions in the southern region suggest a recent southward expansion of the range of the species during the Holocene, supporting the hypothesis of the southward expansion of SDTFs during this period. The central and northern regions of the current distribution of F. bonijesulapensis, which are consistent with arboreal caatinga and rock outcrop floristic units, were potential refugia during Quaternary climatic fluctuations. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 177 , 189–201.     

Hydraulic and photosynthetic co-ordination in seasonally dry tropical forest trees

In the present study the linkage between hydraulic, photosynthetic and phenological properties of tropical dry forest trees were investigated. Seasonal patterns of stem‐specific conductivity (K_SP) described from 12 species, including deciduous, brevi‐deciduous and evergreen species, indicated that only evergreen species were consistent in their response to a dry‐to‐wet season transition. In contrast, K_SP in deciduous and brevi‐deciduous species encompassed a range of responses, from an insignificant increase in K_SP following rains in some species, to a nine‐fold increase in others. Amongst deciduous species, the minimum K_SP during the dry season ranged from 6 to 56% of wet season K_SP, indicating in the latter case that a significant portion of the xylem remained functional during the dry season. In all species and all seasons, leaf‐specific stem conductivity (K_L) was strongly related to the photosynthetic capacity of the supported foliage, although leaf photosynthesis became saturated in species with high K_L. The strength of this correlation was surprising given that much of the whole‐plant resistance appears to be in the leaves. Hydraulic capacity, defined as the product of K_L and the soil–leaf water potential difference, was strongly correlated with the photosynthetic rate of foliage in the dry season, but only weakly correlated in the wet season.     

Host specificity of insect herbivores in tropical forests

Studies of host specificity in tropical insect herbivores are evolving from a focus on insect distribution data obtained by canopy fogging and other mass collecting methods, to a focus on obtaining data on insect rearing and experimentally verified feeding patterns. We review this transition and identify persisting methodological problems. Replicated quantitative surveys of plant-herbivore food webs, based on sampling efforts of an order of magnitude greater than is customary at present, may be cost-effectively achieved by small research teams supported by local assistants. Survey designs that separate historical and ecological determinants of host specificity by studying herbivores feeding on the same plant species exposed to different environmental or experimental conditions are rare. Further, we advocate the use of host-specificity measures based on plant phylogeny. Existing data suggest that a minority of species in herbivore communities feed on a single plant species when alternative congeneric hosts are available. Thus, host plant range limits tend to coincide with those of plant genera, rather than species or suprageneric taxa. Host specificity among tropical herbivore guilds decreases in the sequence: granivores > leaf-miners > fructivore > leaf-chewers = sap-suckers > xylophages > root-feeders, thus paralleling patterns observed in temperate forests. Differences in host specificity between temperate and tropical forests are difficult to assess since data on tropical herbivores originate from recent field studies, whereas their temperate counterparts derive from regional host species lists, assembled over many years. No major increase in host specificity from temperate to tropical communities is evident. This conclusion, together with the recent downward revisions of extremely high estimates of tropical species richness, suggest that tropical ecosystems may not be as biodiverse as previously thought.     

Diversity,composition, and structure of tropical dry forests in Central America

Tropical dry forests have been reduced to less than 0.1% of their original expanse on the Pacific side of Central America and are considered by some to be the most endangered ecosystem in the lowland tropics. Plots 1000 m² were established in seven tropical dry forests in Costa Rica and Nicaragua in order to compare levels of species richness to other Neotropical dry forest sites and to identify environmental variables associated with species richness and abundance. A total of 204 species and 1484 individuals 2.5 cm were encountered. Santa Rosa National Park was the richest site with the highest family (33), genera (69), and species (75) diversity of all sites. Species richness and forest structure were significantly different between sites. Fabaceae was the dominant tree and shrub family at most sites, but no species was repeatably dominant based on number of stems in all fragments of tropical dry forest. Central American dry forests had similar species richness when compared to other Neotropical forests. There was no correlation between forest cover within reserves, or precipitation and plant species richness. There was a significant correlation between anthropogenic disturbance (intensity and frequency of fire, wood collection, grazing) and total species richness, tree and shrub species richness, and liana abundance. These results suggest controlling levels on anthropogenic disturbance within reserves should be a high priority for resource managers in Central America. Further research in forest fragments which examine individual and a combination of disturbance agents would help clarify the importance of anthropogenic disturbance on species richness and abundance.     

Survival and growth of dominant tree seedlings in seasonally tropical dry forests of Yucatan: site and fertilization effects

Aims Seasonally tropical dry forests of the Yucatan Peninsula are typically found in sites with nutrient-poor soils because of the recent geological origin of the region. The landscape is dominated by extensive karstic plates that shape environments where vegetation regeneration through seed germination may be limited by the availability of suitable microsites. In this study, we documented the survival and growth of seedlings from three dominant tree species (Bursera simaruba, Piscidia piscipula and Lysiloma latisiliquum) in seasonally tropical dry forests in Yucatan. Specifically, we evaluated the effect of nutrient addition (N and P, separately and in combination) on seedling survival and growth across three sites with differing levels of precipitation.Methods We conducted a nutrient addition experiment, whereby we established 12 plots of dimensions 10×10 m (100 m 2) at each site, from which three plots were randomly selected to receive one of four treatments: N addition, P addition, N and P addition and no nutrient addition (controls). Prior to treatment application, in each plot, we planted 10 seedlings of each species in October 2010 and subsequently conducted surveys of plant growth and survival every 20 days from November 2010 to April 2011.Important findings Overall, nutrient addition increased seedling survival and the magnitude of this effect was similar among sites. We did not observe an additive effect of the N + P treatment on survival. Similarly, we observed a positive effect of nutrient addition on seedling growth, but this effect was contingent upon site; regarding survival, the effects of N and P on seedling growth were not additive. These results suggest that seedling recruitment and growth in the three dominant species of trees in Yucatan are limited by nutrient availability but that the magnitude of this effect, particularly on seedling growth, is specific for species and site.