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.     

Comparing forest structure and biodiversity on private and public land: secondary tropical dry forests in Costa Rica

Secondary forests constitute a substantial proportion of tropical forestlands. These forests occur on both public and private lands and different underlying environmental variables and management regimes may affect post‐abandonment successional processes and resultant forest structure and biodiversity. We examined whether differences in ownership led to differences in forest structure, tree diversity, and tree species composition across a gradient of soil fertility and forest age. We collected soil samples and surveyed all trees in 82 public and 66 private 0.1‐ha forest plots arrayed across forest age and soil gradients in Guanacaste, Costa Rica. We found that soil fertility appeared to drive the spatial structure of public vs. private ownership; public conservation lands appeared to be non‐randomly located on areas of lower soil fertility. On private lands, areas of crops/pasture appeared to be non‐randomly located on higher soil fertility areas while forests occupied areas of lower soil fertility. We found that forest structure and tree species diversity did not differ significantly between public and private ownership. However, public and private forests differed in tree species composition: 11 percent were more prevalent in public forest and 7 percent were more prevalent in private forest. Swietenia macrophylla, Cedrela odorata, and Astronium graveolens were more prevalent in public forests likely because public forests provide stronger protection for these highly prized timber species. Guazuma ulmifolia was the most abundant tree in private forests likely because this species is widely consumed and dispersed by cattle. Furthermore, some compositional differences appear to result from soil fertility differences due to non‐random placement of public and private land holdings with respect to soil fertility. Land ownership creates a distinctive species composition signature that is likely the result of differences in soil fertility and management between the ownership types. Both biophysical and social variables should be considered to advance understanding of tropical secondary forest structure and biodiversity.     

Diversity and ecology of lianas in tropical dry evergreen forests on the Coromandel Coast of India under various disturbance regimes

In this study we attempted to explore patterns of diversity, abundance, climbing and dispersal mode of lianas in relation to disturbance in 40 Indian subtropical dry forests. The sites were selected to represent four disturbance categories: relatively undisturbed, moderately disturbed, much disturbed and heavily disturbed. All lianas ≥1 cm dbh were counted, which resulted in a total amount of 5689 individuals of lianas, representing 77 species in 62 genera and 32 families. Liana species richness and abundance increased with forest disturbance, but the liana basal area values showed an opposite trend, with high scores in undisturbed sites. Twining was the main climbing mechanism (61.3%) and zoochory (59.6%) was the main dispersal mode in all the four forest categories. Application of Bray–Curtis cluster analysis produced three distinct clusters in which the much disturbed category was more distant from the others. High abundance of large lianas in undisturbed sites and that of the invasive Lantana camara in heavily disturbed site signals the conservation significance of the less disturbed study sites. The predominance of zoochorous dispersal indicates the faunal dependence of lianas, besides of host trees, thus underlining the need for a holistic approach in biodiversity conservation of this and similar tropical forests.     

Liana diversity and distribution in four tropical dry evergreen forests on the Coromandel coast of south India

Liana diversity was inventoried in four tropical dry evergreen forest sites that are characterized by numerous trees, of short stature and small diameter, and a varying degree of anthropogenic disturbance, on the Coromandel coast of south India. A 1-ha plot was established in each of the four sites and was subdivided into 100 quadrats of 10 m× 10 m. All lianas 1 cm diameter at breast height (dbh) rooted within the plot were enumerated. The species richness and density of lianas, with respect to site disturbance and forest stature, varied across the sites. Liana density totaled 3307 individuals (range 497–1163 individuals ha^–1) and species richness totaled 39 species (range 24–29 species ha^–1) representing 34 genera and 24 families. Combretaceae, Asclepiadaceae, Capparaceae and Vitaceae were the well-represented families. The top five species Strychnos minor, Combretum albidum, Derris ovalifolia, Jasminum angustifolium and Reissantia indica contributed 55% of total density. The slopes of the species–area curves were different for each of the four sites and the curve stabilized in only one site. Of the four climbing modes recognized among the total 39 species, 18 were twiners (56% of the total density). Eight species (24% of density) were tendril climbers and 12 species (16% of density) were scramblers. Hugonia mystax was the only hook climber. All the 39 species and 88% of liana density were encountered within a category of 6 cm dbh or less, and a similar pattern prevailed in the individual sites. Of the three diaspore dispersal modes found among the 39 liana species, animal (64%) and wind (23%) dispersal were predominant over the autochorous mode (13%). Liana diversity and distribution in dry forest communities appear to be influenced by forest stature and site disturbance levels. In the light of the extent of liana diversity and sacred grove status of the study sites, the need for forest conservation, involving local people, is emphasized.     

Forest fragmentation and loss reduce richness,availability, and specialization in tropical hummingbird communities

Hummingbirds are important pollinators of many native Neotropical plants but their abundance and diversity in landscapes dominated by intensive human uses such as agriculture have rarely been examined, despite such land‐uses prevailing in the tropics. We examined how tropical deforestation affects hummingbird community structure in premontane forest patches embedded in a tropical countryside of Coto Brus Canton, Costa Rica. We captured hummingbirds in fourteen landscapes representing a gradient in patch size and forest amount, and tested for the effects of these variables on (1) hummingbird captures at flowers (pollinator availability); (2) species richness; and (3) filtering of functional traits. After accounting for sampling effects, both hummingbird availability and species richness declined by 40% and 50%, respectively, across the gradient in deforestation that we observed (9–66% forest within 1000 m). Focal patch size was the strongest predictor, even after statistically accounting for the amount of forest and matrix composition of landscapes. These reductions in availability and richness were well predicted by functional traits; morphologically specialized species with the capacity to transport long‐distance outcrossed pollen and low functional redundancy within the pollinator network showed the greatest sensitivity to landscape change. We hypothesize that declines in hummingbird availability, diversity, and functional traits are important mechanisms driving the observed pollen limitation of ornithophilous flowers in fragmented tropical landscapes. Efforts to conserve large forest patches and enhance matrix permeability are critical for maintaining forest hummingbird communities and pollination services under current and predicted deforestation regimes.     

Tropical dry forests in New Caledonia

Tropical dry forest is the most endangered major vegetation type in the New Caledonia biodiversity hotspot. Vegetation surveys following a transect method used by Gentry were undertaken in two tropical dry forest sites, Ouen-Toro and Pindai, in order to compare species richness, floristic composition, and structure. Pindai contained significantly higher species richness than Ouen-Toro, although there was little difference in forest structure. Tropical dry forest sites in New Caledonia were compared to seven other biodiversity hotspots with tropical dry forest where Gentry's transect method was employed. New Caledonia and other tropical dry forests on islands contain significantly lower species richness than mainland tropical dry forests in biodiversity hotspots. However, New Caledonia contained the highest number of threatened species based on IUCN global conservation categories. Tropical dry forest in New Caledonia appears to be the world's most endangered tropical dry forest based on the extent of forest, number of reserves, and threatened species. Management of tropical dry forests on private and community lands is absolutely imperative to the long-term persistence of this ecosystem.     

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.     

Regeneration of tropical dry forests in Central America,with examples from Nicaragua

Abstract. This paper focuses on the management potential of natural forests in the tropical dry zones of Central America, particularly Nicaragua. Distribution and status of dry forest formations are reviewed. Two case studies from the Pacific coast of Nicaragua are presented to illustrate the disturbance created by traditional utilization, and to show the relatively high potential for natural forest management, with many of the tree species having economic value. Natural forest management, together with conservation and reforestation, is viewed as the main management option for the tropical dry forest areas. In spite of its importance, this option has received little attention or promotion from the governmental forestry sector. Four complementary options which can be devised for silvicultural work in production forests are: encouragement of advanced growth of desirable tree species, inducement of natural regeneration, coppice management, and compensatory planting. These options form different strategies for rehabilitation of the production and conservation functions of the forest and may well be combined in the same area. Research applied to better utilization of the forest resources should form an intrinsic part of rural development programs. Research work in tropical dry forest areas should focus on ecological, as well as socio-cultural and economical aspects related to the management options. Experimental sites are important enabling and securing long-term basic and applied biological research. They may also serve as units for demonstration, training and extension.     

Phylogenetic community patterns suggest Central Indian tropical dry forests are structured by montane climate refuges

Aim

We used an eco-phylogenetic approach to investigate the diversity and assembly patterns of tropical dry forests (TDFs) in Central India. We aimed at informing conservation and restoration practices in these anthropogenically disturbed forests by identifying potential habitats of conservation significance and elements of regional biodiversity most vulnerable to human impact and climate change.

Location

Tropical dry forests of Madhya Pradesh, Central India.

Methods

We analysed the species richness, stem density, basal area and phylogenetic structure (standardized effect size of MNTD, MPD, PD and community evolutionary distinctiveness cED) of 117 tree species assemblages distributed across a ~230 to ~940 m elevational gradient. We examined how these community measures and taxonomic (Sørensen) and phylogenetic (UniFrac) beta diversity varied with elevation, precipitation, temperature and climatic stress.

Results

Species richness, phylogenetic diversity, stem density and basal area were positively correlated with elevation, with high-elevation plots exhibiting cooler temperatures, higher precipitation and lower stress. High-elevation assemblages also trended towards greater phylogenetic dispersion, which diminished at lower elevations and in drier, more stressful plots. Phylogenetic turnover was observed across the elevation gradient, and species evolutionary distinctiveness increased at lower elevations and under harsher abiotic conditions.

Main Conclusions

Harsher abiotic conditions at low elevations may act as a selective filter on plant lineages, leading to phylogenetically clustered low-diversity assemblages. These assemblages contained more evolutionarily distinct species that may contribute disproportionately to biodiversity. Conversely, milder abiotic conditions at high elevations may serve as refuges for drought-sensitive species, resulting in more diverse assemblages. Conservation practices that prioritize both high- and low-elevation habitats could promote the persistence of evolutionarily distinct species and areas of high biodiversity within the Central Indian landscape. Establishing connectivity between these habitats may provide a range of climatic conditions for species to retreat to or persist within as climates change.     

Species Richness,Spatial Variation,and Abundance of the Soil Seed Bank of a Secondary Tropical Rain Forest1

Despite the importance of the soil seed bank in tropical forest regeneration, little is known about spatial variability in species composition and abundance of seeds stored in the soil. To develop sampling methods for comparative studies, we examined species richness, spatial variation, and abundance of germinants from the soil seed bank in a 16 year old secondary, tropical wet forest at La Selva Biological Station, Costa Rica. Surface soil (10 cm deep, 4.7 cm diameter) was collected at the intersection points of a gridded 1 ha plot (10 × 10-m grid, 121 samples) and in a nested 100 m² subplot (2 × 2-m grid, 36 samples). The 1 ha plot had a density of 4535 seeds/m² with 34 species observed. Based on a series of 100 randomized species accumulation curves, a Michaelis-Menten fit predicted a mean species richness of 36.3 species; the number of observed species was close to the predicted asymptote. A nonparametric, first-order jackknife species richness estimator predicted a species richness of 37.0 species. Eighty-five and 95 percent of the observed species richness is contained, on average, within 41 and 74 pooled samples, respectively. Within the 100 m² nested subplot, a density of 5476 seeds/m² was observed, comprising 26 species with an estimated species richness (Michaelis-Menten fit) of 29.1 species. The jackknife species richness estimator predicted a species richness of 36.7 species. For species richness and abundance of both plots, spatial autocorrelation statistics (Moran's I) were not significantly different from zero at lag distances from 2 to 100 m, indicating a random distribution at these spatial scales. For this site, accurate estimates of species composition depend upon the number of samples collected as well as the spatial distribution of sampling effort. Many small samples distributed over a large area provide greater accuracy and precision for estimating species richness of the soil seed bank.     

Environmental control of flowering periodicity in Costa Rican and Mexican tropical dry forests

Aim We analyse the proximate causes of the large variation in flowering periodicity among four tropical dry forests (TDF) and ask whether climatic periodicity or biotic interactions are the ultimate causes of flowering periodicity. Location The four TDFs in Guanacaste (Costa Rica), Yucatan, Jalisco and Sonora (Mexico) are characterized by a 5–7 month long dry season and are located along a gradient of increasing latitude (10–30°N). Methods To dissect the differences in flowering periodicity observed at the community level, individual tree species were assigned to ‘flowering types’, i.e. groups of species with characteristic flowering periods determined by similar combinations of environmental flowering cues and vegetative phenology. Results Large variation in the fraction of species and flowering types blooming during the dry and wet season, respectively, indicates large differences in the severity of seasonal drought among the four forests. In the dry upland forests of Jalisco, flowering of leafless trees remains suppressed during severe seasonal drought and is triggered by the first rains of the wet season. In the other forests, leaf shedding, exceptional rainfall or increasing daylength cause flowering of many deciduous species at various times during the dry season, well before the summer rains. The fraction of deciduous species leafing out during the summer rains and flowering when leafless during the dry season is largest in the Sonoran TDF. Main conclusions In many wide‐ranging species the phenotypic plasticity of flowering periodicity is large. The distinct temporal separation of spring flowering on leafless shoots and subsequent summer flushing represents a unique adaptation of tree development to climates with a relatively short rainy season and a long dry season. Seasonal variation in rainfall and soil water availability apparently constitutes not only the proximate, but also the ultimate cause of flowering periodicity, which is unlikely to have evolved in response to biotic adaptive pressures.     

Convergent structural responses of tropical forests to diverse disturbance regimes

Size frequency distributions of canopy gaps are a hallmark of forest dynamics. But it remains unknown whether legacies of forest disturbance are influencing vertical size structure of landscapes, or space-filling in the canopy volume. We used data from LiDAR remote sensing to quantify distributions of canopy height and sizes of 434 501 canopy gaps in five tropical rain forest landscapes in Costa Rica and Hawaii. The sites represented a wide range of variation in structure and natural disturbance history, from canopy gap dynamics in lowland Costa Rica and Hawaii, to stages and types of stand-level dieback on upland Mauna Kea and Kohala volcanoes. Large differences in vertical canopy structure characterized these five tropical rain forest landscapes, some of which were related to known disturbance events. Although there were quantitative differences in the values of scaling exponents within and among sites, size frequency distributions of canopy gaps followed power laws at all sites and in all canopy height classes. Scaling relationships in gap size at different heights in the canopy were qualitatively similar at all sites, revealing a remarkable similarity despite clearly defined differences in species composition and modes of prevailing disturbance. These findings indicate that power-law gap-size frequency distributions are ubiquitous features of these five tropical rain forest landscapes, and suggest that mechanisms of forest disturbance may be secondary to other processes in determining vertical and horizontal size structure in canopies.     

Genetic population structure and distribution of a fungal polypore, Datronia caperata (Polyporaceae), in mangrove forests of Central America

Aim We examine the genetic structure of a fungal polypore, Datronia caperata (Berk.) Ryvarden (Polyporaceae), colonizing white mangrove, Laguncularia racemosa (L.) Gaertn. f. (Combretaceae), of Central America. Location Mangrove forests of Costa Rica and Panama. Methods Sequences of elongation factor alpha (EFA), beta tubulin (BTUB) and nuclear ribosomal internal transcribed spacer (ITS) regions were obtained from 54 collections of D. caperata collected from Caribbean and Pacific L. racemosa forests in Central America. Measures of haplotype and nucleotide diversity, nested clade analyses and coalescent analyses were used to estimate the direction and extent of migration of the fungus, and the factors promoting population divergence. We also conducted phylogenetic analyses using Bayesian estimation to test whether putative D. caperata collected from L. racemosa was conspecific with D. caperata colonizing other hosts from diverse Neotropical forests. Results Our results demonstrate that there is genetic isolation between D. caperata populations from Caribbean mangroves and those from Pacific mangroves. Our data suggest that the best explanation for the observed haplotype distribution is a recent range expansion from the Caribbean to the Pacific coasts, with subsequent isolation. This is supported by the infrequent overlap of haplotypes, unidirectional migration estimates from the Caribbean to the Pacific and the older estimated age of mutations in the Caribbean low‐copy BTUB and EFA loci. In addition, our data suggest that D. caperata from mangroves are not conspecific with collections from other hosts found in diverse Neotropical forests. Main conclusions The low frequency of shared haplotypes between coasts, coupled with the incomplete lineage sorting after cessation of gene flow, is consistent with isolation during the last Pleistocene glaciation. We hypothesize that the greater haplotype and nucleotide diversity in the Pacific occurs either because larger effective population sizes of D. caperata are maintained in Pacific mangroves or because D. caperata populations underwent a significant bottleneck as a result of local extinction followed by recolonization. In addition, we found that D. caperata found on L. racemosa was not conspecific with D. caperata from non‐mangrove hosts and suggest that D. caperata found on L. racemosa may be a host specialist.     

Tree allometry and improved estimation of carbon stocks and balance in tropical forests

Tropical forests hold large stores of carbon, yet uncertainty remains regarding their quantitative contribution to the global carbon cycle. One approach to quantifying carbon biomass stores consists in inferring changes from long-term forest inventory plots. Regression models are used to convert inventory data into an estimate of aboveground biomass (AGB). We provide a critical reassessment of the quality and the robustness of these models across tropical forest types, using a large dataset of 2,410 trees ≥ 5 cm diameter, directly harvested in 27 study sites across the tropics. Proportional relationships between aboveground biomass and the product of wood density, trunk cross-sectional area, and total height are constructed. We also develop a regression model involving wood density and stem diameter only. Our models were tested for secondary and old-growth forests, for dry, moist and wet forests, for lowland and montane forests, and for mangrove forests. The most important predictors of AGB of a tree were, in decreasing order of importance, its trunk diameter, wood specific gravity, total height, and forest type (dry, moist, or wet). Overestimates prevailed, giving a bias of 0.5–6.5% when errors were averaged across all stands. Our regression models can be used reliably to predict aboveground tree biomass across a broad range of tropical forests. Because they are based on an unprecedented dataset, these models should improve the quality of tropical biomass estimates, and bring consensus about the contribution of the tropical forest biome and tropical deforestation to the global carbon cycle. Electronic Supplementary Material Supplementary material is available for this article at     

Diversity of understory plants in undisturbed and disturbed tropical lowland forests of Little Andaman Island, India

Species richness and density of understory plants were investigated in eight 1 ha plots, distributed one each in undisturbed and disturbed tropical evergreen, semi-evergreen, deciduous and littoral forests of Little Andaman island, India, which falls under one of the eight hottest hotspots of Biodiversity in the world viz. the Indo-Burma. One hundred 1 m⁻² quadrats were established in each 1 ha plot, in which all the understory plants (that include herbs, undershrubs, shrubs and herbaceous climbers) were enumerated. The total density of understory plants was 6,812 individuals (851 ha⁻¹) and species richness was 108 species, representing 104 genera and 50 families. Across the four forest types and eight study plots, the species richness ranged from 10 to 39 species ha⁻¹. All the disturbed sites harbored greater number of species than their undisturbed counterparts. Herbs dominated by species (63%) and density (4,259 individuals). The grass Eragrostis tenella (1,860 individuals; IVI 40), the invasive climber Mikania cordata (803; IVI 20) and the shrub Anaxagorea luzonensis (481; IVI 17.5) were the most abundant species. Poaceae, Asteraceae, Acanthaceae, Orchidaceae and Euphorbiaceae constituted the species-rich families represented by 6 species each. The species-area curves attained an asymptote at 0.8 ha level except in sites DD and DL, indicating 1 ha plot is not sufficient to capture all the understory species in disturbed forests. The alien weeds formed about one-fourth of the species richness (31 species; 28%) and density (1,926 individuals; 28.3%) in the study sites, indicating the extent of weed invasion and the attention required for effective conservation of the native biodiversity of the fragile island forest ecosystem.     

Seed dispersal by carnivores in temperate and tropical dry forests

The seed dispersal mechanisms and regeneration of various forest ecosystems can benefit from the actions of carnivores via endozoochory. This study was aimed to evaluate the role of carnivores in endozoochory and diploendozoochory, as well as their effect on seed viability, scarification, and germination in two forest ecosystems: temperate and tropical dry forest. We collected carnivore scat in the Protected Natural Area of Sierra Fría in Aguascalientes, Mexico, for 2 years to determine the abundance and richness of seeds dispersed by each carnivore species, through scat analysis. We assessed seed viability through optical densitometry using X‐rays, analyzed seed scarification by measuring seed coat thickness using a scanning electron microscope, and evaluated seed germination in an experiment as the percentage of seeds germinated per carnivore disperser, plant species, and forest type. In the temperate forest, four plant species (but mainly Arctostaphylos pungens) were dispersed by four mammal species. The gray fox dispersed the highest average number of seeds per scat (66.8 seeds). Bobcat dispersed seeds through diploendozoochory, which was inferred from rabbit (Sylvilagus floridanus) hair detected in their scats. The tropical dry forest presented higher abundance of seeds and richness of dispersed plant species (four species) than in the temperate forest, and the coati dispersed the highest number of seeds (8,639 seeds). Endozoochory and diploendozoochory did not affect viability in thick‐testa seeds (1,480 µm) in temperate forest and thin‐testa seeds (281 µm) in tropical dry forest. Endozoochory improved the selective germination of seeds. Nine plant species were dispersed by endozoochory, but only one species (Juniperus sp.) by diploendozoochory. These results suggest that carnivores can perform an important ecological function by dispersing a great abundance of seeds, scarifying these seeds causing the formation of holes and cracks in the testas without affecting viability, and promoting the selective germination of seeds.