The intermediate disturbance hypothesis applies to tropical forests, but disturbance contributes little to tree diversity

The intermediate disturbance hypothesis (IDH) predicts local species diversity to be maximal at an intermediate level of disturbance. Developed to explain species maintenance and diversity patterns in species-rich ecosystems such as tropical forests, tests of IDH in tropical forest remain scarce, small-scale and contentious. We use an unprecedented large-scale dataset (2504 one-hectare plots and 331 567 trees) to examine whether IDH explains tree diversity variation within wet, moist and dry tropical forests, and we analyse the underlying mechanism by determining responses within functional species groups. We find that disturbance explains more variation in diversity of dry than wet tropical forests. Pioneer species numbers increase with disturbance, shade-tolerant species decrease and intermediate species are indifferent. While diversity indeed peaks at intermediate disturbance levels little variation is explained outside dry forests, and disturbance is less important for species richness patterns in wet tropical rain forests than previously thought.     

Rainfall seasonality and drought performance shape the distribution of tropical tree species in Ghana

Tree species distribution in lowland tropical forests is strongly associated with rainfall amount and distribution. Not only plant water availability, but also irradiance, soil fertility, and pest pressure covary along rainfall gradients. To assess the role of water availability in shaping species distribution, we carried out a reciprocal transplanting experiment in gaps in a dry and a wet forest site in Ghana, using 2,670 seedlings of 23 tree species belonging to three contrasting rainfall distributions groups (dry species, ubiquitous species, and wet species). We evaluated seasonal patterns in climatic conditions, seedling physiology and performance (survival and growth) over a 2‐year period and related seedling performance to species distribution along Ghana's rainfall gradient. The dry forest site had, compared to the wet forest, higher irradiance, and soil nutrient availability and experienced stronger atmospheric drought (2.0 vs. 0.6 kPa vapor pressure deficit) and reduced soil water potential (?5.0 vs. ?0.6 MPa soil water potential) during the dry season. In both forests, dry species showed significantly higher stomatal conductance and lower leaf water potential, than wet species, and in the dry forest, dry species also realized higher drought survival and growth rate than wet species. Dry species are therefore more drought tolerant, and unlike the wet forest species, they achieve a home advantage. Species drought performance in the dry forest relative to the wet forest significantly predicted species position on the rainfall gradient in Ghana, indicating that the ability to grow and survive better in dry forests and during dry seasons may allow species to occur in low rainfall areas. Drought is therefore an important environmental filter that influences forest composition and dynamics. Currently, many tropical forests experience increase in frequency and intensity of droughts, and our results suggest that this may lead to reduction in tree productivity and shifts in species distribution.     

Annual Rainfall and Seasonality Predict Pan-tropical Patterns of Liana Density and Basal Area

We test the hypotheses proposed by Gentry and Schnitzer that liana density and basal area in tropical forests vary negatively with mean annual precipitation (MAP) and positively with seasonality. Previous studies correlating liana abundance with these climatic variables have produced conflicting results, warranting a new analysis of drivers of liana abundance based on a different dataset. We compiled a pan-tropical dataset containing 28,953 lianas (≥2.5 cm diam.) from studies conducted at 13 Neotropical and 11 Paleotropical dry to wet lowland tropical forests. The ranges in MAP and dry season length (DSL) (number of months with mean rainfall <100 mm) represented by these datasets were 860–7250 mm/yr and 0–7 mo, respectively. Pan-tropically, liana density and basal area decreased significantly with increasing annual rainfall and increased with increasing DSL, supporting the hypotheses of Gentry and Schnitzer. Our results suggest that much of the variation in liana density and basal area in the tropics can be accounted for by the relatively simple metrics of MAP and DSL.     

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

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

Predicting and quantifying the structure of tropical dry forests in South Florida and the Neotropics using spaceborne imagery

Aim This research examines environmental theories and remote sensing methods that have been hypothesized to be associated with tropical dry forest structure. Location Tropical dry forests of South Florida and the Neotropics. Methods Field measurements of stand density, basal area and tree height were collected from 22 stands in South Florida and 30 stands in the Neotropics. In South Florida, field measurements were compared to climatic (temperature, precipitation, hurricane disturbance) and edaphic (rockiness, soil depth) variables, spectral indices (NDVI, IRI, MIRI) from Landsat 7 ETM+, and estimates of tree height from the Shuttle Radar Topography Mission (SRTM) and the National Elevation Dataset (NED). Environmental variables associated with tropical dry forest structure in South Florida were compared to tropical dry forest in other Neotropical sites. Results There were significant correlations among temperature and precipitation, and stand density and tree height in South Florida. There were significant correlations between (i) stand density and mean NDVI and standard deviation of NDVI, (ii) MIRI and stand density, basal area and mean tree height, and (iii) estimates of tree height from SRTM with maximum tree height. In the Neotropics, there were no relationships between temperature or precipitation and tropical dry forest structure, however, Neotropical sites that experience hurricane disturbance had significantly shorter tree heights and higher stand densities. Main conclusions It is possible to predict and quantify the forest structure characteristics of tropical dry forests using climatic data, Landsat 7 ETM+ imagery and SRTM data in South Florida. However, results based on climatic data are region‐specific and not necessarily transferable between tropical dry forests at a continental spatial scale. Spectral indices from Landsat 7 ETM+ can be used to quantify forest structure characteristics, but SRTM data are currently not transferable to other regions. Hurricane disturbance has a significant impact on forest structure in the Neotropics.     

Seasonal Patterns of Flowering and Fruiting in a Dry Tropical Forest in Jamaica1

Tropical dry forests occupy more area and are more endangered than rainforests, yet their regeneration ecology has received less study and is consequently poorly understood. We recorded the flowering and fruiting phenology of a tropical dry forest in Jamaica over a period of 26 mo within ten 15 × 15‐m plots. Community‐wide recruitment reached a maximum in the wet season, whereas no recruitment occurred during the dry season. We observed a unimodal peak in rainfall and fruit production, and the periodicity and intensity of seed production were significantly correlated with rainfall seasonality (the optimal time for germination). Flowering at the community and system levels lagged behind a significant increase and subsequent decrease in rainfall by 7 and 3 mo, respectively, indicating that the dominant factor controlling flowering periodicity is the passage of the major (4‐mo long) rainy season and changes in soil moisture conditions. Fruiting lagged behind flowering by 2 mo and a significant increase in fruiting occurred 2 mo prior to a significant increase in rainfall. At the population level, a correspondence analysis identified a major dichotomy in the patterns of flowering and fruiting between species and indicated two broad species groups based on their time of peak fruiting and the number of times they were in fruit. These were either individuals which were usually in peak fruit 1–2 mo prior to the start of the major rainy season or those that were in fruit more or less continuously throughout the year with no peak fruiting time. This study supports the view that seasonal variation in rainfall and hence soil water availability constitutes both the proximate and the ultimate cause of flowering periodicity in tropical dry forests.     

Responses to Fire in Selected Tropical Dry Forest Trees1

Fire is a frequent disturbance in the tropical dry forests of Central America, yet very little is known about how native species respond to such events. We conducted an experimental burn in a tropical dry forest of western Nicaragua to evaluate plant responses to fire with respect to survivorship and recruitment. Measurements of woody vegetation of all size classes were carried out prior to the prescribed burn and three successive years post fire. We selected the 15 most abundant species <10 cm DBH to assess percent survivorship and sprouting responses post fire. Changes in seedling densities for these 15 most abundant species and the 15 least abundant species were analyzed using a repeated measure ANOVA. We also assessed changes in seedling densities for three species of international conservation concern. We found three major fire‐coping strategies among common dry forests plants: resisters (low fire‐induced mortality), resprouters (vigorous sprouting), and recruiters (increased seeding post‐fire). While survivorship was generally high relative to tropical moist forest species, those species with lower survivorship used either seeding or sprouting as an alternative strategy for persisting in the forest community. Seed dispersal mechanisms, particularly wind dispersal, appear to be an important factor in recruitment success post‐fire. Burn treatment led to a significant increase in the density of seedlings for two species of conservation concern: Guaiacum sanctum and Swietenia humilis. Results of this study suggest that common dry forest species in western Nicaragua are fire tolerant. Further study of individual species and their fire responses is merited.     

Understanding and predicting frost‐induced tropical tree mortality patterns

Extreme climatic and weather events are increasing in frequency and intensity across the world causing episodes of widespread tree mortality in many forested ecosystems. However, we have a limited understanding about which local factors influence tree mortality patterns, restricting our ability to predict tree mortality, especially within topographically complex tropical landscapes with a matrix of mature and secondary forests. We investigated the effects of two major local factors, topography and forest successional type, on climate‐induced tropical tree mortality patterns using an observational and modeling approach. The northernmost Neotropical dry forest endured an unprecedented episode of frost‐induced tree mortality after the historic February 2011 cold wave hit northwestern Mexico. In a moderately hilly landscape covering mature and secondary tropical dry forests, we surveyed 454 sites for the presence or absence of frost‐induced tree mortality. In addition, across forty‐eight 1 ha plots equally split into the two forest types, we examined 6,981 woody plants to estimate a frost‐disturbance severity metric using the density of frost‐killed trees. Elevation is the main factor modulating frost effects regardless of forest type. Higher occurrence probabilities of frost‐induced tree mortality at lowland forests can be explained by the strong influence of elevation on temperature distribution since heavier cold air masses move downhill during advective frosts. Holding elevation constant, the probability of frost‐induced tree mortality in mature forests was twice that of secondary forests but severity showed the opposite pattern, suggesting a cautious use of occurrence probabilities of tree mortality to infer severity of climate‐driven disturbances. Extreme frost events, in addition to altering forest successional pathways and ecosystem services, likely maintain and could ultimately shift latitudinal and altitudinal range margins of Neotropical dry forests.     

Effects of Drought,Pest Pressure and Light Availability on Seedling Establishment and Growth: Their Role for Distribution of Tree Species across a Tropical Rainfall Gradient

Tree species distributions associated with rainfall are among the most prominent patterns in tropical forests. Understanding the mechanisms shaping these patterns is important to project impacts of global climate change on tree distributions and diversity in the tropics. Beside direct effects of water availability, additional factors co-varying with rainfall have been hypothesized to play an important role, including pest pressure and light availability. While low water availability is expected to exclude drought-intolerant wet forest species from drier forests (physiological tolerance hypothesis), high pest pressure or low light availability are hypothesized to exclude dry forest species from wetter forests (pest pressure gradient and light availability hypothesis, respectively). To test these hypotheses at the seed-to-seedling transition, the potentially most critical stage for species discrimination, we conducted a reciprocal transplant experiment combined with a pest exclosure treatment at a wet and a dry forest site in Panama with seeds of 26 species with contrasting origin. Establishment success after one year did not reflect species distribution patterns. However, in the wet forest, wet origin species had a home advantage over dry forest species through higher growth rates. At the same time, drought limited survival of wet origin species in the dry forest, supporting the physiological tolerance hypothesis. Together these processes sort species over longer time frames, and exclude species outside their respective home range. Although we found pronounced effects of pests and some effects of light availability on the seedlings, they did not corroborate the pest pressure nor light availability hypotheses at the seed-to-seedling transition. Our results underline that changes in water availability due to climate change will have direct consequences on tree regeneration and distributions along tropical rainfall gradients, while indirect effects of light and pests are less important.     

Climate‐growth analysis for a Mexican dry forest tree shows strong impact of sea surface temperatures and predicts future growth declines

Tropical forests will experience relatively large changes in temperature and rainfall towards the end of this century. Little is known about how tropical trees will respond to these changes. We used tree rings to establish climate‐growth relations of a pioneer tree, Mimosa acantholoba, occurring in tropical dry secondary forests in southern Mexico. The role of large‐scale climatic drivers in determining interannual growth variation was studied by correlating growth to sea surface temperature anomalies (SSTA) of the Atlantic and Pacific Oceans, including the El Niño‐Southern Oscillation (ENSO). Annual growth varied eightfold over 1970–2007, and was correlated with wet season rainfall (r=0.75). Temperature, cloud cover and solar variation did not affect growth, although these climate variables correlated with growth due to their relations with rainfall. Strong positive correlations between growth and SSTA occurred in the North tropical Atlantic during the first half of the year, and in the Pacific during the second half of the year. The Pacific influence corresponded closely to ENSO‐like influences with negative effects of high SSTA in the eastern Pacific Niño3.4 region on growth due to decreases in rainfall. During El Niño years growth was reduced by 37%. We estimated how growth would be affected by the predicted trend of decreasing rainfall in Central America towards the end of this century. Using rainfall predictions of two sets of climate models, we estimated that growth at the end of this century will be reduced by 12% under a medium (A1B) and 21% under a high (A2) emission scenario. These results suggest that climate change may have repercussions for the carbon sequestration capacity of tropical dry forests in the region.     

Successional Change and Resilience of a Very Dry Tropical Deciduous Forest Following Shifting Agriculture

We analyzed successional patterns in a very dry tropical deciduous forest by using 15 plots differing in age after abandonment and contrasted them to secondary successions elsewhere in the tropics. We used multivariate ordination and nonlinear models to examine changes in composition and structure and to estimate forest recovery rates and resilience. A shrub phase characterized early succession (0–3 yr); afterwards, the tree Mimosa acantholoba became dominant. Below its canopy, sprouts and seed-regenerated individuals of mature forest species slowly accumulated. Canopy height, plant density, and crown cover stabilized in less than 15 yr, whereas species richness, diversity, and basal area continued to increase. The pioneer species group has very low diversity and the long-lived pioneer phase typical of humid forests is absent; species composition may therefore recover soon as suggested by convergence toward mature forest species composition. The time trend of plant density also differed from humid forests for it lacked its characteristic density decline, presumably because of differences in regeneration mechanisms between very dry and other less water-stressed forest types. As opposed to the prevailing hypothesis, resilience was not higher than in moister forests, and thus factors other than structure relative simplicity must be accounted for when assessing resilience.     

Leaf litter arthropod responses to tropical forest restoration

Soil and litter arthropods represent a large proportion of tropical biodiversity and perform important ecosystem functions, but little is known about the efficacy of different tropical forest restoration strategies in facilitating their recovery in degraded habitats. We sampled arthropods in four 7‐ to 8‐year‐old restoration treatments and in nearby reference forests. Sampling was conducted during the wet and dry seasons using extractions from litter and pitfall samples. Restoration treatments were replicated in 50 × 50‐m plots in four former pasture sites in southern Costa Rica: plantation – trees planted throughout the plot; applied nucleation/islands – trees planted in patches of different sizes; and natural regeneration – no tree planting. Arthropod abundance, measures of richness and diversity, and a number of functional groups were greater in the island treatment than in natural regeneration or plantation treatments and, in many cases, were similar to reference forest. Litter and pitfall morphospecies and functional group composition in all three restoration treatments were significantly different than reference sites, but island and plantation treatments showed more recovery than natural regeneration. Abundance and functional group diversity showed a much greater degree of recovery than community composition. Synthesis and applications: The less resource‐intensive restoration strategy of planting tree islands was more effective than tree plantations in restoring arthropod abundance, richness, and functional diversity. None of the restoration strategies, however, resulted in similar community composition as reference forest after 8 years of recovery, highlighting the slow rate of recovery of arthropod communities after disturbance, and underscoring the importance of conservation of remnant forests in fragmented landscapes.     

Vegetation Recovery 16 Years after Feral Pig Removal from a Wet Hawaiian Forest

Nonnative ungulates can alter the structure and function of forest ecosystems. Feral pigs in particular pose a substantial threat to native plant communities throughout their global range. Hawaiian forests are exceptionally vulnerable to feral pig activity because native vegetation evolved in the absence of large mammalian herbivores. A common approach for conserving and restoring forests in Hawaii is fencing and removal of feral pigs. The extent of native plant community recovery and nonnative plant invasion following pig removal, however, is largely unknown. Our objective was to quantify changes in native and nonnative understory vegetation over a 16 yr period in adjacent fenced (pig‐free) vs. unfenced (pig‐present) Hawaiian montane wet forest. Native and nonnative understory vegetation responded strongly to feral pig removal. Density of native woody plants rooted in mineral soil increased sixfold in pig‐free sites over 16 yr, whereas establishment was almost exclusively restricted to epiphytes in pig‐present sites. Stem density of young tree ferns increased significantly (51.2%) in pig‐free, but not pig‐present sites. Herbaceous cover decreased over time in pig‐present sites (67.9%). In both treatments, number of species remained constant and native woody plant establishment was limited to commonly occurring species. The nonnative invasive shrub, Psidium cattleianum, responded positively to release from pig disturbance with a fivefold increase in density in pig‐free sites. These results suggest that while common native understory plants recover within 16 yr of pig removal, control of nonnative plants and outplanting of rarer native species are necessary components of sustainable conservation and restoration efforts in these forests.     

Rapid Liana Colonization along a Secondary Forest Chronosequence

Lianas (woody vines) can have profound effects on tree recruitment, growth, survival, and diversity in tropical forests. However, the dynamics of liana colonization soon after land abandonment are poorly understood, and thus it is unknown whether lianas alter tree regeneration early in succession. We examined the liana community in 43 forests that ranged from 1 to 31 yr old in central Panama to determine how fast lianas colonize young forests and how the liana community changes with forest succession. We found that lianas reached high densities early in succession, commonly exceeding 1000 stems/ha within the first 5 yr of forest regeneration. Lianas also increased rapidly during early succession in terms of basal area but did not show evidence of saturation within the 30 yr of our chronosequence. The relative contribution of lianas to total woody plant community in terms of basal area and density increased rapidly and reached a saturation point within 5 yr (basal area) to 15 yr (density) after land abandonment. Our data demonstrate that lianas recruit early and in high density in tropical forest regeneration, and thus lianas may have a large effect on the way in which secondary forests develop both early and throughout succession.     

Functional traits of lianas in an Australian lowland rainforest align with post‐disturbance rather than dry season advantage

Lianas are an important component of tropical forests; they alter tree mortality and recruitment and impact biogeochemical cycling. Recent evidence suggests that the abundance of lianas in tropical forests is increasing. To understand and predict the effect of lianas on ecosystem processes in tropical forests, it is important to understand the mechanisms through which they compete with trees. In this study, we investigated the functional traits of lianas and trees in a lowland tropical forest in northeast Queensland, Australia. The site is located at 16.1° south latitude and experiences significant seasonality in rainfall, with pronounced wet and dry seasons. It is also subject to relatively frequent disturbance by cyclones. We asked the question of whether the canopy liana community at this site would display functional traits consistent with a competitive advantage over trees in response to disturbance, or in response to dry season water stress. We found that traits that we considered indicative of a dry season advantage (xylem water δ¹⁸O as an indicator of rooting depth; leaf and stem tissue δ¹³C and instantaneous gas exchange as measures of water‐use efficiency) did not differ between canopy lianas and canopy trees. On the other hand, lianas differed from trees in traits that should confer an advantage in response to disturbance (low wood density; low leaf dry matter content; high leaf N concentration; high mass‐based photosynthetic rates). We conclude that the liana community at the study site expressed functional traits geared towards rapid resource acquisition and growth in response to disturbance, rather than outcompeting trees during periods of water stress. These results contribute to a body of literature which will be useful for parameterising a liana functional type in ecosystem models.     

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.     

Vegetation Structure,Composition, and Species Richness Across a 56‐year Chronosequence of Dry Tropical Forest on Providencia Island,Colombia1

We compared vegetation structure and species richness across a 56‐yr chronosequence of six replicated age classes of dry tropical forest on the island of Providencia, Colombia, in the Southwest Caribbean. Stand age classes were determined using sequential, orthorectified panchromatic aerial photos acquired between 1944 and 1996 and Landsat 7 ETM + satellite imagery from 2000. Along the chronosequence we established 59 plots of 2 × 50 m (0.01 ha) to document changes in species richness, basal area, tree height, stem density, and sprouting ability. All woody trees and shrubs >2.5 cm diameter at breast height (DBH) were censused and measured. Although woody species density reached a peak in stands from 32 to 56 yr old, rarefaction analysis showed that species richness increased linearly with stand age and was highest in stands 56 yr old or greater. Nonparametric, abundance‐based estimators of species richness also showed positive and linear associations with age. Basal area and mean tree height were positively associated with age since abandonment, while sprouting ability showed a negative relationship. Our results indicate rapid recovery of woody species richness and structural characteristics along this tropical dry forest chronosequence.     

Evidence for strong seasonality in the carbon storage and carbon use efficiency of an Amazonian forest

The relative contribution of gross primary production and ecosystem respiration to seasonal changes in the net carbon flux of tropical forests remains poorly quantified by both modelling and field studies. We use data assimilation to combine nine ecological time series from an eastern Amazonian forest, with mass balance constraints from an ecosystem carbon cycle model. The resulting analysis quantifies, with uncertainty estimates, the seasonal changes in the net carbon flux of a tropical rainforest which experiences a pronounced dry season. We show that the carbon accumulation in this forest was four times greater in the dry season than in the wet season and that this was accompanied by a 5% increase in the carbon use efficiency. This seasonal response was caused by a dry season increase in gross primary productivity, in response to radiation and a similar magnitude decrease in heterotrophic respiration, in response to drying soils. The analysis also predicts increased carbon allocation to leaves and wood in the wet season, and greater allocation to fine roots in the dry season. This study demonstrates implementation of seasonal variations in parameters better enables models to simulate observed patterns in data. In particular, we highlight the necessity to simulate the seasonal patterns of heterotrophic respiration to accurately simulate the net carbon flux seasonal tropical forest.     

Plant species diversity and tree population structure of a humid tropical forest in Tamil Nadu, India

Vegetation structure and species composition of tropical ecosystems were studied through nine transects at Veerapuli and Kalamalai reserve forests in the Western Ghats of Tamil Nadu, India. Species diversity, dominance, species richness and evenness indices of plant communities and also population structure of woody plants were enumerated. A total of 244 species (183 genera and 76 families) were recorded. Species richness (number of species) were 82,142 and 96 species per 0.3 ha respectively for the study areas of low-elevation forest (LEF), mid-elevation forest (MEF) and high elevation forest (HEF). Species diversity indices were greater in MEF compared to the other two forests except juveniles. In contrast, greater dominance value indices were recorded in LEF than other forests. Density and basal area of the MEF were twice greater than the LEF, while HEF showed greater tree density and low basal area when compared to LEF. The stem density and species richness (number of species) decreased with increased size classes of trees observed in the present study indicated good regeneration status. Population structure of juveniles and seedlings also reflects good regeneration status. Terminalia paniculata (IVI of 99.9) and Hopea parviflora (IVI of 103.8) were dominant tree species respectively in LEF and MEF whereas in HEF Agrostistachys meeboldii (63.65), Cullenia excelsa (63.67) and Drypetes oblongifolia (39.67) share the dominance. Past damage (anthropogenic perturbation) may be one of the reasons for single species dominance in LEF and MEF. Occurrence of alien species such as Eupatorium odoratum and Ageratum conyzoides also indicated the past disturbance in LEF. The variations in plant diversity and population structure are largely due to anthropogenic perturbation and other abiotic factors.     

The direct regeneration hypothesis in northern forests

Question: Can the direct regeneration hypothesis (DRH) be used to predict post‐disturbance regeneration after fire, wind disturbance, and clearcutting in northern forests? Do life‐history traits such as regeneration strategy and shade tolerance influence post‐disturbance regeneration success of tree species? Location: Northern forests in North America. Methods: A meta‐analysis was conducted by collecting published data on pre‐ and post‐disturbance stand compositional characteristics in the northern forests. For each tree species, compositional difference (CD) was calculated as the difference between basal area proportions of the post‐ and pre‐disturbance stands, but for post‐disturbance stands <25 years of age, post‐disturbance proportions were calculated based on relative stem density. Results: Species response to disturbances was best explained by regeneration strategy, while disturbance type had no effect on CD. The proportion of broadleaf trees with either strong or weak vegetative reproduction ability increased after all disturbances. Serotinous species had CD values not significantly different from zero after fire, while CD for semi‐serotinous species was negative. The post‐disturbance proportions of non‐serotinous conifers decreased after all forms of disturbance. Conclusions: All disturbances promote broadleaf trees, regardless of regeneration strategy (suckering, sprouting, or seeding). The DRH is supported for conifers with serotinous cones after fire. Fire causes local extinction of non‐serotinous conifers, while wind and clearcutting only decrease the proportion of non‐serotinous conifers because of partial survival of seed sources and advanced regeneration. This study suggests that increasing stand‐replacing disturbances associated with global climate change will promote broadleaf trees in northern forests.