Regeneration of canopy trees in tropical wet forests

The most diverse tree communities on earth, the tropical wet forests, to a large degree remain ecological enigmas. What accounts for the coexistence of 100 or more tree species per hectare, compared to the 15 or fewer found in most temperate forests? What are the lifespans of tropical forest trees? What factors control their populations through time and space? Do the different species have highly individual regeneration patterns, or are many in fact ecological equivalents? Although we are far from having satisfactory answers to these questions, recent studies of regeneration processes are leading toward new interpretations of these complex communities.     

热带森林植被生态恢复研究进展

热带森林是地球上生物多样性最高和生态功能最为强大的植被类型之一,在维护全球生态平衡中起着至关重要的作用,同时也为人类社会提供着多种多样的物质资源和生态系统服务。然而热带森林是目前生物多样性消失最快和生态功能退化最为严重的生态系统之一,如何有效地保护现存的热带森林不再进一步退化,以及如何使已经退化的生态系统尽快得到恢复是生态学工作者面临的重要议题。不同方式、规模和强度的干扰对热带林的破坏程度及其以后的恢复过程产生的影响不同。除少数大型自然干扰事件外,采伐、刀耕火种、农业开发用地等人为干扰是造成当前热带森林植被大面积退化的主要原因。多种干扰交互作用、杂草与外来物种入侵、退化植被和土壤状况、残存植被组分及土壤种子库、退化植被周围的景观格局以及全球气候变化等因素都能够影响热带森林植被恢复的速度和方向。基于功能群的研究思想将可能为物种丰富的热带森林植被恢复的研究提供一个全新途径。     

Protostelids from tropical forests, woodlands and deserts in Australia

Most of what is known about the distribution of protostelids is limited to results from surveys carried out in North and Central America. To increase our knowledge about protostelid diversity and distribution we surveyed protostelids from 12 study sites in northern Queensland and the Northern Territory of Australia during May-Jun 2003. Aerial litter and ground litter samples were randomly collected along a 200 m transect at each site. Study sites ranged from tropical forests to deserts. We recovered 10 species and two apparently undescribed species from samples of aerial (dead but still attached plant parts) and ground litter. Samples from a woodland site characterized by intermediate moisture conditions had the greatest species richness, followed by samples from dry woodland, tropical forest and desert sites. When species richness for a particular microhabitat was considered, samples of aerial litter yielded more species than samples of ground litter. Percentages of samples colonized with protostelids were similar for the aerial and ground litter microhabitats within a given habitat type except for dry woodlands, in which aerial litter samples were characterized by higher numbers of species than ground litter samples. Two species (Protostelium mycophaga and Soliformovum irregularis) that in temperate North America are associated with aerial litter microhabitats also were recovered from aerial litter in dry habitats in Australia. Schizoplasmodiopsis pseudoendospora, a North American temperate ground litter species, was equally abundant in aerial litter and ground litter in Australia. This study is the first of its kind for protostelid ecology in Australia and the most extensive study of protostelids in the southern hemisphere. These data complement ongoing research on protostelid distribution from around the world.     

Impact of nonnative feral pig removal on soil structure and nutrient availability in Hawaiian tropical montane wet forests

Conservation and restoration of ecosystems impacted by nonnative ungulates increasingly involves their removal and exclusion. While the influence of nonnative ungulate removal on plant communities is commonly monitored, impacts on underlying ecological processes are seldom quantified. Here we examined how nonnative feral pig (Sus scrofa) removal from Hawaiian tropical montane wet forests affects soil physical and chemical properties. Unique to this study, measurements were taken in paired sites inside and outside of five feral pig removal units representing a ~20 year, highly constrained chronosequence where other potentially confounding variables are held constant. Additional targeted measurements were taken inside and outside of a single exclosure in areas characterized by ‘low’ versus ‘high’ feral pig activity. Overall, nonnative feral pig removal increased stable soil aggregates and porosity, and decreased bulk density, water-filled pore space, and soil moisture content. Further, feral pig removal increased soil nutrient regeneration as evidenced by increased extractable cations, increased resin available NO₃ ^? and total inorganic N, and enriched foliar δ¹⁵N. Increasing time since feral pig removal was positively related to net nitrification and total net inorganic N mineralization, and negatively related to pH and net ammonification. Results from both the chronosequence and targeted sampling were consistent in direction and support a central role of feral pig removal in modifying soil physical and chemical properties. Changes in soil properties following ungulate removal coincided with large increases in understory vegetation cover, highlighting the need to better understand aboveground-belowground linkages following nonnative ungulate removal.     

Mechanisms of rainforest persistence and recruitment in frequently burnt wet tropical eucalypt forests

The role of fire in governing rainforest–eucalypt forest ecotone dynamics is of theoretical interest and has conservation management implications. Several eucalypt forests in the Wet Tropics of Australia have an endangered status due to extensive conversion to rainforest. Rainforest plants are known to survive occasional low intensity fires in the eucalypt forest ecotone. However, the ability of rainforest plants to survive frequent fires remains untested. The timing of rainforest expansion is also a subject of interest, and is generally considered to be delayed until fire has been absent for several years. We used 14 years of data collected across 13 plots in the Wet Tropics of north‐eastern Australia to test predictions regarding rainforest seedling recruitment and post‐fire regenerative capacity. The 13 plots received different numbers of fires, between zero and five, over the 14‐year study. The recruitment of new rainforest plants in the ecotone was most abundant in the initial year after fire. If this post‐fire pulse of recruitment is left undisturbed, it can facilitate the subsequent germination of additional rainforest species. The removal of grass cover, whether temporarily in the immediate post‐fire environment or once a developing rainforest mid strata shades out grasses, appears crucial to abundant rainforest recruitment. A variety of tropical rainforest species can persist under a frequent fire regime through resprouting. The difference in the mode of resprouting, between ground‐level coppicing rainforest plants and canopy resprouting eucalypt forest trees, is the critical mechanism that causes regular fire to maintain an open structure in eucalypt forests. The inability of rainforest species to maintain their height when fires fully scorch their crowns, temporarily resets the forest's open structure and delays the rainforest's ability to dominate through shading out grasses to transform the ecosystem into a closed forest.     

Variations in soil N cycling and trace gas emissions in wet tropical forests

We used a previously described precipitation gradient in a tropical montane ecosystem of Hawai’i to evaluate how changes in mean annual precipitation (MAP) affect the processes resulting in the loss of N via trace gases. We evaluated three Hawaiian forests ranging from 2200 to 4050 mm year⁻¹ MAP with constant temperature, parent material, ecosystem age, and vegetation. In situ fluxes of N₂O and NO, soil inorganic nitrogen pools (NH₄⁺ and NO₃⁻), net nitrification, and net mineralization were quantified four times over 2 years. In addition, we performed ¹⁵N-labeling experiments to partition sources of N₂O between nitrification and denitrification, along with assays of nitrification potential and denitrification enzyme activity (DEA). Mean NO and N₂O emissions were highest at the mesic end of the gradient (8.7±4.6 and 1.1±0.3 ng N cm⁻² h⁻¹, respectively) and total oxidized N emitted decreased with increased MAP. At the wettest site, mean trace gas fluxes were at or below detection limit (≤0.2 ng N cm⁻² h⁻¹). Isotopic labeling showed that with increasing MAP, the source of N₂O changed from predominately nitrification to predominately denitrification. There was an increase in extractible NH₄⁺ and decline in NO₃⁻, while mean net mineralization and nitrification did not change from the mesic to intermediate sites but decreased dramatically at the wettest site. Nitrification potential and DEA were highest at the mesic site and lowest at the wet site. MAP exerts strong control N cycling processes and the magnitude and source of N trace gas flux from soil through soil redox conditions and the supply of electron donors and acceptors.     

Tree functional traits as predictors of microburst-associated treefalls in tropical wet forests

On 19 May 2018, a microburst caused 600 isolated forest gaps in a Costa Rican tropical forest. We surveyed fallen and standing trees within gaps to determine whether certain variables are associated with treefalls. Our results highlight considerations for future research to understand the impacts of microbursts in tropical forests.     

Ecological integrity of tropical secondary forests: concepts and indicators

Naturally regenerating forests or secondary forests (SFs) are a promising strategy for restoring large expanses of tropical forests at low cost and with high environmental benefits. This expectation is supported by the high resilience of tropical forests after natural disturbances, yet this resilience can be severely reduced by human impacts. Assessing the characteristics of SFs and their ecological integrity (EI) is essential to evaluating their role for conservation, restoration, and provisioning of ecosystem services. In this study, we aim to propose a concept and indicators that allow the assessment and classification of the EI of SFs. To this end, we review the literature to assess how EI has been addressed in different ecosystems and which indicators of EI are most commonly used for tropical forests. Building upon this knowledge we propose a modification of the concept of EI to embrace SFs and suggest indicators of EI that can be applied to different successional stages or stand ages. Additionally, we relate these indicators to ecosystem service provision in order to support the practical application of the theory. EI is generally defined as the ability of ecosystems to support and maintain composition, structure and function similar to the reference conditions of an undisturbed ecosystem. This definition does not consider the temporal dynamics of recovering ecosystems, such as SFs. Therefore, we suggest incorporation of an optimal successional trajectory as a reference in addition to the old-growth forest reference. The optimal successional trajectory represents the maximum EI that can be attained at each successional stage in a given region and enables the evaluation of EI at any given age class. We further suggest a list of indicators, the main ones being: compositional indicators (species diversity/richness and indicator species); structural indicators (basal area, heterogeneity of basal area and canopy cover); function indicators (tree growth and mortality); and landscape proxies (landscape heterogeneity, landscape connectivity). Finally, we discuss how this approach can assist in defining the value of SF patches to provide ecosystem services, restore forests and contribute to ecosystem conservation.     

Vegetation analysis and tree population structure of tropical wet evergreen forests in and around Namdapha National Park,northeast India

Species composition, diversity and tree population structure were studied in three stands of the tropical wet evergreen forest in and around Namdapha National Park, Arunachal Pradesh, India. Three study stands exposed to different intensities of disturbances were identified, viz., undisturbed (2.4 ha) in the core zone of the park, moderately disturbed (2.1 ha) in the periphery of the park and highly disturbed (2.7 ha) outside the park area. In total 200 plant species belonging to 73 families were recorded in three stands. Tree density and basal area showed a declining trend with the increase in disturbance intensity. The densities of tree saplings and seedlings were lower in the disturbed stands than in the undisturbed stand. Species like Altingia excelsa, Olea dioica, Terminalia chebula, Mesua ferrea and Shorea assamica in the undisturbed stand and Albizia procera alone in the moderately disturbed stand contributed more than 50% of the total tree density in respective stands. The undisturbed stand contained young tree population. In the highly disturbed stand, the tree density was scarce, but had uncut trees of higher girth class (>210 cm GBH). Low shrub density was recorded in both disturbed stands due to frequent human disturbances; the broken canopy and direct sunlight enhanced the abundance of herbs in these stands. With a species rarity (species having <2 individuals) of ca. 50%, the tropical wet evergreenforests of the Namdapha National Park and its adjacent areas warrant more protection from human intervention and also eco-development to meet the livelihood requirements of the local inhabitants in the peripheral areas of the Namdapha National Park in order to reduce the anthropogenic pressure on the natural resources of the park.     

Breeding birds in the tropical rain forests of Kibale National Park, Uganda

A combination of trapping and foot surveys was used to assess the breeding status of birds in unlogged and logged sites in the tropical rain forest of Kibale National Park, Uganda. Breeding of forest edge and gap species was greatly enhanced by logging, whereas crevice and hole nesting specialist breeders were adversely affected. Egg‐laying periods corresponded to higher rainfall regimes at the nearby field station. The relevance of these findings to bird conservation is discussed.     

Above-ground biomass and structure of 260 African tropical forests

We report above-ground biomass (AGB), basal area, stem density and wood mass density estimates from 260 sample plots (mean size: 1.2 ha) in intact closed-canopy tropical forests across 12 African countries. Mean AGB is 395.7 Mg dry mass ha⁻¹ (95% CI: 14.3), substantially higher than Amazonian values, with the Congo Basin and contiguous forest region attaining AGB values (429 Mg ha⁻¹) similar to those of Bornean forests, and significantly greater than East or West African forests. AGB therefore appears generally higher in palaeo- compared with neotropical forests. However, mean stem density is low (426 ± 11 stems ha⁻¹ greater than or equal to 100 mm diameter) compared with both Amazonian and Bornean forests (cf. approx. 600) and is the signature structural feature of African tropical forests. While spatial autocorrelation complicates analyses, AGB shows a positive relationship with rainfall in the driest nine months of the year, and an opposite association with the wettest three months of the year; a negative relationship with temperature; positive relationship with clay-rich soils; and negative relationships with C : N ratio (suggesting a positive soil phosphorus–AGB relationship), and soil fertility computed as the sum of base cations. The results indicate that AGB is mediated by both climate and soils, and suggest that the AGB of African closed-canopy tropical forests may be particularly sensitive to future precipitation and temperature changes.     

Apparent survival of tropical birds in a wet,premontane forest in Costa Rica

Despite the importance of tropical birds in the development of life history theory, we lack information about demographic rates and drivers of population dynamics for most species. We used a 7‐year (2007–2013) capture‐mark‐recapture dataset from an exceptionally wet premontane forest at mid‐elevation in Costa Rica to estimate apparent survival for seven species of tropical passerines. For four of these species, we provide the first published demographic parameters. Recapture probabilities ranged from 0.21 to 0.53, and annual estimates of apparent survival varied from 0.23 to 1.00. We also assessed the consequences of inter‐annual variation in rainfall on demographic rates. Our results are consistent with inter‐annual rainfall increasing estimates of apparent survival for two species and decreasing estimates for three species. For the three species where we could compare our estimates of apparent survival to estimates from drier regions, our estimates were not consistently higher or lower than those published previously. The temporal and spatial variability in demographic rates we document within and among species highlights the difficulties of generalizing life history characteristics across broad biogeographic gradients. Most importantly, this work emphasizes the context‐specific role of precipitation in shaping tropical avian demographic rates and underscores the need for mechanistic studies of environmental drivers of tropical life histories.     

Forest structure and carbon dynamics in Amazonian tropical rain forests

Living trees constitute one of the major stocks of carbon in tropical forests. A better understanding of variations in the dynamics and structure of tropical forests is necessary for predicting the potential for these ecosystems to lose or store carbon, and for understanding how they recover from disturbance. Amazonian tropical forests occur over a vast area that encompasses differences in topography, climate, and geologic substrate. We observed large differences in forest structure, biomass, and tree growth rates in permanent plots situated in the eastern (near Santarém, Pará), central (near Manaus, Amazonas) and southwestern (near Rio Branco, Acre) Amazon, which differed in dry season length, as well as other factors. Forests at the two sites experiencing longer dry seasons, near Rio Branco and Santarém, had lower stem frequencies (460 and 466 ha^–1 respectively), less biodiversity (Shannon–Wiener diversity index), and smaller aboveground C stocks (140.6 and 122.1 Mg C ha^–1) than the Manaus site (626 trees ha^–1, 180.1 Mg C ha^–1), which had less seasonal variation in rainfall. The forests experiencing longer dry seasons also stored a greater proportion of the total biomass in trees with >50 cm diameter (41–45 vs 30% in Manaus). Rates of annual addition of C to living trees calculated from monthly dendrometer band measurements were 1.9 (Manaus), 2.8 (Santarém), and 2.6 (Rio Branco) Mg C ha^–1 year^–1. At all sites, trees in the 10–30 cm diameter class accounted for the highest proportion of annual growth (38, 55 and 56% in Manaus, Rio Branco and Santarém, respectively). Growth showed marked seasonality, with largest stem diameter increment in the wet season and smallest in the dry season, though this may be confounded by seasonal variation in wood water content. Year-to-year variations in C allocated to stem growth ranged from nearly zero in Rio Branco, to 0.8 Mg C ha^–1 year^–1 in Manaus (40% of annual mean) and 0.9 Mg C ha^–1 year^–1 (33% of annual mean) in Santarém, though this variability showed no significant relation with precipitation among years. Initial estimates of the C balance of live wood including recruitment and mortality as well as growth suggests that live wood biomass is at near steady-state in Manaus, but accumulating at about 1.5 Mg C ha^–1 at the other two sites. The causes of C imbalance in living wood pools in Santarém and Rio Branco sites are unknown, but may be related to previous disturbance at these sites. Based on size distribution and growth rate differences in the three sites, we predict that trees in the Manaus forest have greater mean age (~240 years) than those of the other two forests (~140 years).