Ecology and regeneration of tropical dry forests in the Americas: Implications for management

Dry forests represent the most endangered ecosystem in tropical regions and continue to be one of the most sought after environments for human colonization, development and production. In spite of this, dry forests are one of the least well studied tropical habitats. This special issue is dedicated to reviewing much of the information that exists about tropical dry forest in the Americas. The introduction summarizes the articles presented herein, highlighting management strategies that are suggested throughout the special issue that may be useful for conserving tropical dry forest environments.     

Arthropod vertical stratification in temperate deciduous forests: Implications for conservation-oriented management

Studies on the vertical distribution patterns of arthropods in temperate deciduous forests reveal highly stratified (i.e., unevenly vertically distributed) communities. These patterns are determined by multiple factors acting simultaneously, including: (1) time (forest age, season, time of day); (2) forest structure (height, vertical foliage complexity, plant surface textures, tree cavities); (3) plant community composition (plant diversity, invasive species); (4) climatic gradients (light exposure, temperature, wind speed, humidity); (5) resource availability (foliage, sugars, wood, epiphytes, carrion, dung, prey, hosts, mates); (6) inter-specific interactions (predation, interference, competition) and (7) logistics (dispersal abilities, proximity to emergence sites, open flight zones). Several recommendations can be made with respect to incorporating canopy habitats and resources into management plans in order to maintain diverse forest arthropod communities. Efforts should be made to maintain diverse plant communities, for instance, including eliminating or controlling invasive plant competitors. Protecting large diameter trees and snags is also important, especially for a wide variety of canopy arthropod taxa associated with standing or suspended dead wood, tree cavities and epiphytes. Finally, it is essential to ensure adequate spatial and temporal continuity in the availability of these and other key resources. Although how to best achieve this last objective remains an active area of research, it may be preferable to retain clusters of trees as opposed to isolated individuals.     

Carbon and nitrogen in forest soils: Potential indicators for sustainable management of eucalypt forests in south-eastern Australia

Soil organic matter (SOM) has been adopted as an indicator of soil fertility based on the rationale that SOM contributes significantly to soil physical, chemical, and biological properties that affect vital ecosystem processes of forests in Australia. A study was undertaken to evaluate the utility of SOM as an indicator of SFM at two long-term experimental sites in native eucalypt forests, including Silvertop Ash (E. sieberi L. Johnson) and Mountain Ash (E. regnans F. Muell.) in Victoria. This study examines the relative contributions made by various sources of carbon in soil profiles (0–30 cm) of forest soils, viz. mineral soil (<2 mm), plant residues, charcoal (>2 mm), and rock fragments (>2 mm). The long-term changes in these fractions in response to management-induced soil physical disturbance and fire (unburnt, moderate and high intensity) were evaluated. After 10 years, carbon levels in the fine soil fraction (soil <2 mm including fine charcoal) were similar across the range of fire disturbance classes in Mountain Ash forest (20–25 kg/m²) and Silvertop Ash forest (7–8 kg/m²). Likewise differences in carbon associated with other fractions, viz. microbial biomass, labile carbon, plant residues and rock fragments were comparatively small and could not be attributed to fire disturbance. Burning increased the charcoal carbon fraction from 5 to 23 kg/m² in Mountain Ash forest and from 1 to 3 kg/m² in Silvertop Ash forest. Taking into account, the percentage area affected by fire, increases in total soil carbon in these forests were estimated at 25 and 7 t/ha, respectively.

The effects of physical disturbance of soils were examined at one site in Mountain Ash forest where soil cultivation was used as site preparation rather than the standard practice of burning of logging residues. Total carbon in soil profiles decreased from 29 to 21 kg/m² where soil disturbance was severe, i.e. topsoil removed and subsoil disturbed. This was mainly due to a decrease in charcoal carbon from 6.8 to 1.7 kg/m² but severe soil disturbance also increased the amount of carbon associated with rock fragments from 1.6 to 3.5 kg/m².

Management-induced fire increased the coarse charcoal content of soil profiles substantially, thus increasing total carbon content as well as the proportion of recalcitrant carbon in SOM. In contrast, there was little change in the carbon content of the fine soil fraction including the labile and biologically active fractions indicating that these SOM fractions most relevant to ecosystem processes showed little long-term impact from soil disturbance and fire. Conventional sampling of the fine soil fraction (<2 mm) only represented between 50% and 70% of total carbon in the soil profiles. In contrast, total nitrogen in this fraction represented between 75% and 90% of the nitrogen in soil profiles and was less affected by changes in the contributions of N made by coarse fractions. Monitoring of soil N rather than C as an indicator of soil fertility and SFM may be more appropriate for forest soils with significant charcoal content.     

Climate and recent fire history affect fuel loads in Eucalyptus forests: Implications for fire management in a changing climate

Predicted changes to global climates are expected to affect natural fire regimes. Many studies suggest that the impact of these effects could be minimised by reducing fuel loads through prescribed burning. Fuel loads are dynamic and are affected by a range of factors including fire and climate. In this study, we use a 22-year dataset to examine the relative influence of climate and fire history on rates of litterfall and decomposition, and hence fuel loads, in a coastal Eucalypt forest in south-eastern Australia. Litterfall and decomposition were both affected by temperature, recent rainfall and fire history variables. Over the study period prescribed burning immediately reduced fuel loads, with fuel loads reaching pre-burn levels within 3 years of a fire. Modelling fuel loads under predicted climate change scenarios for 2070 suggests that while fuel loads are reduced, the levels are not significantly lower than those recorded in the study. Based on these predictions it is unlikely that the role or value of prescribed burning in these forests will change under the scenarios tested in this study.     

Local variability of stand structural features in beech dominated natural forests of Central Europe: Implications for sampling

Current knowledge of the within-site variability of major stand structural features in beech dominated natural forests is limited. Numerous studies have used just several small plots for characterizations of natural stands, but this may lead to generalizations based on unreliable results. This study shows how major stand structural features vary at the local scale, and how suitable sampling may reflect this variability. Stem position maps of three natural forests in the Czech Republic (Zofin 71 ha, Salajka 19 ha and Zakova hora 17 ha) were used. Each vector stem position map representing all live and dead trees with DBH ≥ 10 cm was intensively analyzed using square sample plots of different sizes (10 × 10 m; 20 × 20 m; 30 × 30 m; 50 × 50 m; 100 × 100 m; 140 × 140 m and 200 × 200 m). Basic statistics (mean, standard deviation, coefficient of variation, min., and max.) were calculated for every plot size and each of six major stand features: density, basal area and volume of living trees, volume of course woody debris, total volume and proportion of course woody debris in total volume.     

Rich deciduous forests in Bia owie a as a dynamic mosaic of developmental phases: premises for nature conservation and restoration management

The Bia owie a Primeval Forest, Poland, is the only forest area in temperate Europe where large tracts of natural forest communities have been conserved. The concept of silvatic mosaic [Oldeman, R.A.A., 1990. Forests: Elements of Silvology. Springer, Berlin] was applied in order to characterise the structure of stands representing oak–lime–hornbeam forest in both, the restricted area of Bia owie a National Park (BNP) and the commercial forests (CF), as well as to indicate a holistic approach towards protection and restoration of natural areas.Six developmental phases of stands were distinguished and defined: regeneration, young, pole, late pole, optimal, and terminal. Compositional and structural differences between the first five phases were also evaluated and visualised. Moreover, the considerable differences between analogous phases in BNP and CF were found. The uniform stand patches were mapped into two plots in BNP (25 ha each) and two in CF (25 and 20 ha), with a resolution of 10 m× 10 m pixels.The mosaic composition of natural communities indicates close-to-the equilibrium stage: the biostatic optimal phase occupies 33–40% of the area, while aggradational younger phases share nearly the same portion of the area as the senescent phase does. Although small, 0.01–0.04 ha eco-units dominate in BNP, there is a considerable portion of intermediate and large patches, indicating a complex character of factors responsible for the mosaic structure of natural stands in BNP. Both, composition and texture analyses indicate that the area of 25 ha is close to the minimum structure area specific for the communities studied.The comparison of natural and managed stands revealed large deficits of the optimal phase (only 19%) and a total absence of the terminal one in the managed area (20–35% in natural forest). Also the mosaic texture of CF plots was simplified because of standardisation of the management unit size. The modification of developmental phases, mosaic composition, and structure must lead to changes in the dynamics of the entire community. The present study demonstrates that the natural forest should be considered as a dynamic supersystem, covering a sufficient area to allow the developmental phases to perform in correct proportions. Because of differences in the durability of various phases, the natural cycle of forest development has an asynchronous and highly unpredictable character. Indeed, the applicability of multipurpose forestry rules as a tool of nature conservation has been questioned. An absolute ban on cutting in the remaining old-growth stands should be a basic rule for the restoration management.     

Post-logging regeneration and recruitment of shihuahuaco (Dipteryx spp.) in Peruvian Amazonia: Implications for management

Over the past decade, shihuahuaco timber - comprising several species of Dipteryx (Fabaceae) traded internationally as “cumarú” or “Brazilian teak” - has become one of the most highly demanded types of hardwood from Peruvian Amazonia, particularly in the Chinese market. Dipteryx is an ecologically important canopy-emergent genus with widespread distribution in neotropical forests. To assess the response of Dipteryx to logging, we conducted inventories in three logging areas in the Regions of Ucayali and Loreto, Peru. The size-class distributions of Dipteryx populations in recently logged sites showed that initial post-logging conditions enhance recruitment of residual seedlings. These conditions are created by a combination of logging gaps and the activities of farmers migrating into logged lands. Through protection and liberation of shihuahuaco seedlings in post-logged forest as well as within and around agricultural fields, local residents and timber companies could favor the recovery of this valued resource. However, as logged land is increasingly converted to agriculture and pastureland, the reestablishment of mature seed trees is not assured.     

Quantitative stand structure of woody components of homestead forests and its implications on silvicultural management: a case study in Sylhet Sadar, Bangladesh

This study determined existing quantitative stand structure and its implication on silvicultural management of homestead forestry. The results showed that fruit and timber species have importance values of 57% and 43%, respectively, in the study area, which is in contrast to the commonly held view of absolute domination of fruit species. The fruit species were only moderately dominant over timber species in relation to the quantitative stand structure of homestead forests. Two fruit species, Mangifera indica and Artocarpus heterophyllus, contribute about one third of the stand structure, while amongst the timber species Samanea saman and Albizia spp. are the most important species. A simulated evaluation of soil expectation value of homestead forest showed that the existing stand structure would not maximize the financial gain perpetually, in contrast, the quantitative stand structure could be effectively regulated to maximize grower benefit without compromising the existing biodiversity. Optimization of the quantitative stand structure of homestead forests could be achieved by changing the species composition, specifically by increasing the percentage of commercially valuable species like Michelia champaca, Tectona grandis, Artocarpus chapalasha, Gmelina arborea, Litchi chinensis, Citrus grandis, Psidium guajava, Lagerstroemia speciosa, Swietenia mahogany, etc., reducing the percentage of species like Mangifera indica, Artocarpus heterophyllus, Cocos nucifera, Samanea saman, Spondias pinnata, Phoenix sylvestris, etc., and eradicating species like Lannea coromandelica and Ficus benghalensis. It was estimated that the optimal relative density of fruit and timber species that would generate optimal financial benefit would be 40.4% and 59.6%, respectively.     

Seed and seedling survival of African mahogany (Entandrophragma spp.) in the Central African Republic: Implications for forest management

African mahoganies of the genus Entandrophragma are among the most valuable and important timber species harvested in Central Africa, representing more than 70% of total export volume from many areas. In spite of the importance of these species, relatively little is known about their regeneration ecology and little effort has gone into understanding the reasons for the consistently reported regeneration failures after logging. I assessed seed survival to germination (Entandrophragma angolense) and seedling survivorship (E. cylindricum) in three different forest types – monodominant Gilbertiodendron, mixed species, and fallow forest – under three different treatments – control, small mesh chicken wire, and large mesh chicken wire – to evaluate the relative importance of different causes of mortality. All seeds were eaten in controls and in both exclosure treatments within Gilbertiodendron forest in a matter of days. Seed survivorship to germination within exclosures in mixed species and fallow forest increased by approximately 10 and 25%, respectively, compared to Gilbertiodendron forest. Six-month seedling survivorship in controls was 37, 12, and 9% in Gilbertiodendron, mixed species and fallow forest, respectively. Seedling mortality was due to different causes in each forest type. In Gilbertiodendron forest controls, an equivalent percentage of seedlings died due to fungal and insect attack (27 and 28%, respectively), while in mixed species forest controls 28 and 55% of seedlings died of these causes, respectively. In fallow forest controls, 48% of seedlings died from predation and/or uprooting by small mammals, all in the first few weeks post-sowing; insect attack (26%) and drought (13%) were other important causes of seedling deaths. Protecting seedlings with exclosures had a dramatic effect on seedling survivorship within the fallow forest, increasing to over 50%.     

Comparison of statistical tests for habitat associations in tropical forests: A case study of sympatric dipterocarp trees in a Bornean forest

Habitat difference is an important mechanism for maintenance of tree diversity in tropical forests. The first step in studies of habitat difference is to statistically analyze whether the spatial distributions of tree populations are skewed to species-specific habitats; this is called a habitat association test. We propose a novel habitat association test on the basis of the probability of tree occurrence along a continuous habitat variable. The test uses torus shift simulations to obtain a statistical significance level. We applied this test to 55 common dipterocarp species in a 52-ha plot of a Bornean forest to assess habitat associations along an elevation gradient. The results were compared to those of three existing habitat association tests using the same torus shift simulations. The results were considerably different from one another. In particular, the results of two existing tests using discrete habitat variables varied with differences in habitat definitions, specifically, differences in elevation break points, and the number of habitat classes. Thus, definitions of habitats must be taken into account when habitat association tests with discrete habitat variables are used. Analyses of artificial populations independent of habitat showed that all of the tests used were robust with respect to spatial autocorrelation in tree distributions, although one existing test had a higher risk of Type I errors, probably due to the use of multiple tests of significance. Power analysis of artificial populations in which distributions were skewed to certain elevations showed that the novel test had comparable statistical power to the most powerful existing test. Statistical power was affected not by the total number of a given tree but by the number of clumps in a plot, suggesting that >5 clumps were required for a reliable result.     

The extraction of rattan-like lianas in the new world tropics: a possible prototype for sustainable forest management

The tropical forests of the Americas contain great biological diversity yet they are being rapidly destroyed by population pressure, injudicious logging, and inappropriate government policies. In this article we explain how the extraction of rattan-like lianas could represent a sustainable system of forest management in which smallholder incomes are increased while relieving deforestation pressures.