Ecological consequences of dead wood extraction in an arid ecosystem

Coarse woody debris (CWD) plays a key role in ecosystems, reducing erosion and affecting soil development, storing nutrients and water, providing a major source of energy and nutrients, serving as a seedbed for plants and as habitat for decomposers and heterotrophs. We asked whether removal of CWD affected the structure and functioning of an arid woodland ecosystem in mid-western Argentina. These woodlands are protected by national laws and inhabited by indigenous local pastoralists who have land rights to use natural resources, including wood for fuel and construction material. We hypothesized that removal of CWD affected negatively the populations of wood-nesting pollinators, the reproductive performance of Prosopis flexuosa (the dominant tree species), plant cover, richness and composition, and nutrient cycling in the soil. We conducted a manipulative experiment consisting of four pairs of 70 m radius circular plots, each pair with an experimental (CWD removal) and a control (no removal) plots. Experimental CWD extraction affected negatively flower visitor abundance, although the magnitude of this effect decreased over time. In contrast, extraction had a significant, positive effect on seed production of P. flexuosa. No significant effects of extraction were found on cover, richness and composition of understory plants and soil properties. Thus, CWD did not have the generally negative effects expected under our hypotheses.     

Monthly survival of African Sylvia warblers in a seasonally arid tropical environment

Latitudinal gradients of life-history traits in animals are thought to be shaped by environmental variables. For example, it has been suggested that the increase in avian clutch size from the tropics towards the northern temperate regions is caused by a reduced survival of adult birds in the north due to increasing environmental seasonality. However, the tropical savannahs of East Africa show pronounced seasonality in resources caused by distinct rainy and dry seasons. This raises the question of whether survival and other life-history traits of birds living in these tropical savannahs are influenced by this seasonality, making them more similar to northern temperate species. We used 2-year monthly resighting data, a multistate modelling approach and the program MARK to test whether survival, transition probabilities between breeding states and other life-history traits of two resident Kenyan Sylvia species (Aves: Passeriformes: Sylviidae) are shaped by seasonality of rainfall in their environment. Contradicting our hypotheses, the two species showed only very slight influence of seasonality of rainfall on their survival. Survival in the dry months was hardly lower than in the rainy months. The species in the more seasonal environment ( S. boehmi , annual survival 71%) survived as well as the one in the more constant environment ( S. lugens , 56%). The observed survival rates correspond well to other life-history traits of the two species and are of similar magnitude to survival rates of other tropical passerines. This implies that either seasonality is not the driving force behind the life-history traits of the two species or the birds do not experience their environment as seasonal, as might be suggested by fluctuations in rainfall.     

Human ecology and the albedo effect in an arid environment

The results of a comparison of agricultural settlement and land-use and local precipitation patterns in the arid region of the Arava in Israel indicated that as the number of settlements and the area of cultivated land has increased since the 1950s, the albedo effect in the region has decreased, and the amounts of precipitation at the first October rains have increased. It is suggested that the albedo effect in desert areas is influenced by extended agricultural land-use.     

Survey ofFusarium species in an arid environment of Bahrain

The prevalence and distribution of soil-inhabitingFusarium species in the soil of vegetable crops grown under custom-made plastic tunnels were studied at three geographical locations on the island of Bahrain. Six species ofFusarium representing a total of 1154 isolates were isolated on modified Komada medium. All species reported in this survey are recorded for the first time from the hot, arid desert of Bahrain.Fusarium solani andF. oxysporum were among the most frequently isolated fungi in all locations and crops.     

Termites like it hot and humid: the ability of arboreal tropical termites to mediate their nest environment against ambient conditions

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Biological control of wood decay in an open tropical environment with Penicillium sp. and Trichoderma viride

The ability of Penicillium sp. and Trichoderma viride to retard the decay of obeche (Triplochiton scleroxylon) wood in the field for 11 months (January–November) covering dry and wet seasons in a tropical environment was investigated using the ‘graveyard’ method. Inoculation of stakes with Gloeophyllum sp. or G. sepiarium (decay fungi) 24 h after treatment with T. viride or Penicillium sp. in January (dry season) did not increase decay after 11 months. Total inhibition of the decay fungi was indicated since the low weight losses of stakes was not markedly different from losses in control stakes biologically treated but not exposed to decay fungi. Inhibition of the activities of other unidentified field fungi was also indicated because weight losses were greater in uninoculated and untreated stakes. However, decay occurred in stakes biologically treated in January but later exposed to decay fungi in May, after the dry season. A repeat application of T. viride treatment in May, to stakes earlier treated in January, markedly reduced decay following exposure to decay fungi in September (wet season). A similar Penicillium sp. application was not as effective as T. viride application because unlike the T. viride protected stakes, decay was greater in stakes twice protected with Penicillium sp. but not exposed to decay fungi. The results indicate that repeated application of biocontrol agents may be important for controlling wood decay following the adverse effect of the dry season.     

Nonlinear stability analyses of vegetative pattern formation in an arid environment

The development of spontaneous stationary vegetative patterns in an arid isotropic homogeneous environment is investigated by means of various weakly nonlinear stability analyses applied to the appropriate governing equation for this phenomenon. In particular, that process can be represented by a fourth-order partial differential time-evolution logistic equation for the total plant biomass per unit area divided by the carrying capacity of its territory and defined on an unbounded flat spatial domain. Those patterns that consist of parallel stripes, labyrinth-like mazes, rhombic arrays of rectangular patches, and hexagonal distributions of spots or gaps are generated by the balance between the effects of short-range facilitation and long-range competition. Then those theoretical predictions are compared with both relevant observational evidence and existing numerical simulations as well as placed in the context of the results from some recent nonlinear pattern formation studies.     

Nonlinear stability analyses of vegetative pattern formation in an arid environment

The development of spontaneous stationary vegetative patterns in an arid isotropic homogeneous environment is investigated by means of various weakly nonlinear stability analyses applied to the appropriate governing equation for this phenomenon. In particular, that process can be represented by a fourth-order partial differential time-evolution logistic equation for the total plant biomass per unit area divided by the carrying capacity of its territory and defined on an unbounded flat spatial domain. Those patterns that consist of parallel stripes, labyrinth-like mazes, rhombic arrays of rectangular patches, and hexagonal distributions of spots or gaps are generated by the balance between the effects of short-range facilitation and long-range competition. Then those theoretical predictions are compared with both relevant observational evidence and existing numerical simulations as well as placed in the context of the results from some recent nonlinear pattern formation studies.     

Relationship between soil nutrient availability and plant species richness in a tropical semi‐arid environment

Question: What is the relationship between soil fertility and plant species richness in the ‘fertile islands’ occurring beneath two species of legume (Cercidium praecox and Prosopis laevigata)? Location: Tehuacán‐Cuicatlán region, central Mexico. Methods: Plant richness was measured in three micro‐environments (below canopies of C. praecox, below canopies of P. laevigata and in areas without canopies). The concentration of soil nutrients (C, N and P), C and N in the microbiota, and processes of ecosystem functioning (net C mineralization rate and N mineralization) were measured. The relationship between soil variables and plant richness were assessed with ANCOVAs. Results: Soil nutrients and species richness increases markedly under fertility islands. There were higher concentrations of C and N in the soil, faster rates of C mineralization, and higher species richness under P. laevigata canopies. The relationship between soil fertility and species richness was always positive except for total N, ammonium and net C mineralization rate under C. praecox, and for available P under P. laevigata. Conclusions: The sign of the relationship between soil fertility and species richness varies according to the nutrient and the micro‐environment. Positive relationships could result from between species complementarity and facilitation. Negative relationships could be explained by a specific limitation threshold for some soil resources (P and N for plants and C for the soil microbiota) which eliminate the possibilities of between species complementarity and facilitation above that threshold. As in all observational studies, these relationships should be considered only correlational.     

Fungal diversity on submerged wood in a tropical stream and an artificial lake

Two collections of submerged wood were made from the Mushroom Research Centre in northern Thailand. One collection comprising 100 samples was made from an artificial lake, which had been made by damming a stream running through a secondary forest. The other collection comprising 90 samples was made from an adjacent non dammed shallow stream running through a similar forest. A total of 68 fungal taxa were recorded during the study and the Shannon–Weiner index (H′) was applied to evaluate the diversities of freshwater fungi. Sørensen’s index (S′) was calculated to evaluate the similarity of different fungal communities. The variation of freshwater fungi on submerged wood between lentic and lotic habitat is discussed and compared with previous studies. A dramatic decrease in species richness and diversity, with significantly changed species composition were observed in the artificial lake as compared to the non dammed stream.     

The seasonal dynamic of ant‐flower networks in a semi‐arid tropical environment

1. Several studies have recently focused on the structure of ecological networks involving ants and plants with extrafloral nectaries; however, little is known about the effects of temporal variation in resource abundance on the structure of ant–plant networks mediated by floral nectar. 2. In this study, it was evaluated how strong seasonality in resource availability in a semi‐arid tropical environment affects the structure of ant–flower networks. We recorded ants collecting floral nectar during two seasons (from December 2009 to January 2013): dry and green seasons. Then, we built interaction networks for flower‐visiting ants in the Brazilian Caatinga separately for each combination of transect and season. 3. In general, strong seasonality directly influenced patterns of ant–flower interactions and the overall complexity of these ecological networks. During the dry season, networks were more connected, less modular, and exhibited greater niche overlap of flower‐visiting ants than during the green season. Moreover, resource utilisation by ants during the dry season tended to be more aggregated. These findings indicate that during the dry season, ant species tended to share many resource bases, probably owing to lower overall resource availability during this season. Species composition of the ant network component was highly season specific; however, a central core of highly generalised ants was present during both seasons. 4. The stability of this central core between seasons could strongly affect the ecological and evolutionary dynamics of these interaction networks. This study contributes to the understanding of the structure and dynamics of ant‐flower interactions in extremely seasonal environments.     

Geigeria alata in the Namib desert: seed heteromorphism in an extremely arid environment

Abstract. Seed heteromorphism in annual Geigeria alata in the Namib Desert was investigated in relation to environmental conditions. Geigeria alata shows heteromorphism in seed dispersal distance depending on the position of the seeds on the parent plant. Seeds produced in capitula on branches (aerial) travel long distances, while seeds produced at the base of the plant remain in the vicinity of the parent plant. The ratio of aerial to basal seed production was investigated under different environmental conditions following an aridity gradient. Although there were significant differences in seed and capitula production between seasons and between different positions on the plant, no overall trend was found with increasing aridity.     

Modeling decay rates of dead wood in a neotropical forest

Variation of dead wood decay rates among tropical trees remains one source of uncertainty in global models of the carbon cycle. Taking advantage of a broad forest plot network surveyed for tree mortality over a 23-year period, we measured the remaining fraction of boles from 367 dead trees from 26 neotropical species widely varying in wood density (0.23–1.24 g cm⁻³) and tree circumference at death time (31.5–272.0 cm). We modeled decay rates within a Bayesian framework assuming a first order differential equation to model the decomposition process and tested for the effects of forest management (selective logging vs. unexploited), of mode of death (standing vs. downed) and of topographical levels (bottomlands vs. hillsides vs. hilltops) on wood decay rates. The general decay model predicts the observed remaining fraction of dead wood (R ² = 60%) with only two biological predictors: tree circumference at death time and wood specific density. Neither selective logging nor local topography had a differential effect on wood decay rates. Including the mode of death into the model revealed that standing dead trees decomposed faster than downed dead trees, but the gain of model accuracy remains rather marginal. Overall, these results suggest that the release of carbon from tropical dead trees to the atmosphere can be simply estimated using tree circumference at death time and wood density.     

The utilization of dead wood resources by woodpeckers in Britain

Dead wood is important for woodpeckers, providing foraging, roost and nest-sites. In this paper, data from long-term studies of woodpeckers and dead wood in oakwoods in southern England are used to examine the dead wood requirements of the three British resident woodpecker species. Both Great Dendrocopos major and Lesser Spotted Woodpeckers Dendrocopos minor select dead trees for nest-sites although the former is able to nest in living trees too. On the other hand a smaller fraction of Lesser Spotted Woodpecker nests are in living trees. Green Woodpecker Picus viridis shows no selection for dead nesting trees. Hence the smallest woodpecker species appears to be most dependent on dead and decaying trees for nest-sites. Great and Lesser Spotted Woodpeckers show no preference for foraging on dead trees although they both make use of dead branches on living trees. Lesser Spotted Woodpeckers forage on smaller branches higher in the tree than Great Spotted Woodpeckers. There has been a trend for increasing dead wood resources in the study woods with both dead wood on the ground and standing dead trees (snags) increasing in the last 20 years. The levels of dead wood are shown to be the result of continual processes of creation and decay. Around 0.5% of oak Quercus spp., Ash Fraxinus excelsior and Hornbeam Carpinus betulus and 3.4% of the birch Betula spp. trees die each year in the woods resulting in a continuity of new dead snags and fallen trees. There is a high turnover of standing dead snags of oak and birch with 95% and 80% annual survival, respectively. Snags are only suitable for nesting Great Spotted Woodpeckers for a few years after their creation. It is suggested that these stand and dead wood dynamics are likely to provide habitats more favourable for the Great Spotted than the Lesser Spotted Woodpecker.     

Richness and diversity of Leguminosae in an altitudinal gradient in the tropical semi‐arid zone of Brazil

Abstract Many studies are based on the premise that, on a local scale, diversity is the result of ecological processes, whereas on a regional scale factors such as the topography, geology, hydrology, and historical and evolutionary events would influence this control. The Baturité Mountain Range (Ceará state), located in the Brazilian semi‐arid zone, is considered an area of extreme importance for conservation with its vegetation varying with the altitude and slope (windward vs. leeward). On the windward (wet) slope, rainforest dominates, whereas the leeward (dry) slope is dominated by seasonal forests and thorny woodland. The aim of this study was to contribute to the knowledge of the patterns of richness and diversity of the family Leguminosae on a local scale (Baturité Mountain Range) as well as a regional scale (northeastern Brazil). The two slopes present quite distinct floras. The dry slope presents higher richness and diversity indices for Leguminosae than the wet slope. The highest diversity of Leguminosae in the dry areas did not corroborate the ideas of other studies carried out in neotropical forests (total flora) that the higher species richness was predicted for wet areas. The present study indicates that the historical and evolutionary processes influence the diversity patterns on a local scale (Baturité Mountain Range), as well as on a regional scale (Brazilian semi‐arid). Our results reinforce the uniqueness of each portion of this area and its importance for conservation.