Slowed decomposition is biotically mediated in an ectomycorrhizal, tropical rain forest

Bacteria and fungi drive the cycling of plant litter in forests, but little is known about their role in tropical rain forest nutrient cycling, despite the high rates of litter decay observed in these ecosystems. However, litter decay rates are not uniform across tropical rain forests. For example, decomposition can differ dramatically over small spatial scales between low-diversity, monodominant rain forests, and species-rich, mixed forests. Because the climatic patterns and soil parent material are identical in co-occurring mixed and monodominant forests, differences in forest floor accumulation, litter production, and decomposition between these forests may be biotically mediated. To test this hypothesis, we conducted field and laboratory studies in a monodominant rain forest in which the ectomycorrhizal tree Dicymbe corymbosa forms >80% of the canopy, and a diverse, mixed forest dominated by arbuscular mycorrhizal trees. After 2 years, decomposition was significantly slower in the monodominant forest (P < 0.001), but litter production was significantly greater in the mixed forest (P < 0.001). In the laboratory, we found microbial community biomass was greater in the mixed forest (P = 0.02), and the composition of fungal communities was distinct between the two rain forest types (P = 0.001). Sequencing of fungal rDNA revealed a significantly lower richness of saprotrophic fungi in the monodominant forest (19 species) relative to the species-rich forest (84 species); moreover, only 4% percent of fungal sequences occurred in both forests. These results show that nutrient cycling patterns in tropical forests can vary dramatically over small spatial scales, and that changes in microbial community structure likely drive the observed differences in decomposition.     

Altered leaf-litter decomposition rates in tropical forest fragments

The effects of forest fragmentation on leaf-litter decomposition rates were investigated for the first time in an experimentally fragmented tropical forest landscape in Central Amazonia. Leaf-litter decomposition rates were measured at seven distances (0–420 m) along forest edge-to-interior transects in two 100-ha fragments, two continuous forest edges, and at an identical series of distances along two deep continuous forest transects, as well as at the centers of two 1-ha and two 10-ha fragments. Decomposition rates increased significantly towards the edge of 100-ha forest fragments. Litter turnover times were 3–4 times faster within 50 m of the edge of 100-ha fragments than normally found in deep continuous forest. In contrast, there was no significant change in the rate of leaf-litter decomposition from the interior to the edge of continuous forest. It is difficult to account for these very different edge responses. Decomposition rates were not correlated with air temperature differentials, evaporative drying rates, litter depth, biomass or moisture content, or with total invertebrate densities, either within individual edge transects or across all sites. The difference in edge response may be due to chance, particularly the patchy removal of vast quantities of litter by litter-feeding termites, or may be a real, area-dependent phenomenon. Clearly, however, forest fragmentation increases the variability and unpredictability of litter decomposition rates near forest edges. In addition to edge effects, decomposition rates were strongly affected by decreasing fragment area. While sites at the centers of 10-ha and 100-ha forest fragments and continuous forest had equivalent decomposition rates, rates were markedly lower at the centers of 1-ha fragments. Litter turnover times were 2–3 times slower in 1-ha fragments than in continuous forest, and up to 13 times slower than at 100-ha edges. Litter structure and nutrient cycling dynamics are inevitably altered by forest fragmentation. Received: 16 October 1997 / Accepted: 14 April 1998     

Savanna soil fertility limits growth but not survival of tropical forest tree seedlings

Background and Aims

Cerradão (Brazilian woodland savannas) and seasonally dry forests (SDF) from southeastern Brazil occur under the same climate but are remarkably distinct in species composition. The objective of this study was to evaluate the role of soil origin in the initial growth and distribution of SDF and Cerradão species.

Methods

We conducted a greenhouse experiment growing Cerradão and SDF tree seedlings over their soil and the soil of the contrasting vegetation type. We evaluated soil nutrient availability and seedling survivorship, growth and leaf functional traits.

Results

Despite the higher nutrient availability in SDF soils, soil origin did not affect seedling survivorship. The three SDF species demonstrated home-soil advantage, enhanced growth with increasing soil nutrient availability and had higher growth rates than Cerradão species, even on Cerradão soils. Growth of Cerradão seedlings was not higher on Cerradão soil and, overall, was not positively correlated with soil nutrient availability.

Conclusions

SDF species are fast-growing species while Cerradão trees tend to be slow-growing species. Although savanna soil reduces growth of forest species, our findings suggest that soil chemical attributes, alone, does not exclude the occurrence of SDF seedlings in Cerradão and vice-versa.     

Root quality and decomposition environment,but not tree species richness,drive root decomposition in tropical forests

Plant and Soil - Tropical forests contribute significantly to the global carbon cycle, yet the relative importance of tree diversity on key ecosystem processes such as root decomposition remains...     

Gamma-glutamyltranspeptidase, but not YwrD, is important in utilization of extracellular blutathione as a sulfur source in Bacillus subtilis

gamma-Glutamyltranspeptidase (EC 2.3.2.2) of Bacillus subtilis, which is an extracellular enzyme, hydrolyzes the gamma-glutamyl linkage of glutathione. YwrD, which is homologous to gamma-glutamyltranspeptidase, was speculated to have a similar physiological role. It was shown that gamma-glutamyltranspeptidase, but not YwrD, is important in utilizing glutathione as the sole sulfur source in Bacillus subtilis.     

Bottom-up and top-down regulation of decomposition in a tropical forest

The soil nutrients, microbes, and arthropods of tropical forests are patchy at multiple scales. We asked how these three factors interact to generate patterns of decomposition in 450 100 cm² litterbags arrayed along a 50 m ridge top in a Panama rainforest. We tested top-down (via grazing by microbivores like collembola and diplopods) and bottom-up (via added N and P) effects on the decomposition of cellulose. By using a 1,000-fold gradient in mesh size we generated a two-fold gradient in arthropod grazing. Microbivore grazing first retarded then ultimately enhanced decomposition rates. Micropulses of N and P (simulating concentrated urine) enhanced neither decomposition rates nor microbivores but increased the abundance of predacious ants. Decomposition rates also varied across the ridge, and were lowest in a plot with the deepest litter and highest soil moisture. These data generate the working hypothesis that N and P cascade upward at grains of 100 cm² to enhance a major predator in the litter; predators then absorb any increases in microbivores attracted to the extra fungal growth. These population interactions are in turn embedded in mesoscale variability generated by individual tree canopies that drive changes in litter quality and soil moisture. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Leaf litter decomposition in a tropical peat swamp forest in Peninsular Malaysia

It has long been assumed that the peat underlying tropical peat swamp forests accumulates because the extreme conditions (water logged, nutrient poor, anaerobic and acidic—pH 2.9–3.5) impede microbial activity. Litterbag studies in a tropical Malaysian peat swamp (North Selangor peat swamp forest) showed that although the sclerophyllous, toxic leaves of endemic peat forest plants (Macaranga pruinosa, Campnosperma coriaceum, Pandanus atrocarpus, Stenochlaena palustris) were barely decomposed by bacteria and fungi (decay rates of only 0.0006–0.0016 k day⁻¹), leaves of M. tanarius, a secondary forest species were almost completely decomposed (decay rates of 0.0047–0.005 k day⁻¹) after 1 year. Thus it is intrinsic properties of the leaves (that are adaptations to deter herbivory in the nutrient poor environment) that impede microbial breakdown. The water of the peat swamp was very high in dissolved organic carbon (70–84 mg l⁻¹ DOC). Laboratory studies revealed initial rapid leaching of DOC from leaves (up to 1,720 mg l⁻¹ from 4 g of leaves in 7 days), but the DOC levels then fell rapidly. The leaching of DOC resulted in weight loss but the physical structure of the leaves remained intact. It is suggested that the DOC is used as a substrate for microbial growth hence lowering the concentration of DOC in the water and transferring energy from the leaves to other trophic levels. This would explain how nutrient poor tropical peatswamps support diverse, abundant flora and fauna despite low nutrient levels and lack of rapid litter cycling such as occurs in other types of tropical rainforests.     

Increased phosphatidylinositol metabolism is an important but not an obligatory early event in B lymphocyte activation

The phosphatidylinositol (PI) response has been implicated in membrane signaling and cell activation. The role of phospholipid metabolism among the early events in B cell activation has not been clear. We have treated murine B cells with anti-Ig antibody and lipopolysaccharide (LPS) and have found that, although anti-IgM induces the PI response, LPS does not. The increase in metabolic labeling of PI is specific to PI, and not the phosphatidylinositols. Anti-IgM unresponsive B cells from CBA/N mice, which may correspond to a specific functional subpopulation of normal B cells, do not increase PI metabolism in response to anti-IgM, nor do they undergo blastogenesis or DNA synthesis. Moreover, when these deficient B cells are given a stimulus sufficient to drive them into S (LPS + anti-IgM), there is still no corresponding activation of PI metabolism. These results are consistent with a two-signal model of xid B cell activation by anti-IgM. One very early signal primes the cells but does not induce the PI response. A second early signal is supplied by LPS. This signal sustains cells in the activated state, allowing them to receive yet other signals to proceed through G1 and progress further along the cell cycle. A similar sequence of events may occur in the normal B cell, with the first signal provided by priming with anti-IgM, and the second signal, the PI response, supported by a sufficiently high dose of anti-IgM to induce PI turnover and maintain the cell in G1.     

Soil does not explain monodominance in a Central African tropical forest

Background

Soil characteristics have been hypothesised as one of the possible mechanisms leading to monodominance of Gilbertiodendron dewerei in some areas of Central Africa where higher-diversity forest would be expected. However, the differences in soil characteristics between the G. dewevrei-dominated forest and its adjacent mixed forest are still poorly understood. Here we present the soil characteristics of the G. dewevrei forest and quantify whether soil physical and chemical properties in this monodominant forest are significantly different from the adjacent mixed forest.

Methodology/Principal Findings

We sampled top soil (0–5, 5–10, 10–20, 20–30 cm) and subsoil (150–200 cm) using an augur in 6 × 1 ha areas of intact central Africa forest in SE Cameroon, three independent patches of G. dewevrei-dominated forest and three adjacent areas (450–800 m apart), all chosen to be topographically homogeneous. Analysis – subjected to Bonferroni correction procedure – revealed no significant differences between the monodominant and mixed forests in terms of soil texture, median particle size, bulk density, pH, carbon (C) content, nitrogen (N) content, C:N ratio, C:total NaOH-extractable P ratio and concentrations of labile phosphorous (P), inorganic NaOH-extractable P, total NaOH-extractable P, aluminium, barium, calcium, copper, iron, magnesium, manganese, molybdenum, nickel, potassium, selenium, silicon, sodium and zinc. Prior to Bonferroni correction procedure, there was a significant lower level of silicon concentration found in the monodominant than mixed forest deep soil; and a significant lower level of nickel concentration in the monodominant than mixed forest top soil. Nevertheless, these were likely to be the results of multiple tests of significance.

Conclusions/Significance

Our results do not provide clear evidence of soil mediation for the location of monodominant forests in relation to adjacent mixed forests. It is also likely that G. dewevrei does not influence soil chemistry in the monodominant forests.     

Habitat moisture is an important driver of patterns of sap flow and water balance in tropical montane cloud forest epiphytes

Microclimate in the tropical montane cloud forest (TMCF) is variable on both spatial and temporal scales and can lead to large fluctuations in both leaf-level transpiration and whole plant water use. While variation in transpiration has been found in TMCFs, the influence of different microclimatic drivers on plant water relations in this ecosystem has been relatively understudied. Within the TMCF, epiphytes may be particularly affected by natural variation in microclimate due to their partial or complete disassociation from soil resources. In this study, we examined the effects of seasonal microclimate on whole plant water balance in epiphytes in both an observational and a manipulative experiment. We also evaluated the effects of different microclimatic drivers using three hierarchical linear (mixed) models. On average, 31 % of total positive sap flow was recovered via foliar water uptake (FWU) over the course of the study. We found that precipitation was the greatest driver of foliar water uptake and nighttime sap flow in our study species and that both VPD and precipitation were important drivers to daytime sap flow. We also found that despite adaptations to withstand seasonal drought, an extended dry period caused severe desiccation in most plants despite a large reduction in leaf-level and whole plant transpiration. Our results indicate that the epiphytes studied rely on FWU to maintain positive water balance in the dry season and that increases in dry periods in the TMCF may be detrimental to these common members of the epiphyte community.     

Entomogenous fungi in tropical forest ecosystems: an appraisal

Abstract. 1. Species of the genus Cordyceps (Ascomycotina; Clavicipitales) are the commonest fungi encountered on arthropods in tropical forests.
2. The asexual states of Cordyceps may occur in conjunction with or separately from the perfect state and are classified in the genera Hirsutella, Hymenostilbe, Nomuraea, Paecilomyces and Verticillium of the imperfect fungi (Deuteromycotina; Moniliales).
3. Most Cordyceps species have a restricted host range and this rigid host specificity, at the generic or tribal level, is especially evident in ant-fungal associations.
4. Pathogenicity was not tested but circumstantial evidence is presented which supports the view that Cordyceps species are primary pathogens of arthropods.
5. Observations of living, infected ants suggest that behavioural patterns are radically altered, possibly altruistically orientated.
6. It is thought that entomogenous fungi are involved in the regulation of arthropod populations and may help to maintain stability in tropical forest ecosystems.
7. The value of these organisms to man may be in the metabolites they produce rather than in their direct usage as biological control agents of insect pests.     

Finding and promoting a local conservation consensus in a globally important tropical forest landscape

Can democratic processes favour conservation outcomes in the tropics? This study focuses on local viewpoints within a forested landscape of high conservation significance in East Kalimantan (Indonesian Borneo). Stakeholders received posters displaying results from a previous study; these posters emphasised local priorities and views regarding local biodiversity. We assess local attitudes to this information, and consider some implications. Knowledge of, and agreement with, poster content increased among villagers, townspeople and civil servants after they received posters. All respondents appreciated the posters and all supported some form of forest conservation. All respondents agreed that biodiversity conservation and local views are vital in land-use planning. All agreed that logging companies need to be better controlled, while 80% consider them a “major environmental threat”. These results bolster our belief that involving communities is not only an ethically defensible way to achieve conservation outcomes, but also a pragmatic opportunity to do so.     

Effect of environmental conditions on decomposition in eight woody species of a dry tropical forest

Litter decay is a significant part of carbon budget. Due to strong environmental control, the changes in the environment may drastically influence the litter decay rates. Litter decomposition of eight dry tropical woody species, viz. Shorea robusta, Buchanania lanzan, Diospyros melanoxylon, Lagerstroemia parviflora, Lannea coromandelica, Terminalia tomentosa, Holarrhena antidysenterica and Lantana camara was studied to document the effect of intra-annual changes in the environment. Litter decomposition was monitored at monthly intervals at five sites using litter bag technique over an annual cycle in a dry tropical deciduous forest of Vindhyan highland, India. Weight loss differed among species and through months, and ranged from 15.38% in L. camara at Kotwa site in January to 30.72% in T. tomentosa at Hathinala site in August. Peak weight loss occurred in August and averaged 46.2% across species and sites. Nitrogen and phosphorus mineralization rates also varied significantly from species to species. T. tomentosa having higher nitrogen content and lower C/N ratio than other species exhibited faster weight loss. Nitrogen and phosphorus contents of litter showed significant positive correlation with weight loss. C/N ratio was negatively related to decay constant, and the weight loss was positively related to the soil surface temperature as well as soil moisture content.     

Testosterone increases perceived dominance but not attractiveness in human males

Recent evidence suggests that certain features on the human face indicate hormonal levels during growth, and that women judge the attractiveness of potential partners based on the appearance of these features. One entrenched notion is male facial features that are affected by testosterone are used as direct cues in mate preference. Testosterone may be particularly revealing as it is purported to be an honest indicator of male fitness. Increased testosterone may impose an immunocompetence handicap on the bearer and only the best males can carry this handicap. To date, tests of this theory have been indirect, and have relied on digital manipulations that represent unrealistic continuums of masculine and feminine faces. We provide a much more direct test by manipulating digitally male faces to mimic known shape variation, caused by varying levels of testosterone through puberty. We produced a continuum of faces that ranged from low to high levels of testosterone in male faces and asked women to choose the points on the continuum that appeared most attractive and most physically dominant. Our data indicate that high testosterone faces reveal dominance. However, there is no evidence of directional selection for increased (or decreased) testosterone in terms of attractiveness to the opposite sex. We discuss the relevance and applicability of evolutionary interpretations of our data and, contrary to predictions, provide evidence of stabilizing selection acting on testosterone through mate preferences.     

The disturbance regime of an old-growth forest in coastal California

This study deals with the disturbance regime of an old-growth, mixed-evergreen forest with a canopy composed of Arbutus menziesii, Lithocarpus densiflora, Pseudotsuga menziesii, Quercus chrysolepis, Q. wislizenii, and Sequoia sempervirens. 80 canopy gaps were randomly selected from throughout a 230-ha watershed. Of the land area sampled, 11.1 to 16.6 % was within gaps. Gap area had a mode of < 50m² and a range of 6–3437 m². Gaps were formed by snags, snaps, tips, and slope failures. Although < 10 % of the gaps sampled were due to slope failures, these accounted for 43 % of the total land area within gaps. All snags resulted from the death of a Pseudotsuga or Arbutus individual, the widely branched trunks of Arbutus individuals accounted for most of the irregularly shaped gaps, and larger gaps resulted from the death of Pseudotsuga and Sequoia individuals, averaging 119 and 111 m² respectively, than from the death of Lithocarpus or Arbutus, averaging 54 and 52 m². Gaps were more frequent over concave sections of slopes and large gaps were more frequent on north-facing slopes. The creation of a gap increased disturbance to the adjacent canopy, with half of the gaps formed through more than one disturbance. The relationship of disturbance regime to topography, the influence of canopy species biology on gap properties, and the repeated events involved in gap formation all indicate a fine scale variation in the pattern of gaps and their characteristics.     

Macrophyte-associated macroinvertebrates as an important food source for wintering waterbirds at Lake Constance

In many lake ecosystems, waterbirds rate among the top consumers. Lake Constance is an important overwintering site for waterbirds in Central Europe because it provides sufficient food sources to meet energy demands during the winter months. In this study we focussed on the submerged vegetation and its associated macroinvertebrate fauna as a food source. We conducted experiments with exclosure cages which protected evergreen charophytes from bird grazing and compared the mass decline throughout the winter to unprotected grazed sites. Furthermore, we assessed the change in macrophyte-associated macroinvertebrates during the course of winter. We found that wintering waterbirds caused a significant decline of charophytes and their associated macroinvertebrates down to a depth of 4 m. We determined the taxa and abundance of macroinvertebrates associated with charophytes and predominantly found Dreissena polymorpha. At sites protected from bird grazing, the biomass of the macroinvertebrate fauna increased relative to the plant biomass during the course of winter. This might also have affected the foraging behaviour of diving waterbirds.     

A non-native invasive grass increases soil carbon flux in a Hawaiian tropical dry forest

Non‐native plants are invading terrestrial ecosystems across the globe, yet little is known about how invasions impact carbon (C) cycling or how these impacts will be influenced by climate change. We quantified the effect of a non‐native C₄ grass invasion on soil C pools and fluxes in a Hawaiian tropical dry forest over 2 years in which annual precipitation was average (Year 1) and ～60% higher than average (Year 2). Work was conducted in a series of forested plots where the grass understory was completely removed (removal plots) or left intact (grass plots) for 3 years before experiment initiation. We hypothesized that grass invasion would: (i) not change total soil C pools, (ii) increase the flux of C into and out of soils, and (iii) increase the sensitivity of soil C flux to variability in precipitation. In grass plots, grasses accounted for 25–34% of litter layer C and ～70% of fine root C. However, no differences were observed between treatments in the size of any soil C pools. Moreover, grass‐derived C constituted a negligible fraction of the large mineral soil C pool (< 3%) despite being present in the system for ≥50 years. Tree litterfall was ～45% lower in grass plots, but grass‐derived litterfall more than compensated for this reduction in both years. Annual cumulative soil‐surface CO₂ efflux (R_soil) was ～40% higher in grass plots in both years, and increased in both treatments by ～36% in the wetter Year 2. Despite minimal grass‐derived mineral soil C, > 75% of R_soil in grass plots was of C₄ (i.e. grass) origin. These results demonstrate that grass invasion in forest ecosystems can increase the flux of C into and out of soils without changing total C pools, at least over the short term and as long as the native tree canopy remains intact, and that invasion‐mediated changes in belowground C cycling are sensitive to precipitation.