Arbuscular mycorrhizal colonization of lime in different agroecosystems of the dry tropics

 Tecomán, in the Mexican state of Colima, had the world's greatest production of lime Citrus aurantifolia Swingle. Typical farming systems in the area include: (a) high-input monoculture, (b) a high-input system in which lime trees grow together with coconut palms, (c) a low-input system called "Family Farms" or "Family Gardens". In the Family Gardens, cultural practices are minimal and other fruit trees (about 16 species) coexist with the lime trees. This traditional minimal input system makes use of locally available resources and they are structurally very diverse. Arbuscular mycorrhizae may be crucial for sustainable production in Family Gardens. Root colonization and spore populations of fungi were scored at 2-week intervals in the three agroecosystems during a 6-month period. First samples were taken after the application of chemical fertilizer and irrigation in the high-input systems. Root colonization of lime was much higher and consistent in the low-input plots than in conventionally farmed plots, with colonization levels of 50–62% that remained the same throughout the sampling time; the high-input systems showed a high variation and lower level of colonization, 36% and 27% in associated and monoculture systems, respectively. Spore abundance was higher in the high-input systems but showed constant variation. The results suggest a strong effect of agroecosystem on mycorrhizal colonization of lime roots. Accepted: 12 September 1995     

Dendrochronology in Southeast Asia

Dendrochronological research in Southeast Asia is under development; however, the amount of tress with potential for dendrochronological studies is restricted. For example, teak trees from India, Myanmar, Thailand, and Java are valuable dendrochronologic studies for ready climate response. Teak from Java is best suited for studying the El Niño-Southern Oscillation and sea-surface temperatures, whereas Indian teak is used to reconstruct periods of drought in India. Further, Thai teak and Vietnamese cypress trees captured the long drought period that led to the demise of the Angkor reign (fourteenth–fifteenth century). Diverse techniques including anatomical observation, cambial markings, cell differentiation, and isotopic analysis prove the age and growth of invisible tropical tree rings. A number of invisible growth rings in trees from both tropical and subtropical forests have been identified, resulting in the advancement of dendrochronology. Climate change is a substantial challenge for most living things and natural resources. A greater understanding of tree species adaptation in this region is necessary. The understanding of long-term paleoclimate can be gained by researching old samples and archaeological materials from this region.     

Poinsettia's wild ancestor in the Mexican dry tropics: Historical, genetic, and environmental evidence

? Premise of the study: The poinsettia (Euphorbia pulcherrima) is the world's most economically important potted plant, but despite its preeminence it is not clear which wild populations are ancestral to the varieties cultivated around the world. Tradition holds that the U.S. envoy to Mexico J. R. Poinsett collected the progenitors of the over 300 varieties in global cultivation on an 1828 excursion to northern Guerrero State, Mexico. It is unknown whether the contemporary cultivars are descended from plants from Guerrero or whether germplasm from other parts of poinsettia's 2000 km long distribution entered into cultivation during the nearly 200 yr of subsequent poinsettia horticulture. ? Methods: To identify the wild populations that likely gave rise to the cultivars and test this historical account, we sequenced plastid and nuclear DNA regions and modeled poinsettia's potential distribution. ? Key results: The combination of nuclear and plastid haplotypes characterizing cultivars was found only in northern Guerrero. Distribution modeling indicated that suitable habitat conditions for wild poinsettias are present in this area, consistent with their likely wild status. ? Conclusions: Our data pinpoint the area of northern Guerrero as the cultivated poinsettia's probable ancestral region, congruent with the traditional account attributing the original collections to Poinsett. Abundant genetic variation likely offers raw material for improving the many shortcomings of cultivars, including vulnerability to cold, stem breakage, and pathogens such as Pythium and Phytophthora. However, genetic differences between populations make conservation of all of poinsettia's diversity difficult.     

Dynamics of habitats and macroinvertebrate assemblages in rivers of the Australian dry tropics

1. The dry tropics are characterised by episodic summer rainfall such that the majority of annual river flow occurs in a short period of time. This dryland hydrological cycle leads to variably connected channels and waterholes along the length of a river bed. 2. We investigated the seasonal changes in biophysical characteristics and macroinvertebrate assemblage composition in dry‐tropics rivers at 15 sites on four rivers, each sampled five times (representing one annual hydrological cycle), in the Burdekin catchment, north Queensland, Australia. 3. Assemblages and their temporal trajectories differed among seasons, sites and habitats, even within the same habitat and/or river. Wet season flooding did not appear to ‘reset’ assemblages, with post‐wet season assemblages differing between years. 4. We found no consistent pattern in taxonomic richness over time, and sites within rivers showed no consistent convergence or divergence (i.e. turnover) in macroinvertebrate assemblage composition. However, biophysical variables associated with the rigours of the late dry season had significant effects on macroinvertebrate assemblages, highlighting the variable and often harsh conditions of dry‐tropics rivers. Underlying these patterns were different resistance and resilience traits of invertebrates (such as colonisation and establishment abilities), as well as the local‐scale effects of biophysical variables. 5. The dynamic nature of dryland rivers presents major challenges to monitoring programmes, and our results suggest a more complex scenario for monitoring and management than previously described.     

Ecophysiological traits of deciduous and evergreen woody species in the seasonally dry tropics

Seasonally dry tropical ecosystems occur in the Americas, Africa, India and Australia. They sustain large human populations, determine regional climate, are sites of biological and cultural conservation, and have significant economic value. Evergreen, deciduous and semi- and brevideciduous trees frequently co-occur. Recent research reveals how these various phenological groups respond to changes in soil and atmospheric water content. Cost-benefit analyses of evergreen and deciduous species show how leaves of deciduous species live fast and die young, whereas leaves of evergreen species live slowly but for longer.     

Dendrochronology In Plains Prehistory: An Assessment

Abstract

Chronologies and climatological interpretations based upon annual growth rings of trees have been available to prehistorians within the Plains for more than half a century. Until the advent of radiocarbon dating, tree rings provided the only “absolute” chronology for most archeological complexes and subsequently, continued as an important adjunct to radiometric methods. Nonetheless, the validity of tree ring dates in the Plains must be questioned. Continuity of research has been lacking, there have been serious methodological problems, and the provenience of many specimens is in doubt. Moreover, there are significant conflicts between tree ring and recent radiocarbon dates.     

Culm recruitment,dry matter dynamics and carbon flux in recently harvested and mature bamboo savannas in the Indian dry tropics

Culm recruitment, standing crop biomass, net production and carbon flux were estimated in mature (5 years after last harvest) and recently harvested bamboo (Dendrocalamus strictus (Roxb.) Nees) savanna sites in the dry tropics. During the 2 study years bamboo shoot recruitment was 1711–3182 and 1432–1510 shoots ha⁻¹ in harvested and mature sites, respectively. Corresponding shoot mortality was 66–93% and 62–69%, respectively. Total biomass was 34.9 t ha⁻¹ at the harvested site and 47.4 t ha⁻¹ at the mature site. Harvesting increased the relative contribution of belowground bamboo biomass. Annual litter input to soil was 2.7 and 5.9 t ha⁻¹ year⁻¹ at the harvested and mature sites, respectively. The bulk of the annual litterfall (78–88%) occurred in the cool dry season (November to February). The mean litter mass on the savanna floor ranged from 3.1 to 3.3 t ha⁻¹; at the harvested site wood litter contributed 70% of the litter mass and at the mature site leaves formed 77% of the litter mass. The mean total net production (TNP) for the two annual cycles was 15.8 t ha⁻¹ year⁻¹ at the harvested site and 19.3 t ha⁻¹ year⁻¹ at the mature site. Nearly half (46–57%) of the TNP was allocated to the belowground parts. Short lived components (leaves and fine roots) contributed about four-fifths of the net production of bamboo. Total carbon storage in the system was 64.4 t ha⁻¹ at the harvested site and 75.4 t ha⁻¹ at the mature site, of which 23–28% was distributed in vegetation, 2% in litter and 70–75% in soil. Annual net carbon deposition was 6.3 and 8.7 t ha⁻¹ year⁻¹ at harvested and mature sites, respectively.     

Soil seed densities and emergence patterns in pastures in the seasonally dry tropics of northeastern Australia

Germinable seed densities in the surface (0–10 cm) soil of pasture communities growing at Lansdown, near Townsville, were measured during the late dry season before the first germinating rain and again during the following wet season after germination but before the input of new seed. Seedlings emerging in the field were counted at approximately weekly intervals during this period to determine emergence patterns. Twelve communities were sampled in 1980–81 and six were re-sampled in 1981–82. During the late dry season germinable seed densities ranged from 5000 to 40 000 seeds m^-2. Seeds of the introduced legume, Stylosanthes hamata, were present in all pastures. There were many seeds of annual grasses (Digitaria ciliaris and Brachiaria miliiformis) and sedges (Cyperus and Fimbristylis spp.) but only few seeds of perennial grasses (both native and introduced). Soil seed densities were much lower during the wet season than during the preceding dry season, particularly for the grasses. Emergence commenced and approximately 70% of all seedlings emerged on the first major rainfall of the wet season. The subsequent emergence pattern varied between years. In 1980–81 there was a gradual and continuous increase in seedling numbers under the continuously moist conditions which prevailed. In 1981–82 further emergence occurred in discrete events related to rainfall and intervening dry periods. Maximum seedling densities exceeded 34 000 seedlings m^-2 including 29 000 grass seedlings (mainly annual species). The implications of these results for species survival and pasture composition are discussed.     

Rodent eradications as ecosystem experiments: a case study from the Mexican tropics

For effective and efficient pest management it is essential to understand the ecology of the target species and recipient ecosystems. The use of rodent eradication as a restoration tool is well established in temperate regions, but less common in the tropics, presenting an opportunity to undertake scientific learning in tandem with rodent eradications. On a dry tropical archipelago, we used a Before-After-Control-Impact framework to document (1) fluctuations in the abundance and demography of invasive Rattus rattus and Mus musculus on three different islands, (2) the trophic niche of all three invasive rodent populations, and (3) changes in the invertebrate community before and after rodent eradication, also comparing with two rodent free islands. While rat density was high and relatively stable throughout the year, the two mouse populations greatly differed in body size and seasonal dynamics, despite their proximity. The rodents in all three populations were generalist and opportunistic feeders, although stable isotope analyses results indicated major differences among them, driven by food availability and rodent species. Seasonal fluctuations in invertebrate communities depended on rodent invasion status, but recovery in the invertebrate communities one year after rodent removal was limited for all islands. Predictions for other tropical ecosystem biomes require long-term research on more tropical islands. Improving our understanding of island and species-specific contexts of rodent eradications can advance island restoration projects and assist the selection of indicator species for ecosystem recovery.     

Dendrochronology and Human Dimensions Issues in Global Change

This introduction briefly outlines the context for global change, the origin of this volume and presents a brief summary of its contents.     

Dendroecology in the tropics: a review

Over the last decade the field of tropical dendroecology has developed rapidly and major achievements have been made. We reviewed the advances in three main themes within the field. First, long chronologies for tropical tree species were constructed which allowed climate reconstructions, revealed sources of climatic variation and clarified climate–growth relations. Other studies combined tree-ring data and stable isotope (¹³C and ¹⁸O) measurements to evaluate the response of tropical trees to climatic variation and changes. A second set of studies assessed long-term growth patterns of individual trees throughout their life. These studies enhanced the understanding of growth trajectories to the canopy, quantified autocorrelated tree growth and yielded new estimates of tree ages. Such studies were also used to reconstruct the disturbance history of tropical forests. The last set of studies applied tree-ring data to growth models. Tree-ring data can replace diameter measurements from research plots, provide additional information to construct population models, improve timber yield models and validate model output. Based on our review, we propose two main directions for future research. (1) An evaluation of the causes and consequences of growth variation within and among trees and their relation to environmental variation. Studies evaluating this directly contribute to improved understanding of tropical tree ecology. (2) The simultaneous measurement of widths and stable isotope fractions in tree rings offers the potential to study responses of trees to climatic change. Given the major role of tropical forests in the global carbon cycle, knowing these responses is of high priority.     

The relationships between salmonellas and faecal indicator concentrations in two pools in the Australian wet/dry tropics

S.A. TOWNSEND. 1992. The relationships between total coliform, faecal coliform, enterococci and salmonella concentrations were investigated at Berry and Howard Pools in the Australian wet/dry tropics. Both pools have catchments with minimal human activity and no major point source of faecal pollution. Forty-five indicator and salmonella enumerations were made from each pool over a 1 year period. Salmonellas were isolated from 69% and 96% of samples collected from Howard and Berry Pools respectively, the maximum (MPN) concentration was 110/100 ml. Native fauna were the primary salmonella source. Spearman rank correlations between indicator organisms and salmonella at Howard Pool were significant at the 5% level and approximated 0.6. At Berry Pool, total coliform and enterococci Spearman rank correlations with salmonella were also statistically significant, approximating 0.3; faecal coliforms and salmonella rankings, however, were unrelated. The higher correlation coefficients at Howard Pool were attributed to its small catchment (4 km²) and the more recent nature of faecal contamination compared with Berry Pool which has a catchment of 130 km². The results highlight the spatial variability of the indicator/pathogen numerical relationship. Total coliform and enterococci counts, as indicators of faecal pollution, were similar and more consistent than faecal coliforms.     

Hydrological connectivity drives patterns of macroinvertebrate biodiversity in floodplain rivers of the Australian wet /dry tropics

1. Floodplain rivers in Australia's wet/dry tropics are regarded as being among the most ecologically intact and bio-diverse lotic ecosystems in the world, yet there have been relatively few community-based studies of their aquatic fauna.
2. To investigate relationships between hydrological connectivity and biodiversity in the region, macroinvertebrates were collected from sites within two contrasting floodplain rivers, the 'tropical' Gregory River and 'dryland' Flinders River systems, during the dry season and analysed at various spatial scales. A subset of sites was re-sampled in the following dry season to explore temporal variation. The fauna consisted of 124 morphotaxa, dominated by gatherers and the Insecta.
3. As predicted, hydrological connectivity (the lotic or lentic status of waterbodies) had a major influence on macroinvertebrate assemblage composition and diversity, both in space and time. Assemblages from waterbodies with similar connection histories were most alike, and beta-diversity between assemblages was greatest between lotic and lentic waterbodies, tending to increase with increasing spatial separation.
4. At smaller spatial scales, a number of within-waterbody, habitat and water quality characteristics were important for explaining variation (61%) in the taxonomic organization of assemblages, and characteristics associated with primary productivity and habitat diversity were important for explaining variation (45%) in the functional organization of assemblages. However, much of the small-scale environmental variation across the study region appeared to be related to broad-scale variation in hydrological connectivity, which had both direct and indirect effects on macroinvertebrate assemblages.
5. Conservation of the biodiversity in Australia's wet/dry tropics may depend on conserving the natural variation in hydrological connectivity and the unregulated flow of floodplain rivers.