Christoph Gehring

Resilience of secondary forest regrowth after slash-and-burn agriculture in central Amazonia

This study describes forest succession following slash-and-burn agriculture in central Amazonia, based on four chronosequences (22 sites) of 2- to 25-y-old secondary regrowth and mature forest. Biomass accumulated in the form of a saturation curve: 25 y of regrowth restored half of the mature-forest biomass, 75% would be restored alter an estimated 17 5 y. Biomass accumulation was accompanied by a rapid decrease in woody plant density and an increase of the positive skew of plant size distribution. Liana biomass share declined from 5.0% in young to 1.9% in old regrowth, and 1.3% in mature forest, whereas the share of palms was low (0.5%) in young regrowth and high in old regrowth (1.8%) and mature forest (2.2%), 25-y-old regrowth was similar with mature forest in respect to woody plant density and growth-form composition, but both the plant, size distribution and the number of stems per plant differed strongly from mature forest. A moderate increase in land use had only minor effects on biomass accumulation, but profoundly changed structural characteristics of regrowth, pointing to a high vulnerability to degradation. Thus, the sustainability of slash-and-burn in central Amazonia may be lower than the rapid initial biomass accumulation would make it seem.

Response of secondary vegetation in Eastern Amazonia to relaxed nutrient availability constraints

This study evaluated the effect of nutrient application on the regrowth dynamics of secondary fallow vegetation in an intensely exploited shifting cultivation area in the eastern Amazon region of Brazil. The importance of N, P, K, Ca, Mg, S and a mixture of micronutrients was tested in a minus-one-trial by comparison with a full complement of nutrients and unfertilized control plots. Fertilizers were applied three times during the experiment and their effects were monitored over a period of 2 1/2 years. Prior to the second fertilization, one third of each experimental plot was cleared of the vegetation cover and planted in maize, prior to the third fertilizer application these subplots were planted in sorghum. Biomass of maize and sorghum were used to indicate nutrient constraints and fertilizing effects due to the different treatments. Both crops were limited by P- and N-availability, with greater responses to P. The initial fertilization did not affect the biomass accumulation of the secondary vegetation during the first 15 months, but two additional applications significantly increased biomass in the complete fertilizer treatment compared to the unfertilized control. Biomass accumulation was primarily P-limited, N-limitation was apparent but not significant. The remaining nutrients did not affect plant growth. Fertilization favored production of nutrient-rich leaves. Application of readily available nutrients gave grasses a competitive edge over slower reacting woody vegetation. Fertilization also caused significant shifts in the contribution of woody species to biomass accumulation, as could be demonstrated for two prominent pioneer tree species. Growth response to fertilization as well as the primary limiting nutrient varied among seven dominant species monitored in the secondary vegetation. We conclude that growth of tropical secondary vegetation can be nutrient limited and it might respond significantly to additional nutrients by increasing biomass production.

Allometry of the babassu palm growing on a slash-and-burn agroecosystem of the eastern periphery of Amazonia

Babassu (Attalea speciosa C.Martius, Arecaceae) is a palm with extraordinary socioeconomic and ecologic importance in large areas of tropical Brazil, especially in frequently burned and degraded landscapes. Nevertheless, surprisingly little is known about this keystone species. This paper investigates the allometry of babassu, in order to improve understanding on palm architecture and to provide researchers with an efficient tool for aboveground biomass estimation of juvenile and adult palms. Juvenile leaf biomass can be accurately predicted with the easily measurable minimum diameter of rachis at 30 cm extension. Adult palm biomass can be estimated based on woody stem height, a variable fairly easily measurable on-field. Leaf biomass of adult palms was highly variable, averaged 31.7% of aboveground biomass and can be estimated only indirectly through the relationships between wood:leaf-ratio and total aboveground biomass. Carbon contents varied little in the babassu palm, without size- or growth-stage related differences, suggesting the general applicability of values (42.5% C for stems, 39.8% C for leaves). As a consequence of the limited secondary diameter growth inherent to palms, stem diameter of adult palms is unrelated to palm height and biomass. Stem tapering decreases with increasing palm height. This is partially compensated by increasing wood density in near cylindrical stems. Nevertheless, maximum babassu palm height of about 30 meters appears to be dictated by mechanical stability constraints. All allometric relationships of babassu described in this study are not affected by vegetation stand age, indicating the general applicability of these relationships.

Species richness increases income in agroforestry systems of eastern Amazonia

Biodiversity is believed to reduce risks (resistance and resilience against perturbations), to increase productivity via niche expansion, and possibly also to improve resource efficiency via mutually benefic species interactions. Agroforestry has been postulated as an ideal pathway of maintaining or restoring biodiversity in a socioeconomically sustainable manner. This study tests the relevance of agroforestry species diversity and richness on socioeconomic performance in a wide range of agroforestry systems in 38 farms aggregated in four clusters of sites in eastern Amazonia. We cover both commercial and subsistence agroforestry, ranging from simply structured plantations to diverse systems (enriched fallows, multi-strata home gardens), as well as pastures and shifting cultivation for comparisons. We quantify (i) all cultivated species, classifying them economically into species with commercial value, primarily subsistence purpose species or ‘non-productive’ species, and (ii) socioeconomic system variables (costs, monetary/non-monetary income, degree of satisfaction). Land-use intensity (per-hectare costs and income) was highest in commercial agroforestry and subsistence home gardens, and lowest in enriched fallows and pastures. All agroforestry systems resulted in higher income:cost ratios and greater satisfaction than pastures and shifting cultivation. Net income, non-monetary income and income:cost ratio were maximum in home gardens. Total species richness was negatively related with costs and monetary income, but not with non-monetary income, due to occupation of space by ‘non-productive’ species (juveniles or species providing ecosystem services). By contrast, productive (combining commercial and subsistence) species richness was positively related with (mainly non-monetary) income, net income and income:cost ratio. According to GLM, both productive species richness and Shannon–Wiener diversity positively affected net income. Future efforts for food security and poverty reduction need to focus more on species-rich agroforestry systems, both in terms of applied research and of extension service programs. Notably, the ubiquitous and successful home gardens merit far more attention.

Exploring the potential for sustainable weed control with integrated rice−fish culture for smallholder irrigated rice agriculture in the Maranhão Lowlands of Amazonia

Combining existing traditions of rice and of fish in an integrated rice−fish (IRF) culture is a promising strategy for sustainably increasing land productivity and diminishing the need for external inputs in smallholder agriculture of Amazonia. This study evaluates the potential of IRF for weed control in irrigated rice production. It was conducted from August 18 to December 5, 2008 in the Maranhao lowlands in the eastern periphery of Amazonia. We compared weed communities in four 3-year-old IRF and four adjacent ‘conventional’ irrigated rice (CIR) fields at 20 and 40 days after transplanting (DAT), at the mid-vegetative stage and at the onset of flowering, which served as an indicator of potential grain yield. Rice–fish fields contained differing mixtures of herbivore and omnivore fish species totaling 4000 fish per ha or 1.7 fish per m 3 . Total weed density was reduced in the IRF system, particularly early in the season, the most critical stage for rice development. The integration of fish into irrigated rice cultivation affected weed species composition, with fish-weeding preferentially reducing monocotyledonous Cyperaceae, one of the more aggressive and problematic weed families in this region. Monocot weed density was negatively correlated with rice aboveground biomass at 40 DAT rice. Although floristic similarity between IRF and CIR fields was low, the impacts of IRF on weed species diversity and weed species richness were not significant. Thus, IRF was not associated with a simplification of the weed community. We conclude that fish-weeding may substitute for manual or chemical weeding in irrigated rice agriculture, an important consideration for resource-poor smallholder agriculture in environmentally sensitive riverine or delta areas of eastern Amazonia.

Arbuscular mycorrhizal fungi associated with the babassu palm (Attalea speciosa) in the eastern periphery of Amazonia, Brazil

Babassu, Attalea speciosa (Arecaceae) is a ruderal palm native to Amazonia, which turned dominant in frequently burned lands throughout the ‘arc of deforestation’ and other degraded lands, in extreme cases attaining complete dominance. This study investigated arbuscular mycorrhizal fungi (AMF) as one possible explanation for the outstanding ecological success of this exceptional palm. We explored the relationships between the babassu palm and native arbuscular mycorrhizal fungi and babassu effects on the AMF richness and mycorrhizal inoculum potential (MIP) in the eastern periphery of Amazonia. For this purpose, we sampled topsoil (0-20 cm) at the onset of the rainy season from a 5-year-old secondary forest regrowth (SEC) area with three levels of babassu dominance (sites with 10, 50 and 70% babassu biomass shares), and at three distances (0, 2.5 and 4 m) from isolated babassu patches within a degraded pasture (PAS), both with five replications per treatment. Glomerospore density varied from 100 to 302 per gram of soil, 56% higher in SEC than PAS. We identified a total of 16 AMF species, with dominance of Acaulospora (six species) followed by Glomus (three species). AMF richness increased with babassu dominance in SEC sites, and reduced with distance from babassu patches within the PAS. The colonization rate of babassu roots was higher in SEC than in PAS, whereas MIP was similar in both areas and without treatment differences. Our study points to strong mycorrhizal association of the babassu palm as a potential mechanism for its outstanding ecological success in degraded lands.