Flávio J. Luizão

ECOLOGICAL RESEARCH IN THE LARGE-SCALE BIOSPHERE– ATMOSPHERE EXPERIMENT IN AMAZONIA: EARLY RESULTS

The Large-scale Biosphere-Atmosphere Experiment in Amazonia (LBA) is a multinational, interdisciplinary research program led by Brazil. Ecological studies in LBA focus on how tropical forest conversion, regrowth, and selective logging influence carbon storage, nutrient dynamics, trace gas fluxes, and the prospect for sustainable land use in the Amazon region. Early results from ecological studies within LBA emphasize the var- iability within the vast Amazon region and the profound effects that land-use and land- cover changes are having on that landscape. The predominant land cover of the Amazon region is evergreen forest; nonetheless, LBA studies have observed strong seasonal patterns in gross primary production, ecosystem respiration, and net ecosystem exchange, as well as phenology and tree growth. The seasonal patterns vary spatially and interannually and evidence suggests that these patterns are driven not only by variations in weather but also by innate biological rhythms of the forest species. Rapid rates of deforestation have marked the forests of the Amazon region over the past three decades. Evidence from ground-based surveys and remote sensing show that substantial areas of forest are being degraded by logging activities and through the collapse of forest edges. Because forest edges and logged forests are susceptible to fire, positive feedback cycles of forest degradation may be initiated by land-use-change events. LBA studies indicate that cleared lands in the Amazon, once released from cultivation or pasture usage, regenerate biomass rapidly. However, the pace of biomass accumulation is dependent upon past land use and the depletion of nutrients by unsustainable land-management practices. The challenge for ongoing research within LBA is to integrate the recognition of diverse patterns and processes into general models for prediction of regional ecosystem function.

LITTER PRODUCTION AND LITTER NUTRIENT CONCENTRATIONS IN A FRAGMENTED AMAZONIAN LANDSCAPE

We analyzed the effects of distance to forest edge and soil texture on fine-litter production and on nutrient concentrations in the leaf fall in an experimentally fragmented landscape in Brazilian Amazonia. Production of fine litter (leaves, twigs 250 m) from forest edges, and in clayey or sandy soils. In total, 28 plots were established, with 10 litter traps per plot. Results reveal a significant effect of distance to forest edge on litter production, but no significant effect of soil type or interaction between soil type and edge distance. On average, annual litter production on edge plots exceeded that on the interior plots by 0.68 Mg/ha (9.50 ± 0.23 vs. 8.82 ± 0.14 Mg·ha−1·yr−1, mean ± 1 se, based on a 3-yr period). With regard to nutrient concentrations in the leaf fall, we detected a significant effect of soil type on three of eight nutrients analyzed. Concentrations of N, Mg, and Mn were greater in leaves on clayey than on sandy soils. Distance to forest edge only significantly affected the concentration of Ca, which was greater near than far from edges, perhaps due to strong Ca mobilization by the roots of pioneer trees. n nSeveral factors may account for the observed increase in litterfall near forest edges, including the greater prevalence of winds, increased plant desiccation stress, and higher rates of tree recruitment, especially of pioneer trees, near edges. Elevated rates of litterfall are likely to have cascading effects on the ecology of fragmented forests, affecting the invertebrate fauna, increasing seed and seedling mortality, and causing forest fragments to be more vulnerable to destructive surface fires.

Soil-induced impacts on forest structure drive coarse woody debris stocks across central Amazonia

Background: Coarse woody debris (CWD) is an essential component in tropical forest ecosystems and its quantity varies widely with forest types. Aims: Relationships among CWD, soil, forest structure and other environmental factors were analysed to understand the drivers of variation in CWD in forests on different soil types across central Amazonia. Methods: To estimate CWD stocks and density of dead wood debris, 75 permanent forest plots of 0.5 ha in size were assessed along a transect that spanned ca. 700 km in undisturbed forests from north of the Rio Negro to south of the Rio Amazonas. Soil physical properties were evaluated by digging 2-m-deep pits and by taking auger samples. Results: Soil physical properties were the best predictors of CWD stocks; 37% of its variation was explained by effective soil depth. CWD stocks had a two-fold variation across a gradient of physical soil constraints (i.e. effective soil depth, anoxia and soil structure). Average biomass per tree was related to physical soil constraints, which, in turn, had a strong relationship with local CWD stocks. Conclusions: Soil physical properties appear to control average biomass per tree (and through this affect forest structure and dynamics), which, in turn, is correlated with CWD production and stocks.

The Biogeochemistry of the Main Forest Vegetation Types in Amazonia

The structure and functioning of natural ecosystems reflect spatial and temporal characteristics of the atmosphere, physiography (geology, topo-hydro-pedo sequences), and biodiversity (vegetation type and associated fauna). Exchanges of gases, water, and organic and inorganic compounds between atmosphere and biosphere (vegetation, soil) strongly contribute to biogeochemical phenomena in Amazonia and to atmospheric phenomena within and outside of Amazonia. Some of these exchanges have importance for cloud formation and rainfall, while others (carbonic and trace gases) contribute to carbon sequestration/greenhouse effects. The Amazon basin encompasses diverse combinations of climate, soil, and hydrology that result in a variety of vegetation types. The current state of knowledge on Amazonian regional biochemistry is mainly based on studies carried out in terra firme type ‘lowland evergreen rainforest’, with a focus on the carbon cycle. For a better understanding of the biogeochemical functioning of the Amazon basin, attention to physiography and its relationship, including feedback mechanism, with vegetation diversity at the landscape scale is needed. This chapter synthesises patterns in biogeochemistry of the main vegetation formations of the Amazon basin. The objective is to demonstrate differences in elemental cycles (stock and fluxes) among the main vegetation formations. The patterns of fixation, storage, and use of nutrients for biomass production, the patterns of internal (re)cycling (leaching of crown nutrients, litterfall, root growth and death) within the main forest types, and the patterns of litter decomposition and soil processes (nutrient availability, nitrogen mineralisation vs. immobilisation) and gas emissions from soil are discussed.