Françoise Yoko Ishida

The effects of partial throughfall exclusion on canopy processes, aboveground production, and biogeochemistry of an Amazon forest

(1) Moist tropical forests in Amazonia and elsewhere are subjected to increasingly severe drought episodes through the El Nino-Southern Oscillation (ENSO) and possibly through deforestation-driven reductions in rainfall. The effects of this trend on tropical forest canopy dynamics, emissions of greenhouse gases, and other ecological functions are potentially large but poorly understood. We established a throughfall exclusion experiment in an east-central Amazon forest (Tapajos National Forest, Brazil) to help understand these effects. After 1-year intercalibration period of two 1-ha forest plots, we installed plastic panels and wooden gutters in the understory of one of the plots, thereby excluding � 890 mm of throughfall during the exclusion period of 2000 (late January to early August) and � 680 mm thus far in the exclusion period of 2001 (early January to late May). Average daily throughfall reaching the soil during the exclusion period in 2000 was 4.9 and 8.3 mm in the treatment and control plots and was 4.8 and 8.1 mm in 2001, respectively. During the first exclusion period, surface soil water content (0-2 m) declined by � 100 mm, while deep soil water (2-11 m) was unaffected. During the second exclusion period, which began shortly after the dry season when soil water content was low, surface and deep soil water content declined by � 140 and 160 mm, respectively. Although this depletion of soil water provoked no detectable increase in leaf drought stress (i.e., no reduction in predawn leaf water potential), photosynthetic capacity declined for some species, the canopy thinned (greater canopy openness and lower leaf area index) during the second exclusion period, stem radial growth of trees <15 m tall declined, and fine litterfall declined in the treatment plot, as did tree fruiting. Aboveground net primary productivity (NPP) (stemwood increment and fine litter production) declined by one fourth, from 15.1 to 11.4 Mg ha � 1 yr � 1 , in the treatment plot and decreased slightly, from 11.9 to 11.5 Mg ha � 1 yr � 1 , in the control plot. Stem respiration varied seasonally and was correlated with stem radial growth but showed no treatment response. The fastest response to the throughfall exclusion, and the surface soil moisture deficits that it provoked, was found in the soil itself. The treatment reduced N2O emissions and increased CH4 consumption relative to the control plot, presumably in response to the improved soil aeration that is associated with soil drying. Our hypothesis that NO emissions would increase following exclusion was not supported. The conductivity and alkalinity of water percolating through the litter layer and through the mineral soil to a depth of 200 cm was higher in the treatment plot, perhaps because of the lower volume of water that was moving through these soil layers in this plot. Decomposition of the litter showed no difference between plots. In sum, the small soil water reductions provoked during the first 2 years of partial throughfall exclusion were sufficient to lower aboveground NPP, including the stemwood increment that determines the amount of carbon stored in the

Recuperation of nitrogen cycling in Amazonian forests following agricultural abandonment.

Phosphorus (P) is generally considered the most common limiting nutrient for productivity of mature tropical lowland forests growing on highly weathered soils. It is often assumed that P limitation also applies to young tropical forests, but nitrogen (N) losses during land-use change may alter the stoichiometric balance of nutrient cycling processes. In the Amazon basin, about 16% of the original forest area has been cleared, and about 30–50% of cleared land is estimated now to be in some stage of secondary forest succession following agricultural abandonment. Here we use forest age chronosequences to demonstrate that young successional forests growing after agricultural abandonment on highly weathered lowland tropical soils exhibit conservative N-cycling properties much like those of N-limited forests on younger soils in temperate latitudes. As secondary succession progresses, N-cycling properties recover and the dominance of a conservative P cycle typical of mature lowland tropical forests re-emerges. These successional shifts in N:P cycling ratios with forest age provide a mechanistic explanation for initially lower and then gradually increasing soil emissions of the greenhouse gas nitrous oxide (N2O). The patterns of N and P cycling during secondary forest succession, demonstrated here over decadal timescales, are similar to N- and P-cycling patterns during primary succession as soils age over thousands and millions of years, thus revealing that N availability in terrestrial ecosystems is ephemeral and can be disrupted by either natural or anthropogenic disturbances at several timescales.

Respostas de pupunheiras (Bactris gasipaes Kunth) jovens ao alagamento

The objective of this work was to study the effects of waterlogged soil on the stomatal conductance, the relative water content, the chlorophyll content and on the N, P, K and soluble sugar concentrations of leaf, bulb and root tissues of young pijuayo palms ( Bactris gasipaes Kunth). Six month old age plants were submitted to flooding by continuous periods of seven, 14 and 21 days. Flooding induced the closure of the stomata, although the leaf tissues have maintained high relative water contents (about 90%). Root anoxia also induced reduction of the contents of total chlorophyll, organic N, P and mainly K in leaf tissues and significant reduction of the root biomass. In the flooded plants, the soluble sugar contents of the leaves, bulb and roots were higher than in the same tissues of the control plants. Although no death of plants have been detected to the end of the experimental period, these metabolic alterations allow to affirm that this species ( Bactris gasipaes Kunth) is sensi- tive to the root flooding.

The Use of Carbon and Nitrogen Stable Isotopes to Track Effects of Land‐Use Changes in the Brazilian Amazon Region

Publisher Summary On an average, 18,000 km2 of Brazilian Amazon terra firme forest vegetation is burned every year. Most of this area is replaced by pastures cultivated to support cattle ranching. The replacement of tropical terra firme forests by cultivated pastures, has introduced different species of African C4 tropical grasses, mainly of the genus Brachiaria. This change opens up a unique opportunity for the use of carbon stable isotope composition to track the fate of introduced C4 grass species since the main matrix of plant species in the Amazon terra firme forests is composed by C3 plants. It has also been documented that tropical terra firme forests generally have an open nitrogen cycle, where losses of N are significantly higher in relation to the inputs of N. As a consequence of these high losses, soils and plants of tropical terra firme forests are characterized by distinctly high values of δ15N. As N availability increases, losses from the ecosystem, such as gaseous N losses and leaching of NO3– as a result of incomplete nitrification, should increase and lead to 15N-enriched plants. Such a fact opens up a second window of opportunity for using the N stable isotopic composition as a tool to investigate changes in the N cycle due to land-use changes that are occurring in the Amazon basin. The stable C and N isotopic composition is used to evaluate changes in ecosystem functioning due to land-use changes that have been occurring in the Amazon region. The chapter discusses isotopic shifts from individual components of the ecosystems, like plants, and shows how the signals of the C4 vegetation have already been incorporated in other reservoirs of C, such as soils, rivers, and atmosphere.