The vertical profile in leaf photosynthetic capacity was investigated in a terra firme rain forest in central Amazonia. Measurements of photosynthesis were made on leaves at five levels in the canopy, and a model was fitted to describe photosynthetic capacity for each level. In addition, vertical profiles of photosynthetic photon flux density, leaf nitrogen concentration and specific leaf area were measured. The derived parameters for maximum rate of electron transport (J(max)) and maximum rate of carboxylation by Rubisco (V(cmax)) increased significantly with canopy height (P < 0.05). The highest J(max) for a single canopy level was measured at the penultimate canopy level (20 m) and was 103.9 &mgr;mol m(-2) s(-1) +/- 24.2 (SE). The highest V(cmax) per canopy height was recorded at the top canopy level (24 m) and was 42.8 +/- 5.9 &mgr;mol m(-2) s(-1). Values of J(max) and V(cmax) at ground level were 35.8 +/- 3.3 and 20.5 +/- 1.3 &mgr;mol m(-2) s(-1), espectively. The increase in photosynthetic capacity with increasing canopy height was strongly correlated with leaf nitrogen concentration when examined on a leaf area basis, but was only weakly correlated on a mass basis. The correlation on an area basis can be largely explained by the concomitant decrease in specific leaf area with increasing height. Apparent daytime leaf respiration, on an area basis, also increased significantly with canopy height (P < 0.05). We conclude that canopy photosynthetic capacity can be represented as an average vertical profile, perturbations of which may be explained by variations in the environmental variables driving photosynthesis.

Abstract. Over the past three decades, large expanses of forest in the Amazon Basin were converted to pasture, many of which later degraded to woody fallows and were abandoned. While the majority of tropical secondary forest (SF) studies have examined post-deforestation or post-agricultural succession, we examined post-pasture forest recovery in 10 forests ranging in age from 0 to 14 years since abandonment. We measured aboveground biomass and soil nutrients to 45 cm depth and computed total site carbon (C) and nutrient stocks to gain an understanding of the dynamics of nutrient and C buildup in regenerating SF in central Amazonia.Aboveground biomass accrual was rapid, 11.0 Mg·ha Ϫ1 ·yr Ϫ1, in the young SFs. Within 12-14 yr, they accumulated up to 128.1 Mg/ha of dry aboveground biomass, equivalent to 25-50% of primary forest biomass in the region. Wood nitrogen (N) and phosphorus (P) concentrations decreased with forest age. Aboveground P and calcium (Ca) stocks accumulated at a rate of 1.2 and 29.4 kg·ha Ϫ1 ·yr Ϫ1 ; extractable soil P stocks declined as forest age increased. Although soil stocks of exchangeable Ca (207.0 Ϯ 23.7 kg/ha) and extractable P (8.3 Ϯ 1.5 kg/ha) were low in the first 45 cm, both were rapidly translocated from soil to plant pools. Soil N stocks increased with forest age, probably due to N fixation, atmospheric deposition, and/or subsoil mining.Total soil C storage to 45 cm depth ranged between 42 and 84 Mg/ha, with the first 15 cm storing 40-45% of the total. Total C accrual (7.04 Mg C·ha Ϫ1 ·yr Ϫ1 ) in both aboveground and soil pools was similar or higher than values reported in other studies. Tropical SFs regrowing on lightly to moderately used pasture rapidly sequester C and rebuild total nutrient capital following pasture abandonment. Translocation of some nutrients from deep soil (Ͼ45 cm depth) may be important to sustaining productivity and continuing biomass accumulation in these forests. The soil pool represents the greatest potential for long-term C gains; however, soil nutrient deficits may limit future productivity.

No abstract

ABSTRACT:The area of secondary forest (SF) regenerating from pastures is increasing in the Amazon basin; however, the return of forest and canopy structure following abandonment is not well understood. This study examined the development of leaf area index (LAI), canopy cover, aboveground biomass, stem density, diameter at breast height (DBH), and basal area (BA) by growth form and diameter class for 10 SFs regenerating from abandoned pastures. Biomass accrual was tree dominated, constituting Ն94% of the total measured biomass in all forests abandoned Ն4 to 6 yr. Vine biomass increased with forest age, but its relative contribution to total biomass decreased with time. The forests were dominated by the tree Vismia spp. (>50%). Tree stem density peaked after 6 to 8 yr (10 320 stems per hectare) before declining by 42% in the 12-to 14-yr-old SFs. Small-diameter tree stems in the 1-5-cm size class composed >58% of the total stems for all forests. After 12 to 14 yr, there was no significant leaf area below 150-cm height. Leaf area return (LAI ‫ס‬ 3.2 after 12 to 14 yr) relative to biomass was slower than literature-reported recovery following slash-and-burn, where LAI can reach primary forest levels (LAI ‫ס‬ 4-6) in 5 yr. After 12 to 14 yr, the colonizing vegetation returned some components of forest structure to values reported for primary forest. Basal area and LAI were 50%-60%, canopy cover and stem density were nearly 100%, and the rapid tree-dominated biomass accrual was 25%-50% of values reported for primary forest. Biomass accumulation may reach an asymptote earlier than expected because of even-aged, monospecific, untiered stand structure. The very slow leaf area accumulation relative to biomass and to reported values for recovery following slash-and-burn indicates a different canopy development pathway that warrants further investigation of causes (e.g., nutrient limitations, competition) and effects on processes such as evapotranspiration and soil water uptake, which would influence long-term recovery rates and have regional implications.

Este estudo avaliou o desempenho da castanha-do-brasil (Bertholletia excelsa) em sistemas agroflorestais implantados em ecossistema de terra firme na Amazônia Central. Foram avaliados 3 sítios de sistemas agroflorestais multi-estratificados, implantados em 1992, em áreas de pastagens degradadas, situadas no km 54 da BR-174, no Campo Experimental da Embrapa Amazônia Ocidental, em Manaus (AM). Os sistemas foram implantados após o processo tradicional de derruba e queima da vegetação secundária estabelecida em pastagens submetidas por 6 anos ao pastejo intensivo e abandonadas por 4 anos, em média, ao processo de regeneração natural. O desempenho da espécie com 12 anos de idade foi avaliado por meio do diâmetro à altura do peito (DAP), da altura total, da taxa de sobrevivência e das variáveis morfométricas "Diâmetro da Copa", "Proporção de Copa", "Grau de Esbeltez", "índice de Saliência", "índice de Abrangência" e "Forma de Copa". Os indivíduos atingiram altura total média de 20,9 m e DAP de 37,9 cm, com incremento médio anual de 1,74 m e 3,16cm, respectivamente. A porcentagem média de sobrevivência foi de 78%, cuja mortalidade foi relacionada às ventanias e raios. Os resultados indicaram a eficiência dessa espécie para reabilitar áreas degradadas e confirmaram-na como uma espécie adequada para formar sistemas agroflorestais.