Relação entre diversidade arbórea e aspectos do ciclo biogeoquímico de uma floresta monodominante de Brosimum Rubescens Taub. e uma floresta mista no Leste Mato-Grossense / Relação entre diversidade arbórea e aspectos do ciclo biogeoquímico de uma floresta monodominante de Brosimum Rubescens Taub. e uma floresta mista no Leste Mato-Grossense

AUTOR(ES)
DATA DE PUBLICAÇÃO

2007

RESUMO

Tropical monodominant forests in which one species represents more than 50% of the above ground biomass are a rare vegetation type. One of these forests is found in the eastern region of the State of Mato Grosso where the tree species Brosimum rubescens Taub. (Moraceae), accounts for more than 80% of the above ground biomass of the forest community. This situation presents the possibility of testing the influence of high dominance and low species diversity on key functions of the ecosystem, such as nutrient cycling. A currently debated topic is the role of biodiversity in functional aspects of the ecosystem, including biogeochemical cycling and the consequences of alterations. To verify if the high dominance of Brosimum rubescens affected the biogeochemical cycle, several aspects of nutrient cycling were investigated and compared between the monodominant forest (MB) (145047 S e 520837 W) and an adjacent mixed species forest (MM) (144932 S e 520620 W), with low dominance of Brosimum rubescens and high tree species diversity. The following hypotheses were raised: B. rubescens influenced the rates of litter production and decomposition and nutrient liberation, with the following consequences, unfavorable Mg/Ca ratio, liberation of Mn in possibly toxic concentration and low concentration of macronutrients as a consequence of a higher nutrient use efficiency (NUE). The following data were collected: a) litter production (six fractions and total) and nutrient demand (N, P, K, Ca, Mg e Mn) during two years; b) translocation rate and NUE for the six commonest tree species that occurred in both forests and the total NUE of the community in both areas and during in dry and wet seasons; c) decomposition rate of leaf litter and liberation of nutrients (macronutrients and Mn) through the year in mixed leaf samples (FM) and leaves of Brosimum (FM); d) biochemical aspects (macronutrients and Mn) and physical aspects (density, humidity and thickness) of the litter layer in dry and wet seasons and e) water dynamics in the soil over one year and hydraulic properties of the soil in the laboratory. Differences in litter production between forests, concentration and nutrient demand were verified in several fractions but not for the total litterfall pattern of each year, two year average (7,41 t.ha-1 for MB and 7,94 t.ha-1 for MM) and total annual amount of nutrients returned, with the exception of Ca that reflects the higher levels of this nutrient in the soil in the MM forest. The total annual nutrient returned by litterfall (demand) was 201.45 kg.ha-1 (MB) and 245,54 kg.ha-1 (MM, including Ca). A similar pattern was found for retranslocation and NUE with various differences among xx species in the two forests for green and senescent leaves, but not in relation to the total NUE in both areas. This pattern was repeated in the decomposition rates, that were not different between areas in the year following the start of the experiment. The mixed leaves decomposition constant (ke) was 0,99 (MB) and 0,98 (MM). The monodominant species did not present evidence of interference in the physical chemical conditions of the litter layer in relation to the adjacent MM. The most important differences between the areas were in the retention of Ca and P in the initial stages of mixed leaf litter decomposition in the MB followed by a strong release of these elements (58,3% and 62,1% of the initial amounts). The same trend was observed for Mn, consistent with the hypothesis of liberation of large amounts of this element in possibly toxic concentrations. The dominance of Brosimum did not seem to be related to soil water conditions, since the water retention curve in the soil had the same pattern in both forests. To explain why the high dominance of Brosimum had so low an influence on the biogeochemical cycle the occurrence of a combination of causal factors of functional groups was suggested resulting in a functional arrangement in the MB similar to that of the MM, a process called a minimum fit, probably a rare combination in nature.

ASSUNTO(S)

funções do ecossistema produção de serapilheira ecosystem functions litter production biodiversidade biodiversity efeito não aditivo decomposição decomposition cerrado cerrado nutrientes amazônia nutrients amazonia ecologia toxidade de mn mn toxicity

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