Germinable soil seedbanks of central Queensland rangelands invaded by the exotic weed Parthenium hysterophorus L.

The germinable soil seedbank was determined at two sites in central Queensland on four separate occasions between February 1995 and October 1996. These sites were infested with parthenium weed (Parthenium hysterophorus L.), a serious invasive exotic weed. During this period, the seedbank varied between 3282 and 5094 seeds m at the Clermont site, and between 20 599 and 44 639 seeds m at the Moolayember Creek site. Parthenium hysterophorus exhibited a very abundant and persistent seedbank, accounting for 47-73% of the seedbank at Clermont and 65-87% of the seedbank at Moolayember Creek. The species richness and species diversity of the seedbank, and the seed abundance of many species, was lower at Moolayember Creek during spring (the time of year when the most dense infestations of the weed originate). Parthenium hysterophorus seedlings also emerged more rapidly from the soil samples than did those of all other species. Hence, it seems that various aspects of the weed's seed ecology, including abundance and the persistence of its seedbank and the rapid emergence of its seedlings, are major factors contributing to its aggressiveness in semiarid rangeland communities in central Queensland. The domination by P. hysterophorus of the seedbanks of these sites suggests that the weed is having a substantial negative impact on the ecology of these plant communities. The diversity of these seedbanks was found to be lower in comparison with that observed in other grassland communities that were not dominated by an invasive weed species. Hence, the prolonged presence of P. hysterophorus may have substantially reduced the diversity of these seedbanks, thereby reducing the ability of some of the native species to regenerate in the future

Nutrientes foliares de espécies arbóreas de três estádios sucessionais de floresta ombrófila densa no sul do Brasil Leaf nutrient content of tree species from three successional stages of tropical rain forest in south Brazil

As concentrações de macro e micronutrientes foliares de espécies arbóreas foram avaliadas em três estádios sucessionais (inicial, intermediário e avançado) de uma floresta ombrófila densa das terras baixas, no litoral do Estado do Paraná, crescendo sobre solo arenoso e oligotrófico. Dentre as espécies arbóreas mais importantes de cada estádio sucessional (quatro iniciais; oito intermediárias; 11 avançadas), coletou-se folhas maduras de cinco diferentes indivíduos de cada estádio sucessional para a análise química dos seguintes elementos: N, P, K, Ca, Mg, C, Zn, Fe, Cu e Mn. As espécies do estádio inicial possuem as menores concentrações médias foliares de N, P, K, Zn, Fe e a maior concentração de Mg e estas concentrações são estatisticamente diferentes (p <0,05), quando comparadas com os valores médios dos demais estádios. Nos estádios intermediário e avançado, as concentrações médias de N, Ca e Mg, assim como os micronutrientes estão dentro dos valores esperados para florestas tropicais. As concentrações médias de K, nos estádios inicial e intermediário, e as concentrações médias de P, em todos os estádios, encontram-se abaixo dos valores reportados para as florestas tropicais. A relação C/N é alta (>25:1), apesar da tendência de diminuir à medida que avança a sucessão. O aumento da concentração da maioria dos nutrientes foliares analisados ao longo do gradiente sucessional está associado, possivelmente, com o incremento do acúmulo da serapilheira sobre o solo, o que proporciona ciclagem de nutrientes mais intensa e maiores concentrações de nutrientes no solo.<br>We investigated the leaf nutrient content of trees species from three distinct successional stages (early, intermediate, late) of a lowland rain forest located near the coast of Paraná State, Brazil. This forest grows on sandy soil with low nutrient content. We collected mature leaves from five different individuals of each sucessional stages of the most important tree species of each successional stage (four of the early; eight of the intermediate and 11 of the late stages) for chemical analysis of following elements: N, P, K, Ca, Mg, C, Zn, Fe, Cu e Mn. The species from early sucessional stage have lower contents of N, P, K, Zn, Fe, and higher contents of Mg. These concentrations are statistically different (p <0,05) when compared to mean values of the other stages. For the intermediate and late successional stages, the average nutrient concentrations of N, Ca, Mg and micronutrients are within the range of values expected for tropical forests. Foliar K concentration, from early and intermediate successional stages, and foliar P concentration, for all successional stages, are below the reported range of values for tropical forests. The C/N ratio is high (>25:1), despite of the tendency to decrease as succession advances. The increment of concentration of the most studied foliar nutrients along the successional gradient is probably related to the accumulation of litter above the soil that provides a more adequate environment for efficient nutrient cycling and higher nutrient concentration in the soil