Allelopathic effects of aqueous extract of leaves of Parthenium hysterophorus were studied on seed germination and seedling growth of three cereal crops (Oryza sativa L., Zea mays L. and Triticum aestivum L.), three cultivated crucifers (Raphanus sativus L., Brassica campestris L. and Brassica oleracea L.) and two wild species of family Asteraceae (Artemisia dubia Wall ex. Besser and Ageratina adenophora (Spreng) King and HE Robins). Seed germination of all crucifer species was completely inhibited at >2% leaf extract of Parthenium hysterophorus but in other species, except maize, complete failure of seed germination was recorded only at >6% in Triticum aestivum and Ageratina adenophora; at 10% in Oryza sativa and Artemisia dubia. Seed germination of Zea mays was not completely inhibited but it was low at high concentration of the extract. The extract had strong inhibitory effect to root elongation of seedling in cereals and to shoot elongation in crucifers and wild Asteraceae. Leaves of Parthenium hysterophorus may be a source of natural weedicide against Ageratina adenophora which will help to control invasive plants.

Plant morphological and physiological traits affect the way plants tolerate environmental stresses and therefore play an important role in shaping species distribution patterns in relation to environmental gradients. Despite our growing knowledge on the role of functional traits in structuring plant communities, few studies have tested their importance at large scales in the wet tropics. Here, we describe the distribution patterns of the most important West African rain forest timber species along the regional rainfall gradient and relate them to their functional traits. We found that the distribution patterns of 25 out of the 31 studied species (80%) were significantly related to mean annual rainfall. Shade tolerance and drought resistance were identified as the main strategy axes of variation. Wood density and leaf deciduousness emerged as the best predictor traits of species position along the rainfall gradient, explaining respectively 32 and 15 percent of the variation. Species traits tended to show stronger relationships with estimated optimum annual rainfall for each species than to the extreme rainfall conditions where they occur. The significant role of rainfall in shaping timber species distribution and the strong relationships between species traits and rainfall indicate that changes in climate, especially declining rainfall, could have strong effects on species composition and abundance in these tropical forests.

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