Vegetation exclusion underCasuarina equisetifoliaL: Does allelopathy play a role?

A study conducted to determine the ecological status of vegetation under aCasuarina equisetifoliaL. plantation revealed that the number of species types, density and biomass of the understorey were drastically reduced compared to an adjoining grassland area. In general, vegetation underCasuarinawas characterised by the presence of a few dominants with a trend towards homogeneity. On the other hand, the vegetation in the grassland was heterogeneous, with conspicuous spatial pattern and, thus, more stable. Indices of richness (indicating numerical strength), evenness (representing spatial distribution), and diversity (combining both richness and evenness) also indicated a marked difference in the vegetation between the two sites. The Shannon index, indices of richness and evenness, and Hill's diversity measures were greater in grassland areas than inCasuarinaplantation, thereby signifying a richer, more diverse and even vegetation in the grassland. In contrast, the Simpson index of dominance which shows an inverse relation with diversity, was greater under theCasuarinaplantation. The reasons for the restrained vegetation underCasuarinawere explored in terms of allelopathic interference of various tree parts (fresh as well as fallen) as one of the major factor. The leaf leachates collected under the canopy ofCasuarinatrees in the plantation were found to have deleterious effect on the growth ofMedicago sativaandAgeratum conyzoidesand were rich in phenolics. The extracts prepared from different tree parts such as needles, female cones and litter adversely affected the growth and dry weight accumulation inM. sativaandA. conyzoides. An appreciable amount of water-soluble phenolics, known phytotoxins, was estimated in the fog leachates and different plant parts under use. It is concluded that phenolics released from the green needles and litter of the tree adversely affect the understorey vegetation.