Abstract
Allelopathic potential, which is characterized by harmful or beneficial effects on a species, is a hypothesis that could explain the success of a vegetable species in process of invasion. For that reason, this study aims to investigate the allelopathic effect of the fresh leaf extract from Azadirachta indica on the germination and early growth of native species (Mimosa caesalpiniifolia, Pityrocarpa moniliformis, Astronium urundeuva, Cenostigma pyramidale, and Libidibia ferrea) in the seasonally dry tropical forest (caatinga). The raw extract was prepared with 200g of fresh leaves in 1L of distilled water, diluted in six treatments. The experiment followed a completely randomized design (CRD), with four repetitions of 25 seeds, and paper towel moistened with extract as substrate. The number of germinated seeds was counted daily. At the end of the experiment, radicle length (RL), germination percentage (GP), germination speed index (GSI), and average germination time (AGT) were evaluated. P. stipulacea’s GP and GSI averages and A. urundeuva’s AGT decreased as extract concentration increased. As far as RL is concerned, in intermediate concentrations the extract hindered P. moniliformis and L. ferrea’s growth but favored C. pyramidale and A. urundeuva’s. Results allow us to infer that A. indica has an allelopathic effect on native species and might interfere with their rooting in the field, which in turn would affect the succession and balance of the forest over time, since the community structure and the ecosystemic processes would be impacted.
Highlights
Impacts caused by invasive exotic species are based on the disturbance or alteration of environmental variables involving mostly biodiversity loss and changes in the provision of some environmental services, especially those developed by native species [1, 2, 3, 4]
In the case of M. caesalpiniifolia there were not enough seeds to calculate weight of a thousand seeds (WTS), 7.23% is its only figure, which corresponds to moisture content (MC)
The negative effect observed for P. moniliformis and L. ferrea confirms the results found in other experiments which used extracts of exotic species, A. indica inclusive
Summary
Impacts caused by invasive exotic species are based on the disturbance or alteration of environmental variables involving mostly biodiversity loss and changes in the provision of some environmental services, especially those developed by native species [1, 2, 3, 4]. There are different hypotheses to explain the success of a plant species in an invasion process [5], such as propagule pressure [6], lack of enemies or predators [7, 8], evolution of increased competitive ability [9], and allelopathic advantage against native species [10, 11] The latter hypothesis proposes that some exotic species develop or already have mechanisms for releasing unprecedented biochemicals – "unique weapons" – that are able to inhibit the germination and growth of neighboring native plants, which in turn impair essential processes for maintaining the natural community diversity [10, 12, 13, 14]. Juss (popularly known as nim indiano), is considered invasive by several authors [15, 16, 17], but there is still a lot be studied in order to measure the potential and impacts of this species invasion in native plant communities [18]
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