Allelopathic potential of rain leachates from Eucalyptus urophylla on four tree species

Allelopathy is considered a major cause of biodiversity reduction in Eucalyptus plantations. Eucalyptus can produce allelochemicals through various ways, such as volatilization and root exudation. However, there are few studies of the roles of rain leachates in mediating eucalyptus allelopathy. Thus, we conducted a greenhouse experiment to assess the allelopathic effects of rain leachates from Eucalyptus urophylla on some vital traits of three native (Leucaena leucocephala, Pterospermum lanceaefolium, and Schefflera octophylla) and one introduced (Albizia lebbeck) tree species. We compared the effects of rain leachates from E. urophylla foliage, litter, and foliage+litter. To identify specific chemicals from rain leachates that can mediate allelopathy, we used gas chromatography coupled with mass spectrometry (GC-MS) to isolate and determine allelochemicals from leachates. The results showed that litter leachates and foliage+litter leachates significantly inhibited seed germination of L. leucocephala, P. lanceaefolium, and S. octophylla. The inhibitory effects on seed germination of litter leachates alone were relatively stronger than that of foliage+litter leachates; however, foliage leachates alone did not inhibit seed germination and seedling survival, except for P. lanceaefolium. We also did not find significant negative effects of rain leachates on biomass, root length, and shoot length of the target species; foliage+litter and foliage leachates even increased fresh weight and dry weight of L. leucocephala and S. octophylla, respectively. Seedling survivorship was lower under the leachate treatments for all the target species except A. lebbeck. GC-MS analysis suggested that phenol acids, ester, alkanes, and alcohol are the main components in leachates. The diversity and content of allelochemical components detected in litter leachates and foliage+litter leachates were much higher than foliage leachates. We concluded that rain leachates from E. urophylla primarily cast a negative impact on seed germination and seedling survival of the receptor species, and allelopathy of E urophylla may be partly responsible for the loss of understory species. Different sources of rain leachates vary significantly in their inhibitory effects, and rain leachates from litter are likely to be an important pathway of allelopathy.