Active restoration possibilities for invasive tree occupied sites: Examples from Melaleuca quinquenervia systems in Florida, USA

Background/Question/Methods

 Thick litter accumulations on forest floors suppress seedling emergence, as has been observed in stands of the invasive tree Melaleuca quinquenervia (melaleuca) in southern Florida, USA. Removal of melaleuca litter enhances seedling emergence in gaps of dense stands. The length of survival of melaleuca seedlings as affected by soil types in the presence of natural enemies is unknown. We hypothesized that where litter is removed melaleuca seedlings will temporarily predominate but will gradually be replaced by other natives when natural enemies are present. We examined these assumptions in mature melaleuca tree stands by conducting sequential experiments in predominantly organic and arenaceous soils. Litter was removed from plots and seeded with a mixture of 200 seeds each of melaleuca and two native plants, Morella cerifera (wax myrtle) and Cladium jamaicense (sawgrass).Experiments were replicated four times in both soil types and repeated four times at 12-wk intervals to determine the effects of litter removal and seeding time. Emergence and survival of seedlings were recorded for 96-wks after which natural enemies were excluded from 50% of the plots on organic soils using insecticide and fungicide. Seedling densities for all species, and height, diameters and canopy coverage of woody perennials were measured for additional 88-wk period.

 Results/Conclusions

 Results showed best seedling recruitment in both soils when litter was removed and plots were seeded during July-November when soils were wet.  Melaleuca was dominant in both soil types during the first 12-wk evaluation period. Melaleuca density decreased whereas other monocots transiently increased in both soil types; other dicots predominated (density-wise) at 96-wk evaluation. There were no changes in sawgrass and wax myrtle densities over the 96-wk, an indication that seeds of these species are rare in the soil seedbank. Seed additions of these two and other native plants will be needed in order to actively restore melaleuca vacated sites. When the plots were protected for additional 88-wk by excluding natural enemies, melaleuca and the native salt-bush (Baccharis spp.) dominated in terms of canopy-coverage, height, and diameter. Suppression of these dominant plant species by natural enemies enhanced the diversity of other native plants. These findings reveal that invasive plant dominated sites with thick litter layers can be actively rehabilitated by combining litter removal process with seeding of desirable native plant species of choice. Furthermore, natural enemies appear to play a positive role by suppressing invasive plants and thereby enhancing native plant diversity.