To assess the potential invasiveness of common eucalyptus species planted for pulpwood, mulch wood or bioenergy crops, field surveys of eucalyptus seedling recruitment were conducted in north and central Florida locations within seed bearing eucalyptus stands and in the proximate plant communities where seed dispersal may occur. Plant communities included non-grazed pasture, intensively site-prepared forestland, abandoned forest road and upland mixed pine- hardwood forest. No eucalyptus seedlings were found in any of the 310 1-m2 survey plots across the two locations. Second, seed addition studies were conducted to determine the relative potential for seedling emergence and survival among Eucalyptus amplifolia, E. camaldulensis and E. grandis added into plots at two seed densities, under disturbed and nondisturbed conditions, in the understory of the eucalyptus stands and in each of the aforementioned proximate plant communities. Overall, the probability of emergence of added seed was very low (P = 0.0 to 0.0032), and seed density effects were not significant. Emergence was significantly greater in disturbed conditions compared to nondisturbed conditions for seedlings originating from natural seed rain from the eucalyptus canopy in central Florida. The amount of time that seedlings survived was greater for E. camaldulensis compared to the other species but no seedlings survived more than 13 wk. These data indicate that under specific favorable conditions, eucalyptus seedlings may establish within or proximate to planted stands, but the overall level of invasiveness demonstrated by E. amplifolia and E. grandis is low for north or central Florida. The demonstrated role of disturbance in facilitating eucalyptus seedling recruitment suggests that a stable perennial plant community (native grasses) should be established instead of bare soil buffer zones to mitigate spread.
Nomenclature: Cabbage gum (Eucalyptus amplifolia Naudin); yellow mallee (Eucalyptus incrassata Labill.); river red gum (Eucalyptus camaldulensis Dehnh.); rose gum [Eucalyptus grandis W. Hill ex Maiden].
Management Implications: Because of its potential for fast growth and high fiber yields, eucalyptus plantations are being established in the southeastern US Coastal Plain as a source of pulpwood, mulch wood and possibly biofuel. Two quantitative study approaches used within seed bearing Eucalyptus amplifolia stands and their proximate native and modified plant communities in north and central Florida found low invasive potential overall. Surveys at the two locations found no eucalyptus recruitment, suggesting limited success for seedling establishment. The subsequent study evaluated the invasive potential of three commercially important eucalyptus species by adding seed at two densities to disturbed (forest litter or vegetation removed to expose mineral soil) and nondisturbed sites. The combined research findings demonstrated that eucalyptus seedling establishment and survival were generally low at the northern and central Florida locations, although greater numbers of seedlings were observed in disturbed conditions, within the eucalyptus stands and for E. camaldulensis as compared to E. grandis or E. amplifolia. Seed addition at 500 or 1000 expected germinants per m2 did result in seedling establishment, but no seedlings survived more than 13 wk. Whereas these results do not support previous weed risk assessment conclusions of high invasion risk for E. camaldulensis, they suggest that caution is warranted regarding the cultivation of E. camaldulensis, especially concerning practices that might increase soil disturbance near eucalyptus stands. The demonstrated role of disturbance in facilitating eucalyptus seedling recruitment suggests that perennial
