Ecology and management of environmental weeds in south-eastern Queensland

While the majority of naturalised exotic species have a relatively minor impact upon ecosystems, some introduced species become the most conspicuous elements of the communities that they invade. These are sometimes known as "engineering" species, owing to their fundamental impacts upon ecosystem structures and processes. It is important to learn how to slow their invasion, as well as how to rehabilitate communities in various stages of invasion, particularly for remnants that are of high conservation value.

Environmental weeds come in many forms, e.g. trees, shrubs, vines, aquatic plants, grasses and other non-woody plants. Chinese celtis (Celtis sinensis), privets (Ligustrum spp.), camphor laurel (Cinnamomum camphora) and broadleaved pepper (Schinus terebinthifolius) are some of the long-lived tall shrubs or trees that invade and eventually dominate riparian and other ecosystems in south-eastern Queensland, displacing native plant species and dependent native animals. All of these exotic invaders, largely dispersed by generalist native frugivores, were originally introduced as ornamentals. This is a pattern that has been repeated for many other invasive species.

This project investigates two ecological aspects that are central to the rehabilitation of sites that are dominated by environmental weeds, namely the dynamics of weed seed banks and the potential for reinvasion of sites where these weeds have been removed and replaced by indigenous native species.

Methods
Seed bank dynamics are investigated by sowing freshly harvested seeds and fruits, either on the soil surface or with shallow burial, in pots sunk into the ground within randomised block designs at the Alan Fletcher Research Station. These designs are replicated in a tunnel, where pots are watered several times daily. Seedling emergence, under both natural rainfall and artificial watering is monitored, as is the viability status of ungerminated seeds.

Reinvasion potential is being assessed by measuring levels of weed seed immigration at rehabilitated sites, in conjunction with monitoring the fates of weed seedlings that appear in plots of various native species, planted following weed control in sites undergoing rehabilitation.

Research results
Chinese celtis (Celtis sinensis)
Seeds and fruits of Chinese celtis were collected from 128 seed traps at a revegetation site at fortnightly intervals between 22 January and 2 April 1997. Peak numbers, averaging approximately 14 viable seeds/m², were recorded for the interval between 22 January and 5 February. Over the entire season, seed fall was much greater where traps were situated below tree canopies (total seed fall of 60.4 viable seeds/m²) than where traps were situated in the open (total seed fall of 0.81 viable seeds/m²). Fruit bats appear to be the major vectors for seed dispersal in this species.

During 1998, seeds and fruits were collected between 22 January and 6 May. Peak numbers were recorded during early January (as compared with late January during the previous year). The latter year’s study showed that seed fall is concentrated under the canopies of native trees, in an annular (ring-like) zone from 20-30% of the distance between the tree trunk and the limits of the tree canopy. This indicates that mulching could be applied more efficiently to a limited area underneath tree canopies, as a means of reducing the rate of invasion by Chinese celtis.

Chinese celtis seedling emergence commenced in the latter half of March 1997. Altogether, 11 cohorts were identified between late March and the middle of October 1997. No further newly emerged seedlings were observed after the latter date. Seedlings survived for much shorter periods in plots that had been mulched with sugar cane trash. The effects of mulching upon seedling growth changed as the season progressed; growth was reduced for cohorts that emerged in mulched plots during the autumn months, but was enhanced, relative to unmulched plots, for cohorts that emerged from late July onwards. It was not possible to determine whether the inhibitory effects of mulching upon early seedling survival lasted into the next year of the study, owing to severe and recurrent vandalism.

Chinese celtis seeds are not long-lived, with a maximum of approximately 10% alive after 12 months in or on the soil, and no seeds alive at 24 months.

Privets (Ligustrum lucidum and L. sinense)
Seeds of both broadleaved and small-leaved privets appear to be relatively short-lived. In the first trial, there was a flush of emergence in the first few months (winter and spring) following sowing, with no further seedlings emerging after 7 months for broadleaved privet and 5 months for small-leaved privet. Emergence for both species in the field plots appeared to be dependent upon shallow burial (10 mm). Viability retention was considerably higher for broadleaved privet. In the second trial, emergence of both species was neither as fast nor as synchronous as in the earlier trial.

Experimental work carried out with privet seeds and fruits under well-watered conditions in a tunnel has shown that germination will occur from intact fruits, albeit delayed in comparison to germination of bare seeds. This is contrary to the results from germination experiments published in the literature.

Camphor laurel (Cinnamomum camphora)
The 1995 experiment examining camphor laurel seedling emergence and seed longevity has been repeated. This 1998 experiment has given substantially different results. After 20 weeks, seedlings have emerged from 56% of the buried seed in the field plots (cf 10% after 52 weeks in the first experiment). It is thought that the immediate removal of the fleshy layer of the fruit in this instance has made a difference; seeds were extracted from entire fruits some months following harvest in the first experiment.

Surface-sown camphor laurel seeds kept constantly moist in a tunnel house have germinated to very high levels, e.g. 92% germination within 20 weeks. Interestingly, fewer buried seeds germinated (30%). This is in marked contrast to the field plot experiment (see above), where none of the surface sown seeds germinated over the same period. It is clear that germination in this species is highly dependent upon extended periods of moisture availability.

Cat’s claw creeper (Macfadyena unguis-cati)
A preliminary study of the germination of cat’s claw creeper seeds showed very low levels of dormancy (between 5-15%) in freshly shed seed. This is consistent with observations indicating that cat’s claw creeper seeds are relatively short-lived, not surviving for 12 months in the field.

A surprising finding was that about 20% of the seeds produced multiple seedlings (sometimes as many as three). This is strong circumstantial evidence that cat’s claw creeper is apomictic, i.e. capable of producing seeds asexually, in addition to the more usual sexual process. Such a strategy enables a weed to provide exact replicates of well-adapted gene combinations, as well as to "experiment" through recombination.

Publications
Panetta, F.D. & McKee, J. (1997) Recruitment of the invasive ornamental, Schinus terebinthifolius, is dependent on frugivores. Aust. J. Ecol. 22, 432-438.

Generalist avian frugivores often play an important role in the processes of naturalization and invasion for plants introduced for ornamental and landscaping purposes. We investigated the potential role of Australian birds in the current invasion of riparian habitats and coastal wetlands by the ornamental Schinus terebinthifolius Raddi. Feeding trials in captivity accurately predicted silvereyes (Zosterops lateralis Latham) as dispersal vectors for this weed. There were no differences in either total germination or germination rate between seeds voided by caged silvereyes and those from which exocarps had been removed manually. Germination of seeds incubated within entire fruits was minimal, as was emergence from intact fruits in a field experiment. Seed banks of S. terebinthifolius were relatively transient under field conditions, with no seeds surviving for 9 months. Since fewer than 5% of the seeds in sown whole fruits gave rise to seedlings (cf. 20-42% for sown bare seeds), we conclude that recruitment potential of S. terebinthifolius is highly dependent upon the consumption of its fruits by frugivores.

Email address
dane.panetta@dnr.qld.gov.au