Individual study: The effect of glyphosate application on the growth of a rare endemic shrub, Pimelea spicata, two common native plants and two invasives under glasshouse conditions, New South Wales, Australia
Matarczyk J.A., Willis A.J., Vranjic J.A. & Ash J.E. (2002) Herbicides, weeds and endangered species: management of bitou bush (Chrysanthemoides monilifera ssp. rotundata). Biological Conservation, 108, 133-141
In Australia, weed invasion has caused the extinction of several endemic plant species and threatens others. Rare plants with small population and range sizes, and which are competitively inferior to invasives, are at particular risk. For example, Pimelea spicata is an endangered endemic shrub threatened by habitat loss and invasion by weeds, including the bitou bush Chrysanthemoides monilifera ssp. rotundata which infests about 70,000 ha of the New South Wales coastline. Bitou was first recorded in New South Wales around 1908, probably introduced through dumping of ships' ballast. From 1946 to 1968 it was planted along the coast to revegetate areas after mining. It is now regarded as one of the worst weeds in Australia because of its invasiveness and associated detrimental economic and environmental impacts.
The winter application of the herbicide glyphosate is used to control bitou, whilst other native plants such as sclerophyllous shrubs e.g. Acacia (wattles), Banksia (banksias) and Leptospermum (tea-trees), and large Lomandra (mat-rush) tussocks are resilient. However, no attempts at bitou control by glyphosphate application had been undertaken in areas with Pimelea as the effects of glyphosate on this shrub were poorly known. Observations at sites where wind had caused glyphosphate to drift over areas where Pimelea was present suggested that it induced reduced growth and productivity. This study investigates the effect of glyphosate application and two different watering regimes on P.spicata, two native plant species and of two weed species under glasshouse conditions.
The experiment, undertaken in a glasshouse, used a fully-factorial block design, with three factors:
1) Species – seedlings of five co-occurring species were used: the endangered endemic shrub Pimelea spicata; the native kangaroo grass Themeda triandra; Kikuyu grass Pennisetum clandestinum (native to East Africa); the invasive bitou C.m.rotundata; and the coastal wattle Acacia sophorae (commom shrub of coastal dunes growing to 1-4 m high);
2) Watering regime – high water or low water;
3) Herbicide – glyphosate or untreated control.
There were six blocks, each containing 20 plants (5 x species, 2 x watering, 2 x herbicide).
Seedlings: Experimental plants were germinated from seed sown onto potting mixture. P.spicata seeds were collected from plants grown outdoors at the Australian National Botanic Gardens, Canberra. The parental stock of these plants originated from the Cumberland Plain and the Illawarra coast, the seeds were pooled to ensure a sufficient sample size. Seeds of the other species came from commercial suppliers (A.sophorae and P.clandestinum) or collected from the field (T.triandra and C.m.rotundata). Seedlings 10-15 cm high were transplanted into 8 cm diameter pots, which were randomly allocated within the experimental design.
Watering: 'High watered' plants were watered every day with 100 ml of tap water; 'low watered' plants were watered every second day with 50 ml of tap water. Individual leaves on each plant in three randomly selected blocks were tagged to monitor stomatal conductance. Plants suffering water stress have lower stomatal conductance than non-stressed plants, because their stomata close to prevent water loss. Stomatal conductance of the tagged leaf was measured every three days from 07:00 hr using a Porometer (ΔTTM) to record the time taken for the relative humidity to increase by 10% within a small chamber clamped onto a leaf.
Herbicide: Glyphosate herbicide (1:180) was applied four weeks after the experiment began (after it was revealed that the stomatal conductance of water-limited plants is greater than that of regularly watered plants). The experiment was run for another seven weeks after herbicide application.
Seedling growth: At the end of the 11 week experiment the oven dried shoot and root mass was determined, each plant was allocated to one of five health categories:
1 = healthy plant
2 = 1-25% of above-ground biomass dead
3 = 26-50% dead
4 = 51-75% dead
5 = 76-100% dead
Survival: All herbicide free plants survived until the end of the experiment, but the application of glyphosate resulted in clear differences. Irrespective of watering regime, the survival of herbicide-treated Pimelea was very low (17%) and all Kikuyu grass survived. For glyphosate treated kangaroo grass, bitou and coastal wattle, survival of well-watered individuals was lower than for 'water-stressed' individuals. Indeed, all coastal wattle (a xeric species) plants died when well-watered and all survived when water-stressed.
Growth: The application of glyphosate reduced plant mass, though the magnitude was affected by water regime and species. Pivotally, the mass of Pimelea and bitou was limited by glyphosate regardless of water treatment. In contrast, the mass of kikuyu and kangaroo grasses, and coastal wattle, was not affected by herbicide when water-limited, but was significantly reduced when well-watered.
Health: Plant health was negatively affected by herbicide application. This negative affect was particularly severe for kikuyu grass, kangaroo grass and coastal wattle under well-watered conditions.
Conclusions: Kikuyu grass, kangaroo grass and coastal wattle plant weight was not affected by glyphosate application when water-limited but it was significantly reduced when these plants were well-watered. However, both Pimelea and bitou were intolerant to glyphosate irrespective of watering regime. Consequently, the authors suggest that weeding rather than herbicide application is the best option for protecting Pimelea from bitou bush encroachment.
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