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Australian Journal of Soil Research, 2004, 42, 197–202

Soil phosphorus enhancement below stormwater outlets in urban bushland: spatial and temporal changes and the relationship with invasive plants Michelle R. LeishmanA,D, Miah T. HughesB,C, and Damian B. GoreB A B

Department of Biological Sciences, Macquarie University, NSW 2109, Australia. Department of Physical Geography, Macquarie University, NSW 2109, Australia. C Present address: NSW Fisheries, PO Box 21, Cronulla, NSW 2230, Australia. D Corresponding author; email: [email protected]

Abstract. Invasion by exotic plant species is a significant problem in urban bushland remnants and is often associated with nutrient enrichment of soils. A major source of nutrient enrichment in urban areas is stormwater runoff, which is transferred from impervious surfaces in urban catchments and discharged at outlets on the residential/bushland interface. We measured the spatial extent of soil total phosphorus (P) enhancement below stormwater outlets on Hawkesbury Sandstone-derived soils in northern Sydney and examined whether total P concentration has increased with time since urban development and extended laterally beyond the stormwater flow path. The average area of soil P enhancement below outlets was 0.24 ± 0.05 ha and was widest 30–50 m downslope from the outlet, where it extended an average 40 m across slope. Catchment area was not significantly related to average soil total P concentration. There was a significant decline in total P across slope from the centre of the flow path and a significant positive relationship between soil total P and proportion of exotic plant cover, with soil P accounting for 77.5% of variation. We found evidence for a buildup in soil total P concentration over time within the run-on zone below outlets, with the rate of enhancement being ~68 mg/kg per decade over a 40-year period. Evidence for lateral transfer of soil P out of the run-on area was more equivocal. There was a significant decline in soil total P across slope from the boundary of the run-on zone, with higher concentrations at distances 0.5 m and 1 m from the boundary compared with >1.5 m. However, this could be due to error in locating the boundary between run-on and non run-on areas. There was no significant relationship between soil P in the non run-on zone and age of development, which would be expected if P was being transferred by biological activity beyond the run-on zone over time. It is clear that the primary areas of concern for management must be the run-on areas below outlets. SeMPtaR.hl0os.3pLh5eorisuhminanurbanbushlandsoils

Additional keywords: soil nutrients, environmental weeds, disturbance, stormwater runoff.

Introduction The substantial reserves and remnant tracts of bushland located within Sydney’s urban area are widely valued for their floristic diversity, habitat for indigenous flora and fauna, and aesthetic relief, as well as the important recreational and educational opportunities they offer (Benson and Howell 1990; Howell and Benson 2000). A significant problem in these urban bushland reserves is invasion by exotic plant species, particularly on soils subject to nutrient addition (Bliss et al. 1983; Leishman 1990; Riley and Banks 1996; King and Buckney 2002). Nutrients enter urban bushland via several paths, including dumping of garden rubbish, overflow and leakage from sewerage pipes, use of introduced soil fill for construction of tracks, and stormwater runoff. The stormwater system transfers runoff from impervious surfaces in urban catchments and © CSIRO 2004

discharges this runoff at outlets that generally drain into previously dry areas of natural bushland at the edge of urban development. Leishman (1990) showed that the major source of soil nutrient enrichment in urban bushland in northern Sydney was stormwater runoff, where areas below roads and stormwater outlets that receive runoff had soil total phosphorus (P) concentrations 6 times higher than nearby hillslope sites. The positive relationship between increased nutrient levels and invasion by exotic plant species in dry sclerophyll bushland is widely recognised and has been qualitatively observed in previous studies (Bliss et al. 1983; Leishman 1990; Riley and Banks 1996; Rose and Fairweather 1997). Phosphorus has been identified as the major limiting factor for plant growth in Australian soils (Beadle 1962; Specht 1963) and work in Hawkesbury Sandstone vegetation 10.1071/SR03035

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communities by Clements (1983), Wright (1984), King and Buckney (2002), and Lake and Leishman (2004) has consistently found that invasion of exotic plants is significantly correlated with high soil P concentrations associated with urban development. The spatial extent of stormwater-affected areas below outlet points is likely to be substantial, given their location high on hillslopes of bushland valleys. Leishman (1990) identified the minimum area of enhancement of soil P below 4 stormwater outlets in the Ku-ring-gai area as 30–100 m wide and extending 40 m downslope. However, in that study, the full spatial extent of P enhancement was not determined. An important management issue is whether the zone of enhanced soil P spreads beyond the run-on zone with time. If the P-enhanced zone is spreading, via biological activity such as nutrient uptake within the run-on zone and litter fall beyond the run-on zone, then invasion by exotic plant species and subsequent degradation of natural bushland is not spatially confined to the flowpaths but will threaten progressively larger areas of urban bushland over time. The questions addressed in this study were as follows. (1) What is the relationship between soil P concentration and percent cover of exotic plant species? (2) What is the spatial extent of soil P enhancement below stormwater outlets? (3) Are soil P concentrations in the run-on areas below stormwater outlets increasing with time, and if so, at what rate? (4) Is soil P enhancement confined to areas receiving runoff or is the additional P spreading through urban bushland soils? Material and methods Study sites All sites were located on soils derived from Hawkesbury Sandstone. Hawkesbury Sandstone is the dominant lithology of the northern Sydney area, covering the southern slopes of the Hornsby Plateau (Fairley and Moore 1989). These soils are quartz-rich with well-drained, acidic, sandy surface textures, and are poor in organic matter and nutrients, particularly phosphorus and nitrogen (Walker 1960; Williams and Raupach 1983). In places, finer textured B and C horizons consisting of sandy clay loam to sandy clay are found (duplex soils, Northcote 1979). All sites were classified as either Hawkesbury or Gymea soil landscape units (Chapman and Murphy 1989). Typical vegetation communities supported by these soils are dry sclerophyll open forests and woodlands (Benson and Howell 1990). Study sites were located in the Lane Cove River and Berowra Creek catchments, both located in northern Sydney. Both catchments constitute a typical Hawkesbury Sandstone landscape, with narrow ridge crests, steep valley slopes, and narrow valley bottoms. The Lane Cove River catchment is largely residential, with a population of more than 182000 residents (Preston 1992; LCRCMC 1995; Riley and Banks 1996). The Berowra catchment has a population of 85000 and includes residential, industrial, and rural land uses. Stormwater outlets were selected for sampling using the following criteria: (1) the stormwater discharged was urban runoff sourced from residential development; (2) the outlet was located on the residential/bushland boundary; and (3) the outlet was located on bushland hillslopes that were independent of natural drainage lines for at least 75 m downslope.

M. R. Leishman et al.

Study design Spatial extent of soil P enhancement below stormwater outlets and relationship to exotic plant success Four stormwater outlets were selected within the Lane Cove River catchment. The catchment size for each outlet ranged from 23 to 68 ha and the age of surrounding development from 22 to 46 years. At each of the 4 sites, four 25-m transects were sampled orthogonal to the slope and direction of stormwater flow. Transects were located 10 m apart, starting 30–45 m below the outlet, depending on the extent of clearing around the boundary of the adjacent property. Soil was sampled at 6 points along each transect, at 2, 5, 10, 15, 20, and 25 m from the centre of the flowpath channel in the run-on zone. Soil cores of 25 mm diameter and 75 mm depth were collected from 5 points within a 1-m2 quadrat centred on the nominated transect distance, using a hand-held auger. Surface litter was removed prior to sampling and the 5 cores from each sample point were bulked then subsampled for analysis of total P. On the first transect of each of the 4 sites, soil from the 2-m and 25-m sample points were field-textured according to the grading described by Paton et al. (1995, p.180). At each sample point, the percent foliage cover for both exotic and native plants was visually estimated within a 1-m2 quadrat. The total percent cover could therefore be >100% when vegetation strata overlapped. Plants were identified according to Buchanan (1981), McLoughlin and Rawling (1991), Fairley and Moore (1989), Carolin and Tindale (1994), and Robinson (1991). For this study, an exotic plant is defined as a plant species that is not indigenous to the local area. Spatial and temporal changes in soil P We used a factorial design with age of surrounding development and distance along each transect as the factors. There were 3 levels of age of surrounding development (1950s, 1960s, and 1970s), with 3 replicate sites for each age giving a total of 9 sites. Sites were located in both the Lane Cove River and Berowra Creek catchments, with catchment area for each stormwater outlet ranging from 15 to 68 ha. At each of the 9 sites, 3 transects orthogonal to the stormwater flow path were sampled. The transects were spaced 5 m apart and were located 30–50 m below the outlet. Soil was sampled from 13 points along each transect. The boundary between the run-on and non run-on zones was defined as the zero point on each transect. This boundary was identified in the field using 2 indicators: (1) high percent cover of exotic plant species as a proxy to indicate enhanced soil P (as determined previously, see results below), and (2) geomorphic and depositional evidence of scour, surface flow, litter such as plastics, and debris. Transects extended 10 m away from the flowpath from the zero point in order to incorporate background soil P levels (the ‘non run-on zone’) and 4 m towards the flowpath from the zero point to incorporate enhanced soil P levels (the ‘run-on zone’). Soil was sampled using the method described above at distances –4, –2, –1.5, –0.5, 0, 0.5, 1, 1.5, 2, 3, 5, and 10 m along each transect. Soil analyses Soils were analysed for total P following the method of Lambert (1982). Total P was selected because it incorporates both organic and inorganic forms, providing a more stable and reliable measure of soil P than available P. Furthermore, total P provides a comparable measure with previous work on enhanced nutrients in urban bushland soils (e.g. Clements 1983; Leishman 1990). Statistical analyses The data were analysed using 1-way analysis of variance and linear regression. The proportion of exotic species was arcsin square root transformed to fulfill assumptions of normality. All analyses were performed using Minitab Version 11 (Minitab Inc. 1996).

Phosphorus in urban bushland soils


Results

We conservatively defined ‘enhanced’ soil P as >150 mg/kg, as background levels for hillslopes are typically 60– 100 mg/kg (Beadle 1962; Leishman 1990). The average area of soil P enhancement covered 0.24 ± 0.05 ha below the outlets, and was broadest 30–50 m downslope of the outlet, where it extended an average of 40 m across the slope. Average soil total P in the enhanced zone was 310 ± 15 mg/kg, with the maximum recorded level of 594 mg/kg. More than 60 m downslope from the stormwater outlet the enhanced soil P zone contracted to a band 10–20 m across the flow path. Based on observations of the presence of exotic plants (see results below), this contracted band of enhanced soil P appeared to extend the length of the flowpath. The surface soil samples taken from within the stormwater flowpath ranged in texture from sandy clay loams to clay loams. Soil samples taken at 25 m distance from the centre of the flowpath contained fewer clays and were field-textured to be sandy loams or loamy sands. Analysis of variance showed that there was a significant difference in soil P concentration with distance from the flowpath (F5,18 = 25.13, P < 0.0001, Fig. 1), with mean soil P declining from 391 ± 30 mg/kg at the flow path to 108 ± 7 mg/kg at 25 m distance. Similarly, there was a significant difference in the proportion of exotic plant species cover with distance from the flowpath (F5,18 = 10.42, P < 0.0001, Fig. 2), with mean proportion of exotic species declining from 86 ± 7% 2 m from the flow path centre to 5 ± 4% at 25 m distance. The dominant exotic plant species recorded in the enhanced soil P zone were Ligustrum sinense (small-leafed privet), Ligustrum lucidum (broad-leafed 450 400

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Fig. 1. Relationship between mean soil total P and distance from the centre of the flowpath channel below 4 stormwater outlets. Error bars represent s.e.

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Spatial extent of soil phosphorus enhancement below stormwater outlets and relationship to exotic plant success

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privet), Lantana camara (lantana), Tradescantia albiflora, and Protasparagus aethiopicus (asparagus fern). The native invasive species Pittosporum undulatum (sweet pittosporum) was also found in the run-on zone, and extended further out across slope than the exotic species. There was a significant positive relationship between soil P and the proportion of exotic species cover (F1,22 = 80.12, P < 0.0001), with soil P accounting for 77.5% of the variation (Fig. 3). Soil with background total P concentrations of 310 mg/kg had a minimum of 50% cover of exotics. Spatial and temporal changes in soil P below stormwater outlets To ensure that the statistical analysis was not confounded by the size of the catchment drained by each stormwater outlet, we initially tested whether the average or maximum soil P concentration of the run-on zone below each outlet was correlated with catchment size. Catchment size was calculated from 1:25000 topographic maps, where catchment boundaries were interpreted as extending perpendicular to contour lines, upslope from each of the outlet locations. We found no significant relationship between catchment size and average soil P (r2 = 23.4%, F1,7 = 3.44, P = 0.106) or maximum soil P (r2 = 0%, F1,7 = 0.15, P = 0.706), indicating that catchment area does not account for significant variability in soil P among sites. A multiple regression of the factors age of development and catchment size on site total P also showed no significant relationship (r2 = 38.5%, F2,6 = 3.50, P = 0.098) Thus, all subsequent analyses did not standardise soil P concentrations for catchment size. The first question we addressed was: is there evidence for a build-up of soil P within the run-on zone below stormwater

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Fig. 3. Relationship between proportion of exotic plant cover and soil total P. Data points represent averages from 6 distances along 3 transects at each of 4 stormwater outlets.

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Fig. 4. Relationship between age of surrounding development and soil total P. Data points are average soil P of the enhanced zone at 9 sites below stormwater outlets.

Fig. 5. Relationship between soil total P and distance from the run-on zone boundary. Data points are averages from 9 sites below stormwater outlets. Error bars represent s.e.

age within the non run-on zone. We found no significant relationship between soil total P in the non run-on zone and age of development (r2 = 0%, F1,7 = 0.26, P = 0.627). In addition, if there was lateral transfer of P into the non run-on zone, we would expect soil P to decline with distance from the boundary with the run-on zone. We found a significant difference in soil P within the non run-on zone with distance (F6,56 = 2.95, P = 0.014), with mean soil P decreasing from 232 ± 25 mg/kg close to the boundary to 117 ± 13 mg/kg 10 m from the boundary (Fig. 5). Fisher’s comparison of means (P = 0.05) showed that the distances 0.5 m and 1 m from the boundary between the run-on and non run-on zones had significantly higher soil P than the other distances further from the boundary. Although this result is consistent with the expectation of enhanced soil P beyond the run-on zone, it could also simply reflect subtly incorrect estimation of the boundary of the run-on zone. Discussion

outlets over time, and if so, at what rate? If this were so, we would expect a significant relationship between soil P concentration and age of development, with the older catchments having higher soil P concentrations. Regression analysis showed that there was a significant positive relationship between soil total P and age of development (r2 = 37.6%, F1,7 = 5.83, P = 0.046) (Fig. 4). The regression equation (average soil total P = 201 + 4.27 age of development) shows that average soil P increased to approximately 286 mg/kg 20 years after development, 329 mg/kg 30 years after development, and 372 mg/kg 40 years after development. The second question we asked was is there evidence for movement of soil P across the boundary from the run-on to the non run-on zone? If there was lateral transfer of soil P, we would expect an increase in mean soil P concentrations with

There was a positive relationship between level of soil P and abundance of exotic plant species. We found that soil total P accounted for 77.5% of the variability in the proportion of exotic plant cover in the vegetation. This correlates reasonably well with the results of King and Buckney (2002), who found that soil P explained 68% of the variation in proportional abundance of exotic species for sites in northern Sydney. Our results show that there is negligible exotic plant cover on soils with 310 mg/kg. This suggests that even relatively small increases in soil P on these low fertility soils can result in significant invasion by exotic plants. Stormwater outlets are a significant cause of P enhancement of soils in urban areas. We found elevated levels of soil P within a zone 40 m wide spanning the

Phosphorus in urban bushland soils

flowpath and extending up to 60 m downslope from the outlet. Below this point, the presence of exotic plant species, indicating enhanced levels of soil P, contracts to a narrow band, 5–10 m wide, which follows the morphology of the flowpath in a manner comparable with that observed along creeks (Buchanan 1983). The average area of enhanced soil P in the wide zone identified below stormwater outlets was 0.24 ± 0.05 ha. If we assume an average size of urban bushland of 10 ha with a minimum of 5 stormwater outlets on the residential/bushland boundary (which is typical for Sydney bushland on Hawkesbury Sandstone, e.g. Buchanan 1983), this corresponds to approximately 12% of a typical bushland remnant being vulnerable to invasion by exotic plants due to nutrient enrichment from stormwater runoff. The average concentration of soil P in the run-on zone below stormwater outlets was 342 ± 20 mg/kg, with the maximum value of 911 mg/kg recorded at any site. Leishman (1990) found a similar average soil P concentration (438 mg/kg) below 3 stormwater outlets in northern Sydney. These enhanced soil P concentrations are associated with relative enrichment of the flowpath below each outlet with clays and organic matter carried in stormwater. The clay loams and sandy clay loams of the flowpath soils were distinctly heavier in texture than the sandy loams and loamy sands of the undisturbed bushland soils. Given the association between soil nutrient enhancement and invasion by exotic plant species in these low fertility soils, a critical question must be: is the zone of enhanced soil P associated with stormwater outlets spreading with time beyond the area of the stormwater flowpath? If this were so, then increasingly large areas of bushland adjacent to stormwater-affected soils would become vulnerable to exotic plant invasion. There are several possible mechanisms for movement of P through soils, both within the soil profile and across the landscape. These include surface erosion and runoff, leaching and sub-surface flow, and biogeochemical cycling. Phosphorus transported in surface runoff is primarily in particulate form, associated with clay particles or other fines such as organic matter (Hairsine and Prosser 1997). Surface runoff moves P progressively downslope with stormwater discharge events. Leaching acts to move P, primarily in dissolved form, down through the soil profile (Letkeman et al. 1996; Sims et al. 1998). Only biogeochemical cycling is capable of moving P across-slope away from the source area; P is absorbed by plant roots, translocated to the growing shoots, and redistributed into the soil with litter fall (Letkeman et al. 1996). As litter fall can extend beyond the source zone of P, this mechanism can effectively redistribute P, independently of site topography and hydrology, and is the most likely mechanism for transfer of soil P beyond the run-on zone below stormwater outlets in urban bushland. There are several plausible models for the enhancement and movement of soil P below stormwater outlets with time.


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Firstly, there may be no further increase in P within the flowpath once a level of P saturation has been reached, and no movement of P across the slope from the flow path. Secondly, P levels in the flowpath may increase with time, with no lateral movement of P across slope. Thirdly, P levels in the flowpath may not increase with time, but P may move laterally across slope. Fourthly, P levels in the flowpath may increase with time, and P may move laterally across slope. We found good evidence for increasing soil P with age of development in the run-on zone below stormwater outlets. Levels of soil P in the run-on zone at sites where the surrounding development is 40 years old are estimated as approximately 372 mg/kg, equivalent to an average increase above background levels of 68 mg/kg per decade. Peak soil P concentrations have increased at more than double this rate, with enhanced levels up to an order of magnitude greater than background concentrations. This result is consistent with models 2 and 4. Surprisingly, differences in catchment area had no apparent effect on the level of soil P enhancement. The evidence for movement of soil P across the boundary between run-on and non run-on zones was more equivocal. We found no evidence for an increase in soil total P with age of development within an area up to 10 m from the boundary of the run-on zone. However, there was a significant decline in soil P away from the boundary between the run-on and non run-on zones, with distances 0.5 m and 1 m from the boundary significantly higher than distances more than 1.5 m from the boundary. This is consistent with lateral movement of soil P away from the run-on zone, but could also simply reflect error associated with the identification of the boundary of the flow path in the field or P deposition from unusually large and infrequent floods. In summary, it appears that it is possible that there is some lateral transfer of P across slope, but if so, the rate of transfer is quite slow and the area affected very limited in extent. If we assume background soil P of 100 mg/kg, then average soil P levels in the non run-on zone have increased around 13 mg/kg (1960s sites) to 19 mg/kg (1950s sites) per decade since development. These enhancement rates compare with >68 mg/kg per decade in the run-on zone. Thus, we believe that the most conservative interpretation is that the data best support the second model proposed i.e. increasing P levels in the flowpath with time, with no lateral movement of P across slope. Given sufficient time, it is possible that a soil P saturation point may be reached (Model 1 and 3), however our evidence suggests this has not yet happened. This study has important implications for management of stormwater to minimise impact on urban bushland and for vegetation restoration in areas below stormwater outlets. It is clear that the primary areas of concern for management must be the run-on areas below stormwater outlets, where P levels can be up to 900 mg/kg and are increasing with time. Urgent measures are required to reduce the amount of nutrients in urban runoff (Bliss et al. 1983; Riley and Field 1996). These

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include community education and awareness (e.g. minimising erosion, washing cars on lawns, street sweeping), managing stormwater to maximise infiltration, removing P from stormwater by physical settling (e.g. detention basins, green filters), biological removal (e.g. via constructed wetlands), or geochemical removal using reagent pillows (Douglas et al. 1998). Rationalising the number of outlets and transferring discharge to existing natural drainage lines is also a useful option (Bliss et al. 1983). Further, methods are needed to remove nutrients from soil that have accumulated since development. These could include bioharvesting using native grasses, microbial uptake, or chemical removal (Cale and Hobbs 1991). Finally it is important to recognise that the soil conditions in areas subject to stormwater runoff have changed so dramatically that vegetation restoration requires creation of new communities of native species that are more suited to high soil moisture and nutrient conditions and are more likely to be competitive against invasive exotic plants. Acknowledgments We thank Lane Cove River Catchment Management Committee for financial support for soil analyses and Hornsby and Ku-ring-gai Councils for information on and access to study sites. Geoff Humphreys provided useful discussion and Vivien Thomson and 2 anonymous referees provided helpful comment on the manuscript. References Beadle NCW (1962) Soil phosphate and the delimitation of plant communities in eastern Australia II. Ecology 43, 281–288. Benson D, Howell J (1990) ‘Taken for granted: The bushland of Sydney and its suburbs.’ (Kangaroo Press: Kenthurst, NSW) Bliss PJ, Riley SJ, Adamson D (1983) Towards rational guidelines for urban stormwater disposal into flora preservation areas. The Shire and Municipal Record 76, 181–185. Buchanan RA (1981) ‘Common weeds of Sydney bushland.’ (Inkata Press: Melbourne) Buchanan RA (1983) ‘Bushland management survey.’ (Ku-ring-gai Municipal Council: Sydney) Cale P, Hobbs RJ (1991) Condition of roadside vegetation in relation to nutrient status. In ‘Nature conservation 2: the role of corridors’. (Eds DA Saunders, RJ Hobbs) pp. 353–362. (Surrey Beatty and Sons: Chipping Norton, NSW) Carolin RC, Tindale MD (1994) ‘Flora of the Sydney region.’ 4th edn (Reed: Chatswood, NSW) Chapman GA, Murphy CL (1989) ‘Soil landscapes of the Sydney 1:100 000 sheet.’ (Soil Conservation Service of NSW: Sydney) Clements A (1983) Suburban development and resultant changes in the vegetation of the bushland of the Sydney region. Australian Journal of Ecology 8, 307–319. Douglas GB, Coad DN, Adeney JA (1998) Reducing phosphorus in aquatic systems using modified clays. Water 25, 42–43. Fairley A, Moore P (1989) ‘Native plants of the Sydney district: an identification guide.’ (Kangaroo Press: Kenthurst, NSW) Hairsine P, Prosser I (1997) Reducing erosion and nutrient loss with perennial grass. Australian Journal of Soil and Water Conservation 10, 8–14.

Howell J, Benson D (2000) ‘Sydney’s bushland. More than meets the eye.’ (Royal Botanic Gardens: Sydney) King SA, Buckney RT (2002) Invasion of exotic plants in nutrient-enriched urban bushland. Austral Ecology 27, 573–583. doi:10.1046/J.1442-9993.2002.01220.X Lake J, Leishman MR (2004) Invasion success of exotic plants in natural ecosystems: the role of disturbance, plant attributes and freedom from herbivores. Biological Conservation 117, 215–226. Lambert MJ (1982) Methods for chemical analysis. Forestry Commission NSW Technical Report 25, 3rd edn. Forestry Commission NSW, West Pennant Hills, NSW. LCRCMC (1995) The Lane Cove River Catchment Management Strategy. Lane Cove River Catchment Management Committee Report, Department of Land and Water Conservation, Parramatta, NSW. Leishman MR (1990) Suburban development and resultant changes in the phosphorus status of soils in the area of Ku-ring-gai, Sydney. Proceedings of the Linnean Society of New South Wales 112, 15–25. Letkeman LP, Tiessen H, Campbell CA (1996) Phosphorus transformations and redistribution during pedogenesis of western Canadian soils. Geoderma 71, 201–218. doi:10.1016/00167061(96)00006-7 McLoughlin L, Rawling J (1991) ‘Making your garden bush friendly. How to recognise and control garden plants which invade Sydney’s bushland.’ 2nd edn (McLoughlin-Rawling Publications: Killara, NSW) Minitab Inc (1996) ‘Minitab user’s guide release 11 for Windows.’ (Minitab Inc.: USA) Northcote KHA (1979) ‘Factual Key for the recognition of Australian Soils.’ (Rellim Technical Publications: Glenside, S. Aust.) Paton TR Humphreys GS, Mitchell PB (1995) ‘Soils: a new global view.’ (UCL Press: London) Preston C (1992) Water quality in the Lane Cove River. AWT Science and Environment Report No. 92/79, Sydney. Riley SJ, Banks RG (1996) The role of phosphorus and heavy metals in the spread of weeds in urban bushland; an example from the Lane Cove Valley, NSW, Australia. The Science of the Total Environment 182, 39–52. doi:10.1016/0048-9697(95)05033-7 Riley SJ, Field R (1996) The protection of bushland at stormwater outlets: guidelines on process. Engineering Report No. CE2, Faculty of Engineering, University of Western Sydney. Robinson L (1991) ‘Field guide to the native plants of Sydney.’ (Kangaroo Press: Kenthurst, NSW) Rose S, Fairweather PJ (1997) Changes in floristic composition of urban bushland invaded by Pittosporum undulatum in Northern Sydney, Australia. Australian Journal of Botany 45, 123–149. Sims JT, Simard RR, Joern BC (1998) Phosphorus loss in agricultural drainage: historical perspective and current research. Journal of Environmental Quality 27, 277–293. Specht RL (1963) Dark Island heath (Ninety-mile Plain, South Australia). VII. The effect of fertilizers on composition and growth, 1950–1960. Australian Journal of Botany 11, 67–94. Walker PH (1960) A soil survey of the County of Cumberland, Sydney Region, NSW. NSW Deptartment of Agriculture Bulletin No. 2, Chemistry Branch, Sydney. Williams CH, Raupach M (1983) Plant nutrients in Australian soils. In ‘Soils: an Australian viewpoint’. (Academic Press: Canberra, ACT) Wright WJ (1984) Non-point source pollution and weed invasion. Master of Environmental Science thesis, Graduate School of Environment, Macquarie University.

Manuscript received 17 March 2003, accepted 12 December 2003

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