differences among plant types in their response to physical disturbance, additional water and nutrients
In this study we tested a number of hypotheses on the success of invasive and non-invasive plants subject to physical disturbance, water and nutrient addition. We found no evidence from the glasshouse experiment for our first hypothesis, that invasive plants have stronger survival and growth responses than non-invasives when grown with additional water. There were no differences in survival or biomass of any of the plant types (native non-invasive, native invasive, exotic non-invasive and exotic invasive) when grown with additional water compared with control conditions. It is possible that the soil moisture availability of the control treatment in the glasshouse was greater than normal field conditions, resulting in no clear difference between control and additional water treatments. However, given the low water retention capacity of the soil used in the experiment and the low frequency of watering of the control, we do not believe this to be the case. Furthermore, pots with the additional water treatment were constantly damp, which we believe is similar to the conditions in the field for soils receiving stormwater runoff. This suggests that in relatively infertile communities, such as the Sydney sandstone vegetation, invasive species are unable to take advantage of additional water due to the overwhelming nutrient limitation of the system.
There have been surprisingly few experimental studies that have shown that invasion success of exotic species is positively related to soil moisture, independent of other factors. Davis & Pelsor (2001) manipulated water and physical disturbance in old fields at Cedar Creek and found that one species of the three studied had increased invasion success with water addition independent of physical disturbance. Many sites within conservation reserves receive additional water through runoff generated by impervious surfaces such as roads and paved areas. Such sites are often subject to invasion by exotic plants (Knops et al. 1995; Kotanen et al. 1998; Gelbard & Belnap 2003). However, our results suggest that additional soil moisture is unlikely to be sufficient to facilitate success of exotic species in nutrient-limited vegetation types.
The second hypothesis tested was that invasive plants have stronger survival and growth responses than non-invasives when grown with additional nutrients. Both the glasshouse and field experiment provided strong evidence that exotic invasives had the highest survival rates compared with other plant types when grown in nutrient-enriched conditions, but native invasives did not differ from native non-invasives. Exotic species generally had higher survival rates than natives in nutrient-enriched conditions. The glasshouse experiment showed that all plant types except native non-invasive species had strong growth responses to added nutrients, while in the field only exotic invasives showed dramatic growth responses to additional nutrients. Exotic plants, whether invasive or non-invasive, had larger biomass and greater shoot length than native non-invasives in both the glasshouse and field experiments. In summary, not all invasive plants reacted strongly to nutrient addition, with only exotic invaders showing consistently strong responses. Thus it would appear that on the relatively infertile Hawkesbury Sandstone soils of the Sydney region, nutrient addition facilitates the success of exotic invaders. Further, exotic species that are naturalized but not currently invasive, may not become problem invaders in the future without changes in environmental conditions or phenotypes.
Thomson & Leishman (2004) showed that nutrient additions of 0.27 g P week−1 in a complete soluble fertilizer for 6 weeks (resulting in total P concentration of 260 mg kg−1) resulted in 100% mortality of native species grown in Hawkesbury Sandstone soils. Leishman et al. (2004) have shown that sites in Hawkesbury Sandstone vegetation with soil total P concentrations greater than 350 mg kg−1 have exotic plant cover of more than 80%. The decline in survival of all plant types grown in the additional nutrient treatment in the glasshouse therefore suggests that nutrient levels were higher than indicated by the results for soil total phosphorus. However, total P of stormwater-affected Hawkesbury Sandstone soils has been recorded at concentrations of up to 900 mg kg−1 (Leishman et al. 2004), suggesting that although soil P availability was higher in the glasshouse than the total P measurements indicated, the nutrient additions were not unrealistic compared with field conditions.
Within each of the four plant types in both experiments we used a range of growth forms. However, the nature of the common species for each plant type in the Hawkesbury Sandstone community resulted in some biases, for example exotics were dominated by herbs while natives were dominated by shrubs. On only two occasions did we find significant interactions between a factor and species within plant type and in neither of these cases could we discern a pattern of response in relation to growth form. Thus we do not believe that our results in relation to plant type were determined by their growth form mix, although we could not test this directly.
Our results are consistent with a number of published studies showing that exotic plant invasions are more common on naturally high-nutrient or nutrient-enriched soils (see Introduction) or that invasion by exotics is more successful where nutrients have been added experimentally (Hobbs & Atkins 1988; Huenneke et al. 1990; Burke & Grime 1996). Previous studies describing experiments where exotic and native species have been grown at varying nutrient levels have also shown that exotic species such as Tamarix ramosissima in riparian systems (Marler et al. 2001), Vulpia bromoides and Echium plantagineum in Eucalypt woodlands (Allcock 2002) and exotic composites in western Australia (Milberg et al. 1999) show greater positive growth responses to high nutrient concentrations than do native species (but see Lowe et al. 2002).
Physical disturbance (hypothesis 3) was found not to provide a relative advantage in survival or growth for invasive compared with non-invasive species. We have treated each of the factors physical disturbance, additional water and nutrients as being independent of each other in these experiments. However, this may not be the case, for example physical disturbance results in increased light on the soil surface and the removal of the boundary layer effect of the vegetation, which may result in reduced topsoil moisture. We attempted to mitigate this in the field experiment by regularly applying water to all plots. Similarly, in some systems additional water or physical disturbance may affect nutrient availability by allowing microbially mediated soil processes to take place. However, in this low fertility sandy soil, phosphorus is the limiting nutrient (Beadle 1962) and is supplied through inorganic forms, and thus is unlikely to be affected by water addition or physical disturbance. Finally, biomass removal associated with physical disturbance may increase nutrient availability by reducing demand. However, the very low concentrations of phosphorus available for plant growth in this system suggests that the additional phosphorus made available would be minimal. This suggests that vegetation communities on phosphorus-limited soils should be resilient to invasion generally, by either exotic or native species. In fact, Lake & Leishman (2004) found in a widespread survey of Hawkesbury Sandstone communities in the Sydney region that sites with only physical disturbance (i.e. adjacent to tracks) did not support exotic or native invasive plants, with the exception of one exotic species, Andropogon virginicus. Furthermore, these vegetation communities have well-developed mechanisms to survive regular fire and consequent biomass removal. Previous work has shown that fire in these communities does not facilitate invasion by exotic species (Thomson & Leishman, in press). Previous experimental studies that have manipulated physical disturbance have found that invasion is most successful where physical disturbance is combined with another factor, such as nutrient addition (Burke & Grime 1996) or high water availability (White et al. 1997), or in areas that are naturally high in nutrients such as wetlands (Rachich & Reader 1999). Thus it would appear that physical disturbance alone in communities limited by other resources such as low soil phosphorus does not facilitate invasion by exotic plants.
Several previous studies have suggested that a combination of different factors (such as physical disturbance, water addition or nutrient addition) results in greater success of exotic invaders (Burke & Grime 1996; Gentle & Duggin 1997; White et al. 1997; Duggin & Gentle 1998). We found that the combination of additional water and nutrients resulted in largest total biomass, shoot biomass and shoot length across all plant types in the glasshouse experiment, while the combination of additional nutrients and physical disturbance resulted in the largest root and shoot biomass across all plant types in the field experiment. However, we only found significant interactions between plant type and nutrient condition in the two experiments, suggesting that it is not the combination of different factors (nutrient addition plus water addition or physical disturbance) that provides an advantage to exotics compared with natives, but simply nutrient addition alone.
Our results are largely consistent with the fluctuating resource hypothesis of invasibility proposed by Davis et al. (2000). Davis et al. (2000) suggest that a plant community is more susceptible to invasion when there is an increase in the amount of unused resources. Our results suggest that an increase in nutrient resources in the low fertility sandstone vegetation communities of Sydney does result in success of invasive exotic species, because these species have better growth and survival with additional nutrients compared with other plant types. However, the inherent nutrient limitation of these communities means that an increase in resources such as additional water or those associated with physical disturbance does not result in successful invasion, as even invasive species are unable to respond to the increased resources at such low rates of nutrient supply.
trait differences among plant types
Exotic species, particularly invasives, had significantly higher root : shoot ratios across all treatments, suggesting that they allocate relatively more to foraging for soil resources than other plant types. The trend, albeit non-significant, to greater specific leaf area in exotic species, is consistent with previous work on species of Hawkesbury Sandstone vegetation communities, which has found higher specific leaf area and foliar nitrogen and phosphorus in exotic species compared with natives (Haslehurst 2002; Lake & Leishman 2004). Large specific leaf area is among the suite of attributes (also including high photosynthetic capacity, high foliar nitrogen and short leaf lifespan, Reich et al. 1997) that is associated with fast growth. Such attributes of exotic plants have been found to differ consistently from co-occurring natives in a range of other habitats (Pattison et al. 1998; Baruch & Goldstein 1999; Durand & Goldstein 2001; Smith & Knapp 2001; Grotkopp et al. 2002), suggesting that fast growth under non-limiting conditions may contribute to the success of some exotic species.
Few previous studies have separated native and exotic species into invasive and non-invasive. Smith & Knapp (2001) examined 13 traits of two exotic invasive, five exotic non-invasive and six native species but did not find consistent differences between invasive and non-invasive exotic species, although this may have been due to the small numbers of species. Lake & Leishman (2004) studied nine traits of exotic invasive, exotic non-invasive and native species from Sydney Hawkesbury Sandstone communities and found that exotic invasives had consistently larger specific leaf area than exotic non-invasive species. Interestingly, the rather smaller difference found here did not translate to greater biomass under high nutrient conditions.
In summary, we found strong evidence that the success of invasive exotic species in these low fertility vegetation communities may be facilitated by the addition of nutrients. Other work in these communities has shown that exotic species are not limited by seed availability (King & Buckney 2001), that nutrient concentrations that are typical of such sites result in high mortality of native species (Thomson & Leishman 2004) and that exotic species are largely confined to areas of nutrient enrichment (Lake & Leishman 2004). Thus we suggest that a combination of the environmental conditions (nutrient-enrichment) and invading species having traits that allow high survival and, possibly, faster growth in response to nutrients, may allow successful invasion in this community.