Results indicate that A. arboreum and A. haworthii are currently near or at their climatic limits on Banks Peninsula. While upward spread is likely to be minimal, continued colonization of coastal headlands and bays (infilling) in the region appears likely, subject to habitat availability and dispersal patterns. As such, the threat from the two Aeonium species to high elevation rock outcrops, where the majority of the regions’ endemic species are found (Wiser & Buxton 2009), is low. The interpretation for C. orbiculata is more nuanced, as persistence at high elevations will depend on whether plants are able to reach sufficient size to survive and recover from winter frosts. It is unknown whether C. orbiculata would be able to complete its life cycle above its current maximum elevation, and although arguably unlikely, marginal populations cannot be discounted. This, in conjunction with the wider range of habitats it is capable of colonizing (Figure A.3, Figure A.7), suggests that C. orbiculata poses a greater threat to the region than either Aeonium species.
Non-climatic factors that vary with increasing elevation were not accounted for in model predictions. Dispersal barriers and pollinator availability are presumed not to be limiting at the scale of Banks Peninsula as the species produce “dust-like” seeds (van Rheede van Oudtshoorn & van Rooyen 1999) and can self-pollinate (Zietsman 1998). Slope and disturbance increase with elevation but this is unlikely to limit species that preferentially occupy vertical, disturbed habitats e.g. cliffs and outcrops. Factors expected to be relevant to the species on Banks Peninsula include soil, land use, and anthropogenic activity. As elevation increases in the region, soil pH tends to become less acidic. Soils at low elevations are predominantly Fragic Pallic and Fluvial Recent, characterized by poor structure, high nutrient content, low organic matter and high erosion, often with loess deposits. At higher elevations these give way to Brown soils (Firm and Mafic) which are stable, with higher clay content and firmer structures and topsoil (Landcare Research 2015). The species generally prefer free-draining soil (Vogan 2003), therefore Brown soils are likely to be less suitable. However, there is no shortage of exposed rock outcrops at high elevations in the region. There is very little difference in land use with increasing elevation; high-production exotic grassland dominates Banks Peninsula, with a slight increase in forested areas and scrubland at higher sites, although anthropogenic disturbance is lower due to fewer roads and homesteads (Boffa Miskell 2007; Ministry for the Environment & Landcare Research 2014). Fire regimes might also vary with elevation as a result of vegetation and soil drainage changes, and though the related Bryophyllum delagoense (Ecklon & Zeyher) is likely facilitated by fire in Australia (Witt & Nongogo 2011), the target species’ responses to fire are unknown and there
are no fine-scale fire data for Banks Peninsula. It seems probable that the non-climatic covariates of elevation will have minimal effects on the upward spread of A. arboreum, A. haworthii or C. orbiculata on Banks Peninsula, and that their eventual distributions will be largely determined by climate and propagule pressure.
3.5.4
Limitations
This study goes into greater detail than many transplant studies, which often investigate a single vital rate [e.g. Paiaro et al. (2007)], do not extrapolate beyond the current distribution [e.g. Kollmann and Bañuelos (2004)], or do not standardize for effects of source population (Hargreaves et al. 2014). Nevertheless, there are limitations to be considered when interpreting the results. The implications of model uncertainty are most severe for seedling establishment, as the three species produce vast numbers of seeds; varieties of C. orbiculata are reported to produce over 1000 viable seeds per flower in the native range, and flowers are numerous (Zietsman 1998). Even if a small percentage of seeds germinate and survive, it is still plausible that populations could establish, and variation in germination could have a large effect on population growth rate. Additionally, predictions assume linear correlations with frost and GDD5, while other factors were set to constant values (e.g. precipitation and canopy cover). Running A. arboreum seedling models under high precipitation (1100 mm p.a.) reduced predicted establishment at 177 MASL by 83%. A similar effect occurs with canopy cover for C. orbiculata models. It is therefore impossible to define absolute elevation limits, and estimates reflect cautious predictions assuming otherwise optimal (i.e. dry, open) conditions. Furthermore, as plants did not flower during the experiment in sufficient numbers, reproductive output could not be tested. Fecundity may also be limiting to population persistence, and knowledge of seed output would allow classification of source and sink populations.
3.5.5
Conclusions
The aims of this study were to investigate whether A. arboreum, A. haworthii and C. orbiculata are close to their climatic limits on Banks Peninsula and to assess the potential for upward spread. There is evidence that climate is strongly limiting to plant performance across the region, and that Banks Peninsula represents a gradient from favourable climates to those beyond the species’ cold tolerances. Aeonium populations have reached almost their maximum elevation, which is likely to be around 300 MASL. Interpretations for C. orbiculata are less certain and it is possible that populations could persist beyond their current maximum elevation. Winter mortality, relative growth and seedling establishment of Aeonium species are all limiting at high elevations, while only relative growth appears to be limiting for C. orbiculata. Because C. orbiculata is less restricted by habitat type and climate than the Aeonium species, it evidently poses the highest risk to Banks Peninsula of the three species.