I'm really excited to finally be able to share some of the work from my PhD, which as of yesterday, is available online at Biological Invasions (this is open access read only - link to full paid version at bottom of page). This wasn't an easy paper to write, as we had a lot of work to try to pack into a relatively short paper, and it took a while to refine it and get it right (thanks to my coauthors, Phil & Richard, for their patience and support!)
In this paper, we focus on the results of a 2-year transplant experiment of two succulent plants along a gradient of elevation and precipitation on Banks Peninsula, NZ. The two species in question, Aeonium arboreum and Aeonium haworthii, are popular garden plants from Macaronesia that were introduced in the late 1800's, when rock gardens were the hot new thing according to European settlers. They have since spread quite a bit but are not currently recognized as unwanted organisms in NZ, though they are locally problematic. They tend to spread prolifically and take over rocky outcrops, which can be super important as refugia for threatened endemics.
One of our field experiment sites on Godley Head, Banks Peninsula, at the end of the 2 year experiment. This was a pretty favourable site - relatively dry and warm. The cage was to protect from sheep grazing, but you can see that a determined sheep managed to nibble the plant on the left!
To test whether these plants might continue to spread locally and within NZ, we planted them within and beyond the climatic conditions we expected might be suitable to them, and measured growth, deaths and seed germination. We modelled how each of these responded to climate and used the predictions to estimate whether the plants would continue to spread, or if they had reached their limits already.
In short, we found that one of the two A. haworthii, was probably at or near its limits already at about 300 m above sea level, and though it might continue spreading into similar environments (infilling), it probably wouldn't get any higher.
Meanwhile, A. arboreum seemed to have similar tolerances to A. haworthii but is currently found only at lower elevations and is not as widespread. So, A. arboreum might be able to continue spreading a little, to around 300 m, similar to A. haworthii.
Predicted relative growth of A. arboreum and A. haworthii with increasing elevation, in years 1 and 2 of the experiment (lines) with means of observed relative growth per field site (points)
These findings are important locally, as the Port Hills Rangers undertake active monitoring of the species and some control, so this information can help them prioritize where to look for new populations. In addition, because A. haworthii is close to equilibrium, we could use its distribution in the region to help predict where else it might occur across NZ. Neat!
Check out the full paper at: https://doi.org/10.1007/s10530-019-01982-2
(P.S. I plan to make a green open access version available on this site and ResearchGate once I figure out the rules. Also, the raw data is here - currently embargoed but just email me if you would like access)