Carpobrotus edulis, a clonal, stoloniferous, succulent plant native to South Africa, is highly invasive in many Mediterranean-climate ecosystems, including coastal California. Numerous studies indicate that clonal integration (resource sharing among stolon-connected ramets growing in different microhabitats) contributes to the invasiveness of this species in summer-drought ecosystems. Intriguingly, prior ex situ studies also indicate C.edulis can switch from C3 photosynthesis to Crassulacean acid metabolism (CAM) under controlled osmotic stress. Although drought-inducible CAM could contribute to this species’ invasiveness in seasonally arid ecosystems, this form of photosynthetic plasticity has never been reported for established C. edulis populations in the field. Our objectives were to determine whether or not CAM induction occurs in situ in established plants and whether this response is affected by clonal integration. Various measures of plant performance (tissue hydration, ramet growth, photosynthetic capacity, and CAM activity) were made on ramets in the field repeatedly during winter and summer months in an existing population near Temecula California. Clonal integration effects were tested by comparing stolon-connected and stolon-severed ramets. We expected CAM activity in summer but not winter plants. We further expected isolated ramets (i.e. with severed stolons) would rely more strongly on CAM than clonally integrated ramets.
Results/Conclusions
Microclimate measurements verified the plants experienced warmer and drier conditions in summer than winter. Plant tissue hydration, photosynthetic activity, and growth were all strongly reduced in C. edulis ramets in the summer. Additionally, connected and isolated ramets alike exhibited a similar summer reduction in plant growth and photosynthesis. Consequently, we found no evidence of a water-sharing benefit due to clonal integration. Our results demonstrated, for the first time, CAM induction in an established C. edulis population under field conditions. Connected and isolated ramets alike exhibited a similar increase in CAM activity in summer compared to winter tissues. This contradicts our expectation that isolated ramets would rely more strongly on CAM-dependent water conservation than connected ramets. Thus, in some cases, water sharing via clonal integration as a means of avoiding severe summer drought-stress may be less important for C. edulis persistence in Mediterranean-climate ecosystems than anticipated. Conversely, water saving via drought-inducible CAM may sometimes be more important for C. edulis persistence in Mediterranean-climate ecosystems than previous studies would indicate. The success of C. edulis as an invasive species in Mediterranean-climate ecosystems may be facilitated by both clonal integration and inducible CAM acting as independent drought-tolerance traits.