Phenotypic plasticity may be an important contributor to the success of many invasive plant species. Shifts in genotype induced by the recipient environment can also lead to a range of phenotypic expression not seen in the native range. Selection for these novel genotypes could lead to local adaptation in the introduced environment. To investigate plasticity and local adaptation in an invaded region, we established three reciprocal transplant gardens using clonal replicates from two common invasive species in New England, Berberis thunbergii and Celastrus orbiculatus. The four source regions for woody cuttings represent points of both species’ invasion history; Japan (native range), Connecticut (near initial introduction point), Vermont (northern invasion front), and North Carolina (southern invasion front). For B. thunbergii, we included horticultural varieties as another “population”. The three garden locations, northern Vermont, Connecticut and central Japan, are proxies for a variety of climatic variables including differences in precipitation and winter and summer temperatures. Phenotypic performance data were analyzed within replicated clones, populations and regions in a hierarchical Bayesian framework allowing us to examine phenotypic, genetic, environmental and GxE variance (and hence plasticity) in performance across species.
Results/Conclusions
Berberis thunbergii populations from Connecticut exhibit home site advantage in the Connecticut gardens, outperforming other wild populations thus supporting the hypothesis of local adaptation. Not surprisingly given their breeding to be locally robust and general purpose across a range of environments, the horticultural varieties perform the best in all gardens. Aside from the horticultural varieties, most other source populations exhibit decreased performance in the native range Japanese garden relative to the other gardens. This may be evidence that novel genotypes from the invaded range are not necessarily at an advantage in the suite of environments found in the native range.
For C. orbiculatus, most populations showed similar reaction norms across the environmental gradient of common gardens suggesting that the success of C. orbiculatus reflects its plasticity across environments. However, genotypes from North Carolina outperform all others across all gardens and are thus able to exploit resources under a variety of conditions.
Despite similar native ranges and invasion histories, B. thunbergii and C. orbiculatus respond uniquely to their novel environments suggesting different factors influencing genotype expression. Evidence of plasticity has important implications for continued introduction and spread, in addition to potential responses to climate change.