Final Report Summary - EVOPLASTINV (Evolving Phenotypic plasticity and Plant Invasiveness: An inter-disciplinary approach)
- The goal of the project EVOPLASTINV was to understand the mechanisms why introduced species become invasive. As study species, we used the introduced Asian annual plant Polygonum cespitosum. P. cespitosum is an introduced annual species found throughout the eastern United States, the mid-western US and parts of Canada. It was introduced to North America from its native range in eastern Asia within the last century, and it has recently become invasive in New England. Although once restricted primarily to disturbed habitats along shaded roadsides or forest trails, it is now beginning to invade more open sites in New England.
- We located 16 populations throughout southern New England and characterised the abiotic environments in terms of light, soil moisture and soil nutrients. We also collected achenes from 25-35 individuals per population of P. cespitosum that we used in greenhouse experiments and molecular analyses. Finally, we obtained material from the native range of the species (four populations from Japan and South Korea).
- We performed several field, glasshouse and molecular studies to gain insights to the invasion process of P. cespitosum in North America. Using field data that had been previously collected together with the detailed information on the populations that we obtained in 2009, we were able to quantitatively measure if there has been ecological range expansion in P. cespitosum. We found that the frequency of sunnier and moister microsites is much higher in 2009 than in 1994. This rapid change has happened in only 15 years.
- We also investigated the existence and distribution of high performance genotypes, i.e. genotypes able to maintain high fitness across diverse habitats. We raised 462 genotypes from 18 populations in the species' introduced and native ranges under resource-rich conditions to identify putative high-performance genotypes (the top 5 % in total reproductive output). We then compared their fitness, life-history and functional traits to a random group of the remaining genotypes (control), in three contrasting environments.
The high-performance genotypes initially identified in favorable conditions also had higher reproductive output in resource-limited environments. Their fitness advantage compared with Control genotypes varied in magnitude from one environment to another, but was significant within all three of the test environments. These High-performance genotypes shared a developmental syndrome characterised by rapid and high germination, faster seedling growth, earlier reproductive onset, and higher reproductive allocation. Surprisingly, these differences did not correspond to differences in other functional traits. P. cespitosum includes a subset of genotypes that have accelerated development and significantly greater fitness in both favorable and stressful conditions. The non-random distribution of these high-performance genotypes among populations in the species' introduced range highlights the importance of both genotypic and population-level variation in invasion dynamics.
- We also investigated the roles of individual and population-level factors (adaptive phenotypic plasticity and local adaptation) in Polygonum cespitosum. We characterised individual fitness, life-history, and functional plasticity in response to two contrasting glasshouse habitat treatments (full sun/ dry soil and understory shade/moist soil) in 165 genotypes sampled from nine geographically separate populations representing the range of light and soil moisture conditions the species inhabits in this region. P. cespitosum genotypes from these introduced-range populations expressed broadly similar plasticity patterns. In response to full sun, dry conditions, genotypes from all populations increased photosynthetic rate, water use efficiency, and allocation to root tissues, dramatically increasing reproductive fitness compared to phenotypes expressed in simulated understory shade. Although there were subtle among population differences in mean trait values as well as in the slope of plastic responses, these population differences did not reflect local adaptation to environmental conditions measured at the population sites of origin. Instead, certain populations expressed higher fitness in both glasshouse habitat treatments. We also compared the introduced-range populations to a single population from the native Asian range, and found that the native population had delayed phenology, limited functional plasticity, and lower fitness in both experimental environments compared with the introduced range populations. Our results indicate that the future spread of P. cespitosum in its introduced range will likely be fueled by populations consisting of individuals able to express high fitness across diverse light and moisture conditions, rather than by the evolution of locally specialised populations.
- Finally, we grew all the different genotypes that were collected during the first stage of the project in common conditions, and collected three leaves of each of them two weeks after germination. After that, we extracted the DNA from all of them (total number of samples = 609). With the help of the Savannah River Ecology Laboratory and the University of Connecticut, we developed primers to amplify microsatellites in Polygonum cespitosum. We found ten polymporphic loci and with them we performed multiplex polymerase chain reaction (PCR) reactions. We found that differences among populations within regions are much larger than differences among regions, likely due to the high selfing-rates of this species. We also observed multiple, independent colonisation events of sunny sites from the shade, which provides insight into the invasion process of this species in the introduced range.