By Katie Sinclair
An invasive reed from Europe is conquering marsh habitat throughout the Chesapeake, displacing native marsh grasses and drastically changing the face of the wetlands. Phragmites australis, a “jack and master” plant grows to nearly 10 feet tall and is adept at extracting nutrients from the soil, outcompeting native Phragmites genotypes. Climate change could increase the spread of this invasive plant. But other human activities, such as development, shoreline hardening and agriculture, could also determine the spread and range of Phragmites.
Climate Change Spurs Phragmites Growth
Rachel Hager, who interned with the Biogeochemistry lab, wanted to see if human activities were giving Phragmites even more of a competitive edge. Excess nitrogen from agriculture and industry, as well as increased CO2 levels, could increase Phragmites growth. Working in the Global Change Research Wetland (GCReW), she tracked the growth of Phragmites under conditions that had more CO2 added, more nitrogen added, and both CO2 and nitrogen added. She found that CO2 and nitrogen led to increased Phragmites growth, and plots with both CO2 and nitrogen grew the most.
Increased growth is only part of the story, however. Rachel wanted to see if taller Phragmites would inhibit other plants’ access to light. She analyzed leaf length, number, thickness and canopy cover to see if Phragmites exposed to additional CO2 and nitrogen were better at blocking light from their competitors. She found that Phragmites exposed to more CO2 and nitrogen had more, thicker and longer leaves, but their canopy cover was the same as control Phragmites plots. Thicker, longer leaves could lead to a longer leaf lifespan and more leaf litter, however, which could still block other plant’s access to light. Rachel hopes to see further research done on the amount of light that makes it through a Phragmites canopy.
Modeling the Spread of an Invader
While his peers were tromping through marshes and forests to collect samples, Ben Sciance, who interned with the Ecological Modeling Lab this summer, used a slightly different approach to study this invasive plant: He undertook the ambitious project of making a computer model that could determine which factors most influenced the distribution of Phragmites.
For 12 weeks, he poured over data sets that charted Phragmites distribution from the Virginia Institute of Marine Science (VIMS) and the Plant Ecology lab at SERC, and analyzed dozens of variables. He looked at Phragmites in both a larger-scale watershed context, as well as on a small, shoreline scale. He faced several challenges along the way: the VIMS and Plant Ecology Lab data didn’t always match up on the shoreline scale, so Ben had to figure out a way to make the two data sets agree. After making several models, Ben did find that some variable were more important in determining the presence of Phragmites than others. Shoreline hardening and near-shore agriculture both positively correlated to Phragmites abundance, while forested shoreline was negatively correlated. Because Phragmites tends to colonize areas with recent disturbance, shoreline hardening, which involves the removal of marsh plants and woody debris, could give Phragmites a chance to establish itself. Nitrogen run-off from agriculture could also encourage Phragmites growth.
Ben ran statistical tests as well to determine how accurate his model was at determining in the presence or absence of Phragmites. Strangely, Ben’s model was more accurate at predicting Phragmites being absent than it was at Phragmites being present, which suggests that there are more factors that affect Phragmites distribution.
While Ben modeled Phragmites distribution, Hope Brooks, who interned with the Plant Ecology Lab, wanted to see if genetics could give any clues to better understanding the spread of this super-invader. By genotyping Phragmites, Hope could see how Phragmites was spreading, and if native and invasive Phragmites were interbreeding.
It helped that plant cells have DNA in two places: in the nucleus and in the cell chloroplasts. Because chloroplast DNA come from the maternal line (seeds) and nuclear DNA from both the paternal (pollen) and maternal, comparing chloroplast DNA to nuclear DNA can determine whether invasive genotypes are spreading through seed or pollen distribution. Hope extracted DNA from leaf samples, and then did several rounds of PCR to amplify the DNA. The samples were then sent off to be sequenced. When she analyzed the data, she found native gene sequences in invasive Phragmites genotypes—evidence that the two strains were in fact interbreeding.
It’s unclear what a hybrid Phragmites would do to the marsh. It could be weaker than the invader. But there’s also the possibility that it could be much stronger. Because of the large number of samples, Hope did not get a chance to compare all the nuclear genotypes to the chloroplast genotypes, but future SERC researchers will be able to continue her project. Understanding the potential of native and invasive Phragmites to hybridize is important for management: A hybrid invasive-native Phragmites could become an even better marsh competitor.