Could Climate Change Affect How An Invasive Grass Colonizes Wetlands?

It’s a common sight along roadsides, in coastal areas, and just about everywhere else: Phragmites australis, or Common Reed, is among the world’s most cosmopolitan species and is found on nearly every continent.  The invasion of non-native lineages of the plant into North American wetlands has been especially well documented.  Wetland ecosystem structure and function may be profoundly impacted by Phragmites invasion, where changes to plant and soil microbial community structure, nutrient cycling, and carbon storage may result.

Phragmites spreads both by seed and asexually (via rhizomes), a combination of reproductive strategies that packs a double punch for invasive potential.  Sexual reproduction by outcrossing increases genetic diversity, which allows adaptation to environmental stressors.  Asexual reproduction allows for rapid encroachment into wetlands, such as salt marshes, where conditions for germination are unfavorable.

One way that climate change may affect the way Phragmites invades wetlands is by altering germination rates, and therefore genetic diversity, within wetland populations of the plant.  I decided to test germination rates of Phragmites, as well as a native coastal marsh species it often displaces, under a temperature scenario the IPCC predicts for the year 2100, using Phragmites and Spartina patens (saltmarsh hay) seeds from populations in the northeastern United States.

I used growth chambers at the University of Rhode Island to simulate present-day and end-of-century temperatures with summer daylight hours and humidity.  I based northeastern US present-day (control) temperatures on regional summer averages (daytime: 32.8°C, nighttime: 22.8°C), and simulated end-of-century conditions by increasing control temperatures by 4.5°C (based on the IPCC projected increase).  I sowed seeds of each species on clean commercial potting mix in replicate nursery pots, administered identical watering regiments, and recorded germination rates for the 4 treatment combinations.

Results indicate distinct germination responses of invasive Phragmites and native Spartina to warming predicted to occur by the end of the next century. Under present-day temperature conditions, Phragmites’ germination rate outpaced that of Phragmites by a factor of six.  Under increased temperature conditions, though, Phragmites’ germination rate more than doubled, while Spartina’ germination rate was significantly reduced.

Changes in germination responses may be linked to success. Already a successful colonizer of disturbed coastal wetland systems at the expense of native vegetation communities, Phragmites may benefit from genetic diversity arising from increased sexual reproduction.  Spartina patens, in contrast, currently faces a confluence of interacting threats from sea level rise, shoreline development, and the expanding Phragmites invasion into the high marsh. Decreased germination rate in response to warming temperatures could potentially affect the response of Spartina to changes affecting its habitat.

The germination responses of the seeds I tested lend insight into possible future effects of climate change on invasive Phragmites and its native salt marsh counterpart Spartina patens.  However, it’s important to note that germination rates and responses to warming may vary among populations and across latitudinal gradients.  The populations tested cannot be assumed to necessarily be representative of either species and so there is a need for future research into climate change impacts on germination rates of Phragmites and native wetland grasses that incorporates populations spanning a variety of latitudinal and environmental gradients.

This study, Effects of Warming on Invasive Phragmites australis and Native Spartina patens Seed Germination Rates and Implications for Response to Climate Change was recently published in the journal Northeastern Naturalist.