PhD Dissertation Defense, Mollie Yacano: The effects of non-native marsh grass (Phragmites australis) on coastal nitrogen cycling

The European haplotype of Phragmites australis is an aggressive and widespread non-native species, capable of growing in fresh, brackish, and fully saline habitats. Invasive species are often considered detrimental to ecosystems, but recent studies have established P. australis performs closely to or outperforms native species in provision of several ecosystem functions. However, P. australis is widely considered a nuisance invader due to negative effects on native biodiversity, and current management practices emphasize suppression and eradication. These attempts are costly and rarely successful due to the resilient nature of the plant, can destabilize already vulnerable systems, and eradication plans rarely consider the trade-off in the loss of ecosystem functions. Marshes play a key role in global nitrogen cycling at the land-water margin, but few studies have directly measured the effect of P. australis on nitrogen processing in North America. Chapter 1 of this dissertation measures annual and seasonal nitrogen cycling associated with P. australis, native marsh grasses, and unvegetated sediments in high- and low-salinity marshes. Chapter 2 uses experimental treatments to determine whether common eradication methods affect nitrogen removal via denitrification. Chapter 3 assesses if P. australis facilitates denitrification equally across the range of land uses it inhabits in coastal urban areas, and provides an economic valuation of associated nitrogen removal. Results indicated denitrification associated with P. australis is on par or greater than adjacent native and unvegetated sediments in both high- and low-salinity marshes, and enhanced denitrification in high-salinity marshes is likely driven by advantageous landscape position of P. australis. Experimental eradication treatments did not affect this ecosystem function on an annual scale; however, seasonal variability suggested there may be effects not captured on the temporal scale of this study. Economic valuation conservatively valued nitrogen removal by P. australis in the study regions as US$1,767,327*yr-1, in contrast to an estimated treatment cost of US$367,772*yr-1. This research demonstrates the important and largely unacknowledged role of non-native P. australis in coastal nitrogen cycling both ecologically and economically. It also provides context to understand potential impacts of invasion and control measures of P. australis across coastal landscapes, which will inform future management practices.