Inundation and precipitation effects on growth and flowering of the high marsh species Juncus gerardii

• We conducted a factorial greenhouse experiment testing the growth and flowering effects of current and future climate change impacts with respect to precipitation and inundation on Juncus gerardii, a cosmopolitian coastal wetland plant species.
• Growth and flowering responded strongly to inundation, but were insensitive to lowered precipitation.
• Growth and flowering are likely being reduced by current flooding patterns.

Accelerated sea level rise threatens coastal wetland plant communities where coastal development restricts transgression, and inundation increases and declining sediment supplies limit the capacity of coastal wetlands to build in elevation. Juncus gerardii Loisel., black needle rush, is a high latitude cosmopolitan plant species and, within salt marshes of the U.S. mid-Atlantic and New England coasts, it occupies a narrow belt along the marsh-upland border. Examination of historic aerial photography, vegetation resurveys, and peat composition analysis for U.S. Northeastern marshes have shown vegetation change patterns indicative of increased inundation, including decline of J. gerardii. To interpret loss patterns for J. gerardii in southern New England, we conducted a factorial experiment to establish its sensitivity to inundation and drought. A strong relationship was found between inundation and growth for J. gerardii, which together with marsh elevation and water level data, suggests that growth is reduced by current flooding patterns. Examination of J. gerardii flowering also indicates that floret and inflorescence density vary with inundation, suggesting that negative impacts of sea level rise on Juncus may extend to seed production. Late spring and summer drought impacted neither J. gerardii growth nor its flowering, implying that J. gerardii is insensitive to below-average precipitation or drought during this time of year. We conclude that current inundation patterns are incompatible with robust growth for J. gerardii, and recommend conservation actions be focused on the marsh-upland border to facilitate the upslope migration of J. gerardii and other transitional high marsh plant species.