The Effects of Precipitation and Environmental Warming on two Salt Marsh Plant Communities

The response of salt marsh function to climate change depends on its ability to keep pace with sea level rise by expanding both horizontally and vertically through peat accumulation and primary productivity. Climate change is expected to regionally warm the air, soil, and water as well as change tide cycles and the intermittency and volume of precipitation. This will strongly alter the ability of salt marsh ecosystems to export biomass and nutrients, filter runoff, sequester carbon, and protect coastlines from flooding and erosion. Charles and Dukes (2009) studied the effect of precipitation and environmental warming by manipulating the habitats of two salt marsh plant communities, marsh hay/spike grass and cod grass. They analyzed the differences between total above ground biomass, stem height, decomposition rates, and flowering patterns for each treatment plot that differed by the amount of precipitation and increase in temperature. Their research found that salt marsh communities are able to withstand slight increases in temperature and large changes in precipitation.Acadia Tucker
Charles, H., Dukes, J. 2009. Effects of warming and altered precipitation on plant and nutrient dynamics of a New England salt marsh. Ecological Applications 9, 1758 –1773.

Healthy salt marsh ecosystems are a balance between salt tolerant and fresh water plants that correspond to a salinity gradient defined by tidal inundations and ground water circulation. Charles and Dukes tested two plant communities, representing salt tolerant and water loving cod grass (Spartina alterniflora) as well as marsh hay and spike grass (Spartina patens, Distichlis spicata) that prefer less saline soils and higher ground. The experiment was preformed in the high marsh of Plum Island, located in Massachusetts. The habitat of each plant community was monitored under five different treatments that consisted of a control (ambient climate), doubled precipitation, no precipitation, warming up to 1.17°C, and the interaction between warming and doubled precipitation.
Open-top warming chambers were created by wrapping “greenhouse plastic” around a PVC pipe frame. This reduced the side effects of closed warming chambers like the increase in humidity but did not allow the warming chambers to maintain heat during the night. The double precipitation treatment was preformed by watering plots directly after storms with the same quantity of water that fell, to double the average amount of precipitation. Drought treatments used plastic shields and funnels to completely divert water away from the designated plant communities. Charles and Dukes measured the response of each treatment by observing the total above ground biomass, height of the tallest stems, number of flowering stems, decomposition rate of leaf litter, and the pore water chemistry related to nutrient availability and soil salinity.
The researchers believed that doubling precipitation would increase plant growth by decreasing the salinity of the soil. However the doubled precipitation treatment showed no significant decrease in the overall salinity, even though the added fresh water did temporarily decrease the salt concentration. The soil salinity was restored to 5 ppt after 2 hours and 10 ppt after 4 hours. This could explain why, with increased precipitation, the overall productivity of each plant community declined. The effects of doubled precipitation reduced overall stem growth and total above ground biomass for both plant communities, effecting S. alterniflora the most significantly. The possible increase in soil waterlogging may have offset any of the positive effects of decreased salinity by means of increased precipitation.
It remains unclear to the researchers why there was an increase in primary productivity during the drought treatment. They postulated that the absence of water allowed for increased soil aeration and nutrient availability because fewer nutrients were leached from the soil due to tidal inundations and runoff. In addition, the interaction between warming and a decrease in precipitation led to a 53% increase in total biomass compared to the 24% increase from warming alone.

The more productive a salt marsh is, the greater ability it has to trap sediment during tidal flushing and directly contribute organic inputs into the soil through the decomposition of leaf litter. A slow rate of decomposition is important for the gradual uplift of salt marsh topography because plant litter helps to trap sediment from the flowing water. The Results showed that, under a period of drought and environmental warming, salt marsh productivity was at its highest. This can be explained by the decrease in decomposition and the increase in above ground biomass, which allow for the salt marsh ecosystem to expand horizontally and vertically. However, precipitation significantly increased the rate of decomposition because microbes decompose wet matter more quickly. Warming had no effect on the rate of decomposition and flowering rates were not affected by the warming or precipitation treatments. As long as salt marshes are not completely submerged as a result of sea level rise, increases in above ground biomass and stem heights suggests that salt marsh plants may become increasingly more productive under future climate projections.

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