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GCE IV - Key Finding in 2019

    Elevation gradients drive microspatial differences in marsh temperature and spring green-up

    Subtle elevation differences in salt marshes are known to cause variation in tidal flooding, with numerous consequences for plant growth, salinity, and other properties. Alber and O’Connell (2019) discovered that elevation differences are also negatively correlated with soil temperature on the marsh platform, irrespective of tidal flooding. Field observations of soil temperature at 10 cm depth demonstrated that elevation increases of 0.5 m corresponded to decreases in average soil temperature of 0.9–1.7°C (Fig. 1). This was corroborated by satellite-based estimates of surface temperatures, which also decreased with increasing elevation. Similar satellite-based findings were also evident in a test marsh in Virginia, suggesting that this phenomenon occurs at broad scales. Biological reactions are temperature-dependent, and these observations indicate that metabolic processes will vary over short distances. This was borne out by our analyses of Spartina alterniflora phenology: O’Connell et al. (2019) found that fine-scale differences in elevation (and hence soil temperature) resulted in green-up onset 1.5–3 weeks earlier in the marsh interior, where elevations were lower than in the mid-marsh or at the channel edge. They suggested that the key driver of plant green-up was winter soil temperature, which has been increasing over the last 60 years. We are following up on these findings by measuring soil temperatures at all of the GCE core sites and planning a controlled experiment in which we will grow Spartina in treatments with different temperatures. These findings have broad implications for accurately estimating marsh metabolism and predicting how changes in temperature will affect future productivity and marsh sustainability.


    Fig. 1 Mean soil temperature versus elevation measured at 10-cm depth along (a) transect 1 during winter, (b) transect 1 during summer, and (c) transect 2 during summer in the GA study area. Dry (closed circles) and flooded (open circles) conditions are plotted separately. Linear models for flooded and dry observations >0.5 m are shown (solid lines) along with confidence intervals (dotted lines) and line equations. The R2 for linear models of the form temperature = elevation * flood status is shown for each deployment. Source: Alber and O’Connell 2019.


This material is based upon work supported by the National Science Foundation under grants OCE-9982133, OCE-0620959, OCE-1237140 and OCE-1832178. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.