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GCE III - Key Finding in 2017

    Disturbance patterns in GCE long-term monitoring plots

    Natural disturbances play important roles in many coastal ecological systems. Experimental studies can determine the effects of disturbance, but long-term studies are necessary to document the natural disturbance regime. Li and Pennings (2016) analyzed disturbance in the GCE domain based on 14 years of annual data from permanent plots, and found that wrack (floating debris) and creekbank slumping were the most common disturbances at the creekbank, whereas snail herbivory was the most common disturbance agent in the mid-marsh (Fig. 1) Disturbance varied up to 14-fold among years as a function of river discharge and sea level. Standing biomass in disturbed plots was sharply reduced, but because fewer than a quarter of the plots on average were disturbed each year, the total effect on standing biomass at the landscape level was only ~18% at the creekbank and ~3% on the marsh platform. A follow-up study (Li and Pennings 2017) demonstrated that the timing of wrack disturbance affected recovery rates, flowering, and the frequency of stem-boring herbivores of Spartina alterniflora. Plots that were experimentally disturbed early in the season had fully recovered by the fall, whereas those disturbed over the summer did not fully recover. Another type of disturbance agent that affects Spartina is the herbivorous crab, Sesarma reticulatum, which can exacerbate headward erosion of tidal creeks. Vu et al. (2017) found that Sesarma excavate large amounts of soil, consume plants at the creekbank (increasing erodibility), and probably also enhance decomposition of soil organic matter through burrow networks that serve to increase oxygen penetration. These studies provide a long-term and landscape context for past studies of disturbance, and have stimulated new hypotheses about how the distribution and effects of disturbance vary across the landscape that we can test in our upcoming GCE 4 proposal.


    Fig. 1 Plant biomass (A) and disturbance frequency (B) vary among years and marsh zones. The frequency of different types of disturbance varies among sites (C, D). Wrack (E), snail (F), and creekbank slumping (G, H) vary in frequency among years. From Li and Pennings 2016. Photos: (I) Heavy wrack (center of photograph) has crushed a stand of creekbank Spartina. (J) Blocks of marsh edge slumping or “calving”.


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.