Disturbance in Georgia salt marshes: variation across space and time

Abstract

Abstract. We documented the frequency and effect on live biomass of five different types of disturbance over 14 years in creekbank and mid-marsh zones of eight salt marshes dominated by Spartina alterniflora in Georgia, USA. Wrack (floating debris) and creekbank slumping were the most common disturbances at the creekbank, and snails were the most common disturbance agent in the mid-marsh. Disturbance frequency varied among sites due to differences in plot elevation and landscape position. Wrack disturbance at the creekbank was positively correlated with plot elevation, and both initial slumping and terminal slumping of creekbank plots were negatively correlated with plot elevation. Wrack disturbance at the creekbank and snail disturbance in the mid-marsh were also most common at barrier island vs. interior marshes. Disturbance varied up to 14-fold among years. Wrack disturbance at the creekbank was negatively correlated with river discharge and sea level, and initial slumping of creekbank plots was also negatively correlated with sea level. The different disturbance types varied in their effects on end-of-year standing plant biomass. At the creekbank, wrack disturbance reduced biomass in affected plots by ~46%, but slumping did not affect biomass until the plot was totally lost. In the mid-marsh, slumping and wrack were not important disturbances, but snail disturbance reduced biomass in affected plots by ~70%. In addition, abiotic conditions (river discharge, maximum monthly temperature, sea level, and precipitation) strongly affected year-to-year variation in biomass. Across the entire landscape, fewer than a quarter of the plots on average were disturbed, and disturbance reduced overall standing biomass by ~18% in the creekbank zone and ~3% in the mid-marsh zone. Our results indicate that wrack has fairly strong effects on end-of-year biomass at the creekbank. Overall, however, variation in abiotic conditions among years had stronger effects on end-of-year standing biomass in both marsh zones than did disturbance.

The effect of a small vegetation dieback event on salt marsh sediment transport

“…The dieback size (e.g. Our site was located near a drainage creek which suggests multidirectional flow, making it particularly vulnerable to wrack deposits (Li and Pennings, 2016). Our site was located near a drainage creek which suggests multidirectional flow, making it particularly vulnerable to wrack deposits (Li and Pennings, 2016).…”

Climate drivers of Zizaniopsis miliacea biomass in a Georgia, U.S.A. tidal fresh marsh

“…They are also similar to estimates of above‐ and below‐ground production of Spartina alterniflora and Spartina cynosuroides in nearby salt and brackish marshes, respectively, using similar methods in the same year (Schubauer and Hopkinson ). End‐of‐year standing biomass of Z. miliacea is greater than that of either Spartina species (Schubauer and Hopkinson ; Więski et al ; Więski and Pennings ; Li and Pennings ), however, suggesting either that we underestimated productivity of Z. miliacea or that it has less seasonal turnover than Spartina . Thus, our data did not conclusively support our first hypothesis.…”

“…These patterns were simpler and more muted than those of S. alterniflora in nearby salt marshes over roughly the same time period, where creekbank and midmarsh biomass showed different temporal patterns, and greater annual variation (∼ 2–3 fold among years) (Więski and Pennings ; Li and Pennings ). In contrast to stands of S. alterniflora , which are regularly affected by disturbance from wrack (floating plant debris), creekbank slumping, and herbivory (Li and Pennings ), Z. miliacea was almost unaffected by disturbance, with only one plot lost due to creekbank slumping over the entire study period. The absence of wrack disturbance in the Z. miliacea marsh is likely due to the lack of seasonal grasses upstream (upstream habitats are floodplain forest) and the steady river flow in this area that prevents wrack produced in salt marshes from moving upstream.…”

Self‐thinning and size‐dependent flowering of the grass Spartina alterniflora across space and time

“…alterniflora (Figure ). Tides are mesotidal, with a range of 2–3 m. To assess the possibility that results would vary as a function of landscape position, we followed Li and Pennings () in categorizing sites as mainland (GCE 1, 4), intermediate (GCE 2, 5, 9, 10) and barrier island (GCE 3, 6), speculating that mainland sites might have more freshwater input and therefore taller plants. At each site, we measured shoot height and flowering status of all shoots each October in 8 permanent plots (0.5 × 0.5 m) along the creekbank and 8 permanent plots (0.25 × 0.25 m) in the mid‐marsh, from 2000 to 2015.…”

“…At each site, we measured shoot height and flowering status of all shoots each October in 8 permanent plots (0.5 × 0.5 m) along the creekbank and 8 permanent plots (0.25 × 0.25 m) in the mid‐marsh, from 2000 to 2015. About 13% of the were disturbed or lost each year due to deposition of floating wrack, creekbank slumping, heavy herbivory or other causes (Li & Pennings, ); these were omitted from the analysis in the year that they were disturbed. Plots that were lost were replaced each year.…”