Mangrove-forest evolution in a sediment-rich estuarine system: opportunists or agents of geomorphic change?

Swales, Andrew, Bentley Sr., Samuel J. and Lovelock, Catherine E. (2015) Mangrove-forest evolution in a sediment-rich estuarine system: opportunists or agents of geomorphic change?. Earth Surface Processes and Landforms, 40 12: 1672-1687. doi:10.1002/esp.3759

Author Swales, Andrew
Bentley Sr., Samuel J.
Lovelock, Catherine E.
Title Mangrove-forest evolution in a sediment-rich estuarine system: opportunists or agents of geomorphic change?
Journal name Earth Surface Processes and Landforms   Check publisher's open access policy
ISSN 1096-9837
Publication date 2015-09-30
Year available 2015
Sub-type Article (original research)
DOI 10.1002/esp.3759
Open Access Status Not Open Access
Volume 40
Issue 12
Start page 1672
End page 1687
Total pages 16
Place of publication Chichester, West Sussex United Kingdom
Publisher John Wiley & Sons
Collection year 2016
Language eng
Formatted abstract
The majority of the world's mangrove forests occur on mostly mineral sediments of fluvial origin. Two perspectives exist on the biogeomorphic development of these forests, i.e. that mangroves are opportunistic, with forest development primarily driven by physical processes, or alternatively that biophysical feedbacks strongly influence sedimentation and resulting geomorphology. On the Firth of Thames coast, New Zealand, we evaluate these two possible scenarios for sediment accumulation and forest development using high-resolution sedimentary records and a detailed chronology of mangrove-forest (Avicennia marina) development since the 1950s. Cores were collected along a shore-normal transect of known elevation relative to mean sea level (MSL). Activities for lead-210 (210Pb), caesium-137 (137Cs) and beryllium-7 (7Be), and sediment properties were analysed, with 210Pb sediment accumulation rates (SARs), compensated for deep subsidence (~8 mm yr−1) used as a proxy for elevation gain. At least four phases of forest development since the 1950s are recognized. An old-growth forest developed by the late-1970s with more recent seaward forest expansion thereafter. Excess 210Pb profiles from the old-growth forest exhibit relatively low SARs near the top (7–12 mm yr−1) and bottom (10–22 mm yr−1) of cores, separated by an interval of higher SARs (33–100 mm yr−1). A general trend of increasing SAR over time characterizes the recent forest. Biogeomorphic evolution of the system is more complex than simple mudflat accretion/progradation and mangrove-forest expansion. Surface-elevation gain in the old-growth forest displays an asymptotic trajectory, with a secondary depocentre developing on the seaward mudflat from the mid-1970s. Two- to ten-fold increases in 210Pb SARs are unambiguously large and occurred years to decades before seedling recruitment, demonstrating that mangroves do not measurably enhance sedimentation over annual to decadal timescales. This suggests that mangrove-forest development is largely dependent on physical processes, with forests occupying mudflats once they reach a suitable elevation in the intertidal.
Keyword Biogeomorphology
Q-Index Code C1
Q-Index Status Confirmed Code
Institutional Status UQ

Document type: Journal Article
Sub-type: Article (original research)
Collections: Official 2016 Collection
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