Cross-shore gradients of physical disturbance in mangroves: implications for seedling establishment

T Balke,T J Bouma,E M Horstman,P M J Herman,C Sudtongkong,E L Webb

doi:10.5194/bg-10-5411-2013

Abstract

Abstract. Mangroves may grow in an active sedimentary environment and are therefore closely linked to physical coastal processes. Seedlings colonize dynamic tidal flats, after which mangroves have the potential to change their physical environment by attenuating hydrodynamic energy and trapping sediments. Disturbance from hydrodynamic energy of waves or currents and the resulting sediment dynamics appear to be a critical bottleneck for seedling establishment on tidal flats and at the forest fringe. However, knowledge about the mechanisms at the single plant level and the spatial pattern of disturbance is limited. By means of a flume study, we demonstrate that a surface erosion threshold of as little as 1–3 cm depth can lead to failure of young seedlings. By monitoring accretion/erosion for 8 months along cross-shore transects in southwest Thailand, we show that, especially on the bare mudflat, the physical sediment disturbance regularly exceeds the critical erosion thresholds derived from the flume study. Physical sediment parameters along the same transects were analysed to deduct patterns of hydrodynamic energy attenuation. Grain size analysis and erosion/accretion data showed only limited energy dissipation within the fringing Avicennia/Sonneratia zone; sediment dynamics only dropped below lethal values for seedlings within the denser Rhizophora zone. Overall, present results emphasize that (i) seedling survival is extremely sensitive to physically driven sediment dynamics and (ii) that such physical disturbances are not only present on the tidal flats but can penetrate a significant distance into the forest. Spatio-temporal patterns in sediment dynamics should hence be considered when conducting restoration of mangrove ecosystems.

Highlights

Seedling establishment in mangroves has been studied in relation to various factors such as salinity (e.g. Ball and Pidsley, 1995), inundation stress (e.g. Ashford and Allaway, 1995), predation on propagules or seedlings
Whereas mortality of Sonneratia caseolaris and Avicennia officinalis seedlings was significantly increased by different rates of burial, Rhizophora mucronata showed no significant response to variable burial rates, natural mortality was higher compared to the most successful Sonneratia caseolaris
Our results highlight the importance of sediment dynamics as an important driver controlling pioneer-seedling establishment

Summary

Introduction

Seedling establishment in mangroves has been studied in relation to various factors such as salinity (e.g. Ball and Pidsley, 1995), inundation stress (e.g. Ashford and Allaway, 1995), predation on propagules or seedlings The few studies on seedling mortality of mangrove pioneer species, due to frequent small physical disturbances of the sediment bed, are scanty and mainly focused on high rates of sedimentation (Terrados et al, 1997; Thampanya et al, 2002). Short-term erosion can still dislodge them (Balke et al, 2011) and excessive sedimentation can kill even several-month old seedlings of both species due to burial (Thampanya et al, 2002). Terrados et al (1997) concluded that areas with sudden sedimentation of > 4 cm on top of the tidal flat are not suitable for Rhizophora apiculata seedling establishment. Whereas mortality of Sonneratia caseolaris and Avicennia officinalis seedlings was significantly increased by different rates of burial, Rhizophora mucronata showed no significant response to variable burial rates, natural mortality was higher compared to the most successful Sonneratia caseolaris

Methods

Results

Conclusion