Building with Nature BwN Guideline Environments Project phases Governance BwN Knowledge base
BwN Building Blocks BwN Toolbox Pilots and cases BwN Knowledge

Log in

Hydrosphere (water on or near the earth)

Salinity: Widely considered one of the most important factors. Most mangroves do not need salt water to grow, they only have a competitive advantage over other plants because they can grow in salt water. The resistance against salinity is depending on the duration of inundation and the life stage of the tree. Seedlings can resist less salinity than adult trees and in general the tolerance to salinity increases with the age of the tree (Kathiresan & Binghan, 2001). Salinity is influenced by precipitation and fresh water supply from rivers. In the table below a range of salinity thresholds is given for certain species.

 

Species

Mangrove life stage

Salinity (ppt)

Qualitative statement

Reference

General

Seedling

3 - 27

Depending on species and life stage, most critical for young halophites

Krauss et al. 2008

Avicennia alba

tree

20 - 30

Relative high tolerance for mature trees

ITTO 2008

Sonneratia alba

seedling

1.75 - 17.5

A season with high freshwater input is required. Needs salt to germinate, however is intolerant of high salinities during germination

Ball & Pidsley 1995

Sonneratia alba

tree

30 - 35

Relative high tolerance for mature trees

ITTO 2008

Rhizophora apiculata

all

15

optimum

Kathiresan & Binghan 2001

Suspended sediment concentration/sedimentation: Mangroves need sediment to sustain their elevation in the tidal range and to be able to keep up with sea level rise and subsidence. Sustaining elevation can only be achieved if sufficient sediment input is available. Damming rivers upstream and coastal structures which interfere with the littoral transport may therefore harm mangroves. Moreover, the forest has to be in good condition in order to be able to retain the sediments and produce organic material. On the other hand, too much sedimentation (especially if it is not gradual) will smother the shoots (seedlings) or pneumatophores (Aerial roots) and cause mortality. Sedimentation is closely linked with the tidal regime, wave climate and morphology (Ellison 1999, Adame et al. 2010, van Santen et al. 2007, Horstman et al. 2011). Suspended sediment concentration in natural forests was measured to be around 300-600 mg/l, sometimes up to 1000 mg/l.

Tidal regime: Less inundation time can be tolerated to a greater extent by seedlings than by adult trees. Tolerance to flooding is species-dependent and can decrease when the plants are already stressed by other factors. Inundation is closely linked to soil/sedimentation processes.

In the table below a range of inundation duration is given for certain species (van Loon et al. 2007).

Species

Mangrove life stage

Inundation (minutes/day)

Sonneratia alba

tree

400 - 800

Rhizophora apiculata

tree

100 - 400

Avicennia alba

tree

400 - 800

Waves: Wave action (and strong water currents) can dislodge seedlings and resuspend sediment causing erosion (Balke et al. 2011). Mangroves can only establish where these hydrodynamics are limited, for example in shallow areas along sheltered coasts as young seedlings are very sensitive to disturbance especially during the first months. Wave action often causes cliff erosion or gradual removal of sediments from around the roots, leading to toppling of the trees. Pioneer species can survive disturbances better than slow growing Rhizophora spp., hence a natural mix and zonation of species allows for a greater resilience of the forest.

Pollution: Mangroves are sensitive to oil spills due to clogging of pheumatophores (Duke et al. 1997) and high nutrient inputs transported along with the sediment (e.g. wastewater). These nutrients increase the biomass above the ground and reduces the resilience of mangroves to changes (Lovelock et al. 2009).

 

 

PDF