Physiological constraints on Mangroves and Salt Marsh plants
Plants which grow in areas of tidal influence must be able to deal with environmental extremes not experienced by other plants.
- Direct effect of the tide
Plants growing in the lower portion of the tidal range have their root systems covered by water at least twice a day. This usually means that the soil is permanently waterlogged. Land plants obtain oxygen for their root systems to carry out respiration by diffusion through the gas spaces in the soil. Waterlogging displaces air from the soil and effectively prevents this movement. (When a gas space in the soil is filled with water, its capacity to transport oxygen is reduced to 1/300000 of what it was before).
- Salinity of water
Most land plants use soil water which contains little salt and which has an osmotic potential which is close to zero. Sea water contains half molar concentrations of sodium and chloride ions and has an osmotic potential close to -2 MPa. ( If we were to place sea water in an osmometer fitted with a semi-permeable membrane in contact with pure water, the pure water would move into the osmometer. If pressure was then applied to the sea water and progressively increased, the pure water would cease entering sea water compartment when the pressure reached 2 MPa - about 20 atmospheres- and water would start to move from the sea water compartment to the pure water if the pressure was further increased.) A normal land plant placed in sea water will loose water to the sea water (and die).
- Water for transpiration
Mangroves, being quite large trees, require a plentiful supply of water. The water which evaporates from their leaves is pure but the water which is available to their roots contains a large amount of salt. The plant must either have a mechanism which enables it to exclude the salt while absorbing the water or it must be able to rid itself of any excess salt which it takes up. Although measurements of the xylem sap suggest that Avicennia marina excludes about 95% of the salt contained by the water it absorbs, this still means that the plant must have a mechanism to excrete excess salt. Some other mangroves exclude salt so successfully that they do not need this special mechanism.