Salt where it does not belong
Most forest trees cannot cope with salt. Mangroves have special adaptations, but the oaks, pines, cypress, palms and many other species of low-lying coastal woodlands do not, or tolerate only a little. Saltwater intrusion is the movement of salty water into places that were previously fresh or only slightly brackish: soils, streams, groundwater and wetlands. When this happens in a forest that is not adapted to it, trees can become stressed and eventually die.
Sea level rise is one driver, but it is not the only one. Understanding the causes helps explain why salt damage can show up even in places where the sea has not risen much.
What causes intrusion
- Higher sea level. A rising sea pushes brackish water farther up estuaries and tidal rivers, and into the groundwater beneath coastal land. Sea level has risen on average around the world, though the local change varies with land sinking or uplift.
- Storm surges. A hurricane or cyclone can flood forests with seawater in a single event. Salt can remain in the soil for months or years if rain does not flush it out.
- Drought and low river flow. Less fresh water flowing from rivers lets salt water move upstream. A period of drought stress can leave trees weaker just as the salt arrives.
- Ditches and canals. Channels dug for drainage, navigation or logging can act as highways that carry salt water inland.
- Groundwater pumping. Heavy extraction near the coast can draw salty water into aquifers.
- Dams and water diversion. Reduced river flow decreases the freshwater push toward the sea.
Often, several of these combine. A woodland already weakened by drainage may be pushed over the edge by a single storm.
How salt harms trees
Salt creates problems in two main ways. First, it makes the water in the soil harder for roots to take up, because a high salt concentration outside the root pulls against water entering the root. The tree can behave as if in drought even when the ground is wet. Second, salt that does enter the tree can build up in leaves and damage cells, causing leaf scorch, early leaf drop and thin crowns.
Seedlings are generally more sensitive than mature trees, so a forest may look fine for years while new trees fail to establish. Eventually the old trees die, and there is no younger generation to take over. Dead or dying trees are more vulnerable to wind, and a coastal storm can cause sudden windthrow among them.
Ghost forests
In several coastal regions, including parts of the eastern United States, stands of dead standing trees have appeared where salt water has entered a freshwater or low-salinity forest. These are often called ghost forests. The gray, bare trunks remain for a time before falling. The ground beneath them may be taken over by salt-tolerant marsh grasses and other plants.
Ghost forests are one of the clearer visible signs of coastal change, and they can be seen on aerial or satellite imagery as patches of dead timber at the landward edge of marshes. Their causes differ from site to site; researchers generally attribute them to a combination of sea level rise, storm events and altered drainage, rather than a single cause.
A shifting boundary
Sea level rise does not simply destroy a coastal forest. It changes the zone of vegetation, and one habitat may replace another:
- Freshwater forest may become brackish marsh.
- Salt marsh may move inland into forest, as marsh plants colonize ground that is newly flooded by tides.
- In warm regions, mangroves may expand into marshes and farther toward the poles as frost becomes rarer, a pattern that matches the broader change in species ranges under a warming climate.
Whether these habitats can move depends on room. If land inland is steep, or blocked by roads, seawalls, farms or buildings, there may be nowhere for the habitat to go, a situation called coastal squeeze. A forest that is lost seaward without a new one forming landward can disappear.
Wider effects
Losing coastal forest has consequences beyond the trees themselves. Wildlife that depends on freshwater or upland forest loses habitat. Storm buffering falls. Soil carbon that was held in waterlogged forest soils may be released when the ground changes, although the effect depends on the setting. People who rely on coastal forests for timber, fuel or farmland may see their land become too salty to use.
At the level of the ecosystem, the result is typically described as a long-term forest ecosystem change rather than a short disturbance. Biodiversity can shift, with some species gaining habitat and others losing it.
What can be done
Options depend on the cause and on local conditions, and none is a complete cure.
- Monitoring. Tracking salinity in soils and wells, and following the health of trees, can give early warning. Simple forest health indicators such as crown thinning or leaf scorch are a starting point.
- Managing water. Restoring freshwater flows, plugging old drainage ditches or installing control structures can reduce the push of salt water.
- Planning for movement. Protecting land inland of coastal forests and wetlands, sometimes in protected areas, gives habitats room to migrate.
- Species choice. In some managed sites, planting salt-tolerant native species may help maintain tree cover, though it is best done with local expertise.
- Accepting change. In some places the most realistic goal may be to support a healthy new habitat, such as a marsh or a mangrove, rather than hold on to the old forest.
Predictions for individual sites are uncertain. They depend on future sea level, rainfall, storms and human decisions, so planning usually works best with ranges rather than single figures.