When speed is a problem in mangrove restoration

When restoring mangrove forests, it’s important to get the right plants, and these aren’t always the fastest growing. Fatih Fazlioglu & Luzhen Chen in Scientific Reports show that non-native mangroves can be both faster growing and more plastic than native species, leading to them becoming invasive. “This is the first meta-analysis on the growth response of mangroves and it has consequential management implications. We suggest that planting of non-native mangrove species should be avoided and their spread should be monitored,” say the researchers.

Mangroves provide an important service in coastal defence. The trees are stress tolerant and can protect intertidal zones from storm damage. However, they are under threat from pollution, coastal development and logging as well as damage from storm strike. It is possible to plant new trees to restore ecosystems, but change in climate adds another stress to the ecosystem, and species do not all respond the same way.

To see how restoration worked, Fazlioglu & Chen conducted a meta-analysis, a study comparing study results from thirty-three different projects to look for common patterns. They found particular problems with two species commonly used in China for restoration, Sonneratia apetala and Laguncularia racemosa.

“L. racemosa was introduced from La Paz, Mexico where is in an arid climatic zone and freshwater availability can increase its growth and productivity,” write Fazlioglu & Chen. “In  Gulf of Mexico, low rainfall and high salinity prevent the movement of mangroves to higher latitudes. However, at its site of introduction (Hainan, China), the mean annual rainfall is almost 9 times higher and salinity is 3 times lower than in La Paz, Mexico. L. racemosa can outcompete native mangrove species (Rhizophora apiculata) in Hainan and it is reported to be spreading outside afforestation sites. Moreover, L. racemosa can express high trait plasticity under environmental stresses including salinity and shade. Therefore, L. racemosa can shift its range towards higher latitudes in China where salinity is lower and become potentially invasive owing to fast-growing characteristics, high germination rate, buoyant propagules, cold resistance, the ability to form dense monospecific stands. These characteristics make L. racemosa a model species for afforestation but invoke questions regarding its invasive potential as it has frequently been used in mangrove afforestation since 2002 in China.”

The problem for native species isn’t simply being crowded out of growth, but never getting the chance to grow in the first place, the authors say. “When an invasive species is introduced to a new habitat, existing native plants may be more vulnerable to allelopathic chemicals originating from invasive species because such biochemicals produced by a mangrove species can prevent the germination, growth and survival of other mangrove species and therefore, determine mangrove forest succession (e.g., Kandelia species). For example, allelopathic properties of introduced mangrove species such as L. racemosa and S. apetala may reduce the germination rate of seeds belonging to native mangrove species and therefore, affect the mangrove ecosystem functioning.”

“We suggest that the invasion potential of non-native species should be considered and the use of introduced non-native mangrove species should be avoided in mangrove afforestation practices. Although ecological repercussions of non-native mangrove species invasion in mangrove forests are uncertain, such introduced non-native species should be monitored for their potential spread. Restoration and management of mangroves are crucial for socio-economic and scientific aspects which can be achieved through a better understanding of how mangrove species respond under global climate change and biological invasion.”