As part of a restoration project to establish new communities on estuarine wetland dominated by the invasive species Spartina alterniflora, we investigated the potential to establish mangrove species after S. alterniflora removal. In order to achieve good establishment it is essential that the newly planted seedlings can survive their first winter where they may be susceptible to cold temperatures. To investigate this we carried out a comparative study of the cold tolerance response of one-year old seedlings of three mangrove species planed in salt marsh cleared of S. alterniflora in the Min River estuary, Southeast China. The species planted included a native species (Kandelia candel) and two alien ones, Leguncalaria racemosa from Mexico and Sonneratia apetala from Bangla Desh. We measured a range of physiological attributes in the leaves of seedlings of the three species over winter (early-October 2010 to late-February 2011), the measurements included: soluble sugar and free proline concentrations, SOD,POD and CAT activities, O₂^- generation rate, MDA and relative electrical conductivity.
Over the entire monitoring period, the leaf soluble sugar concentrations of all three mangrove species seedlings increased, then decreased; K.candel had the greatest concentrations and L. racemosa the least. The rate of O₂^- generation from the leaves of K. candel showed a similar response to the sugar concentrations; S. apetala and L. racemosa on the other hand exhibited the same initial increase but the high rates were maintained at significantly greater rates than K. candel (P<0.01). However, SOD, POD and CAT activities showed the opposite response (P<0.01), with POD and CAT activities in the leaves of L. racemosa and S. apetala seedlings declining significantly in late winter. Free proline concentrations of L. racemosa were higher than that of K. candel and S. apetala; all increased as air temperature reduced.
The contents of MDA and relative electrical conductivity of all three plant species increased as minimum daily air temperature decreased. L. racemosa and S. apetala seedlings had significant higher level of MDA than K. candel seedlings (P<0.01), but there was no obvious difference between L. racemosa and S. apetala seedlings(P>0.05). The relative electrical conductivity of K. candel seedlings was significantly lower than that of L. racemosa and S. apetala seedlings(P<0.01). K. candel seedlings did not show symptoms characteristic of freezing injury during the winter of 2010/11 with damage/death caused mainly by biotic factors (ducks and other water fowl). However, serious damage was detected in seedlings of both alien species (L. racemosa and S. apetala seedlings), indeed all seedlings of these species died in late February 2011.
In short, under current climatic conditions the native species K. candel has a competitive advantage in that its seedlings can persist over winter in the Min River estuary and may be suitable for restoration of salt marsh after Spartina alterniflora removal. The two alien species, L. racemosa and S. apetala, cannot colonize at the moment because they cannot survive over winter but may be able to suitable in the Min River estuary if older seedlings were transplanted which had been acclimatized to the colder condition.