The distribution, ecology, potential impacts and management of exotic plants, Sonneratia apetala and S. caseolaris, in Hong Kong mangroves
香港外來的紅樹林植物 : 無瓣海桑及海桑的分布, 生態, 潛在影響及其管理
Student thesis: Master's Thesis
Invasion is now considered as a global threat to biodiversity as it is more pervasive than loss of natural habitats and anthropogenic pollution. Mangroves in Hong Kong were invaded by two exotic plants, namely Sonneratia apetala Buch.-Ham and S. caseolaris (L.) Engl. The present study investigated the distribution, abundance, reproduction, dispersion and germination of these two species, aiming to evaluate their invasive potential and to examine different removal methods. A territory wide baseline survey of Sonneratia was conducted in 2005 and 2006 to provide the first geographical record in Hong Kong. A total of 1,693 mature individuals (> 1.5m) were found distributing in 14 mangrove stands in Hong Kong. The relative distribution of S. apetala and S. caseolaris was 25.6% (434 individuals) and 74.4% (1,259 individuals), respectively. In terms of geographical location, 99.4% (1,683 individuals) were distributed in Deep Bay area, 0.5% (9 individuals) in Lantau area, and only 0.1% (one individual) in Tolo area, the eastern side of Hong Kong. The survey also showed that both Sonneratia species produced fruits all year round but the peak fruiting seasons varied between two species. S. apetala had two peak fruiting seasons, that is, from September to November and April to June; while the peak fruiting season for S. caseolaris was from July to March. Both species produced more fruits per mature tree and more seeds per fruit in autumn than spring. The greenhouse experiment showed that both species had a significantly higher unfurling numbers in the autumn batch. Comparing with the native mangrove species, Sonneratia had a longer fruiting period and faster germination rate. A desktop analysis was conducted to estimate the spread and the dispersal rate of Sonneratia seeds and to predict the potential affected mangrove stands. Results showed that seeds released from January to April and September to December had a higher tendency to escape from Deep Bay area and flow southward. For seeds released from May to August, they would remain in Deep Bay area for at least one month as there was no net ebbing tide in the first month. The seeds would have a higher chance to strand within Deep Bay area or reached the mangrove stands in the outer Deep Bay. These findings revealed that the spread of Sonneratia would depend on the time of seed released as well as the tidal flow. Greenhouse experiment proved that more than half of the seeds remained viable in 35 parts per thousands (ppt) for more than one month and thus seeds from Deep Bay area could spread to southern part of Hong Kong. Germination experiments showed that both Sonneratia species were salinity sensitive and germination was significantly lower when the salinity was higher than 15 ppt. Both species were sensitive to light intensity but showed no preference to substrate type. Tidal level was another factor affecting germination of S. caseolaris with significantly higher budding and unfurling numbers at high than low tidal levels; but the germination numbers of S. apetala were similar at all tidal levels. Cold period was a factor affecting germination of S. caseolaris, with significantly higher budding and unfurling numbers at 0 hour (without cold treatment) than 48 hours cold treatment; but the budding and unfurling numbers of S. apetala were similar among all cold treatments. The results of the greenhouse experiments were consistent with the current distribution of Sonneratia in Hong Kong, with most colonized in the western (less saline) than the eastern sides of Hong Kong such as Deep Bay and Lantau areas, and were mainly found on the open mudflat (higher light intensity) and absent under closed canopy. A series of field trials comparing the effectiveness of seven removal methods showed that “cut only” method was not reliable as there was 25% of the cut individuals re-sprouted. The non-intrusive “hand pulling” method was most effective in terms of time and money to remove seedlings and saplings with height less than 1.5 m. For the larger saplings and adult trees, “cut and covered by mud” was the best method as it could successfully prevent Sonneratia from re-sprouting and all individuals were dead at the end of the monitoring period. “Cut and apply glyphosate” was the second most effective method, however, the application of herbicide should be avoided due to its uncertain impact to the native flora and fauna. The “cut and covered by plastic bag” method was also effective but it has a disadvantage of putting man-made material into the natural environment and the bags must be removed. “Ring barking” as well as the “frill and apply glyphosate” methods were not recommended as both failed to kill the plants. In summary, the study revealed that the exotic Sonneratia have a high potential to become invasive. It is likely that Sonneratia will be more and more in Hong Kong, these exotic plants must be carefully controlled and monitored. Battle against Sonneratia requires long term commitment, sufficient resources must be allocated for continuous management work.
- Sonneratiaceae, Mangrove plants, Hong Kong, China, Mangrove ecology