Abstract
The introduced Sonneratia apetala Buch. Hamplantation plantations have occupied more than 3800 ha in China. The prevalence, fast growth rate, and high seed production of S. apetala have raised concerns about the risks to native mangrove habitats. Efforts are required to convert these introduced monocultures to mixed or native forests. In this study, we examined native mangrove colonization in the introduced S. apetala plantations at the Qi’ao Island, Zhuhai, China. A 12-month field study was conducted to evaluate the effects of intertidal elevation and light level on the survival and early growth of five native mangrove species, viz., Bruguiera gymnorrhiza (L.) Savigny, Kandelia obovata Sheue, Liu & Yong, Aegiceras corniculatum (L.) Blanco, Avicennia marina (Forssk.) Vierh., and Rhizophora stylosa Griff. Across intertidal elevations and light levels, the survival of B. gymnorrhiza was the highest. All the species had relatively higher survival rates under 30% canopy closure. Although the seedlings survived best at high intertidal elevation, the relative growth rate (RGR) was the highest at low intertidal elevation, and it was promoted by high light level. The stem height at low intertidal elevation was higher than that at high intertidal elevation, and it was the highest under 30% canopy closure. B. gymnorrhiza and R. stylosa at high intertidal elevation had relatively high leaf numbers, whereas K. obovata and A. marina showed a reverse tendency. The growth of stem diameter showed a decreasing trend initially and then increased with better performance at low intertidal elevations, and B. gymnorrhiza presented the best value under 30% canopy closure. Bruguiera gymnorrhiza showed the highest growth rate under similar conditions. Overall, intensive canopy thinning is an effective measure to promote native mangrove growth in S. apetala plantations. Additionally, increasing planting density especially at low intertidal elevations may improve native mangrove establishment and growth. Furthermore, Bruguiera gymnorrhiza is the best choice in the effort to plant native species in S. apetala plantations in the study area.
Highlights
Mangroves are the only forests bridging land and sea with a rich diversity of flora and fauna in the intertidal zones of tropical and subtropical coastlines [1,2]
We found the seedlings of K. obovata and A. marina at the low intertidal elevation improved their ability to capture light energy by increasing the stem height to adapt to flooding stimulation and increased leaf number to increase the ability to capture light energy
The present study showed that the relative growth rate (RGR) of B. gymnorrhiza, K. obovata, and R. stylosa was significantly enhanced by high inundation elevation, especially at high light level, whereas
Summary
Mangroves are the only forests bridging land and sea with a rich diversity of flora and fauna in the intertidal zones of tropical and subtropical coastlines [1,2]. Mangroves provide ecological services to humans, but they are threatened by anthropogenic activities [3,4]. Mangrove habitats are becoming small or fragmented, which is attributable to anthropogenic activities, including aquaculture, agricultural reclamation, urbanization, and pollution explosion [5,6]. China was 250,000 ha, but it was reduced to 42,000 ha by 1956, and further shrunk to 21,000 ha by the end of the 1980s and to 15,000 ha by the end of 1990s, according to the National Investigation of Forest Resource [6]. In the early 1990s, China launched a 10-year mangrove reforestation program in degraded mangrove areas, through which the mangrove forest areas have recovered to 22,000 ha [6]. The planting area of S. apetala has exceeded 3800 ha in China [8]. Spartina alterniflora Loisel. began to invade the Qi’ao
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