华南红树林海岸生物地貌过程研究

STUDY ON BIOGEOMORPHOLOGIC PROCESS OF MANGROVE COASTS IN SOUTH CHINA

红树林生物地貌过程是全球变化海岸带陆海相互作用研究重点内容之一。本文以海南东寨港林市村,广东廉江高桥镇凤地村和车板镇那腮村以及广东深圳福田和锦绣中华3个半定位试验区有关红树林群落结构、潮汐动力、沉积特征和地貌特征的调查资料为基础,探讨红树林生物地貌过程的生物学基础,沉积地貌表现,红树林生态系对人类活动和海平面上升的响应。

The biogeomorphologic process of mangrove coasts is one of the emphasized studies of land-ocean interaction in the coastal zone of the global change. Using the investigation data on community structure, tidal dynamic, coastal sediments and geomorphology from three mangrove sites of the South China in recent years, the biogeomorphologic process of mangrove coasts have been analyzed and summarized. The ecological characteristics of mangroves are the biological basis of their biogeomorphologic functions. The complicated ground and underground root system can not only stabilize plants and increase the absorption and exchange of gases, water and nutrients, but also detain water flow and trap suspended silts. Mangrove has a higher productivity and a higher feed back rate, the litter fall production is 631～1390g/(m^2·a), 35.2%～41.1% of net primary productivity, 2.16%～5.07% of standing biomass, which is the main source of organic detritus and the component of mangrove sediments. Under the control of tide water inundation-adaptability, mangrove can only occupy a specific elevation range on the inter tidal flats, i.e. between mean sea level(or a little higher) and mean tropic higher high water(or mean high water springs), which determines the basic geomorphologic pattern of mangrove coast and its principal mechanism responding to sea level change. The specific sedimentary and geomorphologic features of mangrove coasts concretely reflect the biogeomorphologic function of mangrove detaining water flow and trapping sediments. The tidal dynamic conditions and sedimentation in and out of mangrove are notably different. Dense mangrove assemblage and its complicated root system block water flow so as to decrease tidal current velocity rapidly to 1/4～1/3 less than that in bare flats or 1/13～1/6 less than that in the related creeks, the maximum velocity is no more than 10cm/s, thus offering a favorable environment of sedimentation. Stakes experiment of Rhizophora stylosa forests in Linshicun of Dongzhai Harbor, Hainan Island, taking dense prop roots as its characteristics, showed obvious siltation with a mean rate 4.1mm/a in the thick forests, and erosion with a mean rate of 8.2mm/ a in the sparse woods and bare flats. The seasonal change of sedimentation showed siltation in winter and spring, and erosion in summer. The sedimentary rate also shows decreasing with elevation of tidal flat increasing or inundation frequency decreasing. Regional sedimentation intensity depends on regional sediment supply. In Xijiang River mouth area, Guangdong Province, where sediment supply is rich and tidal flat is silted up rapidly, the sedimentary rate of mangrove tidal flat may reach 33～50mm/a. The specific mangrove sediments are mangrove peat and mangrove mud having high content of silt and clay, high content of organic matter and a smell of H_2 S. The transverse direction changes of grain size of sediments on the whole coastal zone are not apparent. In general, the grain size of sediments is a little courser and the content of organic matter is a little higher in mangrove area than in bare flats out of mangrove in Futian of Shenzhen City, Guangdong Province. The longitudinal and regional differences are more distinctive. The fundamental geomorphologic units of mangrove coasts consist of mangrove tidal flats, bare flats out of mangrove and tide creek system. They have different pattern, sedimentary characteristics, functions and evolution features. While the width of mangrove flat is more than 200m, there often develops tidal creek system, which is favorable for water overflowing and draining. The profile of mangrove tidal flat can be divided into 4 types: (1) concave type, e. g. Nasaicun of Cheban Town, Lianjiang County, Guangdong Province; (2) convex type, e. g. Linshicun of Dongzhai Harbor; (3) steep slope type, e. g. Futian of Shenzhen City; (4) dissected and undulating type, e. g. Fengdicun of Gaoqiao Town in Lianjiang County, special for large tidal range, wide tidal flat and wide mangrove zone. Mangrove forests often respond to human impacts and sea level rise through biogeomorphologic process. The exploiting activities of human in coastal zone are the main cause for sharp decrease of mangrove area, such as reclamation during 1960s and 1970s, aquaculture and urban construction since the 1980s in China. The destroy and ecological decline of mangroves result in the weakening or loss of the ability of coast producing, trapping and keeping organic and inorganic matter, causing net loss of coastal sediment budgets and coast erosion. Mangroves, as one of the most sensitive ecosystems to sea level change, their biogeomorphologic functions can offset to a certain extent the negative influences of sea level rise. Only when the rate of sea level rise is larger than the sedimentary rate in mangrove tidal flat, may mangrove forests be damaged and retreat towards land.