Effects of climate and hydrology on subtropical mangrove growth and community composition

Abstract

Mangroves are of great significance in sequestrating carbon, resisting storm surges and supporting coastal species. However, mangroves face many threats from both humans and nature. Despite many studies on the anthropogenic threats such as deforestation and urbanization, there is still a lack of study that integrates natural effects at both regional and local scales. Unlike tropical mangroves, subtropical mangroves live in environments with highly variable climatic and hydrological conditions. Understanding the climatic and hydrological effects on mangroves is therefore crucial. The first two parts of this thesis identified the key climatic and hydrological factors affecting subtropical mangroves, and the third part of the thesis built a numerical model to investigate the response of mangroves to these identified climatic and hydrological factors and to explore the mangrove composition changes under future sea-level rise scenarios.

In Chapter 2, the regional effects of macro- climatic and hydrological factors such as temperature, precipitation, solar radiation and evapotranspiration on mangrove cover were investigated through statistical analysis for one tropical and six subtropical mangrove stands in southern China. The results indicate that subtropical regions are more influenced than tropical regions by macro- climatic and hydrological factors. Precipitation and temperature are the governing macro- climatic and hydrological factors associated with mangrove area variations. Moreover, the relationship between precipitation and mangrove cover is not necessarily positive at finer temporal resolutions, which may be due to the local groundwater salinity and the salt tolerance of dominant species.

Considering groundwater salinity is an important factor that affects mangrove growth and possibly affects the response of mangroves to external climatic and hydrological changes as proposed in Chapter 2, Chapter 3 performed field measurements to investigate the variations of groundwater piezometric head and salinity at a local subtropical mangrove swamp and analyzed the driving factor. The results indicate that tide is the driving factor that leads the fluctuations in mangrove groundwater, and tidal effects differ horizontally and vertically. The active root zone showed high dependence on the tidal variations due to the presence of macropores, and the groundwater fluctuations, especially the salinity fluctuations, were highly attenuated owing to capillary effects underneath the active root zone.

In Chapter 4 and 5, a three-dimensional (3D) coupled hydro-vegetation model was first developed to simultaneously simulate the mangrove and groundwater variation in temporally varying climatic and hydrological conditions, and was then employed to simulate the spatial and temporal distribution of native and exotic mangroves in a subtropical mangrove forest with and without sea-level rise for 120 years. The results indicate that salinity increase resulted from future sea-level rise is likely to benefit the native mangrove growth and inhibit the exotic mangrove growth.

In general, this thesis systematically evaluated the effects of climate and hydrology on subtropical mangroves. The regional and local study facilitated the monitoring and understanding of the mangrove ecosystem variations in response to climatic and hydrological changes. The developed numerical model advanced the understanding of mangrove composition changes in varying climatic and hydrological conditions, which could further facilitate future research works in this area.