Abstract:

Aims: Mangroves are woody plant communities that thrive in the intertidal zone of tropical and subtropical gulf estuaries, enduring periodic inundation by seawater. Understanding how mangrove plants adapt to this challenging environment and achieve multi-species separation and coexistence has been a focal point of research. Niche theory serves as an important framework for studying species coexistence within communities, with seedlings playing a crucial role in population renewal. The study aimed to construct a miniature mangrove community by selecting six widely distributed species found on Hainan Island and simulating different intertidal environments through manipulation of seawater salinity, flooding time, and light intensity.
Methods: The net photosynthetic rate of these six species’ seedlings was measured under various controlled conditions, while statistical analysis was conducted to examine niche width, overlap, and offset among the seedlings across three resource dimensions: seawater salinity, flooding time, and light intensity. This endeavor seeked to elucidate the mechanisms underlying the separation and coexistence of mangrove plants within the intertidal zone.
Results: The results indicated that in terms of flooded water resources, the Avicennia marina (0.956) was the larger niche width and Ceriops tagal (0.906) was the smallest among the 6 seedlings. The niche overlaps were higher in Avicennia marina, Aegiceras corniculatum and Rhizophora stylosa, while they were higher in Kandelia obovata, Bruguiera sexangula and Ceriops tagal. The degree of niche separation reflected that among the species with a theoretical niche center of 12 h/d, Avicennia marina (0.698) exhibited a wider range of adaptation to flooding; whereas among the species with a theoretical niche center of 8 h/d, Kandelia obovata (0.185) showed a broader range of adaptation to flooding; and among the species with a theoretical niche center of 4 h/d, Bruguiera sexangula (0.115) demonstrated extensive adaptability to flooding. In terms of salinity resources, the Avicennia marina (0.987) exhibited the widest niche width, while Kandelia obovata (0.496) had the smallest niche width. The niche overlaps were high among Avicennia marina, Aegiceras corniculatum and Rhizophora stylosa; similarly, Bruguiera sexangula and Ceriops tagal showed significant overlap, whereas Kandelia obovata and other species displayed low levels of overlap. The degree of niche separation indicated that Avicennia marina (4.357) had a broader range of salinity adaptation in species with a theoretical niche center of 20, whereas Ceriops tagal (10.594) demonstrated greater adaptability to varying salinity levels in species with a theoretical niche center of 10. In terms of light resources, Ceriops tagal (0.995) had the widest niche width, while Avicennia marina (0.828) had the smallest niche width. The mangrove species exhibited higher levels of niche overlap. The degree of niche separation indicated that among species with a theoretical niche center of 100% light, Rhizophora stylosa (23.138) displayed a larger range of light adaptation, whereas among species with a theoretical niche center of 80% light, Ceriops tagal (12.522) showed a greater range of light adaptation.
Conclusion: Combined with the niche analysis of flooding, salinity, and light resources, it is evident that the niche differentiation of each species in terms of flooding and salinity resources occurs relatively early and prominently. Each species occupies distinct ecological niches characterized by low overlap and weak competition for these resources, thereby achieving separation and coexistence in the intertidal zone. However, niche differentiation in light resources appears relatively delayed and inconspicuous. The majority of plants are sun plants exhibiting high niche overlap and strong competition for light.

Key words: mangroves, niche, niche overlap, niche differentiation, species coexistence