The Can Gio mangrove forest is located south of Ho Chi Minh City in Vietnam. It was seriously destroyed during the Vietnam War, but intensely reforested with the mangrove species Rhizophora apiculata afterwards. The UNESCO registered Can Gio as a Biosphere Reserve in January 2000. This story of success has been accompanied by the question whether and how quick the mono-specific plantation will become more diverse in terms of species composition and spatial structure; and thus less vulnerable to insect attacks and synchronous collapsing of even sized plots induced by storms. Since lightning gaps have been identified as a common disturbance phenomenon in mangroves also in other parts of the world, this study has been conducted in order to answer the following questions: (1) How frequent do lightning gaps occur in Can Gio? (2) How are they spatially distributed? (3) Do they significantly contribute to the renaturation of the forest structure? For the first two questions, remote sensing analyses (SPOT and Quickbird: 2003, 2005, 2007), ground truthing, and statistical modeling techniques were combined. The third question was answered by means of simulation experiments carried out with the mangrove simulation model KiWi parameterized for R. apiculata.
Results/Conclusions
The frequency of lightning strikes varied strongly during the period of study. The probability of a lightning strike (0.012 per hectare and year) was smaller than in other areas like Dominican Republic or Florida. The mean size of lightning gaps (246 m²) and their slightly elliptical shape (average eccentricity = 1.34) were in agreement with comparable studies. Statistical modeling revealed that a Matern cluster process with a large range of dispersion (radius = 450m) describes the gap locations. The radii of canopy gaps are similar within a cluster but differ between clusters. The lightning gaps censused were comprised of mixed mangrove species reflecting that seed dispersal from remote forests is not restricted. Simulation results show, however, that canopy disturbances due to lightning are not sufficient for increasing the spatial heterogeneity of the Can Gio forest. Attendant simulation experiments considering the neotropic mangrove species Rhizophora mangle reveal that this features strongly depend on the ratio between canopy disturbances and the overall growth behavior of the species.
