Fine-scale assessment of changes in zonations of species for the management of imperiled mangroves, Pichavaram, India
Résumé
Each mangrove region has a specific history which needs to be understood as it is a central component for adapting current conservation plans to changing coastal conditions induced by increasing natural or human pressures. As these latter impact the functioning, physiognomy and extent of mangroves, it is urgent to design and implement monitoring programs able to monitor changes in zonations of mangrove species (ZMS). Recent studies highlighted the combined potential of very high spatial resolution (VHSR) satellite images and robust field data to map such mangrove areas dominated by a few species.
Here we examined the Pichavaram mangroves, southeast coast of India. The 1100-ha mangrove wetland area was, in 1987, declared to be a Forest Reserve by the Forest Department, Government of Tamil Nadu. Disturbances generated by the 2004-tsunami, decreasing freshwater, and increasing pollutants inflow, combine to modify the geochemistry of the whole region with a potential influence on the zonation of mangrove species. Afforestation programmes have been carried out since the 1980s using the fishbone canal-bank technique.
We inventoried species and measured trunk diameters (DBH), tree heights within forest plots representative of a large range of planted and natural ZMS. Based on this ground expertise, we performed a spectral and textural analysis of five VHSR multispectral images acquired between 2003 and 2018 to generate ZMS maps on a scale of 1:2000. Our results indicate that mangrove cover is increasing while species richness decreases in favour of salt-tolerant species. Our analysis also highlights a shrunk in average tree crown size, suggesting alterations in species succession and plant growth capability. We thank the Tamil Nadu Forest Department for allowing us to undertake this study.