Can Zakat funds be used to plant mangrove trees? Amongst its benefits for the ecosystem include reducing the risk of floods/tsunami, fish breeding ground for fishermen and so on.

The planting of mangrove trees for the reasons above are not part of any Asnaf category. Caring for the environment is a good practice under Islam and the shared responsibility of everyone - from individuals to the ruler. However, funds other than Zakat should be utilised.