Die-Back of the Mangrove Heritiera littoralis Dryand, in the Rufiji Delta (Tanzania) Following El Niño Floods

• Published in: Journal of coastal research no. Special Issue No. 42; pp. 228 - 235
• Main Authors: Erftemeijer, Paul L.A., Hamerlynck, Olivier
• Format: Journal Article
• Language: English
• Published: Coastal Education & Research Foundation (CERF) 01.04.2005
• Subjects: El Nino; Floods; Forest regeneration; Mangrove forests; Mortality; PART III: Holocene Coastal Storms, Lagoons, and Mangroves; Plants; Rain; River deltas; Seedlings; Trees
• ISSN: 0749-0208

The Rufiji Delta is one of the most important wetlands in Tanzania, owing to its significant biodiversity and vast mangrove forest resources, that cover some 53,255 ha or 46% of the country's total mangrove area. The Rufiji River is the largest river in Tanzania, draining 20% of the country. During the El Niño Southern Oscillation Event of 1997–98, heavy rains caused the Rufiji River to swell well beyond its banks resulting in extensive floods, which lasted for at least 6 months. The flow of the Rufiji River during this period had reached an exceptionally high peak maximum of 10,300 m³ s⁻¹, which is about double the maximum peak flows recorded previously over the last 50 years. After the floodwaters had receded, it became apparent that significant tracts of mangrove forest in the upper reaches of the northern delta had died. Analysis of a series of 1999 aerial photographs indicated that an area of approximately 117 ha of mangrove forest had experienced massive tree mortality. A detailed field survey three years after the flood revealed that this mortality was for the major part limited to mature trees of the economically valuable mangrove species Heritiera littoralis. Another tree species, Barringtonia racemosa, co-occurring in the same forest stands, had largely survived the excessive floods. At the time of the survey, the die-back areas were predominantly covered by a dense understory vegetation of vines, climbers, sedges, grasses, ferns and shrubs. Forest regeneration of the affected areas was characterized by a massive development of B. racemosa seedlings and saplings (>9,000 and >4,000 ha⁻¹ respectively). Regrowth of H. littoralis was minimal (20 seedlings ha⁻¹ and 22 saplings ha⁻¹) and viable Heritiera seeds on the forest floor were absent (unlike Barringtonia seeds, which reached densities of over 4,000 ha⁻¹).