Relict mangrove stand on Last Interglacial terrace, Christmas Island, Indian Ocean

Abstract

ABSTRACTA unique stand of mangroves of the genus Bruguiera occurs on the shore terrace at Hosnies Spring, Christmas Island, Indian Ocean. The mangrove stand, of about 0.33 ha, occurs 120 m inland and over an elevational range of 13 m, from 24 to 37 m above sea level. It is flooded by freshwater from a spring which trickles over a partially-cemented calcsinter concretion gravel with mud. Trees reach 30–40 m high, and 80 cm diameter breast height. The size structure is bimodal with few trees in the 10–25 cm dbh size classes, but abundant propagules, seedlings and saplings. The shore terrace contains corals, some in their position of growth, and has been shown by Uranium-series dating to be Last Interglacial in age. Several alternative explanations of how the mangroves reached this site are examined. One explanation is that this stand of mangroves has persisted at the site for approximately 120,000 years since the Last Interglacial. While this cannot be proved it does appear that the stand is relict and is actively regenerating in this unusual location.