Pollen data: Influences of sea level changes and volcanic eruptions on Holocene vegetation in Tonga

Aim: To investigate mid- to late-Holocene vegetation changes on low-lying
coastal areas in Tonga and how changing sea level and recurrent volcanic
eruptions have influenced vegetation dynamics on four islands of the
Tongan Archipelago (South Pacific). Methods: To investigate past
vegetation and environmental change at Ngofe Marsh (‘Uta Vava’u) we
examined palynomorphs (pollen and spores), charcoal (fire), and sediment
characteristics (volcanic activity) from a 6.7-m long sediment core.
Radiocarbon dating indicated the sediments were deposited over the last
7700 years. We integrated the Ngofe Marsh data with similar previously
published data from Avai’o’vuna Swamp on Pangaimotu Island, Lotofoa Swamp
on Foa Island, and Finemui Swamp on Ha’afeva Island. Plant taxa were
categorised as littoral, mangrove, rainforest, successional/ disturbance,
and wetland groups and linear models were used to examine relationships
between vegetation, relative sea-level change, and volcanic eruptions
(tephra). Results: Relative sea-level change has impacted vegetation on
three of the four islands investigated. Volcanic eruptions were not
identified as a driver of vegetation change. Rainforest decline does not
appear to be driven by sea-level changes or volcanic eruptions. From all
sites analysed, vegetation at Finemui Swamp was most sensitive to changes
in relative sea level. Conclusions: While vegetation on low-lying Pacific
islands is sensitive to changing sea levels, island characteristics, such
as size and elevation, are also likely to be important factors that
mediate specific island responses to drivers of change.