Palaeoecological responses to environmental changes on South Pacific islands.

Abstract

South Pacific island ecosystems are exposed to numerous disturbances, which are both abiotic and human-driven in nature. However, little is known about the long-term responses (>100 year) of these ecosystems to such events. Palaeoecological methods can provide information about responses, recovery times, and extend our knowledge of vegetation change beyond recent history through centuries and millennia. Focusing on the South Pacific islands this thesis aims to understand how periods of environmental disturbance such as climate variability, fire regime change, human arrival, relative sea level change, and volcanic eruptions have altered ecosystems.
Specifically, three of the thesis aims are to understand 1) how local vegetation responded to volcanic eruptions and sea-level change during the Mid-Late Holocene in the Tongan archipelago. 2) To quantify at high resolution (~5 years) how the forests surrounding Lake Emaotul, Efate, Vanuatu, and the lake ecosystem responded to the 1450s CE Kuwae eruption and a shift towards drier climate conditions ~1100 cal. years BP. 3) A regional analysis of 15 sites on 13 islands, where the aim is to analyse regional patterns of biotic similarity throughout the past 5000 years.
Methods used to investigate the aims of the thesis are biomarker analysis, charcoal, chironomids, fossil pollen and spore analysis, magnetic susceptibility, radiocarbon dating, tephra analysis, and XRF. Additionally, statistical techniques such as Bray-Curtis similarity, cluster analysis, ordination, and regression models were used. Data from published pollen records was also incorporated.
Results show that in Tonga, relative sea level was a more important driver of change for local vegetation than volcanic eruptions. In addition, smaller, low-elevation island littoral, mangrove, successional, and wetland vegetation may be more vulnerable to sea level changes than vegetation on larger or higher elevation islands or inland vegetation habitats such as rainforest. Analyses from Lake Emaotul, Vanuatu, shows that vegetation turnover increased following the 1450s CE Kuwae eruption. Vegetation and chironomid turnover also increased following a shift towards drier climate conditions ~1100 cal. years BP. The Lake Emaotul analyses also reveal that a recent increase in burning regime led to an increase in vegetation turnover during the last ~85 years. Biotic similarity analyses across South Pacific islands revealed a 5000 year long trend of biotic homogenisation which accelerated ~3150 cal. years BP synchronous with human arrival to many Southwestern Pacific islands.
Four major conclusions can be determined from this work; 1) volcanic eruptions alter vegetation communities in some instances and this can be captured with high-resolution pollen sampling strategies (e.g., the Lake Emaotul record); 2) coastal vegetation communities on low-elevation and/or small islands may be more at risk from sea level rise; 3) vegetation and chironomid communities are sensitive to decreases in precipitation, and 4) human presence on South Pacific islands may have led to enhanced biotic homogenisation. These analyses extend our knowledge of ecological change on South Pacific islands and highlight the importance of environmental disturbances as drivers of ecosystem change.

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Identifiers

ORCID for Nichola Ann Strandberg: orcid.org/0000-0003-1268-2080

ORCID for Sandra Nogue Bosch: orcid.org/0000-0003-0093-4252

ORCID for David Sear: orcid.org/0000-0003-0191-6179

ORCID for Peter Langdon: orcid.org/0000-0003-2724-2643

ORCID for Mary Edwards: orcid.org/0000-0002-3490-6682

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