Island ecosystem responses to the Kuwae eruption and precipitation change over the last 1600 years, Efate, Vanuatu

Abstract

IntroductionIslands of the Southwest Pacific are exposed to geologic and climate-related disturbances that occur on a range of timescales and which probably affect, to varying degrees, their terrestrial ecosystems. Over the past ∼1100 years we know of two major events in the region: the Kuwae eruption which is thought to have occurred ∼500 cal. years BP and a shift to drier conditions which began ∼1100 cal. years BP.MethodsWe investigated terrestrial and lacustrine ecosystem responses to these events and also to a changing fire regime, likely human-caused, using a multi-proxy (C/N, charcoal, chironomids, pollen, and tephra) record from Lake Emaotul, Efate, Vanuatu.ResultsTephra from the Kuwae eruption was found across a 6 cm layer which our age-depth model suggests was deposited 650–510 cal. years BP (95% confidence). Forest and chironomid community turnover increased during the wet-dry shift 1100–1000 cal. years BP; subsequently, chironomid turnover rates decreased again within <135 years and vegetation had partially (but not fully) recovered after ∼80 years. Following Kuwae volcanic tephra deposition, vegetation turnover increased again, reflecting a reduction in small trees and shrubs and an increase in grasses. Subsequently, the forest vegetation did not regain its previous composition, whereas chironomid community composition remained fairly stable before and after tephra deposition. Within the last ∼90 years, enhanced local burning drove another increase in vegetation turnover.DiscussionTerrestrial and freshwater ecosystems in Efate are sensitive to changes in hydroclimate, volcanism, and anthropogenic fires, although to different degrees; while recent human impacts are often obvious, volcanic eruptions and climatic shifts have also structured Pacific-island ecosystems and will continue to do so.

Floristic homogenization of South Pacific islands commenced with human arrival

“…Within the novel drivers of change, burning (for example, as a part of agricultural activities), introduction of non-native taxa and extinctions or extirpations (of both fauna and flora) have been shown to impact island ecosystems 11 , 30 , 40 . First, the increased occurrence and intensity of disturbances is linked to anthropogenic modification of the islands’ landscapes, for example, as a consequence of slash and burn techniques 11 , 36 , 41 . It has also been shown that intense human impacts, such as urbanization and intensively managed agroforestry plantations, can lead to an increase in globally distributed taxa and therefore biotic homogenization 42 , 43 .…”

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

“…Another study from Vanuatu shows increased vegetation turnover associated with a shift from wetter to drier conditions 1100 cal. years BP (Strandberg et al, 2023). In Fiji, a possible dry phase and change in fire regime may indicate a response to an increase in ENSO events 5000 ca.…”

“…In addition, the temporal resolution of pollen records may present a challenge for capturing rapid vegetation changes or recoveries following volcanic eruptions. For example, the Avai’o’vuna Swamp pollen record has one pollen sample every ~440 years, Lotofoa Swamp has one sample every ~300 years, Finemui Swamp has one sample every ~200 years, and Ngofe Marsh has one sample every ~120 years, potentially limiting the identification of vegetation responses and recoveries following short‐lived eruption events, which can occur on shorter timescales (see Strandberg et al, 2023). Our results, however, suggest that volcanic eruptions did not have multicentennial scale impacts on vegetation of these islands.…”

“…How to cite this article: Strandberg, N. A., Edwards, M., Ellison, J. C., Steinbauer, M. J., Walentowitz, A., Fall, P. L., Sear, D., Langdon, P., Cronin, S., Castilla-Beltrán, A., Croudace, I. W., Prebble, M., Gosling, W. D., & Nogué, S. (2023). Influences of sea level changes and volcanic eruptions on Holocene vegetation in Tonga.…”

“…Such layers allow researchers to identify how ecosystems responded to volcanic events, especially when recovery periods exceed the timescales of modern observations, that is, centuries rather than decades. Evidence for the impacts of volcanic eruptions on vegetation from other ecosystems worldwide indicates variable responses, recovery times and initial conditions (Strandberg et al, 2023). For example, in Ecuador, a paleoecological reconstruction showed an increase in grass pollen and a decrease in forest pollen, presumably indicating a shift in vegetation cover (Montoya et al, 2021), while in Chile, Fesq‐Martin et al (2004) recorded a primary succession sequence.…”