Water use rates of tropical atoll vegetation

Slideshow

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Participants

  • Michael Burnett; PhD Candidate, University of California, Santa Barbara, USA
  • Charlie Braman; PhD Candidate, University of California, Santa Barbara, USA
  • Rayna Ruggeri; Conservation Technician, The Nature Conservancy, USA
  • Michael Bliss; PhD Student, Texas A&M University, USA
  • Hillary Young; Professor, University of California, Santa Barbara, USA
  • Leander Anderegg; Assistant Professor, University of California, Santa Barbara, USA

Summary

There are currently 598 atolls documented worldwide. This includes Tetiaroa, which is located approximately 56 kilometers north of Tahiti in French Polynesia. Tetiaroa is composed of 12 small islets, known as motu, surrounded by a coral reef encircling a central lagoon. Atoll ecosystems serve as vital refuges for seabird migration and nesting, support diverse coral reef systems, and maintain important marine–terrestrial interactions that drive nutrient cycling and productivity. Tetiaroa hosts critical seabird colonies including the Brown Booby (Sula leucogaster), Red-footed Booby (Sula sula), Great Crested Tern (Thalasseus bergii), and White Tern (Gygis alba), among others, which rely on its native forests and non-native coconut groves for nesting and foraging. The atoll also supports key endemic and native plants, including Pisonia grandis, Heliotropium foertherianum, and Scaevola taccada, which contribute to its unique terrestrial ecosystem.

Like most island systems, atolls face major threats from invasive species. Islands account for roughly 75 percent of all recorded extinctions of reptiles, birds, amphibians, and mammals, with invasive species implicated in 86 percent of those cases (Island Conservation). On Tetiaroa and nearby Palmyra Atoll, invasive black rats (Rattus rattus) and non-native coconut palms have significantly altered ecological roles and relationships, prompting large-scale eradication efforts. These rats feed on seabird eggs, hatchlings, and crabs, further disrupting the ecosystem. In 2022, Rattus rattus was successfully eradicated from Tetiaroa, although the Polynesian rat (Rattus exulans) remains. Eradication efforts are often paired with studies tracking how ecosystems shift and respond when invasives are removed. Within this broader conservation context, our research will examine how invasive Cocos nucifera influence groundwater use compared to native trees, providing insight into management strategies for the long-term sustainability of atoll ecosystems.

Methods

We measured how much groundwater is consumed by widespread tree species on French Polynesian atolls. Our goal was to determine the average number of liters of water used per day by species such as coconuts, Pisonia, and Heliotrope, and to assess how differences in weather conditions change these rates among species. To do this, we installed sapflow sensors on 60 trees, including Pisonia grandis, Heliotropium foertherianum, Pandanus spp., and Cocos nucifera. We removed a small section of bark at chest height, and drilled three narrow holes into the trunk for the sterilized probes. The probes were then sealed with non-toxic grease to protect the tree, and connected to durable plastic cases holding data loggers and batteries, which we attached to the trunk. These sensors are commonly used in hydrology studies without detectable harm to trees. In addition, we collected a narrow core sample from each tree to measure wood density and water content. All trees were tagged if not already permanently marked.

Techniques

Sap flux sensors, weather monitoring equipment, vegetation surveys, canopy cover photography.

Aim

The data will show how much water different atoll tree species use and how these rates respond to weather variation. This information will provide us data that will guide island managers in decisions about managing groundwater resources, including whether to reduce populations of species that consume large amounts of groundwater. Such strategies are expected to become increasingly important as sea-level rise and drought intensify freshwater challenges for island communities.


Map provided by Tetiaroa Society