Measurement of El Niño Southern Oscillation (ENSO)-related Climate Conditions and Ecosystem Responses in Hawaiʻi

Start Date

2014-09-15

End Date

2017-09-14

Release Date

2014

The upper limits of forests around the world are generally controlled by temperature, especially in temperate regions, i.e., forests cannot survive at higher elevations because it is too cold. In tropical and subtropical regions, however, evidence suggests that moisture availability may be the most important factor controlling forest line position, i.e., forests cannot survive at higher elevations because it is too dry. In particular, moisture availability during short-duration extreme climate events that cause drought (e.g., El Niño events) may control forest line position more directly than long-term average climate. We have evidence that this is the case for the forest line on windward Haleakalā. A recent study found that the position of the cloud forest’s upper limit on windward Haleakalā was most strongly related to relative humidity during a strong El Niño drought and only secondarily with average rainfall. These data suggest the forest line may be particularly responsive to strong, short-duration drought events because plant species here, particularly the dominant canopy tree Metrosideros polymorpha (‘ōhi‘a), are near their physiological limits. The forest line position on the windward Haleakalā slope is not found at the same elevation everywhere, as you would expect if it were controlled by temperature. In our study area, the forest line is highest (2200 m) in the wetter eastern-most end and is 200 m lower in the western end. On the eastern end (NE Rift), annual rainfall averages ~ 6900 mm at the forest line, whereas < 5 km west of there (Pu‘u Alaea), annual rainfall measures ~ 5600 mm at the forest line. These patterns suggest that forest line is not controlled simply by the average climate pattern. Another recent study found clear differences across this forest line ecotone in water relations of ‘ōhi‘a related to microclimate. This baseline study provided an important snapshot of the potential role of water relations in determining forest line position; however, it occurred during a relatively wet window of time over one 10-day period. The objectives of this project are to 1) observe microclimate and ecosystem processes at sites near and above the forest line ecotone during the period from Fall 2014 through Summer 2015, 2) determine the response of ecosystem processes to climate variability within the study period, and 3) determine whether plant responses to climate variability and water stress vary along the cross-slope rainfall gradient. This study will provide crucial data, measured for the first time, on how ecophysiological characteristics of M. polymorpha vary in response to climate variability and how the response differs along the cross-slope gradient.

ScienceBase WMS

• National and Regional Climate Adaptation Science Centers
• Pacific Islands CASC