As elevation increases, both temperature and moisture availability decrease. In many parts of the world, this decrease in temperature is a limiting factor for vegetation—at certain elevations, the temperature becomes too cold for plants to survive. However in the tropics, moisture availability may play a more important role than temperature in determining the altitude at which forests can grow. For example on Haleakalā, a volcano on the Hawaiian Island of Mauʻi, the forest line is not found at the same elevation everywhere, as you would expect if it were controlled by temperature. Rather, the forest line is highest in the wetter eastern-most end and lower on the drier, western end of the volcano.
Research also suggests that short-duration extreme climate events that cause changes in moisture, such as El Niño, can control the upper limit of forests more than long-term average climate does. This is because species found at the forest line are often already near their physiological limits, so extreme changes can have a big effect on their survival.
A strong El Niño occurred in Hawaiʻi in 2015 and provided researchers with a unique opportunity to observe how vegetation responds to ensuing climate variability. Researchers are focusing on Metrosideros polymorpha, a species of flowering evergreen native to Hawaiʻi. This species is the dominant canopy tree found near the forest line and will be used as an indicator for how the forest line responds to El Niño-driven drought. Specifically, the objectives of this study were to:
(1) Observe the climate and ecosystem processes at locations near and above the forest line on Haleakalā during an El Niño event.
(2) Determine how the ecosystem responds to changes in climate, such as drought, caused by El Niño.
(3) Determine whether plant responses to short-term changes in climate and drought vary along the cross-slope rainfall gradient.