Vegetation-climate relationships across space and time at high elevation in Hawai'i.

摘要

Tropical montane cloud forests are expected to be sensitive to ongoing and anticipated climate change. In this thesis, I ask how vegetation patterns around the cloud forest's upper limit in Hawai'i are related to (1) climate and edaphic gradients on the landscape today and (2) climate and fire variability over the past 3,300 years of time. To assess modern vegetation-climate relationships, I installed an array of 12 climate stations, integrated with 134 vegetation plots, from 1900--2400 m in the windward slopes of Haleakal a volcano, where edaphic characteristics are known. To reconstruct vegetation over time, I obtained three paleorecords from lake and bog sediment cores that bracket the modern forest line. I analyzed fossil pollen assemblages and charcoal accumulation rates in each core, spanning the past 3,300 years. To interpret past vegetation, I established a modem pollen database rooted in the vegetation plots. I developed metrics from the modem pollen assemblages to distinguish cloud forest from the sub-alpine and alpine ecosystems above the forest line with the Receiver Operating Characteristic, I applied these metrics to fossil pollen assemblages to provide a probabilistic reconstruction of forest line position over the past 3,300 years. I compared forest line dynamics with fire regimes and published records of Pacific climate variability, including El Nino/Southern Oscillation frequency, Aleutian low strength, and Intertropical Convergence Zone position.;In this Hawaiian cloud forest, vegetation patterns were very strongly related to gradients in moisture availability. The modem data suggest that cloud forest species assemblage is driven by rainfall and the upper forest limit is structured by relative humidity during strong El Nino events. In the past 3,300 years, the upper limit of cloud forest never fell below its modern position, however, it migrated ~ 200 m upslope when circulation modes tended to increase moisture in Hawai'i---during negative stages of the Pacific Decadal Oscillation (PDO), when El Nino event frequency was low and when the Intertropical Convergence Zone (ITCZ) was in a more northerly position.