If the mean trade wind inversion (TWI) altitude descends with increasing CO2, as preliminary models project, high islands in the tropical trade wind belt may be vulnerable to climate change. At the TWI in these tropical montane ecosystems, cloud forest reaches its upper limit and fire frequency peaks. Landscape-level fires are more common during El Niño/Southern Oscillation (ENSO) events and as a consequence, fire may influence vegetation dynamics in changing montane climates. In Hawai‘i, the upper montane cloud forest represents the last remaining habitat for many endangered plants and animals. As increased temperatures push non-native avian malaria and other invasive species uphill, a stronger Hadley Cell may drive the TWI downhill, reducing cloud forest area and watershed yield. What can managers expect as climate change continues? Patterns of species assemblages across modern climate gradients suggest this landscape will respond to changing moisture availability. However, the native fire regime is largely unknown in Hawai‘i, making predictions regarding fire difficult. We compare paleoecological records that span the TWI with published records of paleoclimate variability important in the tropical Pacific to (1) test hypotheses about cloud forest sensitivity to climate change and (2) describe the fire regime across the TWI.
Results/Conclusions
A pollen record from ~200 m below the modern TWI-maintained cloud forest limit showed abrupt vegetation changes. A comparison of fossil assemblages with modern pollen assemblages – from alpine mesic grassland above the TWI, a subalpine shrubland, and a montane cloud forest below the TWI – suggests vegetation responded to a shifting TWI position. Comparison of fossil assemblages with available paleoclimate records suggests vegetation is correlated most strongly with onsite evidence for drought and also with the position of the intertropical convergence zone (ITCZ) and ENSO. The abundance of pollen from Myrsine, the strongest arboreal indicator across modern moisture gradients in the cloud forest, is well correlated with ENSO and the ITCZ (Spearman’s rank order correlations, P <0.001). High rates of vegetation change occurred during peak ENSO activity from ~2200 – 1500 years ago, and these vegetation dynamics occurred without fires. However, fires appeared synchronously at the two highest positions in the landscape, above the modern TWI, from ~500 – 300 years ago, a time of low-moderate ENSO activity. Despite the broad climatic ranges of most Hawaiian plants, vegetation near the TWI will likely change abruptly with climate change, even without the influence of fire, particularly if ENSO intensifies or the ITCZ position shifts.