Dynamics of a tropical forest ecotone in Hawai‘i are driven by changes in large scale climate features and fire

Background/Question/Methods

As climate change continues, landscape conservation cooperatives are beginning to craft adaptation plans that consider the potential impact of climate change.  To aid this effort, much work has been done to elucidate relationships between vegetation and climate across the landscape today.  However, many important ecological drivers vary on time scales that are too long to appear in contemporary ecological studies – such as changes in fire regimes or global climate features.  High-resolution paleorecords offer a window into the importance of variability that occurs over long time scales.  Integrating an understanding of vegetation-climate relationships from the modern landscape with observations of vegetation dynamics and paleoclimate variability over thousands of years will highlight which climate features are important drivers of landscape pattern.  We studied vegetation relationships with climate and edaphic properties across the landscape today, and reconstructed vegetation dynamics over time with pollen records from three sites that span the sharp upper limit of cloud forest on Haleakalā volcano – in the alpine grassland, at the forest line boundary, and in the cloud forest.  We compared these reconstructed vegetation dynamics with charcoal records and published records of variation in important large scale climate features – the Intertropical Convergence (ITC) and El Niño/Southern Oscillation (ENSO).

Results/Conclusions

Across the landscape today, multivariate regression trees show that vegetation is structured primarily by gradients in moisture availability.  Paleoecological records show that vegetation at the three sites has varied over the past ~3,300 years.  Vegetation near the modern-day ecotonal boundary showed the highest degree of dynamism, suggesting forest line has moved over time.  Around 2,500 years ago, when the position of the ITC shifted southward, the Sorensen distance between sites began to diverge, suggesting the sharpness of the ecotone increased.  Multivariate regression trees show that the position of the ITC is highly correlated with vegetation at all sites, and secondarily, frequency of ENSO structures vegetation dynamics.  Around 850 years ago, after a period of frequent drought-inducing ENSO events, two fires occurred that dramatically sharpened the ecotone further – creating the modern alpine grassland and subalpine shrubland that we know today.  Modern data suggest that water availability is important on this tropical mountain and paleorecords show that the ITC and ENSO, which control water availability in Hawai‘i, are particularly strong drivers of vegetation dynamics.  These data underscore the importance of assessing ecological processes and patterns that occur over longer time scales, for more realistic climate change adaptation planning.