Vegetation and fire dynamics in a Hawaiian tropical cloud forest over the early Holocene

Background/Question/Methods

Hawaiian montane vegetation spans some of earth’s steepest climate gradients and houses numerous endemic species.  Anthropogenic climate change’s increasing influence on the montane tropics necessitates greater understanding of cloud forest sensitivity and baseline dynamics. Additionally, the recent fire-induced lowering of the cloud forest limit on Mt. Kilimanjaro indicates the potential role of fire in a changing montane microclimate. Our research seeks to quantify vegetation dynamics and fire regimes in a Hawaiian cloud forest over the early to mid-Holocene.  We analyzed pollen and charcoal particles at high temporal resolution from a small lake (Wai Ele Ele) situated at 1980m a.s.l., ~200m in elevation below the modern cloud forest limit.  To interpret the record in an ecological context, we compare fossil pollen assemblages with a modern library of moss polster and soil assemblages, sampled across a quantitative vegetation study ranging from mesic alpine grassland, through a montane shrubland, and across the forestline into cloud forest.  To increase the taxonomic resolution of our record, we are developing a new proxy for fern community composition – fossil fern sporangia.

Results/Conclusions

Pollen analysis documents notable species assemblage variation, although the site remained forested.  The sediment core lacks charcoal, suggesting that for the past ~4,000 years vegetation varied in the absence of fire. Hierarchical clustering of stratigraphically unconstrained pollen assemblages identified three compositionally distinct groups.  These groups are well-resolved along the primary axis of a non-metric multidimensional scaling ordination, which explains 80% of the variation.  Following the samples stratigraphically through the ordination shows that early in the site’s history, vegetation changed from high fern abundance to greater abundance of two sub-canopy trees (Myrsine sp. and Melicope sp.).  Around 2000 years ago an understory shrub (Broussasia arguta) that exists relatively low on the modern elevational gradient increased in abundance.  Recently, ferns began to increase to levels not seen since ~4000 years ago.  Consequently, the most recent pollen assemblages cluster with the basal pollen assemblage.  Large changes in abundance of fern spores suggest variation in aridity may have induced these vegetation changes. Unfortunately, many fern species produce the same monolete psilate spore type, so taxonomic resolution of spore assemblages is limited.  Preliminary analyses of fern sporangia harvested from herbarium specimens show that sporangia can be identified to family, genus, or species through ordination of <10 sporangium measurements.  A fossil fern sporangia study will increase taxonomic resolution and aid future interpretation of cloud forest dynamics.