Distinguishing the pollen of Dipterocarpaceae from the seasonally dry and moist tropics of south-east Asia using light microscopy

The Dipterocarpaceae family is ubiquitous across mainland and maritime south-east Asia. Species within the family are often so well-adapted to – and prolific within – ecologically distinct forest types, that they are used as habitat indicators within forestry and ecological research. The limited work on the classification of Dipterocarpaceae pollen under light microscopy, however, means that paleoecologists working in the region are currently unable to link fossil pollen to indicator species/assemblages with any confidence. As a consequence, ecologically meaningful and habitat-specific data are homogenized in the paleorecord. This study explores whether the classification of pollen from representative south-east Asian Dipterocarpaceae species can be resolved at a higher taxonomic resolution. This will provide paleoecologists with a tool to (1) better assess shifts in forest types (and hence climate/elevation envelopes) through time, and (2) predict species response to future regional change. Our revised classification scheme is based on an analysis of the pollen morphology of Dipterocarpaceae species from two globally significant ecoregions in south-east Asia – the seasonally dry tropical forests of mainland south-east Asia, and the Sulawesi lowland rainforests of Wallacea – under the light microscope. Results of this analysis show that, using a range of morphological metrics – notably grain size, size-to-wall thickness ratios and exine structure – mainland and Wallacean Dipterocarpaceae pollen can be readily classified into three taxonomic groups – Vatica, Dipterocarpus and Shorea/Hopea/Anisoptera/Pentacme types. At an intra-ecoregional scale, each assemblage can be further broken down into six groups. Some of these groupings appear useful for the classification of certain indicator pollen types within the family, including those taxa representative of key open – and closed – canopy dry forest habitats in the mainland seasonal tropics, the delimitation of peat/swamp forest types in both ecoregions, and the identification of lowland mafic forest types in Sulawesi. Additionally, being able to split the Dipterocarpaceae within these ecologically important ecoregions at a higher taxonomic resolution is of potential conservation significance, permitting long-term assessment of the adaptation capacity of different genera/species to climatic change or disturbance events.