Variability in modern pollen rain from moist and wet tropical forest plots in Ghana, West Africa

Adele C M Julier,Phillip E Jardine,Stephen Adu-Bredu,Angela L Coe,Wesley T Fraser,Barry H Lomax,Yadvinder Malhi,Sam Moore,William D Gosling

doi:10.1080/00173134.2018.1510027

Abstract

How pollen moves within and between ecosystems affects factors such as the genetic structure of populations, how resilient they are to environmental change, and the amount and nature of pollen preserved in the sedimentary record. We set artificial pollen traps in two 100 m by 100 m vegetation plots, one in a wet evergreen forest, and one in a moist semi-deciduous forest in Ghana, West Africa. Five traps from each plot were counted annually from 2011 to 2014, to examine spatial and temporal variation in the pollen rain of the most abundant taxa shared between pollen and vegetation assemblages. Samples from the wet evergreen plot exhibited high variability within years, with the dominant pollen types changing between samples, and many pollen taxa being over-represented relative to their parent plant abundance in some traps whilst being entirely absent from others. The most abundant plant taxa of the wet evergreen plot (Drypetes and Cynometra) do, however, constitute major components of the pollen rain. There is less variation between samples from the moist semi-deciduous plot spatially, as it is dominated by Celtis, which typically comprises >70% of the pollen assemblages. We conclude that pollen rain in these tropical ecosystems is highly heterogeneous, and suggest that pollen assemblages obtained by trapping are susceptible to small-scale variations in forest structure. Conversely, this may mean that current recommendations of more than three years of trapping in tropical systems may be too high, and that space could substitute for time in modern tropical pollen trapping.

Highlights

Record (Overpeck et al 1985; Fægri et al 1989)
We present modern pollen assemblages collected in artificial pollen traps deployed in two forest settings in Ghana; one wet evergreen rainforest (Ankasa) and one moist semi-deciduous forest (Bobiri)
Non-Metric Multidimensional Scaling (NMDS) analysis was undertaken in Vegan, excluding singletons and using double Wisconsin standardisation with dimensions = 3 (Bray & Curtis 1957)

Summary

Introduction

Record (Overpeck et al 1985; Fægri et al 1989). Pollen production has been shown to reflect physical variables such as temperature, precipitation (Nielsen et al 2010) and solar irradiance (Haselhorst et al 2017), as well as pollination syndrome (Bush 1995). Modern pollen studies in tropical forest show high levels of local (within tens of metres of samples) pollen in traps (Bush & Rivera 1998; Gosling et al 2005). The interpretation of the fossil pollen record relies upon an understanding of how plants in modern ecosystems produce pollen The larger the lake, the higher the proportion of regional and anemophilous, taxa it is likely to contain (Janssen 1966; Jacobson & Bradshaw 1981), whereas pollen traps tend to record local signals, meaning that entomophilous and locally abundant taxa can be relatively over-represented. Neotropical pollen studies show that there are clear changes in pollen assemblages along savannah to forest transitions, with shifts from herbaceous and grass dominated assemblages to arboreal dominated forest assemblages (Gosling et al 2009; Alejandra et al 2013)

Methods

Results

Discussion

Conclusion