Modern pollen rain predicts shifts in plant trait composition but not plant diversity along the Andes–Amazon elevational gradient

Masha T Van Der Sande,Mark B Bush,Yadvinder Malhi,William Gosling,Dunia H Urrego,Alexander Shenkin,Crystal H Mcmichael,Karina García Cabrera,William Farfan‐Rios,Miles Silman

doi:10.1111/jvs.12925

Abstract

AbstractAimsTerrestrial ecosystems are changing in biodiversity, species composition and functional trait composition. To understand the underlying causes of these changes and predict the long‐term resilience of the ecosystem to withstand future disturbances, we can evaluate changes in diversity and composition from fossil pollen records. Although diversity can be well estimated from pollen in temperate ecosystems, this is less clear for the hyperdiverse tropics. Moreover, it remains unknown whether functional composition of plant assemblages can be accurately predicted from pollen assemblage composition. Here, we evaluate how community‐weighted mean (CWM) traits and diversity indices change along elevation.LocationAmazon–Andes elevation gradient in Peru.MethodsWe used 82 modern pollen samples and 59 vegetation plots along the elevation gradient, and calculated CWM traits and diversity indices for each pollen sample and vegetation plot. We also quantified the degree to which taxa are over‐ or underrepresented by their pollen, by dividing the relative pollen abundance by the relative basal area abundance in the nearby vegetation survey plots (i.e. the R‐rel values).ResultsWe found that CWM wood density increased, and CWM adult height and leaf area decreased with elevation. This change was well predicted by pollen assemblages, indicating that CWM trait–environment relationships based on pollen abundance data provide meaningful results. Diversity (richness, Shannon and Simpson) decreased with elevation for vegetation plots, but these trends could not be observed from pollen assemblages.ConclusionsOur results demonstrate that more research is needed to develop methods that lead to accurate diversity estimates from pollen data in these tropical ecosystems, but that CWM traits can be calculated from pollen data to assess spatial shifts in functional composition. This opens opportunities to calculate CWM traits from fossil pollen data sets in the tropics, with broad implications for improving our understanding and predictions of forest dynamics, functioning and resilience through time.

Highlights

Many ecosystems are shifting in plant diversity and composition, likely due to human disturbance and climate change (e.g. Harrison et al, 2015; van der Sande et al, 2016; Muelbert et al, 2018)
We focused on three traits that describe different aspects of plant functioning: (a) wood density (WD), which is associated with the growth–survival trade-off and drought resistance; (b) adult height (H), which enhances light exposure and potential growth rates but reduces resistance to drought because of long hydraulic pathways and increases risk of wind breakage (Jackson et al, 2019); and (c) leaf area (LA), which is a measure of light capture and associated with fast growth rates (Poorter and Bongers, 2006)
We found that the community-weighted mean (CWM) wood density increased, and the CWM adult height and leaf area decreased with elevation, for vegetation plots and pollen samples

Summary

| INTRODUCTION

Many ecosystems are shifting in plant diversity and composition, likely due to human disturbance and climate change (e.g. Harrison et al, 2015; van der Sande et al, 2016; Muelbert et al, 2018). Jaramillo et al, 2006; Peros and Gajewski, 2008) and functional composition over time (Brussel et al, 2018; van der Sande et al, 2019), but we yet have a poor understanding of how well such pollen data represent changes in vegetation diversity and composition. Our understanding of the degree to which taxa are over- or underrepresented by pollen data remains poor, especially for species in tropical ecosystems Such information may be necessary to correct for the bias in pollen data and accurately predict taxon composition and diversity. Abundance-weighted species diversity indices may be difficult to predict from pollen data because abundance is often poorly represented by pollen data (Birks and Line, 1992; Odgaard, 1999) This observation implies that variation in diversity would only be captured after correcting for differences in pollen production among taxa. Changes in diversity would be poorly predicted by pollen because of unknown differences in the pollen source area, missing taxa and low taxonomic resolution

| METHODS

Findings

| DISCUSSION

| CONCLUSIONS