Evidence of Range Shifts in Riparian Plant Assemblages in Response to Multidecadal Streamflow Declines

Helen A White,Raphael K Didham,John K Scott

doi:10.3389/fevo.2021.605951

Helen A White, Raphael K Didham + Show 1 more

Open Access

https://doi.org/10.3389/fevo.2021.605951

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Abstract

Riparian corridors are thought to form hydrological refugia that may buffer species and communities against regional climate changes. In regions facing a warming and drying climate, however, the hydrological regime driving riparian communities is also under threat. We examined recruitment in response to streamflow declines for species inhabiting the riparian zone in southwest Western Australia, testing the extent to which the riparian system has buffered riparian communities from the drying climate. We stratified 49 vegetation transects across the &gt;600 mm per annum regional rainfall gradient encompassed by the Warren River Catchment. Local hydrological conditions were estimated over two 10-year periods; 1980–1989, and 2001–2010, to quantify changes in the flood regime. Mixed effects models tested the relationship between rainfall and flooding on the relative frequency of immature to mature individuals of 17 species of trees and shrubs common to the riparian zones. At the low-rainfall extent of their geographic range, the relative frequency of immature riparian species decreased with declining flow, whereas at the high-rainfall extent of their geographic range the relative frequency of immature individuals increased with declining flow. These results suggest that the geographic ranges of riparian species may be contracting at the low-rainfall margin of their range, while at the high-rainfall margin of their geographic range, reduced flooding regimes appear to be opening up new habitat suitable for recruitment and narrowing the river corridor. No such patterns were observed in upland species, suggesting the river may be buffering upland species. We discuss these findings and their implications for ongoing management and species conservation in a region projected to face further, significant rainfall declines.

Highlights

For many species, survival over the coming decades will depend on their ability to adapt to the new climatic conditions in situ, or shift geographic range to maintain their climatic optimum (Parmesan, 2006; Aitken et al, 2008)
Recruitment failure at a species range margin can be indicative of a disconnect between the geographic and climatic ranges of a species and an advanced warning of an impending range shift
Utilizing one of the world’s most striking, geographically stratified rainfall gradients, that has undergone one of the greatest observed declines in rainfall, we tested the effect of streamflow decline on the riparian plant species in southwest Western Australia (SWWA)

Summary

Introduction

Survival over the coming decades will depend on their ability to adapt to the new climatic conditions in situ, or shift geographic range to maintain their climatic optimum (Parmesan, 2006; Aitken et al, 2008). Determination of range shifts in response to climate change requires temporally replicated data (Bertrand et al, 2011; Feeley et al, 2011; Telwala et al, 2013) In lieu of such datasets, range shifts in plant species have been inferred by examining the skew in abundance distributions (Breshears et al, 2008; Murphy et al, 2010), or by exploiting the long generation times and comparing the distribution of seedlings relative to the adult population (Lenoir et al, 2009; Zhu et al, 2012, 2014; Fei et al, 2017). Juvenile recruitment within an existing range, can be much more sensitive to incremental changes in environmental conditions and provide an early indication of a site becoming unsuitable (Lloret et al, 2009; Bell et al, 2014; Garssen et al, 2014)

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