A climate change context for the decline of a foundation tree species in south-western Australia: insights from phylogeography and species distribution modelling

Abstract

Background and aims: A worldwide increase in tree decline and mortality has been linked to climate change and, where these represent foundation species, this can have important implications for ecosystem functions. This study tests a combined approach of phylogeographic analysis and species distribution modelling to provide a climate change context for an observed decline in crown health and an increase in mortality in Eucalyptus wandoo, an endemic tree of south-western Australia.

Methods: Phylogeographic analyses were undertaken using restriction fragment length polymorphism analysis of chloroplast DNA in 26 populations across the species distribution. Parsimony analysis of haplotype relationships was conducted, a haplotype network was prepared, and haplotype and nucleotide diversity were calculated. Species distribution modelling was undertaken using Maxent models based on extant species occurrences and projected to climate models of the last glacial maximum (LGM).

Key results: A structured pattern of diversity was identified, with the presence of two groups that followed a climatic gradient from mesic to semi-arid regions. Most populations were represented by a single haplotype, but many haplotypes were shared among populations, with some having widespread distributions. A putative refugial area with high haplotype diversity was identified at the centre of the species distribution. Species distribution modelling showed high climatic suitability at the LGM and high climatic stability in the central region where higher genetic diversity was found, and low suitability elsewhere, consistent with a pattern of range contraction.

Conclusions: Combination of phylogeography and paleo-distribution modelling can provide an evolutionary context for climate-driven tree decline, as both can be used to cross-validate evidence for refugia and contraction under harsh climatic conditions. This approach identified a central refugial area in the test species E. wandoo, with more recent expansion into peripheral areas from where it had contracted at the LGM. This signature of contraction from lower rainfall areas is consistent with current observations of decline on the semi-arid margin of the range, and indicates low capacity to tolerate forecast climatic change. Identification of a paleo-historical context for current tree decline enables conservation interventions to focus on maintaining genetic diversity, which provides the evolutionary potential for adaptation to climate change.

Keywords: Climate change; Eucalyptus wandoo; LGM; Myrtaceae; evolution; forest decline; haplotypes; last glacial maximum; phylogeography; refugia; species distribution modelling; tree decline..

Fig. 1.

Location of the sampled populations of Eucalyptus wandoo in south-western Australia and their haplotype composition. (A) Sampled populations of E. wandoo are represented with large circles that are coloured according to their haplotype composition. Populations with high haplotype diversity within the refugium area are marked with an outer circle. Species records obtained from NatureMap are represented by light grey circles. Locations of the outgroup species samples are represented with black dots (except for E. angustissima, which is located 300 km east from the arrow, on the coast). The dashed line represents the phylogeographic break between the two lineages. Rainfall isohyets lines (200 mm) are represented in white. (B) Haplotype network for E. wandoo. Haplotypes are identified with the letters A–V, according to Table 2. Interior haplotypes not detected in the samples are represented by small black circles. Each line represents a single polymorphism.

Fig. 2.

Phylogenetic parsimony tree of haplotype relationships in Eucalyptus wandoo. The consensus tree is based on 1000 bootstrap replications. Haplotypes are identified with the letters A–V, according to Table 2. Numbers below lines represent bootstrap values. Numbers in parentheses indicate sample numbers of each provenance. Roman numerals refer to the lineage number.

Fig. 3.

Distribution models of Eucalyptus wandoo for (A) the present, (B) the last glacial maximum (approx. 21 000 years) and (C) 2070 periods. Warmer colours indicate a higher probability of species presence, whereas colder colours indicate a lower probability of species presence (see key). Records of E. wandoo obtained from NatureMap are represented by light grey circles, and sampled populations are represented by black/white circles as indicated in the key in (A). Populations with high haplotype diversity within the refugium area are marked with an outer circle.

Fig. 4.

Climatic stability for E. wandoo (A) in the past, between the LGM and present moment, and (B) in the future, projected for 2070. Records of E. wandoo obtained from NatureMap are represented by light grey circles, and sampled populations are represented by black/white circles as indicated in the key in (A). Populations with high haplotype diversity within the refugium area are marked with an outer circle.