Life histories, ecological tolerance limits, and the evolution of geographic range size in Eucalyptus (Myrtaceae)

Abstract

Current theories explaining variability in species geographic range sizes in plants tend to focus on how traits associated with either physiological tolerance limits or life histories are related to range size. In trees, aspects of both physiological tolerance (e.g. drought tolerance) and life history (e.g. life span and growth rate) are related to stem traits such as wood density and height relative to diameter. We examined how the evolution of stem traits is related to geographic range sizes in Eucalyptus at two spatial scales: across the Australian continent and within the wet forests near the east coast of Australia. Geographic range sizes were estimated from herbarium records. Stem trait data were collected from both natural populations and published sources. We used phylogenetically independent contrasts to test for evolutionary associations between stem traits and geographic range sizes. Across Australia, the evolution of stem traits conferring drought tolerance were not consistently associated greater range sizes. This was surprising since arid and semi-arid environments are geographically expansive. Within the eastern forests, the evolution of stem traits defining slow growing, competitively dominant life histories were associated with greater range sizes. These stem traits should confer both a capacity to disperse into previously unoccupied habitats and the ability to persist in habitats already occupied. Traits associated with physiological tolerance and life history had significant effects on the evolution of range sizes in Eucalyptus. However, we demonstrate that the impact of these traits on range size evolution depends on both environmental conditions and the scale at which these traits are examined.