In studies of the performance of a given population across its species range, the Centre-Periphery Hypothesis (CPH) predicts that greater performance will be seen in the geographic and ecological centres rather than at the periphery. However, in recent years, this has often been found not to be the case. An alternative approach is to investigate the historical centre of a species’ range, which may have long-term effects on how the species grows today.
In a recent article published in Annals of Botany, lead author Gonzalo A. Camps and colleagues used the Chaco tree, Bulnesia sarmientoi, to test this approach. The researchers tested morphological functional traits at both the leaf and tree level at the geographic, ecological, and historical centres and peripheries of the tree’s range. The traits were measured in a total of 24 populations across the range, and results were modelled to reveal which approach best explained trait spatial patterns.
The authors found that the patterns retrieved using each of the centre-periphery approaches were not concordant with one another. The ecological and historical centres were not in the same location, suggesting the niche optimum shifted over time. The historical approach did produce a centre-periphery pattern in which smaller trees were found at the centre, larger ones at the periphery.
“The historical centre encloses a putative climatic refugium for B. sarmientoi, which is currently a hotspot of genetic diversity and has been a climate-stable area from the Last Glacial Maximum (LGM) up to the present,” write the authors. “Regarding environmental conditions, LGM refugium (i.e. historical centre) persists in sites where the soil was formed by Andean sediments from the Pliocene, and where the driest climate conditions in the entire range have occurred over time.”
The authors posit that the difference in tree height at the historical centre could be due to either or both of two factors. First, that the trees are small due to a lack of competition in a stressful environment, and second, that their small size is a direct effect of the stressful conditions. In either case, a historical approach to centre-periphery variation allowed the recovery of historical processes that help to explain tree trait variation.