Does current climate explain plant disjunctions? A test using the New Zealand alpine flora

Abstract

AbstractAimMore than 90% of plant species found in New Zealand's alpine environment are endemic to New Zealand. Two particularly species‐rich regions of high endemism are located at the northern and southern ends of the South Island and several vascular alpine plant species show clear disjunct distributions between these two regions. The aim of this study was to test the ability of a climatic hypothesis to explain these disjunct distributions and to identify if the central South Island acts as a climatic barrier.LocationAlpine South Island, New Zealand.MethodsThe South Island was divided into three regions (northern, central, southern) following an established biogeographical delineation of centres of endemism. Published data were used to quantify current climatic conditions and to obtain occurrence records of all plant species occurring in the alpine areas. We first calculated overall similarity in climate and in vegetation composition above and below the tree line between the three regions. We then assessed climate niche overlap between regions for five congeneric pairs of disjunct and continuous plant species found both above and below the tree line.ResultsWhile there were broad scale similarities in climate between northern and southern centres of endemism, particular above the tree line, the central region does not appear sufficiently dissimilar to the climate of the endemism centres to conclude that it might act as a barrier between populations of the disjunct species investigated here. Instead, we showed that in geographically separated areas, disjunct species occupy less similar climates than species with a continuous range, possibly indicating a niche shift of the disjunct populations in the separated areas.Main conclusionsOur study found no support for current climate constraining the distribution of disjunct species. For all species examined, overlap existed between the climate niche of at least one disjunct northern or southern population and the climatic conditions of the central region. This overlap in climate space suggests that other abiotic and/or biotic factors are preventing the spread of disjunct species into the central South Island endemism gap.