Are Chilean coastal forests pre-Pleistocene relicts? Evidence from foliar physiognomy, palaeoclimate, and phytogeography

Abstract

Aim We ask whether contemporary forests of the Chilean Coastal Range can be considered to be direct and conservative descendants of pre-Pleistocene palaeofloras that occurred in southern South America from the Palaeogene to early Neogene periods (65-10 Ma), maintaining foliar physiognomies that do not match their present-day climate. We also identify the most likely ancestors of present-day coastal forests. Location Coastal Range of south-central Chile (33-40 degrees S).

Methods We compared leaf morphology between five representative modern floras from mid-latitude forests of the Chilean Coastal Range, and 14 Palaeogene-early Neogene palaeofloras from southern South America. We also compared the composition of biogeographical elements (defined by the modern distribution of plant genera) between fossil and present-day assemblages. Palaeoclimatic reconstructions were based on a canonical correspondence analysis between leaf morphology of modern assemblages and eight climatic variables, and tested by a Monte Carlo permutation test. We compared the relative positions of fossil and modern floras on the environmental vector space defined by Canoco, and on axes defined by instrumental and estimated temperature and precipitation data.

Results According to foliar characters, Palaeogene palaeofloras were strikingly divergent from present-day coastal forests of central Chile. In contrast, two extant forest floras of the Chilean Coastal Range have a foliar morphology that resembles some late Eocene to early Miocene mixed palaeofloras, at least 23 Myr older. These two modern sites are representative of an area of the Coastal Range (36-37 degrees S) that has been highlighted for its relictual character. None of the 14 fossil floras corresponded exactly to the modern composition of phytogeographic elements, although correspondence analyses showed that mixed and Neogene subtropical fossil floras were compositionally close to the extant woody floras of coastal forests in central Chile.

Main Conclusions Contemporary forests of the Chilean Coastal Range exhibit strong physiognomic resemblance to the mixed palaeofloras from 33 degrees 57' to 41 degrees 15' S, which may be the closest ancestor of the deciduous and endemic-rich Maulino forest, presently restricted to coastal areas between 36 degrees and 38 degrees S. In turn, the Neogene subtropical palaeoflora that occurred in the Proto-Andean foothills of central Chile is the likely predecessor of Mediterranean-type sclerophyllous forests of central Chile (32-33 degrees S). Despite foliar resemblance between the late pre-Pleistocene and extant forest floras, our palaeoclimatic reconstructions suggest that modern assemblages exist under climatic conditions that do not match their foliar physiognomy. We attribute this convergence in foliar morphology to the 'evolutionary inertia' of surviving lineages, favoured by the buffering effect of the coastal environment on climatic variability.