Incongruent range dynamics between co‐occurring Asian temperate tree species facilitated by life history traits

Abstract

Postglacial expansion to former range limits varies substantially among species of temperate deciduous forests in eastern Asia. Isolation hypotheses (with or without gene flow) have been proposed to explain this variance, but they ignore detailed population dynamics spanning geological time and neglect the role of life history traits. Using population genetics to uncover these dynamics across their Asian range, we infer processes that formed the disjunct distributions of Ginkgo biloba and the co‐occurring Cercidiphyllum japonicum (published data). Phylogenetic, coalescent, and comparative data suggest that Ginkgo population structure is regional, dichotomous (to west–east refugia), and formed ˜51 kya, resulting from random genetic drift during the last glaciation. This split is far younger than the north–south population structure of Cercidiphyllum (~1.89 Mya). Significant (recent) unidirectional gene flow has not homogenized the two Ginkgo refugia, despite 2Nm > 1. Prior to this split, gene flow was potentially higher, resulting in conflicting support for a priori hypotheses that view isolation as an explanation for the variation in postglacial range limits. Isolation hypotheses (with or without gene flow) are thus not necessarily mutually exclusive due to temporal variation of gene flow and genetic drift. In comparison with Cercidiphyllum, the restricted range of Ginkgo has been facilitated by uncompetitive life history traits associated with seed ecology, highlighting the importance of both demography and lifetime reproductive success when interpreting range shifts.

Rethinking Living Fossils

“…Distribution range modeling linked with climatic fluctuations indicates that seasonal aridity more than temperature was a constraining factor (Huang et al 2015 ). Cenozoic co-occurrence of Ginkgo and Cercidiphyllum in disturbed streamside environments suggests prolonged conservation of their habitats (Zhao et al 2016 ). Molecular phylogeography, niche modeling, paleoclimatology, and life-history traits reveal an even more dynamic geographic history through glaciations.…”

“…Finer-scale studies reveal incongruent episodes of retreat, colonization, and expansion linked to changes in preferred habitats, as well as regional temperature and aridity fluctuations, topography, and hydrology. G. biloba's more restrictive, disjoint distribution relative to C. japonicum may relate to longer generation times, more climatically vulnerable reproductive periods, or more limited dispersal of its fleshy seeds compared to wind dispersal (Zhao et al 2016 ), implicating population demographic and life-history traits as potential explanations.…”

Ginkgo biloba’s footprint of dynamic Pleistocene history dates back only 390,000 years ago

“…In our previous contribution analyzing range dynamics between co-occurring Asian temperate trees from temperate deciduous forests [ 16 ], we elaborated on the relevance of three competing hypotheses that may explain the variation in the degree of expansion to former range limits in eastern Asia. The first hypothesis, introduced by Qian & Ricklefs [ 13 ], postulates that during glacial periods populations merged and admixed at lower elevations (isolation with admixture).…”

Phylogenetic age differences in tree assemblages across the Northern Hemisphere increase with long‐term climate stability in unstable regions

“…Previous studies have shown strong evidence that glacial cooling has led to large range contractions in surviving tree species in Europe, albeit a subset of the more hardy species remained relatively widespread (Svenning, Normand, & Kageyama, ; Willis & Andel, ). Although less severe, many tree species have also exhibited range contractions during glacial maxima in North America (Bell, Bradford, & Lauenroth, ; Ordonez & Williams, ) and East Asia (Ni, Cao, Jeltsch, & Herzschuh, ; Zhao et al, ). These dynamics still shape diversity patterns within the regions, affecting endemism (Feng, Mao, Sandel, Swenson, & Svenning, ; Svenning & Skov, ), phylogenetic structure (Ma, Sandel, & Svenning, ) and even functional diversity (Ordonez & Svenning, ).…”

“…Although less severe, many tree species have also exhibited range contractions during glacial maxima in North America (Bell, Bradford, & Lauenroth, 2014;Ordonez & Williams, 2013) and East Asia (Ni, Cao, Jeltsch, & Herzschuh, 2014;Zhao et al, 2016). These dynamics still shape diversity patterns within the regions, affecting endemism (Feng, Mao, Sandel, Swenson, & Svenning, 2016;Svenning & Skov, 2007), phylogenetic structure (Ma, Sandel, & Svenning, 2016) and even functional diversity .…”