Climate sensitivity of Cryptomeria japonica in two contrasting environments: Perspectives from QTL mapping

The growth responses of tree species to climate changes have been evaluated by multiple studies [4–6]. These studies often targeted tree species adapted to dry or extremely cold cli- mates (e.g., Pinus, Larix), such as temperate and boreal forests, and successfully identified cli- matic variables affecting tree growth. Although the number of studies conducted outside those climatic regions is rapidly increasing [7–10], climate sensitivity of tree species adapted to warm and humid climate remains relatively understudied, probably due to their weak growth response to interannual climatic variation. However, the impacts of climate change (e.g., droughts and heatwaves) are expected to rapidly increase globally [1], and the negative impacts of such events (e.g., seasonal drought; [2]) on tree species in humid climates are also likely to increase. Evaluating the mechanisms underlying these species’ growth response to varying cli- matic conditions changes is necessary to predict the impact of climate change in these forest ecosystems. Previously, empirical research of climatic factors was limited on tree growth and consisted of dendrochronological studies, often using mature or old-growth trees to investigate the impact of climate variation over a large time scale (>100 years; [11–13]). In contrast, short- term climatic variation (e.g., decades) may also have a significant impact on tree growth, par- ticularly during the initial growth phase (from seedlings to young trees; [14–17]). It is during

this initial stage of growth that trees exhibit the largest response to environmental conditions [18]. Additionally, the initial growth of trees has been studied as an important trait in tree breeding [19–21]. Evaluating climatic factors which affect initial tree growth is important not only for the fundamental knowledge on growth response of tree species but also for forest management and molecular breeding under changing climate. Phenotypic traits such as wood growth and properties are influenced by environmental and genetic factors [22]. Therefore, investigating the underlying genetic mechanisms of tree growth response to climate change is essential to gain a comprehensive understanding of cli- mate sensitivity of tree species [23]. In this context, genetic analyses such as quantitative trait locus (QTL) mapping and association mapping have been conducted on many forest species to identify important genome regions (QTLs) and genes that influence complex traits, primar- ily growth (stem growth and bud phenology), wood properties (wood density and chemistry) [24], and climate sensitivity (i.e., growth response to climate change; [17]). Identification of the important genome region or genes in terms of climate sensitivity of tree species would be useful for predicting the performance of genotypes under climate change (genomic selection; [25,26]). Cryptomeria japonica (sugi) is a wind-pollinated, allogamous, evergreen conifer species [27]. Sugi is the most widely planted and commercialized tree in Japan, with its straight trunk

and soft wood, and it has many diverse uses, such as in house construction, ship building, and pulp production [28]. Vegetative propagation of C. japonica is easily conducted by cutting shoots, thereby allowing the replication of clones in common garden experiments across mul- tiple environments. A large number of genetic markers have been developed for C. japonica [29,30], which allows the identification of QTLs strongly associated with genes. These charac- teristics make C. japonica suitable for the evaluation of QTLs associated with climate sensitivity. Several studies have evaluated genomic regions that affect the growth and wood properties of C. japonica [30–32]. Long-term growth patterns of C. japonica have been evaluated using tree rings [33,34] and other methods (e.g., dendrometer; [35,36]). However, few studies have assessed the genetic mechanisms of growth patterns linked with climatic variability in C. japonica. In this study, we aimed to evaluate the growth patterns during temporal variations in temperature, precipitation, and dry period using tree rings (climate sensitivity traits; Housset et al. 2018); we also attempted to identify the genomic regions (QTLs) influencing climate Climate sensitivity of Cryptomeria japonica PLOS ONE | https://doi.org/10.1371/journal.pone.0228278 January 28, 2020 2 / 14