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

despite the Kumamoto trees being 1 year younger. This indicates that Kumamoto is more suit- able for tree growth and is probably more robust to drought conditions than the Chiba site, which may explain the contrasting growth patterns observed in our study. Another possible explanation might be that there was a significant difference in precipitation between the two sites during the growing season: the amount of rainfall from May to August was 2.5-fold higher in Kumamoto than in Chiba. The Chiba population therefore experienced more drought stress during the growing season than the Kumamoto population. This might have caused a delayed effect of drought stress in early spring at the Chiba site [60,61], which partly explains the lack of a significant growth response to drought at the Kumamoto site. A relatively intense growth response to temperature and precipitation was found in July of the year prior to tree-ring formation at the Chiba site; however, the sensitivity to temperature and precipitation was relatively weak compared to sensitivity to drought conditions, as indi- cated by the number of significant correlations (i.e., temperature: 23; precipitation: 25; and dry period: 50). This indicates that climatic factors influencing the growth of C. japonica may be difficult to identify using monthly temperature and precipitation data alone. The lack of sensi- tivity to temperature can partially be explained by the wide distribution range of C. japonica, which is widely adapted to various environments. Natural forests of this species range from the

Aomori prefecture (40˚420N, 140˚120E) to Yakushima Island (30˚150N, 130˚300E) [27,62]. As the study sites were spread over a wide latitudinal range (within the species’ natural distribu- tion range), the temporal variation in temperature observed may not sufficiently influence tree growth. However, the observed lack of sensitivity to precipitation could be due to more extreme climatic events, such as a large amount of rainfall during a short period (e.g., typhoons). Such climatic events can significantly increase monthly precipitation but have a limited influence on water availability for tree growth. This indicates that water availability for the growth of C. japonica cannot be fully explained by monthly precipitation alone and high- lights the importance of using other indicators more likely to reflect water availability and its influence on tree growth [63]. This may be particularly important for tree species adapted to humid climates because such species’ climate sensitivity is often difficult to assess compared with those adapted to drier habitat (e.g., Pinus, [5]; Larix, [4]). 4.2. Climate sensitivity QTLs The QTL located in linkage group 3 in the YI38 map contributed significantly to the climate sensitivity of C. japonica (PVE = 12.4%). The PVE magnitude of the QTL is considered to be relatively high; in comparison, the generally reported effect of QTLs in other tree species is often less than 5% (reviewed in [24]). The large contribution of this QTL to climate sensitivity

indicates that it plays a significant role in tree growth under drought conditions. Additionally, this QTL could be closely related to genes associated with growth response to drought stress. Key genes in drought tolerance and adaptation of plant model species (e.g., Arabidopsis thali- ana, Populus) and other forest tree species (e.g., Pinus, Picea) have been reported in previous research (reviewed in [64]), and further research with similar approaches on C. japonica is essential to identify the same genes. Furthermore, identifying genes associated with drought stress is of importance for future forest management as climate change is expected to increase the frequency of extreme climatic events, such as drought and heatwaves [1]. Apart from the strongest QTL, there were a number of significant QTLs identified with a relatively small contribution to overall climate sensitivity (<3%). This implies that drought sensitivity in C. japonica is influenced by a number of genes, each with a different effect on the overall climate sensitivity phenotype. This observation was consistent with the findings of pre- vious QTL mapping studies on the growth traits of major tree species (e.g., Pinus taeda, [65]; Climate sensitivity of Cryptomeria japonica PLOS ONE | https://doi.org/10.1371/journal.pone.0228278 January 28, 2020 9 / 14