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

Eucalyptus globulus, [66]). These studies indicate that, similar to growth, climate sensitivity is also influenced by many QTLs with relatively small effect sizes. It is important to note that the contribution to climate sensitivity by all identified QTLs (35.9%) was almost three-fold larger than the strongest QTL alone (12.4%), and the intermediate prediction accuracy was found using identified QTLs (r = 0.57, P < 0.001). This indicates that the 13 QTLs identified in this study might be useful in predicting the drought sensitivity of C. japonica. Likewise, using geno- mic data to predict climate change sensitivity is important to inform future forest management practices, particularly for long-lived forest tree species. 5. Conclusions This study found that dry periods (longer than 4 days) strongly influenced the growth response of C. japonica mapping population at the Chiba site. Moreover, we found one QTL with a large contribution to overall climate sensitivity, possibly related to a key gene associated with growth response to drought stress. In contrast, we did not find a significant growth response to drought stress at the Kumamoto site, which was the original habitat of the mapping progenies. This indi- cates that climate sensitivity of the study population is probably influenced by surrounding environmental conditions, which highlights the need for further research on climate sensitivity of C. japonica under various conditions. This includes research on life history stages, climate

(i.e., experiments covering wider geographical locations), and other environmental conditions (e.g., soil conditions; [67]). The epigenetic effects on genotypes might also be important for the growth response observed in the present study [68, 69]. Additionally, because our study was conducted using local varieties from the Kyushu region, additional studies capturing wider genetic variation (e.g., populations from natural forests) with different approaches—such as genome-wide association study (GWAS)—are necessary to generalize the findings of this study. It is also important to note that a further study using the mapping progenies derived from the parents in different provenances (e.g. Honshu region), could provide significant insights into the growth response of Cryptomeria japonica found in the present study. Furthermore, it is pos- sible that the climate adaptation QTLs have been fixed in the Kyushu region, and were mono- morphic in the mapping progenies. This could be tested by crossing C. japonica trees in and outside the Kyushu region. Finally, it is important to note that, while heavy rainfall events (100 mm/day) are increasing, the number of rainfall days in Japan has decreased over the past 4 decades [70]. If this tendency continues in the future, the frequency of dry periods > 4 days (which had significant influence on tree growth at the Chiba site) may also increase. Although C. japonica is currently planted or naturally distributed in humid climates, the negative impact of climate change on the growth of C. japonica via short-term droughts may become more

severe, which requires more research on the climate sensitivity of this species in the near future. Supporting information S1 Fig. Clonal averages of raw (A, B) and relative (C, D) basal area increment (BAI) at the Kumamoto site (A, C) and Chiba site (B, D). (PDF) S2 Fig. Continuous dry (non-raining) days at (A) the Chiba site and (B) the Kumamoto site. The horizontal dashed line indicates the threshold of 4 days of dry period. (PDF) S3 Fig. Varying thresholds and the ratio of zero data (no dry period in a given month). The horizontal dashed line represents 50% of the ratio of zero data. (PDF) Climate sensitivity of Cryptomeria japonica PLOS ONE | https://doi.org/10.1371/journal.pone.0228278 January 28, 2020 10 / 14