Intra-Annual Dynamics of Cambial and Xylem Phenology in Subalpine

Park et al. J Wood Sci (2021) 67:22 https://doi.org/10.1186/s10086-021-01950-2 Journal of Wood Science ORIGINAL ARTICLE Open Access Intra-annual dynamics of cambial and xylem phenology in subalpine conifers at Deogyusan National Park in the Republic of Korea Jun‑Hui Park1, En‑Bi Choi1, Hong‑Chul Park2, Na‑Yeon Lee2 and Jeong‑Wook Seo3* Abstract Subalpine conifers are highly sensitive to climatic changes. In these trees, the intra‑annual dynamics of cambial activ‑ ity and phenological process in xylem development are closely associated with climatic conditions. However, these scientifc fndings have not been verifed for subalpine conifers in the Republic of Korea. Therefore, we initiated such a study with four subalpine conifers, viz. Abies koreana, Pinus koraiensis, Taxus cuspidata, and Picea jezoensis, grow‑ ing between 1573 and 1594 m a.s.l. at Deogyusan National Park. Microcores (Ø 2 mm) of these trees were obtained using a mini borer, called as Trephor, every week between April 7 and September 25 in 2017 to monitor their growing seasons, the intra‑annual dynamics of the cambial activity, and the number of cells during phenological phases of cell enlargement and cell‑wall thickening. For the study, fve trees were selected for each conifer species. Results showed that the cambial activity of A. koreana, P. koraiensis, and P. jezoensis required at least 73.8 heat‑sum values, whereas the T. cuspidata needed 109.6 heat‑sum. The durations of cambial activity of A. koreana, P. koraiensis, T. cuspidata, and P. jezoensis were 134 (127–144), 113 (92–128), 113 (106–120), and 100 (76–128) days, respectively. The intra‑annual variations of the number of cells in the cambium, during the cell enlargement phase and cell‑wall thickening phase showed predominantly a bell‑shaped curve, with a delay of approximately 2–4 weeks between each other. On the other hand, the number of cumulated mature cells showed an S‑shaped curve. Through this study, the frst funda‑ mental data on phenological process in xylem development of subalpine conifers in the Republic of Korea have been successfully presented. Keywords: Cambial activity, Xylem cell development, Phenology, Subalpine conifers, Degree‑days Introduction role in reducing the vast amount of carbon emissions Te total forest area in the Republic of Korea is from the atmosphere [2, 3]. However, the trees growing 6383000 ha which corresponds to 64% of the total land at high elevations and/or latitudes face mortality risk and of Korea (Korea Forest Service 2018). Furthermore, the extinction threat due to the ever-increasing global warm- net absorption of greenhouse gases due to land use, land ing and drought risks due to climatic changes [4–8]. use change and by the forestry (LULUCF) sector is 41.6 Subalpine forest, with harsh conditions for tree growth million metric tons CO2 eq. or 5.9% of the total national [9, 10], serves as an ecotone between mixed deciduous emissions in 2017, with forests accounting for 99.9% of forest and alpine vegetation communities [11]. In the the total absorption [1]. Terefore, it can be seen that Republic of Korea, the subalpine forests start from 800 to trees, the main component of forests, play the most vital 1000 m a.s.l. [12], and trees such as Abies koreana, Abies nephrolepis, Picea jezoensis, Juniperus chinensis, Pinus *Correspondence: [email protected] pumila and Taxus cuspidata dominantly occupy the for- 3 Department of Wood & Paper Science, Chungbuk National University, ests [13–16]. Although the conifer species in the subal- Cheongju 28644, Republic of Korea pine zone can grow under extreme growth conditions, Full list of author information is available at the end of the article these species are vulnerable to the efect of rapid climate © The Author(s) 2021. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/. Park et al. J Wood Sci (2021) 67:22 Page 2 of 10 change [7, 17–19]. In a recent publication [16], it has climate change. To evaluate the infuence of temperature been reported that the area of conifers in the subalpine on the initiation of their cambial activities, heat-sum val- zones of the Republic of Korea had reduced by 25%, from ues, the so-called degree-days, were applied. Te results 93.27 to 69.90 km2, over the last 21 years (1994–2015). are expected to serve as fundamental data to evaluate the Deogyusan, the fourth highest mountain peak (1614 m growth conditions of subalpine conifer tree species under a.s.l.) of the Republic of Korea, is one of the important climate change as well as to understand their growth habitats of plants, containing 811 diferent plant spe- phenology at the cellular level. cies [20]. Te Deogyusan National park serves as a habitat for subalpine conifers; however, their spread is only Materials and methods restricted within 3 km2 [12]. Although the subalpine Study sites and tree species conifers are under the threat of extinction due to cli- Te study site was chosen between the peaks of the matic change, surprisingly, only a handful of studies have Hyangjeokbong (1614 m a.s.l.) and Jungbong (1594 m been done so far on the climatic efect on their growth or a.s.l.) in Deogyusan National Park (35°51ʹN, 127°44ʹE) growth strategy. Moreover, most of the research focused (Fig. 1). Te natural vegetation between the Hyangjeok- on the subalpine ecosystem [21–23]. bong and Jungbong peaks consists of Quercus mongolica, Tree growth mainly occurs due to the activities of mer- Pinus koraiensis, Taxus cuspidata, Abies koreana, and istem tissues, viz. apical meristem and vascular cambium Rhododendron schlippenbachii. Among these, the tree [24, 25]. Te growth of shoot is induced by the former species older than 100 years are Taxus cuspidata and and is an externally visible phenology, whereas radial Abies koreana at Hyangjeokbong [54, 55]. As recorded growth is due to the latter and remains hidden [26, 27]. by a meteorological station close to the study site, the Te radial growth integrates exogenous infuences as well mean annual temperature and total precipitation dur- as endogenous factors, which are archived in each annual ing 2011–2017 were 5.2 °C and 1677 mm, respectively ring [28, 29]. Unlike the annual rings, the cambial activ- (Fig. 2). Te coldest and warmest months were January ity and xylem formation can be monitored in a short (− 9.6 °C) and July (17.8 °C), whereas the driest and wet- time rather than a growing season [30–33]. Based on test months were January (54 mm) and August (348 mm), these advantages, researchers explored the intra-annual respectively. dynamics of cambium, the xylem phenology and their In the present study, four conifer tree species, viz. Abies growth rates to achieve a deeper insight into the tree- koreana, Taxus cuspidata, Picea jezoensis and Pinus growth strategy depending upon climate [32, 34–37]. koraiensis were selected for experiments. Among these, Trees at cold temperatures, such as subalpines, are A. koreana, T. cuspidata and P. jezoensis were the rep- particularly vulnerable to frost damage during winter resentative subalpine species whose habitats are higher season at the beginning and cessation of meristem tis- than 1300 m a.s.l. at the Deogyusan National Park. By sue growth [10, 38–40]. Terefore, temperature plays an contrast, the primary habitat of P. koraiensis is between important role in activating their growth. A large volume low and high altitudes (550–1500 m a.s.l.), and only a of research [41–43] confrmed that trees growing under few P. koraiensis are distributed at the study site [13, 56]. cold conditions can initiate cambial activity with lower Considering the efect of size and age of the trees on the heat-sum value than those in warmer environment. Te cambial activity [31, 57], the trees were selected accord- heat-sum value is the amount of heat accumulated by a ing to their vitality and diameter at breast height. Due to plant during its growth or up to its the maturity [44]. Fur- limited population of each species at the study site, only thermore, the trees in cold environment have a shorter fve trees were monitored and no trees of the same age growing season than those at warmer locations [45–51]. were selected (Table 1). Te tree ages were verifed using Some research groups presented that even if the grow- increment cores extracted from portions of the test trees ing conditions remain the same, the growing seasons do at 1.2 m from the ground after fnishing micro sampling not remain the same and vary according to tree species at the end of October, 2017. [32, 46, 50–53]. However, these general fndings, have not been verifed to-date for the conifer trees in the subalpine Microcore sampling and preparation zones of the Republic of Korea.

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