Projected changes in temperature and precipitation in mid-latitude wet regions are expected to significantly affect forest ecosystems. We studied the physiological and shoot growth responses of Abies holophylla and Abies koreana seedlings to warming (3 °C above ambient temperature) and increased precipitation (irrigation with 40% of rainfall) treatments under open-field conditions. The physiological parameters, quantified by the net photosynthetic rate, transpiration rate, stomatal conductance, and total chlorophyll content, were monitored from July to October 2018. Shoot growth (i.e., root collar diameter and height) was assessed in August and December 2018. Irrespective of the treatments, the physiological parameters of both species decreased from July to August under warming treatment due to heat stress before recovering in September and October. Warming alone (W) and warming along with increased precipitation (W*P) decreased the physiological activities of both species in July, August, and September, with more pronounced effects on A. koreana compared with A. holophylla. Increased precipitation resulted in the increased chlorophyll content of both species in October. Shoot growth was not generally affected by the treatments, except for a subtle reduction in height under W*P for A. koreana. A. holophylla had consistently higher values for the physiological parameters and shoot growth than A. koreana. Our results indicate that the physiological activities of the Abies species could be seriously reduced under climate change, with a more severe impact on A. koreana. Among the two species, A. holophylla appears to be a more robust candidate for future forest planting.

High temperatures and droughts following winter dormancy can negatively affect seedling growth and mortality. An open-field experiment was conducted to study the growth and mortality of Larix kaempferi seedlings in response to spring warming and drought treatments and to determine whether seedlings could regain their growth capability once the treatments were discontinued. In May 2020, 1-year-old seedlings were exposed to four treatments: control, warming-only, drought-only, and the combined warming and drought. Drought treatment reduced the seedling height and root collar diameter and increased the mortality rate. The combined warming and drought treatments had the highest mortality rates, followed by the drought, control, and warming treatments. However, after the cessation of the treatments, the combined warming and drought treatments increased seedling height, root collar diameter, and individual seedling biomass because the high mortality rate relaxed competition among seedlings. This suggests that the effects of low competition on the surviving seedlings may mitigate the negative effects of warming and drought on seedling growth. Our study demonstrates that despite the high mortality and decreased growth during the treatment period, seedlings subjected to combined high temperature and drought stress showed short-term high levels of growth compared to seedlings subjected to a single stress.

This study was conducted to investigate abnormal shoot responses in Pinus densiflora seedlings to open-field warming and precipitation manipulation. In May 2017, 18 plots (2 temperature levels × 3 precipitation levels × 3 replicates) were constructed and 90 seedlings of 1-year-old P. densiflora were planted in each plot. The temperature in the temperature warming (TW) plots was set to 3℃ higher than that in the temperature control (TC) plots, and precipitation was set to decrease (PD) or increase (PI) by 40% compared to the precipitation control (PC) plots. The manipulation experiment started in June, 2017. The abnormal shoot occurrence rates in September and November, 2017; seedling number in each needle length growth stage in September, 2017; and length of abnormal shoots in November, 2017 were measured. Warming significantly increased the occurrence rate and developed the needle length growth stage of abnormal shoots in P. densiflora seedlings, whereas precipitation and interaction effects between the two treatments were not significant. The mean occurrence rates in September and November were 86.17 ± 11.80% and 90.78 ± 7.92% in the TW plots and 58.59 ± 16.13% and 64.80 ± 14.71% in the TC plots, respectively. Thus, the abnormal conditions derived from increased temperature stimulated the occurrence of abnormal shoots. Warming also elongated the needle length of the abnormal shoots, which might affect future photosynthesis. However, there was no significant difference in abnormal shoot length under warming and precipitation manipulation. Thus, warming stimulated the occurrence and needle development of abnormal shoots in P. densiflora seedlings, which could lead to the inhibition of subsequent growth by the use of reserved nutrients and degradation of seedling quality.

Urban trees grow in diverse environments where site conditions and human management may influence their growth patterns. However, few allometric equations (AEs) have been developed for urban trees, and the effects of environmental variations across urban land use categories on tree biomass remain largely unexplored. Therefore, this study developed urban land-use-specific AEs for major urban tree species in South Korea. We selected eight major urban tree species groups (at genus level), harvested 201 trees, and non-destructively measured the stem volumes of 1995 trees using a laser dendrometer. Species-specific and generalized AEs to estimate stem volume were developed under three urban land use categories: street trees, urban parks, and others. The results indicated that differences in stem volume across urban land use categories varied by species, with street trees generally showing smaller stem volumes. Furthermore, due to the high variation in stem volume within species, sampling designs that encompass diverse size distributions are necessary when developing AEs for urban trees. Our findings indicate that various factors in urban environments influence tree volume and considering these differences is essential for improving biomass estimation accuracy.

The national level carbon sequestration estimation for urban trees generally relies on diameter at breast height (DBH) increments. However, limited research on urban tree growth has led to uncertain estimates. In this study, we determined the DBH increment of six major urban tree species in South Korea using three analytical methods (mean, linear regression, and generalized additive model) and two data classifications (total and segmented). Additionally, wedeterminded the aboveground woody biomass (AGWB) increment, which was calculated using the estimated DBH increment, and the accuracy of the estimates using different methods. We found wide variations in DBH increment (0.616 to 1.002 cm yr⁻¹) among the assessed tree species. Metasequoia glyptostroboides exhibited the highest mean growth, while Pinus densiflora showed the lowest. The peak DBH increment occurred at a median of 25.4 cm for Quercus spp. and 40.0 cm for Prunus yedoensis. The total mean method led to overestimations of AGWB increment, especially for the largest trees (i.e., those in the top 5% of DBH). Our study suggests that relying on a simple total mean method can lead to significant uncertainties in estimating AGWB increment. Therefore, more accurate methods for estimating DBH increment, especially in urban environments, are needed.

Soil microbial communities are essential to the terrestrial ecosystem processes by mediating nutrient cycling, and their function and composition may be altered under climate change. In this study, the effects of extreme climate events (extreme warming and precipitation pattern) on the microbial communities and extracellular enzyme activities in the soils planted with 1-year-old Pinus densiflora and Larix kaempferi seedlings were investigated. Open-field warming (+3 °C and +6 °C) and precipitation manipulation including drought induced by the complete interception of rainfall and heavy rainfall (113 mm per day) were applied from 13 July to 20 August 2020. The activities of soil enzymes, including β-glucosidase, acid phosphatase, N-acetyl-glucosaminidase, and leucine aminopeptidase, microbial biomass carbon and nitrogen, and changes in microbial community composition were determined. The microbial biomass carbon was 15.26% higher in Larix kaempferi-planted soils than in Pinus densiflora-planted soils. Fungal Chao 1 in the heavy rainfall and drought plots were 53.86% and 0.84% lower than the precipitation control, respectively, and 49.32% higher in the Larix kaempferi plots than under the Pinus densiflora. The fungal Shannon index was 46.61% higher in plots planted with Larix kaempferi than in those planted with Pinus densiflora. Regarding the dominant phyla, the relative abundance of Ascomycota in heavy rainfall plots was 14.16% and 13.10% higher than in the control and drought plots, respectively, and the relative abundance of Mortierllomycota was 55.48% higher under Larix kaempferi than under Pinus densiflora. The overall results are considered to reflect the microbial sensibility to environmental conditions and interaction with the planted species. Since the current study observed only short-term responses to extreme climate events, further study is required to determine the continuous effects of environmental changes on the associations between plants and soil microbes.