Introduction

Eucalyptus (Eucalyptus spp.) evergreen trees are native to Tasmania in Australia, and approximately 700 species are distributed worldwide (Brooker and Kleinig, 2006). These trees are cultivated on more than 20 million hectares in most subtropical regions (Coppen et al., 2002; Batish et al., 2008; Grattapaglia et al., 2012; Butnor et al., 2019; Hyun et al., 2024). They grow to heights of 70 to 100 m and live between 200 and 400 years. With their high production of biomass, they have been used extensively as roadside trees and in a variety of handicrafts, textiles, soaps, perfumes, cosmetics, aromatic oils, natural medicines, functional foods, pesticides, wood products, paper pulp, and as windbreaks (Coppen et al., 2002; Pacifici et al., 2007; Batish et al., 2008; Grattapaglia et al., 2012; Hyun et al., 2024). There has been a rapid expansion in the sales of Eucalyptus spp. for indoor decoration in South Korea; however, few have been cultivated as field-grown ornamental trees (aT, 2023; Hyun et al., 2024). Approximately 218,093 bunches of flowers of E. pulverulenta were sold in 2022, leading the cut flower market of Eucalyptus spp.; this type was followed by E. gunnii (115,783) and E. parvula (112,673), with less than 40,000 distributed for E. polyanthemos, E. pauciflora, E. ficifolia, E. nitens, and E. globulus.

Eucalyptus spp. trees grow well under annual average temperatures between 5 and 25°C and precipitation levels between 250 and 1,700 mm, requiring six to eight hours of daily sunlight and temperatures higher than –5°C (Coppen et al., 2002; Pacifici et al., 2007; Teulières et al., 2007; Hyun et al., 2024). Gyeongsan-si, one of the warmest regions of S. Korea, reached average temperature of 14.5°C with annual precipitation of 1080.8 mm, with the temperature not dropping below –2.9°C in January for the last 30 years, suggesting that it may be an optimal climate zone in which to grow Eucalyptus spp. (KMA, 2023). However, transplanted E. websteriana and E. kruseana seedlings are very sensitive to the cold winters, showing less than 50% of survival in this region (Teulières et al., 2007; Hyun et al., 2024). E. gunnii and E. pauciflora are tolerant to temperatures as low as –18°C and are thus enhance to be suitable for cultivation in temperate zones given the global warming that has occurred in recent decades (Bowman et al., 2014; Schimpl et al., 2018; Butnor et al., 2019; Hyun et al., 2024).

Agricultural mulches are used to cover the soil surface to decrease weed density, insect and pest populations, runoff, and frost, and to increase soil water conservation. This method can effectively improve crop productivity and soil health by enhancing soil organic matter and nutrient status levels (Fang et al., 2011; Gill and McSorley, 2012; Iqbal et al., 2020; Jourgholami et al., 2020; Song and Du, 2023). In contrast, organic mulching of woody biomass does not affect the tree height, leaf area index or leaf characteristics in hardwood plantations during the establishment stage but increases these values in poplar (Populus spp.) plantations with degraded soils (Fang et al., 2011). Little information is available on the eco-physiological traits and survival rates of Eucalyptus spp. in fields that respond to straw mulching, which is commonly used for ornamental plants (Huang et al., 2008). Cold-tolerant Eucalyptus spp. with organic mulching can be hardened and de-hardened from abrupt chilling or frost under warmer winter weather, which has frequently occurred in recent years and has induced serious photo-oxidative damage (Schimpl et al., 2018; Butnor et al., 2019).

This study was performed to evaluate the suitability of the Eucalyptus tree spp. types E. gunnii, E. parvula, and E. pulverulenta based on the adaptation potential and to evaluate the effects of a straw mulch treatment on early tree establishment in a temperate region subjected to warm and humid conditions in summer and cold and dry conditions in spring and winter in southern South Korea in 2022 and 2023.

Materials and Methods

Experimental layout

An experiment with three species of Eucalyptus seedlings, E. gunnii, E. parvula, and E. pulverulenta, grown under no mulch (NM) and straw mulch (SM) conditions, was initiated at a University-affiliated experimental farm in Gyoengsan-si, South Korea (35°N, 128°E). Experimental plots were formed in a mixture of silt loam at 6.73 pH, electrical conductivity (EC) of 0.05 dS·m^-1 , an organic matter (OM) content of 13.0 g·kg^-1 , 5.83 mg·kg^-1 of NO₃, 674.2 mg·kg^-1 of P₂O₅, 0.25 cmolc·kg^-1 of K₂O, 6.1 cmolc·kg^-1 of CaO, and 1.6 cmolc·kg^-1 of MgO starting on April 30, 2022, 0 days after planting (DAP). One-hundred and twenty seedlings of E. gunnii, E. parvula, and E. pulverulenta (approximately 10.0 cm long and 1.1 cm in diameter) were purchased from a wholesale nursery (World Flower Corp., Jincheon-gun, S. Korea). The Eucalyptus seedlings were planted 5 cm deep at a spacing of 30 cm between the trees and were supported by a single plastic pole support stake. These were all harvested on August 31, 2023, i.e., 480 days after planting (DAP).

Weeds around the trees in the NM plots were managed by weeding by hand three times a year, and the mown clippings were returned to the areas under the trees. A 10 cm-thick layer of rice straw was applied to the soil surface around the trees planted in the SM plots at 0 DAP, with the mulching depth maintained through repeated mowing and blowing of the grass clippings during the growing season. The clippings contained approximately 2.94% N, 0.45% P, 3.03% K, 0.64% Ca, and 0.45% Mg, while the straw contents consisted of 0.97% N, 0.11% P, 1.22% K, 0.44% Ca, and 0.15% Mg.

Crop nutrient management and protection

Experimental plots were fertilized with manure compost (poultry 30%, cow 30%, pig 5%, saw dust 30%, and zeolite 5%; Taesan Farming Corp., Gyeongju-si, S. Korea) and with pelletized oil cake (castor bean 58%, sesame cake 22%, and rice bran 20%; Muge Corp., Yeongcheon-si, S. Korea) at a rate of 6.0 g available total nitrogen (T-N) per tree per year of the Eucalyptus tree age based on the recommended amount of fertilization (Ferreira et al., 2018). The oil cake was applied annually in split doses in April, July, and October of 2022 and in April, May, and June of 2023 to reduce the toxicity of fertilization for the growth of juvenile Eucalyptus. Soil mineral nutrition in the NM plots was as follows: 7.17 pH, 0.07 dS·m^-1 EC, 38.9 g·kg^-1 OM, 2.70 mg·kg^-1 NO₃, 960.0 mg·kg^-1 P₂O₅, 0.26 cmolc·kg^-1 K₂O, 7.28 cmolc·kg^-1 CaO, and 1.61 cmolc·kg^-1 MgO at 30 DAP.

The Eucalyptus spp. trees were watered by hand with a hose every day until the roots were newly established and twice per week during spring, summer, and fall, and a few times during the winters of 2022 and 2023.

Minimum air temperature and precipitation data from 0 to 480 DAT were obtained from annual climatological reports and compared with averages over last 30 years (KMA, 2023; Fig. 1). Soil moisture levels and temperatures 10 cm below the soil surface were monitored hourly using a wireless data logger (Efento Co., Ltd., Wroclaw, Poland) installed in February and May of 2023.

Fig. 1.

Air temperature (Panel A) and precipitation (Panel B) levels in a Eucalyptus plantation in Gyeongsan-si at 60-day intervals from 0 to 480 days after planting (DAP) in 2022–2023.

Beet armyworm larvae (Spodoptera exigua (Hübner)) first appeared on leaves at 50 DAP (Fig. 2), and any foliar damage symptoms were visually assessed and expressed as a percentage of leaf damage at 50 and 70 DAP. An organic pesticide (Bacillus thuringiensis subsp. aizawai GB413; GREENfarmer Co., Inc., Yeoju-si, S. Korea) and chemical pesticide (10% Chlorfenapyr; FarmHannong Co., Inc., Yeoju-si, S. Korea) were sprayed under the Eucalyptus tree canopy to control pest incidence nine times during the periods in which pests were present. A black woven weed mat was applied to the furrow to suppress the occurrence of insects and weed vegetation.

White non-woven fabric fences were temporarily installed at a height of 60 cm to protect the Eucalyptus trees against low temperatures and frost damage from 220 to 310 DAP (Fig. 2). Any brown spot regions inspected on the leaves were recorded monthly from 220 to 310 DAP (Hyun et al., 2024). Severe browning and wilting of leaves without resprouting were considered cases of mortality (Butnor et al., 2019) and were scored at 60-day intervals from 0 to 480 DAP (Fig. 2).

Fig. 2.

Images of leaf-feeding beet armyworms (left), installed white non-woven fabric fences to protect trees against cold temperatures (middle), and brown leaves (right) of Eucalyptus species in a plantation in Gyeongsan-si.

Tree measurement variables

The tree height, trunk thickness 10 cm above the soil surface, and the number of shoots and leaves of the Eucalyptus spp. trees were measured at 60-day intervals from 0 to 480 DAP. A SPAD-502 meter was used to measure the fully expanded leaves on mid-positioned shoots of each tree (Minolta Co., Tokyo, Japan).

A complete Eucalyptus tree was destructively harvested by excavating the entire root systems at 480 DAP, and the roots were washed with tap water to remove the soils embedded on the root surface (Kuyah et al., 2013). The whole tree was then dried in a dry oven at 65°C for seven days and was weighed using a weighing balance scale (EB-430HU, Shimadzu Co., Ltd., Tokyo, Japan) to determine the dry mass.

Five middle-positioned shoots with fully expanded leaves without soft tips at the mature stage on each treated Eucalyptus were cut, measuring 30–50 cm long at 480 DAP (Pacifici et al., 2007). The weights of the leaves on the stem cuttings were recorded at three-day intervals from 0 days after harvest (DAH) to 15 DAH, using a weighing balance scale (EB-430HU, Shimadzu Co., Ltd., Tokyo, Japan), the foliar SPAD value was determined using a SPAD-502 chlorophyll meter, (Minolta Co., Tokyo, Japan). The percentage of weight loss was determined by the difference in the weight of the leaves on the cuttings at 0 DAH and the weight recorded at 15 DAH and then dividing the result by the weight at 0 DAH.

Experimental design and analysis

Six treatments were randomly allocated to the experimental units within each block (plot), with each treatment consisting of four replicates (4 plots, 1 plot = 7 trees). Middle-positioned trees in each plot were chosen as data trees to evaluate the mean tree measurement variables, and the other six trees were used as guard trees or potential data trees if the original data tree died. All values of numeric variables in the data were analyzed by means of a one-way analysis of variance with SAS Version 14.1, testing for the p < 0.05 level of significance.

Results and Discussion

Climate and soil conditions

The minimum air temperature was higher from 0 to 210 DAP and 270 to 480 DAP and lower from 210 to 270 DAP in 2022 and 2023 compared to those over the last 30 years (Fig. 1A). The lowest temperature (–14.2°C) was observed at 265 DAP, which could reduce tree acclimation and recovery in winter and spring (Schimpl et al., 2018; Butnor et al., 2019). Heavy precipitation frequently occurred from 0 to 120 DAP and from 360 to 480 DAP during spring and summer (Fig. 1B) in 2022–2023, associated with seasonal monsoon climate swings in northeast Asia (Loo et al., 2015; Butnor et al., 2019), with drought between, i.e., from 120 to 360 DAP.

The minimum and maximum soil temperatures at a depth of 10 cm were similar to the NM and SM plots in August; however, the minimum moisture content in the SM plots was slightly higher than that in the NM plots (Fig. 3A and 3C). The minimum soil temperature in the SM treatments dropped slightly below 0°C in February (Fig. 3B and 3D) due to the retention of soil moisture and the limited solar heat energy caused by the soil surface insulated by the straw mass (Chen et al., 2007; Zhang et al., 2009).

Fig. 3.

Temperature and soil moisture content at a depth of 0–10 cm in soil specimens treated with non-mulching (NM) and straw mulching (SM) treatments in a Eucalyptus plantation in Gyeongsan-si in August of 2022 and in February–March of 2023.

Soil hardness values reached 11.8 kgf·cm^-2 in the NM plots and 8.7 kgf·cm^-2 in the SM plots at 60 DAP and 16.7 kgf·cm^-2 in the NM plots and 9.7 kgf·cm^-2 in the SM plots at 90 DAP (Fig. 4). Soil compaction around the tree root zone in the SM case would have prevented the creation of large particles of aggregates that formed due to the degradation of the straw mass and favorable habitat environments for soil fauna; it was also shown to enhance the physical properties of the soil and easily elongate the root branching of mulched-Caucasian alder (Alnus subcordata) trees (Jourgholami et al., 2020; Song and Du, 2023).

Fig. 4.

Soil hardness at a depth of 0–10 cm of soil specimens treated with non-mulching (NM) and straw mulching (SM) treatments in a Eucalyptus plantation in Gyeongsan-si at 0, 60, 90, 120, and 450 days after planting (DAP) in 2022–2023: ns, not significantly different. ** and ***indicate a significant difference between the NM and SM at the p < 0.01 and p < 0.001 levels, respectively.

Tree growth

Tree heights were lower for the E. parvula-NM and -SM specimens between 0 and 120 DAP as compared to those of the E. gunnii and E. parvula trees (Fig. 5A). The heights of the E. parvula-SMtreesincreased significantly from 180 to 480 DAP (175.3 cm), while the growth of E. parvula-NM trees slowed between 120 and 360 DAP but rapidly increased between 360 and 480 DAP (148.5 cm). E. pulverulenta trees grown under NM showed severe dieback with dark, discolored, and dried tissues of the outer bark and leaves at 480 DAP, with a reduction in apical growth observed for the E. pulverulenta-SM trees (102.0 cm) and stagnation for the E. gunnii-NM (82.0 cm) and -SM (132.0 cm) cases. None of the trees increased between 180 and 360 DAP, indicating inactive growth during the fall to spring periods with a mean temperature of less than 10.0°C, an outcome similar to the seasonal growth patterns of the tree trunks (Fig. 5B), the most critical factor to consider when predicting vegetative growth of young Eucalyptus spp. trees (Sillett et al., 2015; Butnor et al., 2019; Hyun et al., 2024). E. parvula-SM trees had the greatest trunk thickness of 20.9 mm at 480 DAP, followed by E. parvula-NM trees (15.9 mm), for which the thickness was approximately 150% greater than those of other Eucalyptus trees treated here. The presence of straw materials prevents soil water evaporation, enhances water use efficiency, reduces nutrient competition, and maximizes the vegetative growth of other Eucalyptus spp. trees during the establishment stage (Adams et al., 2003; Chen et al., 2007; Zhang et al., 2009; Jourgholami et al., 2020; Song and Du, 2023).

Fig. 5.

Tree height (Panel A) and trunk thickness (Panel B) of Eucalyptus gunnii, E. parvula, and E. pulverulenta with non-mulching (NM) and straw mulching (SM) treatments in a plantation in Gyeongsan-si at 60-day intervals from 0 to 480 days after planting (DAP) in 2022–2023. Different lowercase letters above each phase indicate significant differences as determined by Duncan’s multiple-range test at p < 0.05.

The number of shoots produced was highest in E. parvula-SM (122.7) and -NM (80.0) trees, followed by E. gunnii-SM (57.0), E. pulverulenta-SM (26.0), E. gunnii-NM (19.0), and E. pulverulenta-NM (0.0) (Fig. 6A). A large number of leaves per tree also occurred in E. parvula-SM (2,079.0) and -NM (1,388.0) trees, at a rate approximately three times higher than that in E. pulverulenta and E. gunnii trees (Fig. 6B). The development of the leaves and shoots of E. parvula could have been stimulated by genetic contributions, turgor pressure generated from the strong tree trunks, the extended tree heights, and the shoots (Merchant et al., 2007; Hyun et al., 2024).

Seasonal SPAD values increased from 0 to 240 DAP for all leaves and declined slightly from 240 to 300 DAP during the cold winter (December in 2022 to February in 2023) (Fig. 6C). SPAD values generally indicate the status of the photosynthetic capacity and T-N and are correlated with the health and quality of Eucalyptus foliage (Pacifici et al., 2007; Ribeiro et al., 2009; Ferreira et al., 2015; Hyun et al., 2024). The SPAD readings gradually increased or remained stable between 240 and 480 DAP for all leaves, being lowest for E. pulverulenta and highest for E. gunnii trees, which reportedly are resistant to cold and can maintain photosynthetic responses (Schimpl et al., 2018).

Fig. 6.

Number of shoots (Panel A), number of leaves (Panel B), and SPAD values (Panel C) of Eucalyptus gunnii, E. parvula, and E. pulverulenta trees treated with non-mulching (NM) and straw mulching (SM) treatments in a plantation in Gyeongsan-si at 60-day intervals from 0 to 480 days after planting (DAP) in 2022–2023. Different lowercase letters above each phase indicate significant differences as determined by Duncan’s multiple-range test at p < 0.05.

Total plant biomass was highest in the E. parvula-SM trees (290.8 g) and lowest in the E. parvula- (0.0 g) and E. gunnii-NM trees (29.3 g; Fig. 7). Living or straw mulches increase OM in degraded soils and increase the growth of young poplar (Populus spp.) trees, which could partially substitute the supplementation of essential nutrient requirements for the growth of young Eucalyptus spp. trees (Adams et al., 2003; Fang et al., 2011; Jourgholami et al., 2020; Song and Du, 2023). However, there was little significance observed with regard to plant biomass among the treatments due to the wide variations within the data, as shown in the figure. The seedlings used in this experiment were not segregated genetically, and individual progenies could have shown large variations among the treatments, even among the same population.

Fig. 7.

Dry weight (DW) of Eucalyptus gunnii, E. parvula, and E. pulverulenta with non-mulching (NM) and straw mulching (SM) treatments in a plantation in Gyeongsan-si at 480 days after planting in 2023. Different lowercase letters above each phase indicate significant differences as determined by Duncan’s multiple-range test at p < 0.05.

Tree damage

High levels of brown coloration, such as necrosis and scorching, occurred in all Eucalyptus foliage from 220 to 310 DAP, i.e., from November to February (Fig. 8A). The E. pulverulenta and E. gunnii trees showed browning at a rate of more than 80% of the leaves, whereas browning decreased to 58.6% in the E. parvula-NM trees and 58.2% in the E. parvula-SM trees. Seasonal tree survival worsened from 0 to 480 DAP, with the highest survival rate observed for E. parvula-SM (75%) and -NM (50%) trees, followed by E. gunnii-SM (27%), E. pulverulenta-SM (25%), E. gunnii-NM (23%), and E. pulverulenta-NM trees (0%) (Fig. 8B). There was gradual freezing from winter to early spring (2022 – 2023), which frequently occurred during abrupt swings of low temperature and dry conditions. This could have contributed to a decline in the acclimation and survival of juvenile E. gunnii and E. pulverulenta trees exposed to chilling stress (Loo et al., 2015; Schimpl et al., 2018; Butnor et al., 2019). However, regarding the selection of lines suitable for the environment in southern South Korea, data on cold-tolerance, such as electrolyte leakage due to low temperatures and low carbohydrate contents in the stems, should be provided for more complete scientific information.

Fig. 8.

Leaf browning (Panel A) and survival (Panel B) of Eucalyptus gunnii, E. parvula, and E. pulverulenta with non-mulching (NM) and straw mulching (SM) treatments in a plantation in Gyeongsan-si at days after planting (DAP) in 2022–2023. Different lowercase letters above each phase indicate significant differences as determined by Duncan’s multiple-range test at p < 0.05.

Populations of beet armyworms developed on all Eucalyptus spp. specimens from 50 to 70 DAP and did not spread further after 70 DAP (Fig. 9). They also did not appear during the following season, presumably due to the higher allelopathic concentrations in the leaves in 2023 (Batish et al., 2008). Foliar damage by armyworms increased by 10–20% in E. pulverulenta-NM trees as compared to that in E. gunnii or E. parvula at 70 DAP but was slightly reduced in E. pulverulenta and E. parvula trees treated with SM. The SM application would provide resources to predatory insects seeking beet armyworm larva compared to NM given the shallow cover of grass clippings containing low C:N ratios in the latter case, quickly decomposing and reducing the amount of residue on the soil surface (Gill and McSorley, 2012; Iqbal et al., 2020). Feeding preferences among the Eucalyptus spp. were determined considering the degree of leaves with glaucousness as well as the age, specific weight, proportions of water, T-N value, T-C value, volatiles, and instances of secondary metabolism (Horgan, 2011), which should have offered more favorable foliar feeding in the E. parvula case given its non-glaucous leaves and lower aromatic quality. Nevertheless, the high vegetation of E. pulverulenta trees that were established early would be the preferred hosts for the eggs and larvae of armyworms (Horgan, 2011).

Fig. 9.

Insects, beet armyworms, and damaged leaves of Eucalyptus gunnii, E. parvula, and E. pulverulenta with non-mulching (NM) and straw mulching (SM) treatments in a plantation in Gyeongsan-si at 50 and 70 days after planting (DAP) in year 2022. Different lower-case letters above each datum point indicate significant differences as determined by Duncan’s new multiple-range test at p < 0.05.

There was a sharp increase in the weight loss of all leaves on the cuttings stored without water from 0 to 6 DAH, and this slowed with a water treatment, particularly for the cuttings of SM trees (Figs. 10 and 11). Approximately 50% of the weight loss of leaves on the cuttings, considered as measure of the vase life with desiccation and browning, was recorded at 4.5 DAH in all Eucalyptus spp. trees without water and did not appear in water-added cuttings at 15 DAH, a finding that was previously reported with regard to the longevity of cut foliage of E. parvifolia at room temperature (Delaporte et al., 2000; Pacifici et al., 2007). The foliar SPAD value of cuttings stored without water increased slightly at 9 DAH and then decreased at 15 DAH, except for E. gunnii-SM leaves, whereas increasing values were observed in water-stored Eucalyptus leaves at 15 DAH. Overall, E. gunnii-SM leaves showed the lowest weight loss and the highest SPAD values at 15 DAH, minimizing postharvest deterioration. This would reduce economic losses during storage and transportation and attract consumers' attention in floriculture markets (Delaporte et al., 2000; Pacifici et al., 2007).

Fig. 10.

Images of non-watered- or watered cuttings of Eucalyptus gunnii, E. parvula, and E. pulverulenta with non-mulching and straw mulching treatments in a plantation in Gyeongsan-si at three-day intervals from 0 to 15 days after harvest (DAH) in 2023.

Fig. 11.

Weight loss and SPAD outcomes of non-watered or watered cuttings of Eucalyptus gunnii, E. parvula, and E. pulverulenta with non-mulching and straw mulching treatments in a plantation in Gyeongsan-si at three-day intervals from 0 to 15 days after harvest (DAH) in 2023. Different lowercase letters above each phase indicate significant differences as determined using Duncan’s multiple-range test at p < 0.05.

Conclusion

For field-grown Eucalyptus spp., widely distributed in South Korea in the cut flower market, for all genotypes straw mulching effectively improved soil conditions, tree vigor, and postharvest longevity and reduced frost damage, confirming the high mortality of E. pulverulenta without SM. E. gunnii, which grows naturally in cold regions,was expected to be highly resistant to freezing; however, chilling tolerance was not consistent. This was presumably due to low acclimation under gradual chilling events but frequent abrupt swings in temperature (Schimpl et al., 2018). E. parvula-SM trees are likely to be a suitable species if cultivated with straw mulch for soil management to enhance overall tree productivity, conferring an adaptation potential upon exposure to chilling stress during the first two years of its establishment in southern South Korea. However, the scant biochemical and/or genetical understanding of the interactions between the different responses to Eucalyptus spp. should be offset by monitoring electrolyte leakage levels, carbohydrate and mineral nutrients, and photosynthetic activity within this cultivar of Eucalyptus spp. when subjected to different long-term soil environmental conditions in a range of climatic regions.