Initial Growth of Large, Outplanted, Container-Grown Rooted Cuttings of Sugi (Cryptomeria japonica) with Leaf Removal Treatment for Alleviating Transplant Shock and Stem Incline

Abstract

We examined the effectiveness of partial leaf removal for alleviating transplant shock and reducing the risk of stem tilting in large-rooted cuttings of container-grown sugi (Cryptomeria japonica) with high shoot:root ratios. Different intensities of leaf removal (0%, 25%, 50% and 75%) were applied to large sugi rooted cuttings immediately after outplanting, and the stomatal conductance (g_s), growth, and degree of stem incline (DSI) during the first growing season were compared to short-rooted cuttings. The 75% removal treatment was associated with high g_s values (ca. 1.7–3.0 times of the other treatments) in the early stage of plantation establishment indicating the alleviation of water stress; however, this advantage disappeared within three months after planting. The stems with lower defoliation rates (i.e., 0% and 25%) tended to have large DSI values (about twice those of the 50% and 75% treatments) at an early stage of plantation establishment; however, this effect had almost disappeared within three months. The change in the DSI values and the height-to-diameter ratio (H/D) demonstrated that stem tilting was closely related to a high stem slenderness. The results of the present study showed that the effect of leaf removal on the alleviation of transplant shock and stem tilting was limited. Conversely, both stem elongation and diameter growth tended to be delayed at higher defoliation rates (27%–38% declines in elongation rate and 69%–30% declines in diameter growth compared to the 0% leaf removal treatment, respectively) due to the loss of photosynthetic organs. The large initial H/D also decreased the elongation rate immediately after planting. We consider that it would be better to reduce the initial H/D at the production of the cuttings before outplanting, rather than reducing the risk of stem tilting by leaf removal at the time of outplanting.

1. Introduction

At the early stages of afforestation or reforestation with timber trees, alleviating inter-species competition with weed plants and preventing browsing damage by herbivores are crucial for the successful establishment and growth of the planted target trees [1,2]. Planting large seedlings or rooted cuttings is expected to facilitate their establishment by giving them an advantage in interspecific competition with weed plants, e.g., [2], and reducing the risk of fatal browsing damage to stem apexes [2,3,4,5].

However, using large seedlings can also increase the risk of unsuccessful or delayed establishment [6,7,8]. Compared to a developed leaf mass, the relatively smaller root mass of young seedlings/rooted cuttings, which arises due to the relatively small volume of the pots or container cavities used for rearing (or the cut roots of bare-root seedlings), can adversely affect the water balance of seedlings/rooted cuttings. This disruption to the water balance arises due to a decrease in water uptake relative to the transpiration loss that occurs after outplanting; this phenomenon is commonly referred to as ‘transplant shock’ [9,10,11,12]. Transplant shock can be problematic even in seedlings that develop shoots under favorable nursery conditions with appropriate fertilization or irrigation. For example, numerous studies have reported that high shoot (top):root ratios in seedlings at the time of outplanting resulted in high mortality and/or low growth under harsh field conditions, e.g., [1,12,13,14]. Although container-grown seedlings/rooted cuttings have been reported to have a higher tolerance to transplant shock compared to bare-root seedlings/rooted cuttings, e.g., [15,16,17,18,19,20], large seedlings with larger shoot:root ratios that are grown in the same cavity size as normal-sized seedlings [21] may be at considerable risk of transplant shock, which is common even under humid conditions such as the monsoon climate in Japan [12,22].

Further, using large seedlings may have another disadvantage in terms of mechanical stability; for example, large seedlings grown in multi-cavity containers with a high cavity density (such as those that are typically used for normal-size seedlings) are susceptible to tilting or falling over [2]. For sugi (Cryptomeria japonica D. Don) and hinoki (Chamaecyparis obtusa Sieb. et Zucc.), which are representative timber species in Japan, seedlings grown in containers with high cavity densities have been reported to have large height-to-diameter (H/D) ratios, which cause a delay in height increase after outplanting [23,24]. This delay in height increase likely occurs due to a growth response in which diameter growth is enhanced in order to decrease the H/D ratio and increase the mechanical stability of stems. In contrast to seedlings, rooted cuttings generally have low initial H/D ratios because the cutting shoots usually have a large initial diameter shoot base compared to seedlings [25]. Due to density effects, however, rooted cuttings that are grown to a taller height at the same cavity density used for normal-sized rooted cuttings will typically have large H/D ratios. In such cases, the risk of instability after outplanting will be the same as for large seedlings, c.f., [26]. However, in terms of the advantage in competition with weed plants, this instability after outplanting would reduce the merit of using large seedlings/rooted cuttings.

It has previously been proposed that leaf removal may be an effective means of improving the balance between the above- and below-ground parts of hinoki seedlings [12] as appropriate leaf removal would improve the water balance of seedlings by reducing the water loss via transpiration [27]. A study of leaf removal in bare-root and container-grown hinoki seedlings found that partial leaf removal was effective for reducing the mortality of bare-root seedlings after outplanting [12]. They also reported that the mortality of container-grown seedlings was relatively low, even without leaf removal, indicating that container-grown seedlings were less susceptible to transplant shock in hinoki. A lower mortality was also reported for container-grown sugi rooted cuttings under stressed conditions compared to bare-root cuttings subjected to different watering regimes [28]. Given the potential risks of transplant shock reported in large seedlings of longleaf pine and other many woody plants, c.f., [6,8], leaf removal could therefore be effective for large container-grown seedlings/rooted cuttings. However, relatively little information is available on the effectiveness of leaf removal in container-grown large seedlings/rooted cuttings for alleviating transplant shock and reducing the mechanical instability (tilting) of aboveground parts.

Further, leaf removal has been demonstrated to have a negative effect on seedling growth. Several studies have reported that the decrease in photosynthesis associated with leaf removal directly resulted in a decrease in seedling growth and/or an increase in mortality of several woody plant species such as scarlet oak, green ash, and Turkish hazelnut [29,30]. Defoliation on selected branches in loblolly pine significantly affected the diameter growth and stem profile [16]. Conversely, removing leaves from low on the trunk of hinoki seedlings did not significantly affect their growth after outplanting, suggesting a lesser role for these lower leaves in the growth of seedlings of this species [12]. In addition, as a physiological response to leaf removal, it has been demonstrated that lower leaf removal of Lebanese cucumber is compensated for by an increase in the photosynthetic capacity of the remaining leaves [31]. The findings of these studies suggest that the negative effects of leaf removal, in terms of decreased growth due to a reduction in photosynthetic organs, can vary depending on the species characteristics and/or size of the seedlings/rooted cuttings.

In this study, we sought to clarify if and how partial leaf removal in large container-grown sugi rooted cuttings can (1) alleviate transplant shock and (2) reduce the risk of stem tilting with high shoot:root ratios. The leaf-removal treatments, which differed in intensity (percentage of removal), were performed using large sugi rooted cuttings grown at the same cavity density as normal-sized cuttings. Stomatal conductance (g_s) at an early establishment stage after outplanting and growth during the first growing season were measured.

2. Methods

2.1. Plant Materials

The sugi cultivar “Ken-Aira-20”, one of the plus trees from Kagoshima Prefecture, southern Kyushu, southwestern Japan, was used in this study. One-year-old, large, rooted cuttings (hereafter referred to as large cuttings) of this cultivar were prepared using multi-cavity containers with 24 cavities per container (JFA-300). Each cavity had a volume of 300 mL and six ribs on the inner wall for preventing root looping. Cutting shoots (40 cm in length) were collected in November 2015 from a private scion garden, planted into a container cavity filled with coconut husk as the soil medium, and grown for 16 months until February 2017. From June 2016, an organic fertilizer (N:P:K = 8%:7%:7%) was applied every week (2 g/cavity) as the cuttings grew. The average size of the large-rooted cuttings at outplanting in February 2017 was 90 cm in height and 8.9 mm in root collar diameter. The height of the large cuttings (90 cm) was approximately 1.7–2 times the size of rooted cuttings ordinarily used for outplanting in the study region. In the field measurements and data analyses, in addition to the large cuttings, normal-sized bare-root cuttings (hereafter referred to as small cuttings (SC), which measured 55 cm in height and 7.3 mm in root collar diameter) of the same cultivar grown in the same nursery were used to compare the stock types, as described below.

2.2. Study Site

The experiment was conducted in a clear-cut stand that was previously used as a hinoki plantation on a gentle NW-facing slope of Mt. Takatsuka (32°09′ N, 130°44′ E) in Kumamoto Prefecture, southwestern Japan. The study site is situated in a warm-temperate zone (ca. 500 m above sea level). The mean annual temperature and precipitation recorded at the nearest weather station in Hitoyoshi City (ca. 7 km from the study site at 146 m a.s.l.) were 15.8 °C and 2535 mm, respectively. The soil at the site comprised light-colored andosols with volcanic ash soil at the surface. The volcanic ash soil is classified as Brown Forest Soil [32] and is included in Cambisols of the World Reference Base for Soil Resources 2006 [33,34]. The creeping moist soil of the study site is suitable for planting sugi.

2.3. Experimental Design

On 23 February 2017, the 35 large sugi cuttings were outplanted at the study site in a 5 × 7 configuration and a planting density of 2000/ha. The site was fenced to prevent browsing by sika deer (Cervus nippon). Leaf removal was performed on 13 March. The 35 large cuttings were randomly divided into four groups, and four different leaf removal treatments were applied: 75% leaf removal (R75), 50% leaf removal (R50), 25% leaf removal (R25), and no leaf removal (R0) as the control, using 10, 8, 9, and 8 cuttings, respectively (Figure 1). The percentage of leaf removal was based on crown length (=tree height after planting), and the lower 1st-order branches corresponding to the removal percentage in the crown length were removed at their branch base (on the main stem). The percentage of removed branches’ mass was estimated based on oven-drying 10 similarly sized samples of the same cultivar. For the R75, R50 and R25 treatments, 90%, 70% and 40% of the leaves on the basis of dry weight were removed, respectively. Fungicide was not applied on the trimmed scars prophylactically, but no symptoms of decay or other injuries were observed during the experimental period.

2.4. Field Measurements

The tree height and root collar diameter (on the ground surface after planting) were measured immediately after outplanting by the staff at Kyushu Regional Forest Management Office on 17 March 2017; we used these data as the initial size of the trees in our experiment. On 15 April 2017, we measured the natural height of the stem, stem length, stem base diameter (at 5 cm above the ground surface), and the lateral (horizontal) deviation of the stem apex from the stem base position. The stem length and natural height were measured to evaluate the pure elongation of the stem and the decline of height by stem incline. The measurements were conducted at least once a month until 4 December 2017. The degree of stem incline (DSI, °) was calculated using the natural stem height and lateral (horizontal) deviation of the stem apex from the stem base. The height to diameter ratio (H/D) was calculated on the basis of the stem length and the stem base diameter (tree height and root collar diameter for the initial value on 17 March) for every tree at every measurement.

In order to evaluate the physiological stress of the planted trees, stomatal conductance (g_s) was measured from 15 April to 8 August in 2017 using a steady-state porometer (LI-1600; Li-Cor Inc., Lincoln, NE, USA). The measurements were performed on all sample trees from 12:00 to 14:00 on a sunny day. The same parts of the needle leaves of the upper crown were used for measuring g_s throughout the measurement period.

For comparison, we added small cuttings (SC) of the same cultivar planted at a different site (350 m away from the site used for the large cuttings, but with a similar topography and soil conditions) from 25 April 2017. Five planted SC trees were selected and subjected to the same measurement as the large cuttings.

Figure 2 shows daily mean air temperature and daily rainfall during the experimental period, and mean vapor pressure deficit (VPD) for each measurement of g_s.

2.5. Data Analyses

The stem length, stem height, stem base diameter, and g_s were compared among five categories, i.e., four leaf removal treatments for the large cuttings (R75 to R0) and the small cuttings, for every measurement date (Tukey–Kramer multiple comparison test). Similarly, comparisons of H/D and DSI among the treatments and stock types were performed using Scheffé’s and Stee–Dwass multiple comparison tests, respectively. The change in the relationship between DSI and H/D were analyzed using the Kendall rank correlation test by pooling all of the treatment samples and stock types.

3. Results

3.1. Survival, Stem Length, Stem Height and Diameter Growth

None of the individuals died over the course of the study, and no natural defoliation was observed for any of the treatments during the experimental period. Stem elongation in the large-planted cuttings was only slight until June, before increasing thereafter (Figure 3A). The increase in stem length after July tended to be high in the R0 and R25 treatments (average: 24 cm) compared to the R50 and R75 treatments, though the differences among the four treatments were not significant (p > 0.05). The stem length of SC individuals increased markedly from July to October, and the initial difference in stem length of SC individuals and R0 and R25 individuals (40 cm) decreased to 27 cm at the end of the first growing season. The significant difference between the R75 treatment and the SC individuals disappeared after October. The stem height of the large cuttings decreased once during April to May due to stem tilting, particularly in the R0 and R25 treatments, before increasing until November (Figure 3B). Increments in height tended toward small in R75 (a 38% decline compared to R0), which resulted in no significant difference of the stem height compared to SC individuals from October.

In contrast to the slight height growth and stem elongation until July, the stem base diameter of the large cuttings in treatments R0, R25, and R50 started to increase in April, showing progressively larger increases with the lower rates of leaf removal (but no significant differences were observed among the R0, R25, and R50 treatments; p > 0.05) until the end of the first growing season (Figure 3C). Growth in the diameter of the R75 individuals was slow (a 69% decline compared to R0), with no significant difference observed compared to the SC individuals throughout the experimental period; as a result, the R75 individuals had a small final size (12.7 mm), which was significantly smaller than the R0 (17.9 mm) and R25 (16.8 m) individuals (p < 0.05), and similar to the SC individuals (12.3 mm) (p > 0.05).

The mean initial H/D of the large cuttings was in the range 100–120 (m/m). The mean H/D of the R0, R25, and R50 individuals decreased to 90 in July (Figure 3D), reflecting the early-started diameter growth (Figure 3C) compared to the slight growth in stem height (Figure 3A), and decreased to 65–75 at the end of the first growing season, while R75 individuals had a significantly higher final H/D (85) (p < 0.05) than R0 (66) and R25 (75) individuals (Figure 3D). The H/D of SC individuals was smaller than those of the large cuttings until July (p < 0.05), and did not show a particular increasing or decreasing trend, remaining in the range of 60–80 throughout the experimental period.

3.2. Stomatal Conductance

Stomatal conductance (g_s) was significantly higher in the R75 treatment than in the other treatments (ca. 1.7–3.0 times of R0, R25 and R50) in April and May, and in the R0 and R25 treatments in June (p < 0.05) (Figure 3E). SC individuals tended to have lower g_s values until June, and these values were significantly lower than those of R75, R50, and R25 individuals in late April. In July, g_s values were high in all treatments, including in SC individuals, and no significant differences among treatments were observed thereafter.

3.3. Stem Incline

On 15 April, one month after planting, large DSI values of more than 20° were observed in the R0 and R25 treatments. Although a maximum DSI value of 60° was observed in R0 individuals (Figure 3F), no significant differences in DSI values were observed among treatments (p > 0.05). The DSI values of R0 and R25 individuals decreased rapidly for two months until June, along with the decrease in H/D (Figure 3D), and showed similar values to R50 and R75 individuals, ranging from 5 to 10° after July. SC individuals had small DSI values (<7°) throughout the experimental period.

A positive correlation was observed between H/D and DSI on 15 April (τ = 0.346, p = 0.004), with extremely large DSI values (>40°) observed in stems with high H/D (>130), including individuals in the R75 treatment (Figure 4). The initial DSI for the same H/D on 15 April tended to be lower in the R75 than in other treatments, especially in those with high H/D (>125). The positive correlation between H/D and DSI was also observed on 8 August (τ = 0.285, p = 0.013), but this correlation had disappeared by 5 October along with the decrease in both DSI and H/D. Stems with a small H/D (ca < 110) had a low DSI (<20°). After August, a large DSI value (>20°) was only observed in one individual in the R75 treatment.

4. Discussion

4.1. Alleviation of Transplant Shock by Defoliation

The findings of the present study showed that partial defoliation had a positive effect on alleviating transplant shock in sugi at a certain level. Leaf removal of 75% of the crown length (R75) was associated with high g_s values––approximately twice that of the other treatments––in the early stage of plantation establishment (Figure 3E), indicating the short-term effectiveness of partial defoliation for alleviating water stress by improving the shoot:root ratio [9,10,11]. In a study on hinoki seedlings, partial defoliation was reported to be only effective for alleviating transplant shock in bare-root seedlings and not in container-grown hinoki seedlings [12]. In contrast, our results suggested that the defoliation of large, container-grown sugi rooted cuttings, which have higher relative proportions of above-ground organs relative to their roots, can reduce water stress by maintaining water balance as a short-term effect (until June).

However, the advantage conferred by the relatively higher g_s values in R75 individuals compared to the other treatments (i.e., R50, R25 R0 and the SC individuals) disappeared within three months after planting in the present study. This indicated that the long-term effect of leaf removal was limited when evaluated over the entire growing season. The greatly improved g_s in July under a VPD similar to the April measurements implied that the planted cuttings, especially of R50, R25, R0, and SC individuals, successfully developed their roots enough to improve their shoot:root balance, and that the stress conditions in the present study were less severe even for bare-root cuttings. Further, increases in height and diameter growth in all treatments during the first three months (i.e., until June) were lower than in the periods thereafter (Figure 3A–C), and no dead individuals were observed throughout the experimental period. This also suggested that severe transplant shock resulting in a decrease in growth, or the death of planted trees, might not occur under the conditions employed in the present study (i.e., stock size, suitable site environment, and planting season). As a result, the effects of the partial defoliation treatment are considered to have been limited in terms of the long-term effects, even in the case of the highest defoliation rate (R75). Meanwhile, the lowest g_s values of bare-root cuttings (SC) until June suggested that the root ball of the container-grown cuttings played a role in reducing water stress compared to bare-root stocks as reported by the previous studies [15,17,18,19,20,28], even for unbalanced shoot:root ratio of large cuttings (e.g., R0). We consider that leaf removal could be more extensive if larger stocks are used (i.e., stocks with more unbalanced above- and below-ground organs) at drier sites or seasons, c.f., [35,36].

4.2. Initial Growth and Stem Tilting of Large Rooted Cuttings

In the present study, stems with a low defoliation rate (R0 and R25) were likely to have larger DSI values compared to a high defoliation rate (R50 and R75) in the early stages of plantation establishment (Figure 3F), suggesting that the leaf removal treatment was partly effective in preventing stem tilting by improving the mechanical stability of the large cuttings. However, this effect almost disappeared within three months; for example, markedly high DSI values (>30°) were rarely observed after June, even in the R0 and R25 treatments. Further, the changes in the DSI values and H/D demonstrated that stem tilting was closely related to high stem slenderness (Figure 4), as reported previously for sugi [4]. The rapid decreases in DSI in the R0 and R25 treatments in our results were also associated with a decline in H/D, which was derived by the early-started vigorous diameter growth (Figure 3C) in contrast to the late-started and slow growth in stem height (Figure 3A). These results suggested that the mechanical stability of the stem (i.e., the risk of tilting) was more strongly dependent on H/D than on the leaf and branch mass in this study. The large variations in DSI within the same treatment at the early stage of plantation establishment is considered to be due to the initial variation in H/D at planting that had arisen before outplanting regardless of the leaf removal treatments. Further, although the simple average of DSI (regardless of their H/D) showed no significant difference among treatments because of its large variation (Figure 3F), the initial DSI tended to be lower in R75 than in other treatments in a high range of H/D (Figure 4). These results also implied that stem tilting was fundamentally influenced by H/D, and intensive leaf removal (reduction of the load) may reduce the risk of stem tilting under the high H/D conditions. However, the effect of leaf removal on the alleviation of stem tilting, as well as on transplant shock, was limited in the present study to the early establishment stage and only by a high leaf removal rate.

On the other hand, both the stem elongation and diameter growth tended to be delayed in response to an increase in the leaf removal rate (Figure 3A,C). In particular, the highest leaf removal rate treatment (R75) showed a relatively short stem length at the end of the measurement period, which was not significantly different from the stem length of the SC individuals. The growth in stem base diameter showed a clearer and earlier trend in growth decline of R75 individuals after June, when R75 individuals have a similar stem base diameter to the SC individuals. Since the g_s values of R75 individuals were higher than those of the other treatments/stock types until June (Figure 3E), the impact of the transplant shock is considered to be lowest in the R75 treatment; consequently, the decline observed in the growth of the R75 individuals could be attributed to these individuals having fewer photosynthetic organs than the other groups. The stem length and stem base diameter at the end of the experimental periods tended to be small as the defoliation rate increased, although the differences were not significant. Thus, for the large cuttings and planting conditions used in the present study, leaf removal had a negative effect on growth in the long term (over the entire growing season), rather than alleviating transplant shock in the short term (immediately after outplanting). Numerous studies have examined the effect of reduced leaf mass by defoliation on the growth of different plant species such as scarlet oak, green ash, and Turkish hazelnut [29,30].

It was reported that the growth of hinoki seedlings did not decrease in response to defoliation, and it was suggested that the lower older leaves of hinoki seedlings contributed little to carbon gain compared to the younger higher stem leaves probably because they did not adapt to the transplant environment [12]. In contrast to their results for hinoki, the growth of the large sugi cuttings used in the present study decreased after the removal of the lower older leaves. Our results therefore suggest that even the lower leaves play an important role in contributing to the carbon gain of planted sugi during the first growing season after outplanting. Further, the findings show that species characteristics as well as the stock type/size may affect the responses of planting stocks to leaf removal.

A large H/D in planting stock is known to delay the initial elongation rate after planting because stems that are too slender often undergo relatively higher rates of diameter growth [23,24,37] to improve the mechanical stability of the stem. In our experiment, stems other than in the R75 treatment first showed diameter growth from April, and then the elongation rate increased after the H/D was in the range of 80–90 in July (Figure 3). The delay in the elongation of R50 individuals was likely attributed to the reduction in leaf mass due to the leaf removal treatment. For R75 individuals, it is possible that the reduction in leaf mass was insufficient for inducing stem diameter growth (Figure 3C) to decrease the H/D (Figure 3D) sufficiently and accelerate elongation, resulting in further delays in elongation over the course of the study.

The difference in the stem elongation and diameter growth observed between stock types (i.e., large, container-grown rooted cuttings and bare-root short cuttings) could be explained by the difference in their initial H/D (Figure 3D). Thus, in order to effectively maximize the potential of large cuttings/seedlings, having individuals with a lower initial H/D (e.g., <110) would ensure a fast increase in height immediately after planting, as well as avoiding transplant shock based on the appropriate shoot:root ratio.

5. Conclusions

The results of the present study suggested that very intensive (75%) leaf removal in large-rooted cuttings of sugi (90 cm tall) grown in containers with 300 mL cavities is not required for alleviating transplant shock when evaluated in the long term (over the entire growing season), provided that the rooted cuttings are planted under appropriate conditions, i.e., in a suitable habitat and when water stress will be low. The effectiveness of leaf removal, and the optimal leaf removal rate, for larger stocks under severe conditions should be examined further in the future.

When large-rooted cuttings with high H/D are grown at the same densities as shorter cuttings, the risk of stem tilting after outplanting is increased. Our results suggest that very intensive leaf removal in large-rooted cuttings with high H/D can reduce the risk of stem tilting. However, it can also result in a decline in growth after outplanting due to individuals having fewer photosynthetic organs. It would be better practice to reduce the initial H/D by growing cuttings at lower densities before outplanting, rather than reducing the risk of stem tilting due to leaf removal at the time of outplanting.