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Article

Influence of Slope Aspect and Position on Xylem Formation Dynamics in Subtropical Chinese Fir Plantations

by
Yingni Huang
1,2,†,
Qianlin Li
1,2,†,
Chunmei Bai
1,2,
Wendi Zhao
1,2,
Diego Ismael Rodríguez-Hernández
3 and
Xiali Guo
1,2,*
1
Guangxi Colleges and Universities Key Laboratory for Cultivation and Utilization of Subtropical Forest Plantation, Guangxi Key Laboratory of Forest Ecology and Conservation, College of Forestry, Guangxi University, Nanning 530004, China
2
Guangxi Youyiguan Forest Ecosystem National Observation and Research Station, Youyiguan Forest Ecosystems Observation and Research Station of Guangxi, Pingxiang 532600, China
3
School of Biological Sciences, University of Bristol, 24 Tyndall Avenue, Bristol BS8 1TQ, UK
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Forests 2024, 15(7), 1193; https://doi.org/10.3390/f15071193
Submission received: 13 May 2024 / Revised: 14 June 2024 / Accepted: 7 July 2024 / Published: 10 July 2024
(This article belongs to the Special Issue Dendrochronology beyond the Ordinary: Understanding the Wood Formation and Climate Change)
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Abstract

:
Recent studies on the intra-annual dynamics of trees were mainly focused on mature trees in natural forests; however, less is known about how topography (e.g., slope’s position and aspect) shape the intra-annual xylem formation dynamics of young trees in plantation forests. We monitored intra-annual xylem formation dynamics of 6-year-old Chinese fir (Cunninghamia lanceolata (Lamb.)) trees across two different aspects (northeast and southwest) and three different positions (upper, middle, and lower) of the slope in a planted forest in subtropical China. We found that the intra-annual xylem formation of trees on the northeast slope aspect (176.98 ± 34.52 cells) was significantly higher than that on the southwest slope aspect (140.19 ± 36.07 cells) due to the higher growth rate (0.67 ± 0.11 cells/day vs. 0.53 ± 0.10 cells/day). In the middle slope position, xylem formation (187.89 ± 19.81 cells) was also significantly higher than that of the upper (147.35 ± 29.08 cells) and lower slope positions (140.51 ± 48.36 cells), resulting from higher growth rate rather than longer growing season length. Our study demonstrated that intra-annual xylem formation dynamics of Chinese fir were altered by different topographic features and therefore encourage the implementation of management strategies that consider different slope aspects and positions to maximize forest productivity.

1. Introduction

Global warming is increasing rapidly because of intensive human activities, including fuel combustion, deforestation, and industrial emissions. By the end of the 21st century, the average global temperature is predicted to increase by approximately 1.5 °C [1]. This could significantly impact tree growth, leading to alterations in the structure and functioning of forest ecosystems [2]. As critical carbon reservoirs, forests could effectively regulate the atmospheric carbon cycle, mitigating the impacts of global warming [3,4]. However, due to a compound of drought and heat waves, there is an increasing rate of tree mortality worldwide [5,6]. Thus, it is urgent to understand how trees respond to their local changing environment.
As an important part of trees, the xylem not only provides a stable structural supportation but also facilitates the transport of water and nutrients between roots and canopy [5,7,8,9]. It originates from the division of vascular cambium and gradually develops into mature cells through cell enlargement and cell wall thickening [7], further permanently fixed as structural carbon [10]. Therefore, understanding intra-annual xylem formation dynamics is crucial for predicting the carbon sequestration potential in forests. Previous studies have also revealed that certain external disturbances, such as insect outbreaks, and various climatic factors, including drought, floods, snowfall, and frost, could lead to intra-annual density fluctuations (IADFs) in the xylem [11,12]. The IADF is often correlated with tree age, ring width, and altitude, reflecting trees’ adaptation to climate change [13,14,15]. In tropical forests, cambium activity may continue throughout the year [16]. Whereas in temperate and boreal forests, cambium activity and xylem formation show a typical shift between dormancy and activity period [17,18,19,20], corresponding to the seasonality of temperature and photoperiod [10,21,22]. Specifically, trees in these zones tend to show the highest xylem cell production rates around the summer solstice [21,22]. Although there are several studies in these ecosystems, compared with boreal and temperate forests, less related studies have been carried out in subtropical montane forests, especially for young trees in plantation [23,24], hindering a clear understanding of the regulatory mechanism behind xylem formation dynamics for young trees.
Intra-annual xylem formation dynamics of trees are influenced by both internal and external factors, including endogenous hormones and nutrients [25,26,27], as well as temperature, precipitation, and photoperiod [28,29,30,31]. Among these, temperature is the primary factor regulating cambium activity in trees, which consequently affects xylem cell development [30]. Warmer spring temperatures can induce an earlier start and extended duration of the growing season [24,32], resulting in a longer period for carbon uptake and increasing forest productivity [33]. However, rising temperatures can also increase the risk of droughts [34], leading to xylem embolism, reduced water transport [35], tree decay, and die-off [36]. On the flip side, photoperiod could also control tree growth by affecting the onset of cambium activity and the maximum daily growth rate [30,31,37,38]. The topography including elevation and slope aspect also affects the dynamics of xylem formation by altering micro-environments [32,39,40]. However, the interplay effect of slope aspect and position on xylem formation has been neglected, especially in mountainous regions [41,42]. Therefore, further investigations are warranted to provide valuable insights into tree physiology and ecosystem functioning in mountainous environments.
The aspect of the slope can impact the micro-environment [43] by modifying the light incidence, humidity, and air temperature that a tree receives [44,45], which in turn affects its relative growth rates. In mountainous and hilly regions of the northern hemisphere, trees growing on the south-facing slope could receive higher solar radiation and soil water evaporation compared to those growing on the north slope aspect [46,47]. For instance, a previous study found that higher tree growth on tropical trees was due to the increased solar radiation and soil water availability that was reaching the south slope aspect [48]. Moreover, the micro-environment on the same slope aspect (e.g., soil moisture and nutrients) may vary according to different slope positions, especially when coupled with different levels of vegetation density [43]. Previous studies have shown that soil moisture in the upper slope position is lower due to the high solar radiation [47,49,50]. By contrast, the lower slope position has stronger soil moisture retention capacity due to lower solar radiation and evapotranspiration rates [51,52]. In addition, the redistribution of rainfall across different slope positions is also one of the reasons for variations in soil moisture over the same slope [53]. Thus, studying the effects of both slope aspect and position on the xylem formation dynamics of young trees can provide new insights into tree growth rates and forest management strategies.
Guangxi Zhuang Autonomous Region is located in the subtropical monsoon climate zone; 61.45% of this region is dominated by mountains and hills, leaving few flat areas. As a result, this area is particularly prone to soil erosion, making it more sensitive and fragile to water shortage [54,55], which could further result in a decrease in the number of tree populations, hindering tree growth, and may even lead to tree mortality in extreme cases. Chinese fir (Cunninghamia lanceolata (Lamb.)), a widely distributed fast-growing tree species, has high economic value in southern China and plays an important role in ecosystem services, such as water supply and organic matter storage [56,57,58,59]. Given the ecological importance of Chinese fir and the high topographic variability present in the region, studying the effect of the mountain slope aspect and position on xylem formation in this area is conducive to developing targeted management strategies to promote Chinese fir growth. In this study, we monitored the intra-annual xylem formation dynamics of young Chinese fir trees (6 years old) across two different slope aspects (northeast and southwest) and three different positions (upper, middle, and lower) of the slope in a planted forest in subtropical China. We therefore aim to (1) describe the intra-annual xylem formation dynamics of young Chinese fir plantations in subtropical regions and (2) elucidate the effects of the different slope aspects and positions on intra-annual xylem formation of young Chinese fir.

2. Methods

2.1. Study Area

This study was conducted in the Guangxi Zhuang Autonomous Region, southwest China (108°17′58′′ E, 22°58′14′′ N, Figure 1A). The area is located between the southern side of the Daming Arc Mountains and the northern margin of the Nanning Basin. The landforms are mainly hills and low mountains, with an altitude of 150–500 m and an average slope grade of 20–30°. The area experiences a subtropical humid monsoon climate, characterized by an average annual temperature of 22.11 °C, with the lowest temperature being in January at 6.97 °C, and the highest in July at 36.22 °C. The total annual rainfall in this region is 3444.34 mm. In the traditional classification of dry and wet seasons, approximately 72% of the annual rainfall in the study area in 2022 was concentrated during the wet season, spanning from April to September (ERA-5, Figure 1C). The altitude of the experimental site is 226 m (in a lower slope position). In 2022, the Chinese fir plantation (originally planted in 2017) was divided into three different slope positions (upper, middle, and lower) based on northeast and southwest slope aspects. Four forest plots were set up with a distance of 25 m between each slope position, with a total of 24 forest plots (20 m × 20 m) in all slope aspects and positions (Figure 1B). Within each plot, one representative healthy, upright tree was selected to monitor the intra-annual xylem formation dynamics throughout the year (Table 1).

2.2. Samples Collecting and Lab Preparation

Tree sampling was conducted during the early growing season from 8 March to December 2022. Wood microcores were collected every three days in March to accurately observe the onset of xylem formation in young Chinese fir trees (6 years old). While from April to December, samples were collected once every 1 or 2 weeks. Microcores were collected at breast height (1.3 m) using a trephor (D&J (Shanghai) Technology Co., Ltd., Shanghai, China) [60] with a distance of approximately 3–5 cm from the previous sampling location. After collection, wood samples were immediately fixed in centrifuge tubes containing 50% ethanol solution (Tianjin Damao chemicals reag ent factory, Tianjin, China) and further stored in a refrigerator at 4 °C [18].
A total of 552 microcores were collected for the study, and these were subsequently dehydrated in a series of ethanol solutions with increasing concentrations and then transparentized in a dipentene solution (Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China). Samples were then embedded in paraffin (Fujian Aowei Medical Equipment Co., Ltd., Ningde, China) at a melting point of 58–60 °C and split into 8 μm-thick xylem sections using a rotary microtome (Leica RM 2235, Leica Biosystems Nussloch GmbH, Nußloch, Germany). After drying the xylem sections on a hot plate, they were dewaxed twice with xylene (Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China) and anhydrous ethanol sequentially. The sections were stained with 0.06% toluidine blue (Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China) for 1 min and then observed under a Leica microscope (Leica Biosystems Nussloch GmbH, Nußloch, Germany) at 4× and 10× magnifications [12,13]. In each microcore section, the number of cells at the different developmental stages of the xylem (cambium, enlarging, wall-thickening, and mature) was recorded based on three columns along the radial direction. Lastly, we summed the total number of xylem cells (sum of enlarging, wall-thickening, and mature).

2.3. Data Analysis

The Gompertz function was used to simulate the xylem formation dynamics for each Chinese fir tree [61,62]. The formula for the Gompertz function is as follows:
y = A e − e β − κ t
where y represents the total number of cells accumulated per week, A is the upper asymptotic line, β is the position parameter on the x-axis, κ is the rate of change parameter, and t is the day of the year (DOY). The following parameters were extracted from the Gompertz function for further analysis: (1) annual xylem production (A); (2) onset and end of xylem growth, which were determined when the newly formed xylem growth reached 5% and 95% of annual xylem production, respectively [63]; and (3) the duration of the growing season. The derivative of simulated xylem growth was then calculated to obtain the daily xylem growth rates. Analysis of variance (ANOVA) was used to compare whether there was a significant difference among the two slope aspects and three slope positions of the abovementioned xylem growth parameters. The duration between the onset and end of the growing season was defined as the growing season length, and the average growth rate during the whole growing season was the division of xylem cell numbers during the whole growing season to the growing season length. All statistical analyses were performed in R version 4.2.1 [64].

3. Results

3.1. Comparison of Cambium Activity and Xylem Formation Dynamic of Chinese Fir in Different Slope Aspects and Slope Positions

There was no significant difference in the changing trend of cambium cells observed in Chinese fir in different slope aspects and positions (Figure 2A and Figure 3A), with cambium cell layers ranging from 4 to 7 without obvious rule. On 8 March 2022 (DOY 67) (the first sampling date), the activation of cambium cells in Chinese fir was observed, along with the emergence of enlarging cells. Except for 8 March (DOY 67), the enlarging cells were between 3 and 8 layers (Figure 2B and Figure 3B). Two weeks later (DOY 81), wall-thickening and mature cells appeared in Chinese fir across various slope aspects and positions (Figure 2B,C and Figure 3B,C). The changing trend of wall-thickening cells of Chinese fir generally increased first and then decreased. The northeast and southwest slope aspects reached the maximum on 16 August (DOY 228) and 8 July (DOY 189), respectively (Figure 2C). The upper, middle, and lower slope positions reached the maximum values on 13 October (DOY 286), 2 August (DOY 214), and 16 August (DOY 228), respectively (Figure 3C). The number of maturation phase cells showed an increasing trend, with the northeast slope aspect approaching a maximum of nearly 170 layers, while the southwest slope aspect reached 130 layers (Figure 2D). In different slope positions, the maximum layers of upper, middle, and lower slope positions were 130 layers, 180 layers, and 140 layers, respectively (Figure 3D).

3.2. Comparison of Formation Dynamics and Development Patterns of Chinese Fir Xylem in Different Slope Aspects and Slope Positions

After Gompertz model fitting, the growth of Chinese fir xylem showed similar growth patterns in different slope aspects and positions (Figure 4A and Figure 5A). The overall growth trend followed a sigmoidal shape, reaching maximum xylem growth in mid-December. Specifically, the northeast slope aspect showed a significantly higher growth count of 177 cells compared to the southwest slope aspect (140 cells) (Table 2). Similarly, within the same slope position, the growth count on the northeast slope aspect surpassed that of the southwest slope aspect (upper: 158 vs. 136 cells; middle: 200 vs. 176 cells; lower: 173 vs. 108 cells) (Figure 6A), and in the lower slope position, the northeast slope aspect was significantly higher than the southwest slope aspect (Table 3). The middle slope position exhibited the highest growth count at 188 cells, significantly higher than the upper (147 cells) and lower (141 cells) slope positions (Table 2), consistently maintaining higher values throughout the monitoring period compared to both upper and lower slope positions (Figure 5A). Within the same slope aspect, the xylem growth count on the middle slope position also exceeded that of both the upper and lower slope positions (Figure 7A). In the northeast slope aspect, it was shown as middle slope position (200 cells) > lower slope position (173 cells) > upper slope position (158 cells), and in the southwest slope aspect, it was shown as middle slope position (176 cells) > upper slope position (136 cells) > lower slope position (108 cells). A significant difference was observed between the lower slope position and the middle slope position of the southwest slope (Table 3).
The growth rate curves displayed similar trends across different slope aspects and positions, characterized by an initial increase followed by a decrease (Figure 4B and Figure 5B). The average growth rates for the northeast and southwest slope aspects were 0.67 cells/day and 0.53 cells/day, respectively, showing a significant difference (Table 1). Similarly, within the same slope position, the growth rate on the northeast slope aspect exceeded that of the southwest slope aspect (upper: 0.61 vs. 0.52 cells/day; middle: 0.74 vs. 0.63 cells/day; lower: 0.67 vs. 0.45 cells/day) (Figure 6A). In the lower and middle slope positions, the northeast slope aspect was significantly higher than the southwest slope aspect (Table 3, Figure 6B). Furthermore, the average growth rate on the middle slope position (0.69 cells/day) significantly exceeded that of the upper (0.56 cells/day) and lower (0.56 cells/day) slope positions (Table 1). Within the same slope aspect, during the early growing period, the growth rates across three slope positions were similar, but after the middle growing period, the growth rate on the middle slope position notably exceeded that of both upper and lower slope positions (Figure 7B). In the northeast slope aspect, it was shown as middle slope position (0.74 cells/day) > lower slope position (0.67 cells/day) > upper slope position (0.61 cells/day). In the southwest slope aspect, it was shown as middle slope position (0.63 cells/day) > upper slope position (0.52 cells/day) > lower slope position (0.45 cells/day). The middle slope position of the southwest slope aspect was significantly higher than that in other slope positions (Table 3).

3.3. Comparison of Simulation Parameters of Chinese Fir Xylem Formation in Different Slope Aspects and Slope Positions

According to the Gompertz function fitting results of the xylem formation dynamics of Chinese fir in the year (Table 2), it was found that the onset dates of xylem growth on the northeast and southwest slope aspects were 28 February (DOY 59) and 11 February (DOY 42), respectively, and the end dates were 18 November (DOY 322) and 31 October (DOY 304), respectively. The length of the growing season was 262 days for both slope aspects. The xylem growth for the upper, middle, and lower slope positions began on 19 February (DOY 50), 20 February (DOY 51), and 19 February (DOY 50), respectively, and ended on 9 November (DOY 313), 21 November (DOY 325), and 27 October (DOY 300), respectively. The length of the growing season was 262 days, 274 days, and 250 days, respectively. Significant differences were observed in the onset dates of xylem growth between the northeast slope aspect and the southwest slope aspect, as well as in the end date of xylem growth between the middle and lower slope positions. In the middle slope position, the onset on the northeast slope aspect was significantly different from that on the southwest slope aspect (Table 3).

4. Discussion

In previous studies, the effects of topographic factors on xylem formation dynamics were concentrated in the elevation and aspect of large-scale mountains [32,39,40,65,66], while related studies in subtropical hilly areas have been overlooked. In fact, in mountainous areas, trunks or branches try to resist external forces and maintain a vertical upward growth posture, thus forming reaction wood with significantly different anatomy and chemical composition [67]. In this study, we selected upright and healthy trees with consistent slopes and monitored their xylem formation rather than focusing on the reaction wood. By monitoring the intra-annual xylem formation dynamics of different slope aspects and the positions of fast-growing Chinese fir in the Guangxi Zhuang Autonomous Region, our results could offer useful suggestions for forest management.

4.1. Effect of Slope Aspect on Xylem Growth of Chinese Fir

Our results showed that the intra-annual xylem growth of Chinese fir on the northeast slope aspect (shady slope) was higher than that on the southwest slope aspect (sunny slope), even when comparing each slope position. This was consistent with the results of Guo et al. [68], who found that the ring width of Picea purpurea on the northwest slope aspect was significantly higher than that on the southeast slope aspect. In addition, Kraus et al. [40] also demonstrated that European beech (Fagus sylvatica) and Norwegian spruce (Picea abies) had larger increments of xylem toward the north. Iszkulo et al. [69] also found that the annual ring width of Juniperus thurifera on the east slope aspect was significantly higher than that on the west slope aspect. This is because different slope aspects lead to differences in the micro-environment, including light, humidity, wind speed, and soil characteristics [70,71,72], thus affecting the growth of the xylem.
Compared with the southwest slope aspect, the solar radiation of the northeast slope is weaker, the soil water evaporation is less [47], and the soil water availability is higher. The higher soil water availability makes it easier for roots to absorb sufficient water, which enhances the availability of water during the growth of tree cells. Water is one of the main components in plant cells and provides the pressure required for cell expansion and elongation [73]. When water is absorbed into the cell, the cell wall will expand, resulting in cell expansion and elongation, thereby promoting the growth of trees [74]. In other words, the expansion-driven cell enlargement process may be mainly affected by water availability. In addition, water availability also indirectly affects cambium cell division and xylem formation by regulating photosynthetic rate and carbohydrate storage [28]. Therefore, the availability of water is also considered to be an important factor affecting the total yield of xylem [75]. A previous study had shown that trees grown in soil with higher water availability exhibit higher average annual trunk radial growth, higher daily growth rate, and longer duration of xylem productivity [76].
Our results also showed that the growth rate determined the growth of the xylem rather than the growing season length. In general, the average growth rate of the northeast slope aspect was significantly higher than that of the southwest slope aspect. Furthermore, for different slope aspects at the same slope position, the average growth rate of the lower position in the northeast slope aspect was significantly higher than that in the southwest slope aspect. Previous studies have shown that the length of the xylem growth season and growth rate jointly determine the growth of the xylem [77,78,79]. In our results, we found that there was no significant difference in the length of the growing season from the comparison of slope aspect nor in slope position. Therefore, we believe that the higher xylem growth in the northeast slope is mainly due to the higher growth rate.
In addition to water, soil quality also plays an important role in regulating the growth of Chinese fir. Chinese fir grows fast and has high requirements for soil texture and nutrients. It is suitable for growing in fertile, deep, and humid acidic soil. Gong et al. [80] pointed out that the contents of total nitrogen, organic matter, and carbon–nitrogen ratio in the shady slope were significantly higher than those in the sunny slope, indicating that the shady slope had more abundant soil resources. These nutrients are necessary for plant growth and cell division, which can promote the normal growth and metabolism of Chinese fir cells and increase the growth rate of Chinese fir. Therefore, we believe that water and fertilizer conditions may be key factors in determining the intra-annual xylem growth of Chinese fir. Under the combined effects of various factors such as light, soil, and water availability, the northeast slope aspect showed greater annual xylem growth than the southwest slope aspect.

4.2. Effect of Slope Position on Xylem Growth of Chinese Fir

Our results show that the intra-annual growth of Chinese fir in the middle slope position is greater than that in the upper and lower slope positions, indicating that the micro-environment of the middle slope position is more conducive to the growth of Chinese fir. Different slope positions lead to the redistribution of precipitation, temperature, and heat [45,81]. Although the response of the upper slope position to precipitation is faster, the attenuation rate is also faster, which may be due to the redistribution of water in the lower slope position. At the same time, the soil moisture in the downhill decreased slowly [82]. The water flow rate of the upper slope position is fast, which may take away part of the soil’s organic matter, thus reducing the soil fertility. At the same time, soil organic matter is transported downhill on the slope, which is helpful for the accumulation of nutrients in the downhill, thus improving soil fertility. On the other hand, with the increase in slope position, the intensity of solar radiation increases [47,49,50,52]. The upper slope position receives more light, and the large evaporation and low soil moisture content are conducive to the decomposition of organic matter, resulting in less soil organic carbon content than the lower slope position. In summary, the soil moisture content and fertility of the upper slope position are the lowest, the lower slope position is the highest, and the middle slope position is between the two. The solar radiation on the upper slope position is the largest, but the soil moisture content and fertility are the lowest. The soil moisture content and fertility of the lower slope position were the highest, but the solar radiation was the smallest. In the above two extreme cases, the growth of Chinese fir may be limited and cannot reach the best state. In contrast, the soil moisture, fertility, and light conditions in the middle slope position may be suitable for tree growth, which is helpful in improving the growth rate of Chinese fir xylem. However, from the perspective of different slope positions on the same slope aspect, the growth rate of Chinese fir in the middle slope position was significantly greater than that in the lower position, especially for the southwest slope aspect. This indicated that the interplay of slope aspect and position may result in a complex micro-environment and jointly affect the xylem growth. Studies have shown that the differences in climate indicators such as light intensity, wind speed, and human disturbances at different slope positions will have an impact on plants [83]. Thus, these factors in the lower slope position of the southwest slope aspect may reduce the xylem growth rate and further xylem production.

4.3. The Application for Forest Management

Our results offered practical management strategies for Chinese fir plantations regarding tree growth across different slope aspects and positions in mountainous areas. Previous studies also highlighted the importance of considering the effect of abiotic factors including topography and soil resource availability on tree growth [84]. In general, for young Chinese fir seedlings, we encourage planting them in the northeast slope aspect and middle slope position areas to improve the xylem growth and yield. Sunny and upper slope positions could be replaced with more drought-tolerant and light- demanding species, while in lower slope positions, shade-tolerant species with higher water demand are encouraged for planting. However, given the complex interplay between slope aspect and position, more cautions experiments that consider these topographic features are needed. In addition, it is recommended to establish a regular monitoring system to track the growth of Chinese fir in each slope aspect and slope position in the long term and adjust the management strategy timely according to the monitoring results [85]. In this way, we can better grasp the dynamic changes in forest resources, adjust the forest structure, and ensure the sustainable utilization and protection of forest resources. Also, we found that the xylem growth rate of young Chinese fir was higher from April to September, which corresponds to the high temperature and rainfall. This phenomenon was observed in different slope positions of two slope aspects, indicating the dominant control of climate effect on xylem formation. Under climate change, the increasing frequency of extreme events including drought and frost will strongly alter the xylem formation dynamics of trees; thus, more comprehensive forest management studies that consider the effects of both biotic and abiotic factors should be made to reduce the challenges of climate change [86].

5. Conclusions

At present, although there are many studies on the formation dynamics of xylem, they have mainly focused on mature trees in natural forests. Considering that young trees are more sensitive to the environmental changes, it is very important to conduct an in-depth study on them. In addition, the existing studies mainly focus on the large-scale environment, and there are relatively few studies on the micro-environment. Therefore, we chose to study young Chinese fir in different slope aspects and slope positions to explore the effect of the microenvironment on xylem formation dynamics. The results showed that the xylem growth of Chinese fir on the northeast slope aspect was significantly higher than that on the southwest slope aspect, and the xylem growth of Chinese fir on the middle slope position was significantly higher than that on the upper and lower slope positions. Further analysis found that the intra-annual growth of the northeast slope aspect and the middle slope position was mainly affected by the higher growth rate, indicating that the microenvironment of the northeast slope aspect and the middle slope position was conducive to the growth of young Chinese fir. These findings highlight the important role of different slope aspects and slope positions in shaping the formation dynamics of Chinese fir xylem and provide an important reference for further optimizing forest management.

Author Contributions

Conceptualization, Y.H. and X.G.; Methodology, C.B., W.Z. and D.I.R.-H.; Formal analysis, Q.L. and C.B.; Writing—original draft, Y.H. and Q.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the National Natural Science Foundation of China (32201543) and the Natural Science Foundation of Guangxi Province (2021AC19325).

Data Availability Statement

The data that support the findings of this study are available from the corresponding author, upon reasonable request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

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Figure 1. Location of the study site in the central part of Guangxi Zhuang Autonomous Region of southern China: (A) Geographic location of the study site; (B) distribution of plots on two slope aspects; (C) daily average temperature, maximum temperature, minimum temperature, and rainfall in the study area. SW: southwest slope aspect; NE: northeast slope aspect; U: upper slope position; M: middle slope position; L: lower slope position.
Figure 1. Location of the study site in the central part of Guangxi Zhuang Autonomous Region of southern China: (A) Geographic location of the study site; (B) distribution of plots on two slope aspects; (C) daily average temperature, maximum temperature, minimum temperature, and rainfall in the study area. SW: southwest slope aspect; NE: northeast slope aspect; U: upper slope position; M: middle slope position; L: lower slope position.
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Figure 2. Number of cells (A) in the cambial zone, (B) radial enlarging phase, (C) wall-thickening phase, and (D) mature cells in different slope aspects of Cunninghamia lanceolata (Lamb.) Hook in 2022.
Figure 2. Number of cells (A) in the cambial zone, (B) radial enlarging phase, (C) wall-thickening phase, and (D) mature cells in different slope aspects of Cunninghamia lanceolata (Lamb.) Hook in 2022.
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Figure 3. Number of cells in (A) the cambial zone, (B) radial enlarging phase, (C) wall-thickening phase, and (D) mature cells in different slope positions of Cunninghamia lanceolata (Lamb.) Hook in 2022.
Figure 3. Number of cells in (A) the cambial zone, (B) radial enlarging phase, (C) wall-thickening phase, and (D) mature cells in different slope positions of Cunninghamia lanceolata (Lamb.) Hook in 2022.
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Figure 4. Simulated (A) number of xylem cells and the (B) growth rate of Cunninghamia lanceolata (Lamb.) Hook in different slope aspects.
Figure 4. Simulated (A) number of xylem cells and the (B) growth rate of Cunninghamia lanceolata (Lamb.) Hook in different slope aspects.
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Figure 5. Simulated (A) number of xylem cells and the (B) growth rate of Cunninghamia lanceolata (Lamb.) Hook in different slope positions.
Figure 5. Simulated (A) number of xylem cells and the (B) growth rate of Cunninghamia lanceolata (Lamb.) Hook in different slope positions.
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Figure 6. Simulated (A) number of xylem cells and the (B) growth rate of Cunninghamia lanceolata (Lamb.) Hook in different slope aspects at the same slope position.
Figure 6. Simulated (A) number of xylem cells and the (B) growth rate of Cunninghamia lanceolata (Lamb.) Hook in different slope aspects at the same slope position.
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Figure 7. Simulated (A) number of xylem cells and the (B) growth rate of Cunninghamia lanceolata (Lamb.) Hook in different slope positions at the same slope aspect.
Figure 7. Simulated (A) number of xylem cells and the (B) growth rate of Cunninghamia lanceolata (Lamb.) Hook in different slope positions at the same slope aspect.
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Table 1. Basic information about the sampling trees. SW: southwest slope aspect; NE: northeast slope aspect; U: upper slope position; M: middle slope position; L: lower slope position.
Table 1. Basic information about the sampling trees. SW: southwest slope aspect; NE: northeast slope aspect; U: upper slope position; M: middle slope position; L: lower slope position.
Tree NumberSlope AspectSlope PositionPlot NumberElevation (m a.s.l)Gradient (°)DBHc (m)Height (m)
1NEU12572311.28.8
2NEU22582311.08.8
3NEU32582410.38.1
4NEU42592311.89.0
5NEM52422411.38.3
6NEM62432214.110.7
7NEM72412510.87.2
8NEM82422510.98.0
9NEL92262413.09.8
10NEL102272412.58.9
11NEL112262513.69.3
12NEL122252513.09.3
13SWU132572510.77.6
14SWU142582410.27.3
15SWU152572410.27.5
16SWU162602511.08.0
17SWM172432410.07.1
18SWM182432311.48.5
19SWM192402311.78.4
20SWM202422411.18.6
21SWL212252512.18.5
22SWL222252412.09
23SWL232272513.59.6
24SWL242272311.48.3
Table 2. Analysis of variance (ANOVA) results of the key parameters of xylem formation between different slope aspects and positions of Cunninghamia lanceolata (Lamb.).
Table 2. Analysis of variance (ANOVA) results of the key parameters of xylem formation between different slope aspects and positions of Cunninghamia lanceolata (Lamb.).
VariablesF Value
Slope AspectSlope Position
NESWUML
Onset (DOY)59 ± 10 A42 ± 13 B50 ± 15 a51 ± 9 a50 ± 20 a
End (DOY)322 ± 14 A304 ± 29 A313 ± 16 ab325 ± 11 a300 ± 34 b
Growing season length (days)262 ± 20 A262 ± 37 A262 ± 24 a274 ± 11 a250 ± 41 a
Average growth rate (cells/day)0.67 ± 0.11 A0.53 ± 0.10 B0.56 ± 0.11 b0.69 ± 0.07 a0.56 ± 0.15 b
Xylem cell production (N)176.98 ± 34.52 A140.19 ± 36.07 B147.35 ± 29.08 b187.89 ± 19.81 a140.51 ± 48.36 b
Note: Different uppercase letters indicate significant differences between slope aspects (p < 0.05), and different lowercase letters indicate significant differences between slope positions (p < 0.05). SW: southwest slope aspect; NE: northeast slope aspect; U: upper slope position; M: middle slope position; L: lower slope position.
Table 3. Analysis of variance (ANOVA) results of the key parameters of xylem formation with different slope positions in the same slope aspect and the different slope aspects in the same slope position of Cunninghamia lanceolata (Lamb.).
Table 3. Analysis of variance (ANOVA) results of the key parameters of xylem formation with different slope positions in the same slope aspect and the different slope aspects in the same slope position of Cunninghamia lanceolata (Lamb.).
VariablesF Value
NESW
UMLUML
Onset (DOY)57 ± 15 Aa59 ± 3 Aa62 ± 10 Aa44 ± 14 Aa43 ± 2 Ab38 ± 19 Aa
End (DOY)316 ± 16 Aa330 ± 8 Aa319 ± 15 Aa309 ± 17 Aa321 ± 14 Aa280 ± 37 Aa
Growing season length (days)260 ± 27 Aa271 ± 5 Aa257 ± 24 Aa265 ± 24 Aa278 ± 15 Aa243 ± 57 Aa
Average growth rate (cells/day)0.61 ± 0.12 Aa0.74 ± 0.03 Aa0.67 ± 0.14 Aa0.52 ± 0.09 Ba0.63 ± 0.05 Ab0.45 ± 0.04 Bb
Xylem cell production (N)158.41 ± 35.73 Aa199.50 ± 6.63 Aa173.03 ± 43.48 Aa136.29 ± 19.22 ABa176.29 ± 22.64 Aa108.00 ± 27.35 Bb
Note: Different uppercase letters indicate significant differences between different slope positions on the same slope aspect (p < 0.05), and different lowercase letters indicate significant differences between different slope aspects under the same slope position (p < 0.05). SW: southwest slope aspect; NE: northeast slope aspect; U: upper slope position; M: middle slope position; L: lower slope position.
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MDPI and ACS Style

Huang, Y.; Li, Q.; Bai, C.; Zhao, W.; Rodríguez-Hernández, D.I.; Guo, X. Influence of Slope Aspect and Position on Xylem Formation Dynamics in Subtropical Chinese Fir Plantations. Forests 2024, 15, 1193. https://doi.org/10.3390/f15071193

AMA Style

Huang Y, Li Q, Bai C, Zhao W, Rodríguez-Hernández DI, Guo X. Influence of Slope Aspect and Position on Xylem Formation Dynamics in Subtropical Chinese Fir Plantations. Forests. 2024; 15(7):1193. https://doi.org/10.3390/f15071193

Chicago/Turabian Style

Huang, Yingni, Qianlin Li, Chunmei Bai, Wendi Zhao, Diego Ismael Rodríguez-Hernández, and Xiali Guo. 2024. "Influence of Slope Aspect and Position on Xylem Formation Dynamics in Subtropical Chinese Fir Plantations" Forests 15, no. 7: 1193. https://doi.org/10.3390/f15071193

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