Changes in climate and other environmental conditions can cause influence on the growth condition of forests (Metsaranta & Bhatti, 2016). Alterations in the climate and environmental conditions affect the structure and function of vegetation (Ding et al., 2019; Anderson-Teixeira et al., 2013). As a result of a variation in soil properties (Soong et al., 2020; Wilcke et al., 2008), changes in temperature and precipitation patterns and increases of carbon dioxide concentration are likely to drive significant modifications in forests (Kirilenko & Sedjo, 2007). Seasonal patterns of wood growth are related to water availability (Worbes, 1999), temperature and soil water (Beedlow et al., 2013), and the length of the growing season, implying that soil properties and climate change related factors are likely to affect forest growth. In this regard, silvicultural systems based on the concept of sustainable use require knowledge of the long term diameter increment of tropical forest trees and their response to climate (Worbes, 1999). Thus, understanding the factors influencing tree growth is the essential component of forest ecology.
In order to get information about the growth of trees, growth analysis is a unique method to obtain growth information (Husch et al., 2003). Among the growth evaluation methods, analysis of annual tree-rings is a potential source of long term information (Haines et al., 2018). Soil and tree-ring analysis is also often used to assess long-term trends in tree growth (Peters et al., 2015; Park et al., 2015). Those methods are powerful tools to reconstruct historical growth changes and have been widely used to assess tree responses to global warming (Duchesne et al., 2019). Study on soil and tree-ring analysis provides important contributions in understanding the climate sensitivity of trees and the effects of global change on forests (Brienen et al., 2016). On the other hand, investigation of annual tree ring is best in estimating the ages of the trees (Brienen et al., 2009), so as to examine the pattern of growth of trees over a long period of time (Rozendaal et al., 2011).
The forms of growth, activity and renewal of stems and branches are primary determinants of ecosystem function and strongly influence net primary productivity (Rossatto et al., 2009). Thus, this study was aimed to evaluate the growth patterns of C. obtusa species along longevity focusing on the relationship between growth of the species and soil and ecological factors; climate variables mainly change in temperature and precipitation.
2. Materials and Methods
2.1. Study Areas
The study was conducted at two cities and one county of Gyeongnam province (Figure 1). Sample site selection was made based on the regional representation of cities and counties considering the south, east and western Gyeongnam province. In a 20 * 20 m area a total of 11 quadrats were laid under the plantation stand of C. obtusa. A number of quadrats located at Hadong (three quadrats), Gimhae (four quadrats) and Tonyeong (four quadrats) respectively. Mature stands with an estimated age of greater than 25 years were considered. To avoid the biasness of data quality, all the sample quadrats were placed away from the margin and road sides of the stands and trees with defect were excluded. Table 1 shows the background information of the stands included in the study.
Table 1. Background information (environment, climatic conditions and tree status)a of the study areas.
aaverages of DBH, height, volume, age, annual growth, spacing, altitude and (ten years averages of annual temperature and total precipitation). The values in parentheses are standard errors of the mean.
Figure 1. Locations of the study areas (shaded areas represent sample study cities and county).
2.2. Data Collection and Analysis
A total of 141 trees were selected for the data collection. Mora Swedish wood increment borer was used to collect data of annual growth rings of trees to study lifetime growth patterns and age of the species. In addition, Vernier caliper, Hypsometer and meter were used to measure tree height and DBH. Ten years of annual average temperature and total precipitation were gathered, and soil sample was also collected from each quadrat.
Growth pattern evaluation of the mature stand was evaluated by the annual growth rings collected from diameter at the breast height (DBH). All the core samples were air-dried and polished with the sand paper to get the fine grains. Age determination processing was done by using a computer based software WinDENDRO version 6 - 36 Canada for automatic annual ring detection with the accuracy of 1/1000 mm measurement. Age of a tree was identified from the pith of the trees. To study the properties of the soils, the samples were air-dried and passed through 2 mm-sieves to obtain fine earth separates. From each soil sample of different study areas, over 5 mg of fine soil samples were prepared and measured for the moisture content and dried at 105˚C for 12 hrs and measured again for their dry weight. The organic matter content (%) of the soil was measured. Determination of soil pH was made from 1:5 ratio of soil and water (5 g: 25 ml) after shaking for 30 minutes by using pH meter, Orion, USA. The cations exchange capacity (CEC, cmol (+)/kg) was determined by adding Ammonium Acetate (NH4OAc) saturation of pH 7. From the prepared sample analysis of soil moisture content, pH, organic matter (%), EC (cmol + /kg of soil), and available phosphorous was conducted. A statistical software (SPSS version 25) was used for data analysis in order to evaluate the significance of variables and its relation to trends of growth of the tree species.
From the results of the analysis of the annual growth rings, it was identified that trends of growth patterns of C. obtusa species were declined while the age increase (Figure 2). Development of growth was higher at young age (ages from time of planting to 30’s) but the growth condition changed while the trees mature. This implies the impact of tree age on the annual growth and its cause of growth decline along longevity. Annual average temperature was positively correlated (p < 0.01) with height growth indicating that changes in the condition of climate resulted in impacts on the growth pattern of the trees (Table 2).
Statistically, there was no significant correlation between growth factors (height and DBH) to annual total precipitation, some soil properties such as moisture content, organic matter, and available phosphorous, and slope, and tree age (Table 3). However, soil pH significantly was correlated to DBH growth indicating that C. obtusa growth was influenced by level of pH in the soil.
Figure 2. The growth patterns of stands along age at the studied cities and county.
Table 2. Pearson’s correlation in between tree growth to soil, climate and ecological factors.
**Correlation is significant at the 0.01 level (2-tailed), Av. P = available Phosphorous, OM = organic matter, MC = moisture content, Temp (˚C) = annual average temperature, precipitation (mm), annual total precipitation.
Table 3. Averages of soils properties of the study sites.
The values in parentheses are standard errors of the mean.
Moreover, there was a negative significant relation (p < 0.01) between DBH growth and altitude. Also it was revealed that the average annual growth of DBH was relatively higher for younger stands than old mature stand aged above 70 years (Table 1).
Spacing was found to be one of the factors related to the growth patterns of C. obtusa trees. From the Pearson’s correlation, there was a positive and significant relationship (p < 0.01) in between tree height and spacing. Trees planted at narrower spacing showed higher mean total height growth than at wider spacing, however, the volume growth increased as the spacing increased.
Factors Influencing the Growth Patterns of the C. obtusa Stands
To obtain a measure of growth pattern, one might first think of examining the correlation between growth, environmental, soil, and climatic factors. From the study results, it was identified that growth trends of C. obtusa species declined while the age increased. The same result was reported by different studies showing an inverse relationship between annual growth rate and age along the longevity, as the age increase, the amount of annual growth decrease (Johnson & Abrams, 2009; Binkley et al., 2002; McMurtrie et al., 1995). A study on another species (Pinus koraiensis) in S. Korea resulted in the way of growth percentages of DBH, height and volume tended to decrease with age particularly rapidly at age 20 - 30 (Seo et al., 2014, 2018). Likewise, in the eastern region of S. Korea the diameter and height growth of Pinus densiflora species felled down rapidly with longevity until age 30 (Seo et al., 2015). These indicate that tree growth pattern variation after maturation and changes through life time. The pattern of tree growth varies through life time. Plantation tree grew fast at the young age (under 20 years) but annual increments then decreased (Worbes, 1999). Consequently, although the productivity of forest is basically regulated by the genetic traits of the species and the site conditions, it also differs according to the stand development stage or stand age (Fujimori, 2001; Smith & Long, 2001).
By investigating the determinant causes that may result growth decline along longevity, different studies suggested various reasons. For instance, a study by (Black et al., 2008) showed that long-lived species maintained slower radial growth rates than shorter lived species on the same sites, showing the influence of tree age on growth of stands. The gradual decline of productivity of forest is assumed to be aging mechanisms (Murty & McMurtrie, 2000), which is due to the nutrient availabilities declines with age (McMurtrie et al., 1995), and change in climate conditions (He et al., 2012), causing growth decline. On the other hand, part of the universal age-related decline in forest growth derives from competition-related changes in stand structure and the resource-use efficiencies of individual trees (Binkley et al., 2002; Alen, 1986). However, competition related changes might be much higher at the young age of the stand but as forest get mature, it develops self-thinning and reduce potential competent on the available resources, and thus the possibility of competition in reducing DBH growth is real (Buechling et al., 2017), but might not be stronger as to the young stands.
Similarly, there is a strong positive relationship between soil properties and forest dynamics of growth (Soong et al., 2020) and growth decline occurs when there are changes in the supply rate of required soil resources (Ryan et al., 1997). Fast-growing species seem to benefit from initially higher pH in the soil indicating that soil properties favor the growth of some species (Carrasco-Carballido et al., 2019; Martínez-Garza et al., 2016). In this study the growth decline was resulted where there was higher soil moisture content, organic matter and available phosphorous; implying that the relationship between some soil properties and growth decline of C. obtusa along longevity was insignificant. However, soil pH was found to be an influential factor in the DBH growth pattern of the species. Previous studies also noted that the species prefers moist but well-drained, loamy, somehow acidic soil for optimum health and growth (Larum, 2018; Park et al., 2015).
Temperature is one of the main factors regulating the growth of trees as it might induce growth declines due to drought stress at lower elevations particularly for old trees (Primicia et al., 2015) . Understanding the effect of temperature on individual tree species growth is one of the primary elements of the response to climate change and designing method of suitable forest management. The effect of temperature on tree growth varies as it influences the amount and exposure to sunlight, moisture evaporation and related. Increasing temperature directly affects plant growth through effects on photosynthetic and respiration rates (Kirschbaum, 2000). Variation in the temperature influences the growth as it affects the soil hydrological and thermal regimes (Hagedorn et al., 2014); water and nutrient availability, the timing of the warming and rising atmospheric CO2 (Way & Oren, 2010). Thus, the significant relationship between C. obtusa growth and annual average temperature indicates the intimate relation of growth decline to the climate variability (Aubry-Kientz et al., 2019).
Trees cannot grow everywhere; meaning growth of trees is limited by its growth requirements. Altitude is among the limiting factor of growth of trees, despite it varies with the type of species and their adaptation capacity. Some trees become small and have more open canopies at high altitudes, growth pattern varies when the altitude changes; and it declines with the altitude (Coomes & Allen, 2007; Büntgen et al., 2019). This is related with the temperature fluctuation along the altitude caused by climate changes (Paulsen et al., 2000), as the altitude increases, air condition varies, and soil properties change which affect life of trees and its growth condition.
Furthermore, growth form of a tree depends on the spacing. Widening of spacing results higher DBH growth and trees planted at narrower spacing grow taller in height. This is related with the number of plants growing at a certain areas and the form of competition. The result revealed that at a close spacing the average height can be increased substantially and also the volume growth increased as the spacing increased. One of the possible reasons for such kind of relationship is the result of intraspecific competition (Akers et al., 2013; Zhu et al., 2007; Watt et al., 2006).
Despite a wide range of results of studies reporting about the decline of tree growth along longevity, it is still not clear whether tree longevity depends on slow growth rates and whether or not this relationship is species-specific, genetic and/or environmentally controlled (Büntgen et al., 2019). Growth patterns of most studied plantation and confer trees tend to decline along lifetime; however, trees in the natural forest showed a positive trend of diameter growth (Kohl et al., 2017). Thus, specific studies that focus on the relationship between environmental, and/or genetic and species-specific factors and forest growth decline along the longevity could provide a sort of information about the causes. According to the results of the current study, different factors were found to be responsible for the growth decline of C. obtusa species along the longevity. Age, soil pH, change in temperature and altitude were the main factors that were identified for their effects on the growth pattern along the life of the stands.
We are grateful to the lab members of silviculture in the department of Forest Resources, Gyeongsang National University for their unreserved support during collection and processing of data for the study.
Data Availability Statement
Necessary data would be made available when requested.
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