Original articleDendroecological assessment of Korshinsk peashrub (Caragana korshinskii Kom.) from the perspective of interactions among growth, climate, and topography in the western Loess Plateau, China
Introduction
Soil erosion in terrestrial ecosystems is an important global environmental problem that significantly affects environmental quality and socioeconomic development (Fu et al., 2011). China's Loess Plateau, which lies between 100°54′E and 114°33′E and between 33°43′N and 41°16′N, suffers from some of the most severe soil erosion in the world. As the largest area of loess in the world, it covers more than 600,000 km2 in China. The Loess Plateau covers seven provinces or autonomous regions (Shaanxi, Gansu, Shanxi, Inner Mongolia, Ningxia, Henan, and Qinghai). Since the 1950s, a series of government-backed projects, including extensive tree planting, watershed erosion control measures, and the “Grain-for-Green” program, have been implemented to control soil erosion and restore vegetation (Chen et al., 2007a, Chen et al., 2007b, Chen et al., 2008, Wang et al., 2011). Under these programs, most of the traditional farmlands on steep slopes have been converted into pasture grasslands, shrublands, and forests during this process (Yang et al., 2012b).
However, both local and external factors, including climate change, drought, the introduction of unsuitable species, inappropriate planting density, and failure to account for the slope aspect, has led to soil desiccation and vegetation degradation in many areas; the ultimate result of these processes has been a decline in the water generation and soil conservation functions of the artificial vegetation (Li et al., 2007, Chen et al., 2008, Fu et al., 2012, Jia and Shao, 2013). Studies have shown that controlling vegetation density at reasonable levels, combined with periodic coppicing of the shrubs, could relieve the soil moisture deficits, prevent vegetation degradation, and promote the ecological functions provided by the vegetation (Li and Guo, 2011, Yang et al., 2012a). The appropriate plant density and shrub coppicing period are determined mainly by interactions among the soil moisture conditions at a site, the planted species, the planting density, the slope aspect, and plant age (Chen et al., 2005, Bi et al., 2006, Cheng et al., 2009, Cheng et al., 2013b, Guo, 2009, Huang and Zhang, 2013, Jia and Shao, 2013, Mo et al., 2013). However, little long-term data is available on plant growth and the related climatic and environmental response patterns.
Understanding these interactions requires long-term data, and the annual growth rings of woody vegetation can provide such information when more direct measurements are unavailable. The variation in the characteristics of these growth rings results from a complex set of interactions. This pattern of variation is a function of interactions between a plant's genotype and the environment for the whole tree, as these interactions affect the factors that control cambial growth (Schweingruber, 1996, Downes et al., 2002). Tree rings provide precise information on past growth of trees and on their response to the environmental changes (Spiecker, 2002). Through analysis of relationship between climate and tree growth, dendroecology can provide insights into the factors that limit tree growth (Antos et al., 2008).
It is well known that the growth potential of trees depends on their genetic composition and their age. Generally, the tree ring width and the radial stem increment both increase during an initial juvenile phase, then decrease until they reach a relatively stable increment (Schweingruber, 1996). Thus, tree-ring growth patterns can provide an indication of both tree vigor and periods of growth suppression and damage caused by environmental factors, and have been used to predict the decline and mortality of trees and guide forest management (Bigler et al., 2004, Antos et al., 2008), such as thinning and harvesting of forests and coppicing of shrubs, especially in artificial forests in arid regions.
The slope aspect at a site is an important topographic variable because it affects the daily, seasonal, and annual amount of solar radiation that plants and soil receive. As a consequence, topography has a strong influence on the microclimate of a site, and especially the air temperature, relative humidity, and soil moisture, each of which affects the relationships between climate and vegetation growth. However, there have been conflicting research results for different regions and species. For example, ring-width variations of Empetrum hermaphroditum sp. growing on ridges, north-facing slopes and south-facing slopes in an alpine environment reflected the dominant regional climate signals rather than topographic differences (Bär et al., 2008). Conversely, slope aspect critically influenced the radial growth of Qilian juniper (Sabina przewalskii Kom.) and Qinghai spruce (Picea crassifolia Kom.) on the northeastern Tibetan Plateau (Liang et al., 2006). Similar results have been found for Pinus sylvestris L. in northern Norway (Kirchhefer, 2000), Pinus tabulaeformis Carr. artificial forest in China's western Loess Plateau, China (Wang and Zhang, 2009), and four tree species, namely yellow-poplar (Liriodendron tulipifera L.), northern red oak (Quercus rubra L.), chestnut oak (Quercus prinus L.), and red maple (Acer rubrum L.) in an Appalachian watershed (Fekedulegn et al., 2003). However, Graumlich (1993) reported that species differences were more important than site differences in the mixed coniferous and deciduous forests of Wisconsin and upper Michigan. Therefore, it is essential to understand the interactions among growth, climate, and topography for each species, particularly when a species is used in artificial afforestation in arid areas.
In recent decades, dendrochronological studies of shrubs and dwarf shrubs at high latitudes (Bär et al., 2007, Bär et al., 2008), in semi-arid and arid areas (Xiao et al., 2005, Xiao et al., 2006, Xiao et al., 2012), and at high altitudes (Xiao et al., 2007, Liang and Eckstein, 2009, Liang et al., 2012) have increased. These studies demonstrated the high potential of shrub species for extending the present tree-ring network, and represent a highly promising research direction (Lu and Liang, 2013).
Korshinsk peashrub (Caragana korshinskii Kom.) is a drought-tolerant mesquite that can grow in regions with annual precipitation ranging from 100 mm to 550 mm (Cheng et al., 2013a). As a result, it is naturally distributed in areas such as the Tengger, Badain Jaran, and Ulan Buh deserts, and Mu Us sandland (Liu et al., 1987). During the last 30 years, its strong tolerance of harsh environmental conditions and its high economic and ecological value have led to widespread planting of this shrub in the Loess Plateau to reduce soil erosion, control desertification and provide animal feed (Li et al., 2006, Xia et al., 2006, Fang et al., 2008).
In the present study, we obtained growth-ring data for this species, and examined its relationship with various topographic and climatic parameters. Our goals were to examine whether the radial growth varied in response to regional climatic and topographic conditions, determine whether there was a most appropriate slope aspect for afforestation with Korshinsk peashrub, and identify the optimal age for coppicing of the shrub and rehabilitation of degraded land in the western Loess Plateau of northwestern China.
Section snippets
Study area
This study was conducted north of Lanzhou City (103°56′E, 36°06′N, H 1716 m a.s.l.), in China's Gansu Province (Fig. 1). It is located in the western part of the Loess Plateau, which has a semi-arid temperate climate (Fig. 2). Annual mean rainfall averages 249.0 ± 59.1 mm (1961 to 2013), with more than 84% falling from May to September. Annual pan evaporation ranges from 1400 to 1800 mm. The mean annual air temperature is 7.2 ± 0.6 °C, with a mean daily maximum temperature of 20.6 °C in July, and a mean
Characteristics of stem radial growth and the ring chronology
The stem radial growth trends at the three sampling sites were similar, although the maximum growth rate occurred on the semi-shady slope (Fig. 3). The radial growth increased until an age of 3 to 4 years, reaching a maximum of 1.2 to 1.5 mm at this time; thereafter, ring width decreased gradually to about 0.4 mm at 8 years, followed by a slower decrease, with stabilization at around 0.2 mm occurring by around 10 years.
After removal of the age-related radial growth trend, we established three
Influence of the regional climate and topography on radial growth
In arid areas, the recharge of soil moisture by precipitation is crucial to sustain plant growth. In the central Loess Plateau, where average annual precipitation of 414.9 mm, soil desiccation, which is generally defined as the soil dried to a moisture content of less than 5% to a depth of more than 20 cm (Xiao et al., 2006a), has occurred to a depth of 100 cm in 3-year-old Korshinsk peashrub plantations. In the 5-year old plantations the depth of desiccation increased to 300 cm, with the density
Conclusions
In the western Loess Plateau, the radial growth of Korshinsk peashrub on shady, semi-shady, and sunny slopes was influenced by both the regional climate and by site topography. The mean temperature in May was significantly positively correlated with ring growth on all three slopes. Precipitation in September and October was also significantly correlated with ring growth, although the different soil moisture conditions on the shady slope resulted in no significant correlation for that site. Our
Acknowledgments
Our research was funded by the National Natural Science Foundation of China (41471082) and National Key Technology R & D Program of China (2011BAC07B05, 2011BAC07B04). We are grateful to the journal's editor and anonymous reviewers for their valuable comments and language improvement.
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