Dendroecological assessment of Korshinsk peashrub (Caragana korshinskii Kom.) from the perspective of interactions among growth, climate, and topography in the western Loess Plateau, China

Abstract

Dendroecology is a useful tool to provide insights into the growth of woody plants and their response to climate change and topographical factors, since the results can provide guidance for the management of artificial forests. Using dendrochronological methods, we examined the relationships between slope aspect, climatic variation, and radial growth of Korshinsk peashrub (Caragana korshinskii Kom.), a primary shrub species used in afforestation in arid and semi-arid regions of the Loess Plateau, in northwestern China. Mean May temperature is the primary factor that limits radial growth at all study sites, followed by September and October precipitation at the shady and sunny sites. Our surveys of canopy area, shrub height, number of branches and average ring width showed that, except for the number of branches, the value on semi-shady slopes was larger than those on shady and sunny slopes. No evidence of soil desiccation was found, and the best water status occurred on semi-shady slopes at depth from 10 to 60 cm within the 1.8 m profile. The age-related average ring width curves indicate that radial growth increased to about 4 years of age, declined rapidly until 8 years of age, and then stabilized. Our results suggest that Korshinsk peashrub is a suitable species for afforestation in the western Loess Plateau, especially on the semi-shady slopes, and that a suitable age to begin branch coppicing is around 8 to 10 years. However, shrub density should be maintained at low levels (around 5000 shrubs per hectare).

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.