INTRODUCTION

Aral Sea has been desiccated by water abstractions from the contributing rivers to supply water for the irrigated cotton and rice cultivation since 1960 (Breckle et al. 2001). Furthermore, the dried sea floor has transformed to the desert Aralkum since the 1980s (Breckle et al. 2001). The soil in Aralkum is often high in salinity and low water content, organic matter, and nutrients (Breckle et al. 2001). The microclimate in Aralkum had changed by desiccation of Aral Sea, resulting in increasing salt and dust storm and deteriorating soil physical and chemical properties (Qadir et al. 2009). Afforestation has been used to rehabilitate the degraded area by reducing the wind erosion and improving soil quality (Khamzina 2006), since longer time is otherwise required for the natural vegetation introduction (Ravindran et al. 2007).

The choice of candidate species for afforestation in the Aralkum desert is limited by dryness, salinity, and large annual and seasonal variations in air temperature. Haloxylon species are introduced naturally under such conditions and are thus used for the afforestation on arid and saline soil (Orlovsky and Birnbaum 2002). These species have high tolerance to salinity, dryness, and low nutrients in the soil (Huang et al. 2003). They can accumulate high concentration Na⁺ and K⁺ ions in photosynthetic tissues, and can uptake the amount of water necessary for photosynthesis (Wang et al. 2004). Black saxaul (Haloxylon aphyllum) is a dominant woody species in the Central Asian deserts due to its xerophytic and halophytic characteristics. Afforestation with Haloxylon aphyllum may stimulate the introduction of other species, resulting in the expansion of vegetation cover (Novitskiy 2012). However, severe drought stress could decrease the photosynthetic activity, resulting in the reduction of Haloxylon species growth (Su et al. 2007).

Physical and chemical soil properties such as salinity, sodium absorption ratio, and sand ratio can determine the growth and mortality of H. aphyllum in the Aralkum (Matsui et al. 2019). Furthermore, climatic factors such as the amount of precipitation, seasonal temperature, and spring frosts could affect the growth and survival of Haloxylon species in arid desert regions (Kuzmina and Treshkin 2012). High wind speed stimulates the desiccation of vegetation, and sand accumulation by dust storms can induce the mortality of seedlings and small shrubs (Okin et al. 2001).

The carbon storage in arid land is estimated at 36% of the global carbon storage, and Kazakhstan has one of the world’s largest dryland areas (Trumper et al. 2008). However, the carbon storage and sequestration in Central Asia and their contributions to the global carbon cycle have not been specified in spite of its large area and contribution to the global carbon cycle (Li et al. 2015). In particular, land areas naturally dominated or afforested by H. aphyllum may contain a considerable carbon stock (Thevs et al. 2013), but current estimates are uncertain because of the scarcity of field-based data (Yohe et al. 2006).

The aim of this research was to investigate the growth and carbon storage of H. aphyllum in afforested areas on the dry Aral Sea bed, Kazakhstan. We investigated (i) the growth dynamics of H. aphyllum through examining the chronosequence of afforestation sites, (ii) assessed the relationship of the plant growth with afforestation year and soil properties, and (iii) estimated the biomass and carbon storage in the H. aphyllum afforestation areas.

MATERIALS AND METHODS

Six sites of the afforested area were selected near Kazalinsk, southern part of the North Aralkum (Table 1; Fig. 1). The annual mean temperature and total precipitation measured 8.9°C and 123 mm by Kazalinsk meteorological station (Breckle and Wucherer 2012). Afforestation with H. aphyllum was conducted in 2000, 2005, 2009, 2010, 2013 and 2017 (Table 1). In August 2018, three 30 m×44 m sized plots were randomly selected in each site for plant measurements. The plot size reflected the typical planting distance in Aralkum afforestation sites.

Root collar diameter (RCD, cm) was measured at 10 cm above ground using a digital caliper, and height (cm) was measured from ground to top of tree using a ruler for all H. aphyllum plants in each plot. The number of H. aphyllum plants was also counted and density of H. aphyllum plants (shrub ha^-1) was calculated in each plot.

Above-ground biomass (AGB), below-ground biomass (BGB), and total biomass (TB) of individual H. aphyllum were estimated using measured height (H) data in allometric equations (1), (2), and (3) (Xu et al. 2017):

Biomass accumulation (g m^-2) was calculated by dividing the sum of total biomass of all H. aphyllum by the plot area, and carbon storage (C g m^-2) was calculated by multiplying the biomass of each component by mean carbon content of Haloxylon species (above-ground component: 48.05% and below-ground component: 47.05%; Buras et al. 2012).

The differences in RCD, height, and number of H. aphyllum, as well as biomass accumulation and carbon storage in H. aphyllum among study sites were analyzed using ANOVA and Tukey post-hoc test. The relationship of diameter and height of H. aphyllum with afforestation year and soil properties in the depth of 0-10 cm (unpublished data, Appendix 1) were determined using correlation analysis with SAS 9.4.

RESULTS AND DISCUSSION

RCD (cm) was 2.9±0.1 in S₂₀₀₀, 3.6±0.4 in S₂₀₀₅, 3.9± 0.2 in S₂₀₀₉, 4.0±0.4 in S₂₀₁₀, 4.3±0.4 in S₂₀₁₃, and 2.5±0.1 in S₂₀₁₇, respectively (Fig. 2). Height (cm) was 106.2±14.0 in S₂₀₀₀, 112.7±10.6 in S₂₀₀₅, 149.8±14.5 in S₂₀₀₉, 189.8± 2.3 in S₂₀₁₀, 223.7±8.9 in S₂₀₁₃, and 121.2±3.5 in S₂₀₁₇, respectively (Fig. 2). There was a significant difference only between RCD in S₂₀₁₃ and that in S₂₀₀₀ or S₂₀₁₇. Height was significantly higher in S₂₀₁₃ than in the other sites except for S₂₀₁₀, and height in S₂₀₁₀ was significantly higher than that in the oldest (S₂₀₀₀, S₂₀₀₅) and in the youngest (S₂₀₁₇) sites.

The height of H. aphyllum in the current study was within the range of results previously reported for Haloxylon species; 170 cm height of H. aphyllum in the Gurbantonggut Desert of China (Xu et al. 2011) and 15-160 cm of H. aphyllum in the Aral region of Kazakhstan (Matsui et al. 2019). Breckle (2013) reported that the height of psammophytic vegetation was 100-200 cm, and that of halophytic vegetation was below 100 cm in the Aralkum.

According to Zhu and Jia (2011), the height of H. aphyllum increased with the period after plantation in the afforested area in China (2 years to 30 years). Besides, the growth of Haloxylon species was related to soil properties such as salinity and nutrient content, and the properties of deeper soil layer had particularly high correlations with the growth of Haloxylon species (Matsui et al. 2019). However, in the current study, the growth of H. aphyllum showed a different tendency from afforestation year and soil properties. The correlation coefficients of RCD and height were not significant with afforestation year (0.05 and 0.46, respectively) and soil properties (- 0.68 to - 0.18 and - 0.29 to 0.31, re-spectively) (p<0.05; Table 2).

Thus, the growth of H. aphyllum might be influenced by microclimate variability of the sites such as temperature and precipitation rather than age and soil properties in this region. It was reported that spatial variations in microclimate and soil moisture conditions could influence the growth of Haloxylon species (Breckle 2013). Vegetation in some regions is under the influence of severe hot summer, cold winter and low precipitation. Moreover, the study areas are located in northern Aralkum, the region largely affected by dust storms that limit the plant growth (Breckle 2013). The intensity and frequency of dust storms vary spatially and temporally (Spivak et al. 2012). Therefore, it seems that there was a great variation in the growth of H. aphyllum among sites. Further study is needed to investigate the effect of microclimate on the growth of vegetation in Aralkum.

Density of H. aphyllum was 58.1±16.6 in S₂₀₀₀, 164.1± 24.1 in S₂₀₀₅, 65.7±6.7 in S₂₀₀₉, 80.8±11.0 in S₂₀₁₀, 116.2± 11.0 in S₂₀₁₃, and 108.6±11.0 in S₂₀₁₇, respectively (Fig. 3). The number of H. aphyllum in S₂₀₀₅ was highest, and was significantly higher than that in S₂₀₀₀, S₂₀₀₉, and S₂₀₁₀ in contrast to the growth pattern. The number of saxaul may vary among sites due to mortality of originally planted H. aphyllum as well as their self-propagation. Mean total biomass accumulation in afforested areas was 20.57 g m^-2 (AGB: 10.45 g m^-2 and BGB: 9.93 g m^-2), ranging from 2.42 g m^-2 to 64.53 g m^-2 (AGB: 1.21-32.98 g m^-2 and BGB: 1.15-31.38 g m-2) (Table 3). Mean carbon storage in afforested areas was 9.70 C g m^-2 (AGB: 5.02 C g m^-2 and BGB: 4.67 C g m^-2), ranging from 1.12 C g m^-2 to 30.61 C g m^-2 (AGB: 0.58-15.84 C g m^-2 and BGB: 0.54-14.77 C g m^-2) (Table 3). Among the study sites, biomass accumulation and carbon storage were both highest in S₂₀₁₃, and lowest in S₂₀₀₀. Biomass accumulation and carbon storage had a tendency similar to that of height of H. aphyllum, despite the exceptionally large density H. aphyllum plants at S₂₀₀₅ site.

Biomass accumulation and carbon storage estimates for H. aphyllum in the current study were smaller than those in other regions in Kazakhstan and in Mongolia. Eisfelder (2017) reported that AGB of shrublands varied from 10 to 300 g m^-2 in Kazakhstan. Zhaglovskaya (2017) reported that TB was 1,663 g m^-2 in natural saxaul stand in the Ili River delta area, Kazakhstan. Batsaikhan (2018) reported that biomass of H. aphyllum was 35.8 to 290.8 g m^-2 for AGB and 46.3 to 252.3 g m^-2 for BGB in southern desert region, Mongolia. Relatively low biomass and carbon storage were akso related to extreme conditions in Aralkum as mentioned above. However, despite lower carbon storage compared to the other regions, Aralkum afforestation increased the vegetation cover and biomass carbon storage of otherwise sparsely vegetated land. Moreover, it was reported that afforested vegetation could increase soil carbon content and storage in saline and dry soil (Hbirkou et al. 2011). Thus, total carbon storage in Aralkum might be expected to increase in the long-term.

In the previous studies, various allometric equations for Haloxylon species were reported to estimate biomass and carbon storage (Buras et al. 2012;Xu et al. 2017). However, the growth of H. aphyllum in the current study was different from the other regions by the unique environment of the Aralkum. Thus, to estimate more accurate carbon storage, the site-specific allometric equations should be developed for Haloxylon species in the Aralkum.

CONCLUSION

This study investigated the growth and carbon storage of H. aphyllum in afforested areas in the Aralkum Desert, Kazakhstan. The growth of H. aphyllum varied among afforested sites, and might have been affected by the site microclimate and the strength or frequency of strong wind. Carbon storage estimates of 9.70 C g m^-2 in total biomass of H. aphyllum in afforested areas imply the carbon sequestration through increasing vegetation cover of the desiccated Aral Sea bed. Longer-term monitoring on the Aralkum vegetation growth and further studies on the relationship between Haloxylon species and environmental variations are needed.