Baitian Wang

Afforestation plays an important role in controlling soil erosion and nutrient loss on the Loess Plateau in China. However, previous studies on soil nutrient changes have mainly focused on the effects of tree species, whereas little is known about how changes in shrubs and herbs caused by afforestation drive soil nutrient changes. In this study, we examined the variation characteristics of understory vegetation and soil nutrients for different vegetation types. The results showed that compared to abandoned farmland, plantations significantly increased soil organic carbon and total nitrogen but had no significant effect on total phosphorus. Robinia pseudoacacia L. forests were more effective than Pinus tabuliformis Carr. forests in increasing soil nutrient content. In addition, herbaceous vegetation in the R. pseudoacacia forest better explained the soil nutrient variation, and herb abundance was the best explanatory variable; however, shrub vegetation contributed more to soil nutrient variation in the P. tabuliformis forest, and shrub abundance contributed the most. Accordingly, we determined that understory plant abundance, rather than species richness, may be the most important factor driving soil nutrient changes. Specifically, herb abundance in the R. pseudoacacia forest may drive soil nutrient changes mainly by regulating herb biomass and litter biomass. By contrast, shrub abundance in the P. tabuliformis forest indirectly affected soil organic carbon mainly by altering shrub biomass. Furthermore, although the phylogenetic relationships had less effect on soil nutrients than species composition, they also made important contributions. Therefore, the phylogenetic relationships should also be considered in addition to species composition when assessing the impact of vegetation on soil properties in the future.

With climate change, understanding tree responses to climate is important for predicting trees’ growth, and plant competition as a nonnegligible biotic factor plays a key role in such response. However, few studies have investigated how competition affects the response of Pinus tabulaeformis plantations to climate . In our study, we investigated nine 29-year-old P. tabulaeformis plantation plots (three density gradients). The dendroecological method was used to analyze the impact of competition on trees response to drought and interannual climate variation. Stand density index was used to indicate the intensity of competition. The results showed that competition modified the climate-growth relationship. Competition increased trees’ sensitivity to drought but the relationship between competition and sensitivity to drought was nonlinear. The competition effect slightly increased under intense competition conditions. Additionally, competition reduced trees’ sensitivity to interannual climate variation. After 1999, the effect of competition was obvious. The sensitivity of small-diameter trees, especially those in middle- and high-density stands, declined. Thus, in the future these trees presumably may exhibit a reduced sensitivity to interannual climate variation and a greater sensitivity to drought.

The inappropriate selection of measurement points and measurement times in an ecosystem may easily lead to the underestimation or overestimation of soil respiration due to spatial and temporal heterogeneity. To assess the law of spatial and temporal heterogeneity and more accurately determine the soil respiration rate, we measured the soil respiration rate of a forest in the plant growing season from 2011 to 2013 on Changbai Mountain in 8 directions and 7 distances from each tree trunk. Neglecting the direction of the measuring point may overestimate or underestimate the soil respiration rate by 29.81% and 26.09%, respectively; neglecting the distance may overestimate or underestimate the soil respiration rate by 41.36% and 20.28%, respectively; and ignoring the measurement time may overestimate and underestimate the soil respiration rate by 41.71% and 57.64%, respectively. In addition, choosing a measurement point in the eastern direction at a 1.8 m distance and conducting the measurement in September may relatively accurately reflect the soil respiration rate of the ecosystem. These findings can deepen our understanding of soil respiration rate heterogeneity and may provide new ideas for improving the measurement method of soil respiration.

Understanding the growth adaptive strategies of Amorpha fruticosa in response to the changes of saline soil environment under different mulch types (plastic mulch [PM]; sand mulch [SM]; rice straw mulch [RSM]; and control with non‐mulching [CK]) is very important, as this determines afforestation success in a moderately saline soil. The species' soil microenvironments, fine root traits, and biomass allocation patterns were investigated in 2016 and 2017. Compared to CK, electrical conductivity of saturated paste extracts (EC_e) reduced significantly under the mulching treatments especially under PM. Both PM and SM effectively increased soil temperature (T_soil) and available nitrogen (AN), but reduced soil organic carbon (SOC) in 2016; while RSM increased soil nutrients, but reduced T_soil. The A. fruticosa under RSM (following PM and SM) invested more biomass, to shoots relative to roots, to coarse roots relative to fine roots, and to stems and leaves at the expense of coarse roots and fine roots from 2016 to 2017, as its saline soil environment especially the low T_soil improved. In both years, the fine roots of A. fruticosa under PM employed exploitation potential or ‘extensive strategy’ (fine root mass density; fine root length density) more successfully at the 0–40 cm, while the fine roots under CK used higher exploitation efficiency or ‘intensive strategy’ (specific root length; and specific root area). The indicators of intensive strategy at the 40–60 cm were higher under PM than SM and RSM. Therefore, PM will better help A. fruticosa adapt to the harsh soil environment during afforestation in a moderately saline wasteland.

The abandonment and cultivation of croplands in the Eurasian Steppe has become the focus of global and regional food security and agricultural policy-making. A large area of cropland in some post-Soviet countries has proven to be abandoned with the disintegration of the Soviet Union; however, it is unclear as to whether Kazakhstan also experienced a similar change as one of the main food providers for the former Soviet Union. In this study, we used the annual land cover dataset (1992–2015) from the European Space Agency, Climate Change Initiative (ESA-CCI) to detect spatio-temporal characteristics of rainfed and irrigated cropland changes in Kazakhstan. The Mann–Kendall test and regime shift analysis showed that rainfed and irrigated cropland at national level had a significant increasing trend with a significant rising up to 1999 and stagnation during 2000–2015, which was further confirmed with analysis at 14 regions. The greatest contributor to rainfed and irrigated croplands was steppes, followed by shrinkage of water bodies since 2005 to a great extent, rapid urbanization process resulted in losses of a part of irrigated oases. The trend surface analysis indicated that reorganized stable pattern characterized by rainfed cropland in north and irrigated cropland in south was driven by the strategy of the gradual agricultural development of oases. The nonexistence of cycle between the abandonment and recultivation proved that newly-gained cropland from steppe may be less degraded and more productive for sustainable land use in Kazakhstan. In conclusion, this study can provide strong evidence for sustainable land use and a basis for food security policy-making in Kazakhstan, and even all of the Central Asian countries in the future.

Based on the daily precipitation data from 53 meteorological stations, 11 extreme precipitation indices were selected, categorized and calculated; the temporal and spatial patterns in these indices and their teleconnections with the large-scale circulations were analyzed by the non-parametric Mann-Kendall test; and Sen’s slope estimator and linear regression for the period of 1960–2014 were calculated. The results indicated that all extreme precipitation indices had spatial patterns decreasing from the southeastern to the northwestern parts of the Beijing-Tianjin Sand Source Region (BTSSR), except for the consecutive dry days (CDD), which exhibited a reverse spatial pattern. At the whole-region scale, most extreme precipitation indices showed an insignificant decreasing trend, with exceptions in the intensity indices (RX1day and RX5day) with a statistical significance at the 90% confidence level. The total annual precipitation showed a general shift towards a drier climate in the study area. Spatially, all indices for extreme precipitation showed decreasing trends at most stations, except for simple daily intensity index (SDII) and heavy precipitation days (R10). The change in extreme precipitation may be affected by the El Niño-Southern Oscillation (ENSO), East Asian Summer Monsoon (EASM) and Pacific Decadal Oscillation (PDO). Better understanding of extreme precipitation for the BTSSR may be useful in the regional planning for ecological restoration and water management.