1. Introduction
Soil water plays an important role in the terrestrial water cycle and the soil−plant−atmosphere continuum (SPAC), it is a key parameter in the hydrological and climatic models [
1,
2]. Due to the wide distribution and high chemical dissolution of the bedrock such as the carbonate rocks, these are usually characterized by numerous fissures and rich underground channels in the humid karst areas. The soil layers are very thin and with low water-holding capacity: most precipitation and surface water immediately infiltrate into the underground. Therefore, the soil moisture is usually deficient, which results in it being one of the restricting factors for vegetation restoration, crop growth and ecological construction in the humid karst areas [
3,
4,
5].
Against the background of the karst’s fragile ecological environment, the soil erosion and rocky desertification were serious in the humid karst areas [
6,
7,
8]. In recent decades, to protect the local ecological conditions, the state and local governments in China have launched a series of ecological restoration projects, including the Natural Forest Protection Project, the Grain for Green Project, and the Karst Rocky Desertification Comprehensive Control and Restoration Project [
9,
10,
11,
12]. Although there was remarkable regional vegetation restoration in this area, it also greatly changed local hydrological processes such as evapotranspiration and rainfall interception, and ultimately directly or indirectly affected the soil moisture content [
13,
14]. Therefore, the study of the effects of vegetation restoration on soil moisture content is of key importance for the management of water resources and vegetation restoration practices, and agricultural industrialization in the humid karst areas.
At present, the effects of vegetation restoration on soil moisture content in the humid karst areas have received increasing attention, and previous studies have mainly focused on the plot scale and slope scale. On the plot scale, the results of many researchers indicated that the soil moisture content was high in the grassland stage. For example, Li et al. found that the soil moisture content in grassland was higher than that in farmland [
15]. Zhou et al. pointed out that, influenced by the characteristics of grassland such as shallow root systems, large vegetation coverage and slow water evaporation resulted in the grassland showed higher soil moisture holding capacity than bare land, woodland and shrubland [
16]. On the slope scale, most of the results showed that the soil moisture content varied in different vegetation restoration stages. For example, Chen et al. found that the soil moisture content of forested land was higher than that of shrubland and grassland [
17]. Chen et al. found that the soil moisture content of shrubland was higher than that of cultivated land and artificial forested land [
18]. The publication by Sun et al. pointed out that soil moisture content was highest in shrubland, followed by cultivated land and grass land, lowest in forested land [
19].
Influenced by the complex topography and diverse niche types of the ecological environment, the distribution of soil moisture content and its influential factors in the humid karst areas are features of strong spatial heterogeneity [
20,
21,
22,
23]. However, these conclusions obtained by field observations conducted at plot scale and slope scale can only represent the conditions of soil moisture content on a small scale and in a short time period, so they are unclear and fail to clarify the characteristics of the effects of vegetation restoration on soil moisture content in the humid karst areas at regional scale [
24,
25,
26]. Therefore, it is necessary to study the effects of vegetation restoration on soil moisture content at regional scale, which is vital for vegetation restoration practices in humid karst areas.
In general, few studies have researched the effects of vegetation restoration on soil moisture content carried out at the regional scale in humid karst areas. Based on soil moisture content from reanalysis and ground stations, Deng et al. found that the soil moisture content was dominated by a drying trend during 1982–2015 [
27]. In addition, the soil moisture content under different land cover types showed highest in forest, followed by wetland, agriculture, grassland, sparse vegetation, urban and bare area. Overall, soil moisture content experienced a decreasing trend in 1979–2017, with the soil moisture content of agricultural land and forest areas significantly reduced [
28]. However, these studies mainly focused on the characteristics of the soil moisture content differences under different land cover types, which did not represent the process of vegetation restoration. Generally, the effects of vegetation restoration on regional soil moisture content are still unclear in humid karst areas.
Therefore, the purpose of this manuscript is to explore the effects of vegetation restoration on soil moisture content at the regional scale in humid karst areas. Based on the soil moisture content, EVI, and precipitation data set during 2002–2018, the spatiotemporal variation characteristics of EVI, soil moisture content, and precipitation were first analyzed. Then, the effects of vegetation afforestation on regional soil moisture content were quantitatively evaluated under similar precipitation conditions. The results of this study may not only provide scientific reference for evaluating the regional soil moisture-carrying capacity of vegetation restoration in humid karst areas, but also provide scientific guidance for further practices of vegetation planting density and spatial layout in the ecological construction.
4. Discussion
4.1. Cause of Negative Effects of Vegetation Restoration on Regional Soil Moisture Content
Many previous studies have analyzed the effects of vegetation restoration on regional soil moisture content; however, it is still unclear whether vegetation restoration positively or negatively influences the soil moisture content. Some of the previous studies pointed out that vegetation restoration has a positive correlation with soil moisture content. Because the plant canopy can decrease the land surface temperature, reduce the soil evaporation and increase the precipitation infiltration, these factors finally improve the soil moisture content [
36]. In the present study, we found that vegetation restoration has contributed to a drying trend of regional soil moisture content, which is in line with most previous studies undertaken in the karst areas [
37,
38,
39]. For example, the similar study published by Kovačič et al. [
40], in which they pointed out that multidecadal vegetation overgrowth significantly contributed to the reduction in the karst soil moisture content. Another study suggests that the increasing trend in global leaf area index (LAI), documented by Zhu et al. [
41], could be reflected in the significant impact on groundwater recharge and on the partitioning between green and blue water, which finally resulted in a drying trend of soil moisture content in the karst areas.
The reasons for the negative effects of vegetation restoration on regional soil moisture content in humid karst areas are possibly caused by this: due to the bedrock being characterized by high penetrability and many fissures in humid karst areas, once the precipitation reaches the land surface, it immediately infiltrates underground. So, although precipitation in humid karst areas is abundant, surface water resources are in short supply due to the special hydrogeological structures, and there are insufficient water resources, in particular little soil water, for vegetation restoration consumption. With large-scale vegetation restoration, the increasing leaf area will lead to more canopy interception, water consumption and evapotranspiration, which largely decrease the soil moisture content. At the same time, due to the litter layer, the soil moisture is not easily supplemented by precipitation, which results in a reduction in soil moisture. Consequently, vegetation restoration leads to a drying trend of the regional soil moisture content.
4.2. The Effects of Vegetation Restoration on Soil Moisture Content Varied Under Different Lithological and Climatological Conditions
According to
Figure 1c and
Figure 4d, the extremely significant decreasing trend in the soil moisture was mainly distributed in the homogenous limestone area [
42]. The nonsignificant decrease and nonsignificant increasing trend of soil moisture content was mainly distributed in the homogenous dolomite area, while the extremely significant increasing trend of soil moisture content was mainly distributed in the clastic rock. The reason might be that the homogenous limestone area mainly consists of highly soluble mineral components, such as calcium carbonate [
42]. These components are easily dissolved by rainwater, which leads to the formation of crevices within the bedrock surface [
43], these crevices limit the retention of water and subsequently result in low soil moisture content, thus vegetation restoration might extensively decrease the soil moisture content. The homogenous dolomite areas mainly consist of magnesium carbonate and usually feature a thick soil layer, which can not only hold more water but also is beneficial for vegetation growth. Thus, vegetation restoration will not lead to an obvious influence on soil moisture change. However, in the clastic rock areas, where the land surface is distributed by a thicker soil layer and higher water-holding capacity [
44], the vegetation restoration does not lead to a reduction in soil moisture content, but also can accumulate the precipitation to increase the soil moisture content.
In addition, precipitation generally decreased from the southeast to the northwest of the study area (
Figure 5b). The soil moisture change in the humid karst areas was mainly due to precipitation, and the results showed the high precipitation distribution in the southeast, where the main increasing trend of soil moisture content was distributed. Generally, it was found that the spatial distribution of soil moisture content was explained by the lithology and precipitation, thus the effects of vegetation restoration on soil moisture content might be different under different lithology and precipitation conditions. Therefore, the results suggest that the effects of vegetation restoration on soil moisture content under different lithological and climatological conditions should not be ignored in future vegetation restoration practices in humid karst areas.
4.3. Reasons for Differences in Effects of Vegetation Restoration on Soil Moisture Content in Different Spatial Scales
According to the conclusions in previous studies, the effects of vegetation restoration on soil moisture content showed significantly different characteristics in different spatial scales, these effects particularly varied at plot scale and slope scale. The reasons may be as follows: on the one hand, because the comparisons between experimental groups are inconsistent and most of the observation experiments are mixed with external environmental factors, such as plant community structures, topographic features, and climatic conditions. These interactions impact the various influential factors on soil moisture content and can easily lead to different research results. On the other hand, due to the influences of the strong heterogeneity of the ecological environment and the thin and discontinuous soil layer, the soil moisture and its influential factors feature strong spatial heterogeneity and vary in different spatial scales in the humid karst areas. Therefore, the conclusions are significantly different on the effects of vegetation restoration on soil moisture content in different spatial scales. Thus, against the background of the greening of the Earth, the humid karst area has been confirmed to be one of the remarkable vegetation restoration efforts in the world. Therefore, it is important to conduct the study on the effects of vegetation restoration on soil moisture at regional scale. As such, the results of the present study can provide knowledge of the effects of vegetation restoration on soil moisture content at the regional scale, and be of key importance for balancing the relationship between the water needs of vegetation and the water supplies of the soil in further vegetation restoration practices in humid karst areas.
4.4. The Rationality and Limitation of the Method to Define Similar Precipitation Conditions
With a series of Ecological Restoration Projects that have greatly improved landsurface vegetation cover, how to quantitatively analyze the impact of vegetation restoration on soil moisture content has become the hot topic in recent years. Precipitation was a critical factor affecting soil moisture content, and showed different spatiotemporal variation characteristics in the early and later stages of vegetation restoration. Therefore, the influence of precipitation should be eliminated when analyzing the soil moisture content changes in different periods. Since the paired years with similar climate conditions was proposed, it has been widely applied to separate the influence of human activities from climate factors on river runoff and sediment discharge. In previous studies, similar precipitation conditions were identified by a similar amount of precipitation and similar processes of precipitation in the paired years between early and later stages. However, there is still no uniform determination of this method, including similar amount of precipitation, similar process of precipitation and interval year in the paired years.
In this study, we defined the similar precipitation conditions with three rules that need to be simultaneously applied: first, the interval year in the paired years is ≥5 years; second, the relative variation rate of annual precipitation in the paired years is ≤20%; third, the correlation coefficient between monthly precipitation in the paired years should pass the significance level of 0.05. Based on the method of the paired years with similar precipitation conditions, this study evaluated the effects of vegetation restoration on soil moisture by eliminating the influence of precipitation by pixel scale. Compared with the previous studies, this method in the present study can not only reveal the effects of vegetation restoration on regional soil moisture content more accurately, but also is of great significance for future soil moisture content research and vegetation restoration practices.
4.5. The Contribution and Application of the Present Study
Influenced by the fragile ecological environment, of which the resulting soil moisture content is the main restricting factor in the humid karst areas. In recent years, due to the influence of accelerating climate change and the implementation of a series of ecological restoration projects, although the vegetation coverage has been greatly increased, it has also greatly changed the spatiotemporal soil moisture content. Against the unique geological and hydrological background, such as thin soil layer and space mismatch between soil and rocks, soil moisture content is a feature of strong spatial heterogeneity in the humid karst areas. However, the conclusions in previous research were mainly conducted at plot scale and slope scale, consequently, these conclusions also varied in different studies and are unclear at the regional scale. In the present study, our results reveal that vegetation restoration has led to a drying trend of the regional soil moisture content in humid karst areas during 2002–2018, which is not conducive to vegetation restoration and increases the risk of drought in this area. Therefore, the findings suggest that in the future practice of vegetation restoration in humid karst areas, we should pay attention to balancing the relationship between vegetative water consumption and soil moisture content. According to the different land surface climate conditions, choosing appropriate vegetation types and controlling reasonable vegetation planting density can realize sustainable vegetation restoration. Although this study selected Southwest China as a case study area, the results of this study can be transferred to other similar studies aiming to evaluate the effects of vegetation restoration on soil moisture.
In this study, the effects of vegetation reconstruction on regional soil moisture content were analyzed under similar precipitation conditions, which can be good for eliminating the influence of precipitation conditions, so as to make the research results more scientific and convincing. There are some limitations in our present study, for example, due to the land cover types in humid karst regions usually being characterized by strong spatial heterogeneity, the soil layer is shallow and discontinuous so the soil moisture varied even in the case of 1 km2 areas. However, the remote sensing data in our study with a spatial resolution of 25 km, low spatial resolution remote sensing images bring errors into the land surface conditions because a single pixel covers a larger area and there are a lot of mixed pixels. Therefore, the surface soil moisture content might be different within one pixel, which has limitations for the monitoring of actual soil moisture content conditions in the humid karst areas. Although the present study looks at the effects of vegetation restoration on regional soil moisture content by controlling similar precipitation condition, there are still some differences of precipitation in the paired years between early and later stages of vegetation afforestation, which could impact the accuracy of this study results. Despite some limitations to our present study, in general, the results of the present study can reveal the effects of vegetation restoration on regional soil moisture content in humid karst areas, and can also provide important references for the management of water resources, vegetation restoration practices and agricultural industrialization at regional scale in future.