The utilization and regulation of water by terrestrial plants have always been the core focus of ecohydrology research (Li et al., 2022; Wang et al., 2021a). Plant water source refers to the water required for plants to maintain normal growth and physiological activities, and affects the distribution of plant populations; it usually includes rainfall, condensation, soil water, surface water and groundwater (Ehleringer and Dawson, 1992; Nie et al., 2011; Song et al., 2016). The concept of plant water use efficiency is used to describe the relationship between plant growth and water consumption, reflecting the carbon and water cycles along the soil–plant–atmosphere continuum (De Deurwaerder et al., 2020). As an important factor contributing to maintaining the stability of regional ecosystems in karst regions, the water use of plants in different seasons reflects the hydrological and environmental characteristics of ecosystems (Wang et al., 2020a). Understanding the sources, strategies, and efficiency of plant water use will yield an improved vision of soil–vegetation–atmosphere hydrological processes, as well as facilitate ecological management and plants’ adaptation to climate change.
Karst is a relatively widely distributed landform in the world, formed by soluble rocks (carbonate, sulfate, halite, etc.), accounting for 15% of the world's land area (Jiang et al., 2014). Due to its unique geological background, the karst landscape is strongly developed, with high bedrock exposure, high permeability, fragmented soils, shallow soils, and high spatial heterogeneity, resulting in very limited water storage for plant uptake and growth in rock fissures and shallow soils (Jiang et al., 2022; Jiang et al., 2014; Shi et al., 2019). It has caused the spatial distribution pattern of soil and water resources of "soil and water upstairs and water downstairs". Previous studies on the supply sources of nutrient elements, photosynthesis and transpiration, and root systems have indirectly shown that the sources of water uptake by karst plants are multitudinous (Huang et al., 2009; Zhongfeng et al., 2008). With the use of stable isotope technology to study plant water in karst areas, this area has been greatly advanced. The study of plant water sources in karst areas is currently of increasing interest to researchers (Shi et al., 2022). Several studies have shown that plant water sources in karst areas are related to season, plant type, the depth of water table burial, and rock outcrop (Deng et al., 2020; Liu et al., 2019; Nie et al., 2011; Nie et al., 2012; Wang et al., 2020b). Rose et al. found that water stored in granite bedrock was an important and necessary source of water for plants in shallow soils in karst areas, helping them to survive in the dry season (Rose et al., 2003). Jose et al. found that water in the soil–rock interface at 2–3 m was an important source for evergreens in shallow soils in karst areas during the dry season (Querejeta et al., 2007). In areas where the bedrock comprises dolomite or impure carbonate rock, the bedrock weathering layer yields an important source of water for plants in the dry season (Rong et al., 2012). In contrast, for plants that develop on hard and pure tuffs, fractured soil water is an important source for plants in rocky desertification areas, while pore water, fractured water, or pipe water are their main sources in the dry season (Cao et al., 2014). The enrichment degree of groundwater in karst areas is mainly controlled by aqueous medium, stratigraphic lithology, and geological structure, which together contribute to a lack of surface runoff and abundant underground runoff,(Hylander and Dynesius, 2006; Liu et al., 2014). Although previous research in this area is well-developed, it is generally sparse. However, bibliometric analysis analyses on the elements related to vegetation water use in karst areas are still very limited (Li et al., 2008; Li et al., 2007).
Bibliometrics is a quantitative evaluation method. It can be used to study the distribution characteristics, quantitative relationships and change patterns of literature by constructing a data matrix based on the external characteristics of literature through statistical analysis of a large amount of literature data, and then visualize the relationships between information units or groups(Zhu and Hua, 2017). It is mainly used to study academic research; it objectively and quantitatively reflects the overall layout, hotspots, frontier dynamics and development trends of a specific discipline or field through mathematical and statistical methods (Gaede and Rowlands, 2018; Zou et al., 2018). Zhao et al. analyzed karst research from a bibliometric perspective, in which authors, countries, journals, and disciplines of karst research were considered, and showed that stable isotopes and karst hydrology have been the focus of research in recent years (Zhao et al., 2021). Sohel et al. analyzed research on woody plant water sources from the past three decades from a bibliometric perspective and discussed the research directions and development trends of woody plant water sources in the context of global warming (Sohel et al., 2019). Some researchers have discussed the directions and development trends of studies on water resources used by woody plants in the context of global warming, but have not addressed the analysis of plant water use in karst areas. Aleixandre et al. examined the trends in the research on global water use efficiency and Wang et al. examined the trends in research on heat tracer-based sap flow methods (Aleixandre-Tudo et al., 2019; Wang et al., 2022).
Based on the core database of Web of Science (WoS) and using the VOSviewer software, this paper considers the number of publications, the major countries and research institutions, leading scholars, subject categories and keyword clusters in the field of plant water use in karst areas over the past 40 years (1984–2022). This study aims to (1) identify the developmental lineage and leading journals in the field; (2) identify the most productive authors, countries, and institutions; and (3) summarize the knowledge base, research hotspots, and research trends in the context of plant water research in karst areas.
In this study, we examine the contributions made to English language research over the past 40 years. In this section, we present the most important findings on the topic and specify the goals of this study. In Section 2, we explain how the goals of this study were achieved, via the three phases of data processing and use methodology (source and topic selection, processing software, and data analysis and visualization). In Section 3, the results of the data analysis are presented, which consist mainly of analyses of the numbers and sources of publications, the patterns of collaboration, and the most salient knowledge areas. Section 4 presents a discussion of future research trends on the topic. Section 5 presents the main conclusions and findings of this study.