212 • Journal of Cave and Karst Studies, December 2018 Ting-Yong Li, Chun-Xia Huang, Lijun Tian, Marina B. Suarez, and Yongli Gao. Variation of  13 C in plant-soil-cave systems in karst regions with different degrees of rocky desertifcation in southwest China. Journal of Cave and Karst Studies, v. 80, no. 4, p. 212-228. DOI:10.4311/2018ES0107 VARIATION OF  13 C IN PLANT-SOIL-CAVE SYSTEMS IN KARST REGIONS WITH DIFFERENT DEGREES OF ROCKY DESERTIFICATION IN SOUTHWEST CHINA AND IMPLICATIONS FOR PALEOENVIRONMENT RECONSTRUCTION Ting-Yong Li 1, 2, C , Chun-Xia Huang 1 , Lijun Tian 3 , Marina B. Suarez 3 , Yongli Gao 3 Abstract Speleothem  13 C has been taken as an indicator of the history of rocky desertifcation, and changes in  13 C have been thought to refect the transition between C3 and C4 sur face vegetation types. In this study the  13 C values of plants, soil organic matter (SOM), dissolved inorganic carbon (DIC) of waters and modern calcite deposits in caves were in- vestigated at fve sites with different rocky desertifcation degree (RDD) in Southwest China. The main results can be summarized as follows: (1) dominant vegetation was the C3 type, with average plant  13 C values ranging from 26 ‰ to 32 ‰ (V-PDB), and SOM  13 C values ranging from 20 ‰ to 25 ‰ (V-PDB) for all the sites; (2) large variation for the  13 C of DIC from drip water and modern calcite deposits in caves, which must be the result of multiple, inorganic factors in the epikarst zone and not the local vegetation type; (3) a proposed conceptual model to demonstrate that the evolution of Asian summer monsoon (ASM) can be recorded in speleothem  13 C due to changes in epikarst zone hy- drological conditions, exerting infuence on stable carbon isotopes’ fractionation, and not necessarily due to changing vegetation types in the subtropical zone of Southwest China. Introduction Karst is one of the most fragile and vulnerable environments because of the low soil formation rate and high per- meability of carbonate rocks. Rocky desertifcation is defned as the transformation of vegetation- and soil-covered karst areas into rocky landscape under the action of natural processes of hydrology and ecology, and through human activities (Yuan 1997; Jiang et al., 2014). The causes for initiation and development of rocky desertifcation, including the contributions from natural processes and human activities, are still unknown. To prevent and rehabilitate rocky de- sertifcation of karst areas, it is essential to know the history of rocky desertifcation for a specifc region. Speleothems in karst caves record climatic and ecologic information in geochemical proxies, such as the stable isotopes of oxygen and carbon ( 18 O and  13 C), and the concentrations of several elements (Fairchild et al., 2006).  13 C and  18 O have been the most popular proxies in the reconstruction of paleoclimate and paleoenvironmental changes (Dorale et al., 1992; Genty et al., 2003; Fairchild et al., 2006).  13 C of soil CO 2 is controlled by the proportion of biomass from C3 and C4 plants and the CO 2 respiration of soil in different climatic conditions (Cerling, 1984; Mattey et al., 2016). It is considered that the variations of speleothem  13 C can refect the changes of regional vegetation, because the differ- ent photosynthetic pathways of different plant types result in different  13 C values: C4 vegetation has much higher  13 C (typically around 12 ‰), while carbon isotopic composition of C3 vegetation is close to 25 ‰ (McDermott, 2004); the  13 C of associated speleothems is 6 ‰ to 12 ‰ with overlying C4 plants, and 14 ‰ to 6 ‰ with overlying C3 plants (Salomons and Mook, 1986, p. 241–269; McDermott, 2004). However, direct correlation of the speleothem  13 C with regional vegetation is a simplifed or idealized understanding. In reality, complex processes in soil and the epikarst zone will infuence the migration of carbon isotopes and disturb the correlation between the speleothem geochemistry and overlying vegetation (Hendy, 1971; Salomons and Mook, 1986, p. 241–269; Coplen et al., 1994; Bar-Matthews et al., 1996; Baker et al., 1997; Fairchild et al., 2006; Frisia et al., 2011). The most recent publications have emphasized the infuence of regional, hydrological circulation and the soil humidity balance on stalagmite  13 C (Li et al., 2007; Liu et al., 2016). Rocky desertifcation is a serious ecological and environmental problem in Southwest China (Jiang et al., 2014). In order to understand the origin and history of regional, rocky desertifcation, research on the mechanism of carbon iso- tope migration in karst systems, based on cave monitoring, has been carried out (Luo et al., 2009; Li et al., 2011, 2012). As well, reconstruction of the evolution of regional vegetation via the  13 C records of speleothems has been done (Li et al., 1998). Li et al. (2012) studied the transition of  13 C signals from overlying plants, soil, and bedrock, in cave water and modern deposits in Furong Cave, Southwest China. However, investigation of the  13 C characteristics of modern plants and soils in karst regions with different rocky desertifcation degree (RDD) has been rare. 1 Chongqing Key Laboratory of Karst Environment, School of Geographical Sciences, Southwest University, Chongqing, 400715 2 Field Scientifc Observation & Research Base of Karst Eco-environments at Nanchuan in Chongqing, Ministry of Land and Resources of China, Chongqing 408435, China 3 Department of Geological Sciences, University of Texas at San Antonio, San Antonio, TX 78249, USA C Corresponding author: cdlty@swu.edu.cn