The two naturally occurring isotopes of carbon, C13 and C14, are found in slightly different proportions. These proportions are stable and can be recovered from the soils in which the plant material decayed. Photosynthesis is one chemical process that impacts the ratio of C13 to C14 found in plants, which can be identified from the decayed organic matter present in the soil. There are three distinct photosynthetic pathways that leave different isotope signatures: C3, C4, and CAM (Crassulacean acid metabolism). Most plants use the C3 pathway. Only about 5 percent of all plants use the C4 pathway. Nearly half of the plant species using this pathway are tropical grasses. While C4 plants may exhibit sensitivity to extreme temperature changes and a more efficient use of CO2 in conditions of reduced availability of that gas, neither condition was present in the study region after the Last Glacial Maximum (LGM). Finally, the CAM pathway of plant photosynthesis is found in about 10 percent of all plant species. Prominent among the families with the CAM pathway are the Cactaceae, the Bromeliaceae, the Euphorbiaceae, and other succulents. Many are adapted to arid environments, closing their stomata during the day and taking in CO2 at night to conserve water (Boutton 1996). Stable isotope values for C3 plants range between −32 percent and −22 percent. The range of values for C4 plants and most CAM plants is −17 percent to −9 percent (Boutton 1996).