A broad compilation of modern carbon isotope compositions in all C3 plant types shows a monotonic increase in δ 13 C with decreasing mean annual precipitation (MAP) that differs from previous models. Corrections for temperature, altitude, or latitude are smaller than previously estimated. As corrected for altitude, latitude, and the δ 13 C of atmospheric CO 2 , these data permit refined interpretation of MAP, paleodiet, and paleoecology of ecosystems dominated by C3 plants, either prior to 7-8 million years ago (Ma), or more recently at mid-to high latitudes. Twenty-nine published paleontological studies suggest preservational or scientific bias toward dry ecosystems, although wet ecosystems are also represented. Unambiguous isotopic evidence for C4 plants is lacking prior to 7-8 Ma, and hominid ecosystems at 4.4 Ma show no isotopic evidence for dense forests. Consideration of global plant biomass indicates that average δ 13 C of C3 plants is commonly overestimated by approximately 2‰.aridity | carbon cycle | closed canopy | paleoprecipitation P lants exhibiting C3 photosynthesis have dominated the history of terrestrial vegetation, as CAM plants occupy only a small percentage of typical ecosystems (e.g., 1), and C4 plants became abundant in grasslands only within the last 7-8 million years (2). C3 plants exhibit a large range of carbon isotope compositions (−20 to −37‰, V-PDB; Fig. 1), generally reflecting a physiological response to aridity (anomalously high δ 13 C) and a combination of low light levels plus leaf litter recycling (anomalously low δ 13 C; 3). Past studies have conflicted on the dependence of δ 13 C on mean annual precipitation (MAP). Most studies support a negative correlation (e.g., 4), but some have reported no correlation (5) or even a positive correlation (6).This study explores carbon isotope systematics of C3 plants through a comprehensive compilation of literature data, minimizing local differences that have given rise to disparate interpretations. This work directly benefits paleodietary and paleoecological studies by providing estimates of MAP in ancient environments as well as clearer boundaries for identifying the understory of closed-canopy forests and the contribution of non-C3 plants to diet, which is used to gain insights into C4 origins (2). The new compilation also helps refine models of the modern carbon cycle by providing a better estimate of global C3 δ 13 C values (7). The present analysis differs from a recent investigation of correlations between tree þ shrub δ 13 C and MAP or plant functional type (8) in that it is several times larger on a site-by-site basis, spans the full range of plant growth forms (including trees, bushes, grasses, and herbs), averages data from each site to minimize sampling bias (4), and regresses an arguably more appropriate function to the data. Selection criteria of climate and isotopic data also differ slightly. This broader approach of considering all C3 plants and averaging compositions is important when evaluating herbivore paleodiets, whic...
