The weathering of calcium and magnesium silicate minerals on the continents has exerted a major control on atmospheric CO2 over geologic time, and vascular plants may have played an important role in this process. In western Iceland, we have examined the role of plants in weathering by measuring the chemistry of waters draining adjacent areas of basaltic rocks that are either barren (having a partial cover of mosses and lichens) or populated by trees. The study area was chosen to maximize vegetational differences and to minimize differences in microclimate, slope, and lithology, while avoiding hydrothermal waters and anthropogenic acid rain. Results, including data on cation uptake by growing trees, indicate that the rate of weathering release of Ca and Mg to streams and vegetation is two to five times higher in vegetated areas than the release of Ca and Mg to streams in barren areas. This finding suggests a major role for vascular plants in accelerating weathering and thereby lowering atmospheric CO2 as they invaded upland areas of the continents between 380 and 350 Ma.