1. Leaf area index (LAI), a measure of canopy density, is a key variable for modelling and understanding primary productivity, and also water use and energy exchange in forest ecosystems. However, LAI varies considerably with phenology and disturbance patterns, so alternative approaches to quantifying stand-level processes should be considered. The carbon isotope composition of soil organic matter (d13CSOM) provides a time-integrated, productivity-weighted measure of physiological and stand-level processes, reflecting biomass deposition from seasonal to decadal time scales. 2. Our primary aim was to explore how well LAI correlates with d13CSOM across biomes. 3. Using a global data set spanning large environmental gradients in tropical, temperate and boreal forest and woodland, we assess the strength of the correlation between LAI and d13CSOM; we also assess climatic variables derived from the WorldClim database. 4. We found that LAI was strongly correlated with d13CSOM, but was also correlated with Mean Temperature of the Wettest Quarter, Mean Precipitation of Warmest Quarter and Annual Solar Radiation across and within biomes. 5. Synthesis. Our results demonstrate that d13CSOM values can provide spatially explicit estimates of leaf area index (LAI) and could therefore serve as a surrogate for productivity and water use. While d13CSOM has traditionally been used to reconstruct the relative abundance of C3 versus C4 species, the results of this study demonstrate that within stable C3- or C4-dominated biomes, d13CSOM can provide additional insights. The fact that LAI is strongly correlated to d13CSOM may allow for a more nuanced interpretation of ecosystem properties of palaeoecosystems based on palaeosol 13C values.