Spatial heterogeneity in plant and soil properties is known to influence ecosystem functions, but the linkages between spatial variation and ecosystem services in grasslands are unclear. Here we examine within-field variation in sward structure (vegetation height) and test whether indices of spatial heterogeneity can be used as a simple indicator of aboveground production services or carbon stocks over time in upland grasslands. Two upland pastures with continuous cattle grazing (high versus low stocking rate and N inputs) were studied over a six-year period using a spatially explicit sampling scheme, as part of a long-term field management trial in central France. We found that grassland height showed significant spatial and temporal variation during the study period in both grazing management treatments. Contrary to expectations, stocking rate did not affect the degree of within-field variation in vegetation height recorded during the study. Mean annual biomass in both grazed paddocks showed a positive relationship with variation in end-of-season vegetation height, but was unrelated to vegetation properties at the start of the growing season. Simple metrics of height variation appear to be a useful proxy of grassland standing biomass, and hence carbon stocks, at broader spatial and temporal scales, and could improve the prediction of field-scale function in a changing environment.