The 2008 Tejon Ranch Conservation and Land Use Agreement required an adaptive management plan for over 200,000 acres of conserved lands, including vast expanses of rangelands on which the landowner retained the right to graze cattle. Ecological Site Descriptions (ESDs) and State and Transition Models (STMs) were developed for over 100,000 acres of grasslands using a field-based, data-driven approach. In the southern San Joaquin Valley side of the ranch, we identified ecological sites that support states dominated by native plant communities with high native forb species diversity and abundance. These forb-dominated communities have naturally lower fall biomass, as measured by Residual Dry Matter (RDM), than nonnative annual grass-dominated communities, and support several vertebrate species of conservation concern (e.g., San Joaquin kit fox and blunt-nosed leopard lizard) that are hypothesized to prefer low plant biomass conditions. By linking STMs with field data about temporally dynamic environmental conditions, we identified the interactions between annual Mediterranean weather patterns and ecological sites as drivers of native and nonnative plant-dominated community phases and states, which in turn influence conditions for other conservation targets. Our adaptive management plan used ESDs paired with STMs to identify the spatial distribution of grassland types and focal species, understand drivers of the temporal transitions of specific grassland types and the feasibility of achieving desired conditions, and hypothesize grazing strategies to promote conservation values. For instance, a recommended practice to achieve conservation objectives in the Holocene Alluvial Flats Ecological Site is to maintain RDM <500 lbs/acre, particularly in years with high levels of rain early in the growing season that encourages nonnative annual grass growth and high RDM conditions. To help facilitate this practice, the Conservancy and the ranchers cooperated to install new fences and waters in pastures supporting Holocene Alluvial Flats Ecological Sites.