Mediterranean oak-dominated agro-silvo-pastoral systems of southwestern Europe (called montado in Portugal and dehesa in Spain) are semi-natural, savannah-style High Nature Value farmlands (HNVfs) shaped by centuries of anthropogenic (e.g., cultivation, grazing) and natural (e.g., drought) disturbances. Therefore, changes in grazing and precipitation may alter their vegetation composition and ecosystem properties and impact upon their long-term viability. We quantified the responses of soil organic carbon (SOC) and plant taxonomic and functional trait diversities to cattle grazing and inter-annual changes in precipitation across three open-woodland sites within the montado landscape in Alentejo, southern Portugal. The sites are characterised by the presence or absence of cattle grazing and different amounts of mean annual precipitation (ranging from semiarid to dry subhumid). Three different precipitation periods were used: a shorter autumn-to-spring period and longer 12- and 18-month periods before sampling. Specific leaf area, plant height and seed mass were used to estimate single-trait community weighted means and a multi-trait measure of community functional diversity (Rao’s Q). SOC and plant species richness responded negatively to the presence of cattle grazing but positively to increase in precipitation (and its interaction with cattle grazing), while trait-based measures were largely unresponsive to cattle grazing and precipitation (though RLQ analysis revealed strong controls of the environmental variables on plant life cycle, growth form, leaf phenology and dispersal strategies). SOC was most responsive to longer-term (18-month) changes in precipitation, whereas plant species richness was most responsive to shorter-term (autumn-to-spring) changes in precipitation. These results suggest that different components of plant diversity respond differently to external drivers in montado HNVfs, while the response time of soil properties may be longer than that of plant taxonomic diversity.