Grassland afforestation promotes carbon sequestration at the expense of consuming water resources. With the development of forest plantations, the increase in aboveground biomass could aggravate the limitations of soil moisture and nutrients, and thus alter understory succession. However, it is unclear how these changes influence the mineral element cycle in plant-soil systems. We conducted a four-year experiment in young, middle-aged, and mature stands in a larch plantation and investigated how stand development affects macronutrient (K, Ca, and Mg) and micronutrient (Na, Fe, and Zn) concentrations in soils and leaves of larch and understory plants. Our results showed that as stand age increased, soil moisture and soil pH decreased. The reduced soil moisture directly led to an increase in soil total sodium (Na) and potassium (K) concentrations. There was no significant difference in mineral element concentrations in the larch leaves among the three stands. However, the understory vegetation in the mature stands had higher foliar K concentration but lower foliar Na concentration than in the young stands. Furthermore, plant calcium (Ca) concentration was positively correlated with plant K:Na ratio. The stoichiometric coupling between leaf [Ca] and K:Na ratio became tighter with increased stand age, which could improve the tolerance of plants to drought and soil salinization. Our findings suggested that water limitation and soil salinization caused by grassland afforestation intensified with increased stand age. To adapt to these stresses, understory vegetation often shifts to species with high K but low Na leaf concentrations.