Young spruce and beech trees were planted in field plots and exposed to 2450-MHz microwave radiation generated by a 600-W magnetron source. Power flux densities (PFD) ranged from 0.007 to 300 W/m², depending on the location within the plots. Microwave exposure was permanent during 3.5 years. Heating effects of the microwave radiation were estimated by measuring the hydrolysis rate of exposed sucrose solutions. This method revealed a temperature increase of ∼4°C at the highest PFDs. During the whole exposure period, the microwave radiation caused no visual symptoms of damage. PFD had no effect on crown transparency, a parameter commonly estimated in forest condition inventories. Height growth was similar at all PFDs for both species, as was photosynthesis in beech leaves, determined by chlorophyll fluorescence measurements. A negative relationship existed between foliar concentrations of calcium and sulfur in beech and PFD during the first 2 years of exposure. However, the concentrations of both nutrients remained within the sufficiency range. In the third year of exposure, the effect was absent. The microwave effect on calcium is discussed in connection with literature data.