The tree foliage (needles and non-lignified shoots) of the main Latvian tree species – the Scots pine (Pinus sylvestris L.) and the Norway spruce (Picea abies (L.) Karst.) – contains essential oils, a product with useful properties. Prior to this, there has been no research done in Latvia about the yields and quality indices of essential oils during the different seasons. This study examines oil content and quality during the autumn, winter, spring and summer periods. Essential oil distillation dynamics were investigated to determine the optimum distillation length for the recovery of essential oil content in tree foliage over the course of a year. Samples of tree foliage were collected twice per month during a one year investigation period, and the content of essential oils was determined. The obtained essential oils were analyzed, determining the density, refractive index and the amount of the main terpenes (α-pinene, camphene, β-pinene, 3-carene, limonene and bornyl acetate). The pycnometer, refractometer and gas chromatography-mass spectrometry (GC-MS) methods were used to determine the density, refractive index and terpene content, respectively. The results show that oil amount and physicochemical properties are variable during the course of a year. This study demonstrates that it is possible to produce pine and spruce essential oils throughout the whole year, and that the product corresponds to the Latvian standards for an industrial product – pine and spruce essential oils.

Key words: Scots pine, Norway spruce, tree foliage, essential oils, seasonal dynamics, terpenes, distillation dynamics.