The study concerns sustainable supply of primary energy from biomass and considers the interrelation between the amount of energy captured in biomass by photosynthesis and the total land area under perennial species grown for the purpose. The authors analyse available experimental data statistically relevant to natural growths comprising a large number of individual trees of grey alder (Alnus incana), a well-known fast-growing species broadly spread in Latvia and for centuries being used as firewood. By graphical approximation of the growth-rate data available for growths up to 50 years of age the optimum age for harvesting dependent on the age at which the maximum growth-rate of biomass is reached is shown to be 18 years confirming traditional popular knowledge. With account for long-term sustainable supply of energy under condition of 18-year rotation, the average yield of energy from highest quality sites of the total land area permanently occupied by alder is calculated to be ca. 85 GJ/ha and the required land equivalent - slightly less than 12 ha per TJ of primary energy from photosynthesis.
