LONG-TERM DEVELOPMENT OF NEEDLE MASS, RADIATION INTERCEPTION AND STEMWOOD PRODUCTION IN NATURALLY REGENERATED PINUS-SYLVESTRIS STANDS ON EMPETRUM-VACCINIUM SITE TYPE IN THE NORTHERN BOREAL ZONE IN FINLAND - AN ANALYSIS BASED ON AN EMPIRICAL-STUDY AND SIMULATION

The development of stand needle mass, interception of photosynthetically active radiation (PAR) and production of stemwood were examined in Scots pine (Pinus sylvestris L.) stands growing on Empetrum-Vaccinium site type in the northern boreal zone in Finland. In the analysis, Ilvessalo's classical study on the growth and development of Scots pine stands was completed by: (1) a needle biomass equation to facilitate the analysis of the structural development and stemwood production of the stands in relation to needle biomass, and (2) a process model of radiation interception to simulate the pattern of PAR interception by the stands with age. The analysis covered a period of 150 years, from stand age 50 to 150 years. According to the simulations, the stand needle biomass culminated around age 90 years and decreased thereafter. The stage of maximum stemwood production was reached at the age of 50 years, when the stand needle mass had increased to approximately two-thirds of the maximum. The amount of intercepted PAR changed in parallel with stand needle mass, but the amount of PAR intercepted per unit weight of foliage was reciprocally related to stand needle mass. With the increase in stand needle mass, and the concomitant increase in the rate of self-thinning, the needle efficiency in stemwood production decreased. After the culmination of needle mass the stemwood production efficiency of needles settled to a nearly stable level, in spite of the later decline in total needle mass and the consequent increase in needle efficiency in PAR interception. The estimated proportion of intercepted PAR stored to stemwood decreased throughout the period of analysis. Possible mechanisms underlying the observed patterns of stand development are discussed with reference to the literature.