Growth Variation in a Naturally Established Population of Pinus Sylvestris

Abstract

We studied the variation in growth of individual trees in a population of Pinus sylvestris near Zurich in relation to tree age, size, and local competition. This population established naturally when the drainage pattern in a peat bog was altered after the Second World War. An unusual opportunity to study stand development was afforded when the trees were harvested in 1987 as part of a restoration program. All trees stumps were mapped, and growth since establishment was measured as cross—sectional area of wood in 5—yr intervals, using disks from the base of each tree. The distribution of cross—sectional area was lognormal over most of the period of growth. Size at the beginning of a time interval was the primary determinant of individual growth during that interval, but age and local competition also had significant effects. Younger, smaller trees were better fitted by an exponential growth model, whereas older, larger trees were better fitted by a Gompertz (sigmoidal) model. Some of the result were very different from previous studies: (1) The spatial distribution of trees was not significantly different from random. (2) Size variability decreased during the course of stand development, even though there was no self—thinning. This was because (3) the density of natural establishment was low, and competition, although clearly detectable, was still relatively weak and symmetric after 45 yr of stand development. Thus, there was no initial advantage in competition, and the younger trees, which were still growing exponentially, were able to catch up with the older trees, whose relative growth rates were declining.