Patterns of inter-annual variation in the size asymmetry of growth in Pinus banksiana.

A large body of literature suggests that asymmetric competition, where large individuals suppress the growth of smaller individuals by intercepting a disproportionate share of incoming light, is a dominant process in tree population development. This has not been examined extensively for long-lived tree species that accumulate growth over many years under varying growing conditions. Using dendrochronological techniques, we reconstructed annual growth and mortality rates at ten stands of jack pine (Pinus banksiana Lamb.) in Western Canada. We used these data to calculate an annual index of the size asymmetry of growth for each stand for the last 50 years. Jack pine is a shade-intolerant species found in even-aged monoculture stands, so the simple hypothesis is that large trees should consistently perform relatively better than small trees. Inter-annual variation in the index of size-asymmetric growth was positively associated with interannual variation in stand productivity at eight of ten sites. The size asymmetry of growth also showed a positive trend with age at eight of ten sites, even though all sites were in a period of declining leaf area. This should have reduced the intensity of asymmetric competition for light and reduced the size asymmetry of growth over time. Alternate hypotheses for this trend are (1) that physical collisions between crowns result in asymmetric competition for growing space because they are more damaging to small trees, or (2) that a differential build up of diseases in susceptible trees suppresses their growth, even in the absence of competition.