Altitudinal variation in lifetime growth trajectory and reproductive schedule of a sub-alpine conifer, Abies mariesii

To determine whether forest trees are differentiated in terms of reproductive schedules among populations under different environmental conditions within a species, we studied three populations of Japanese subalpine snow-fir, Abies mariesii, located at different altitudes (1000, 1250 and 1400 in) on Mt. Hakkoda in northern Japan. We examined life-history schedules, including lifetime growth trajectories, reproductive maturation timing and size-dependent resource allocation to reproduction. With increasing altitude, the asymptotic maximum size of trees decreased and trees approached their maximum size at younger ages: a substantial reduction in tree growth occurred earlier and life span tended to become shorter with increasing altitude. We found that trees advance their reproductive schedules at higher sites in relation to both maturation timing (size, age and whole-tree growth rate at typical reproductive onset) and resource allocation (reproductive biomass and reproductive effort), coinciding with a general prediction of life-history theory. The rate of growth in height, which was increasing, tended to decrease at around the height at which most trees produced cones, and this height was much less with increased altitude. We propose a new hypothesis that life historical adaptation - that is, earlier resource allocation to reproduction at higher sites - is one of the reasons why trees are smaller at higher altitudes.