Interrelation between shoot characteristics and solar irradiance in the crown of Pinus contorta ssp. latifolia.

In Pinus contorta Doug. ssp. latifolia (Engelm.), annual shoot length increment, annual foliage production per shoot, and the daily integral of photosynthetically active radiation (DPAR) at shoot tips increased with height in the crown, whereas leaf longevity decreased. There was a decrease in DPAR along the shoot from the tip to beyond the oldest leaves, and the value reached at the point just proximal to the oldest leaf cohort was the same for all shoots from all crown locations, regardless of the age of the oldest leaves. The results suggest that shoot elongation, leaf production and leaf retention are controlled by the light environment within the crown. Because high irradiance promotes leaf production and low irradiance promotes leaf shedding, foliage is not randomly distributed in the crown.