ABOVEGROUND NET PRIMARY PRODUCTION DECLINE WITH STAND AGE - POTENTIALCAUSES

Aboveground net primary production (ANPP) commonly reaches a maximum i n young forest stands and decreases by 0-76% as stands mature. However , the mechanism(s) responsible for the decline are not well understood . Current hypotheses for declining ANPP with stand age include: (1) an altered balance between photosynthetic and respiring tissues, (2) dec reasing soil nutrient availability, and (3) increasing stomatal limita tion leading to reduced photosynthetic rates. Recent empirical and mod eling studies reveal that mechanisms (2) and (3) are largely responsib le for age-related decline in ANPP for forests in cold environments. I ncreasing respiratory costs appear to be relatively unimportant in exp laining declining productivity in ageing stands.