Dark respiration of pines

Plant respiration is a large, environmentally sensitive component of the carbon balance for pine ecosystems and can consume >60% of the carbon fixed in photosynthesis. If climate, genetics, or carbon allocation affect the balance between assimilation and respiration, respiration will affect net production. Respiration rates for tissues within a tree vary with the number of living cells and their metabolic activity. For pines, foliage and fine roots have similar respiration rates, with rates for seedlings (60 420 nmol C (mol C biomass)-1 s-1 at 15?C) higher than those for mature trees (20 70 nmol C (mol C biomass)-1 s-1 at 15?C). Woody tissue respiration is low compared with other tissues (<10 nmol C (mol C biomass)-1 s-1 at 15?C, for dormant large stems; and 4-60 nmol C (mol C biomass)-1 s-1 at 15?C, for small stems, branches, twigs and coarse roots). Reported annual total respiration for the living parts of pine trees uses 32-64% of the annual total of net daytime carbon fixation. The ratio of annual respiration to photosyn thesis increased linearly with stand biomass for young pine stands. Simulations of respiration and assimilation for Pinus elliottii and P. contorta forests support the hypothesis that pines growing in warmer climates have lower leaf area index because temperature shifts the canopy compensation point. Simulations of these same stands with increased air temperature in situ suggest that pines growing in cool climates might offset increased foliar respiration and maintain assimilation by reducing leaf area. Future research on the role of respiration in forest productivity should concentrate on producing annual budgets at the stand level.