PARENT SESSION
Oral Session #22: Plant function and response to stress. Presiding: K. Miyanishi.
Tuesday, August 7, 2001. 8:00 AM to 11:45 AM. Hall of Ideas P&Q.
Does shade tolerance change with plant size? A test of tissue and whole plant respiration in deep shade.
Machado, Jose Luis1, Reich, Peter1, 1
ABSTRACT- Since in deeply shaded environments, minimizing dark respiration (Rd) can be an effective behavior vis-a-vis carbon balance, differences among species, across light levels or along plant size gradients can be important but are not well understood, especially in combination. In this work, we evaluate the relationships between plant size and tissue or whole-plant Rd of juveniles (10-18 years old) of three cold-temperate tree species with different rankings in shade tolerance: (1) the very tolerant balsam fir (2) the tolerant red maple; and (3) the mid-tolerant white pine, varying in size from 25 to 150 cm height and growing in deeply shaded understories (2 to 10% of full sun) in northern Minnesota, USA. We measured specific Rd rates, biomass distribution, and N concentration of leaves, stems, and roots of saplings in a complete factorial design (3 size classes x 3 light environments). Consistent with our hypotheses, as plant grew larger, whole plant N concentrations declined due to a combination of changes in tissue %N and shifts in biomass distribution patterns. However, contrary to our hypotheses, tissue and respiration rates per unit N increased markedly with plant size in all species, such that tissue and whole plant respiration rates increased with size. Overall, our data suggest that plants have limited capacity to further down-regulate carbon losses when comparing low light to very low light conditions. Finally, we found evidence that different responses in biomass distribution and tissue maintenance cost influence the species' ability to grow and perhaps survive in deeply shaded environments.
KEY WORDS: dark respiration, shade tolerance, low light environments, plant size