Variation in leaf and fine stem CO2 flux as a function of plant size: A comparison of seedlings, saplings, and trees

Background/Question/Methods

Tissue level traits may vary as individual trees grow older and larger, but evidence to assess such changes remains limited, especially for trees larger than sapling size.  In this study, we measured functional traits of leaves and fine stems of three cold-temperate deciduous tree species (Prunus serotina, Acer rubrum, and Quercus ellipsoidalis) to determine whether variations occur with tree height and light environment.  Our hypotheses were that metabolic rates and specific leaf area would decrease with plant size, reflecting an overall decline in metabolic rate with increasing size and age.  Maximum photosynthetic rates, leaf and fine stem dark respiration rates (measured at 25° C), and specific leaf area were quantified in trees growing across a range of sizes (0.5 to 20 m in height).  In all three species, individuals in each of the size classes were sampled in both high and low light environments to eliminate light differences as a potential driver of size-related differences in leaf and stem properties.  All individuals were growing close to one another in similar soils in dense or sparse tree patches in a woodland at Cedar Creek Ecosystem Science Center in Minnesota.

Results/Conclusions

Ontogenetic trends in maximum photosynthesis and leaf respiration varied between species.  P. serotina individuals exhibited a decline in rates of photosynthesis and respiration with tree size, A. rubrum individuals exhibited an increase, while rates in Q. ellipsoidalis individuals did not vary significantly.  However, both stem respiration rates and specific leaf area responded similarly to increasing tree height in all three species, declining significantly as trees grew larger in size.  This suggests that gas exchange rates do not scale similarly with tree size in discrete tissue types (i.e. leaves versus fine stems), nor in all species.