Ontogenetic variation in whole-plant carbon balance of juvenile evergreen trees

Background/Question/Methods

Shade tolerance is often assumed to be a fixed trait of a species, though recent work has indicated that larger individuals are less able to tolerate shade than smaller individuals of the same species.  We hypothesized that whole-plant light compensation points and carbon flux per gram plant of shade tolerant species will be more stable during juvenile ontogeny than those of shade intolerant species.  The whole-plant light compensation point (WPLCP) has been suggested as a conceptually simple measure of the light requirements of species, and is estimated as the x-intercept of the relationship between growth and light environment over a long time interval.  Here we compare WPLCP for growth and survival of four size classes (<50 cm, 50-100 cm, 100-150 cm, and >150 cm) of juvenile evergreen tree species differing in shade tolerance.  We also examine ontogenetic variation in light capture and simulated whole-plant carbon gain by combining 3D-digitizing with the architecture model YPLANT.  

Results/Conclusions

Preliminary results indicate that light requirements and WPLCP increase as trees grow larger, although this ontogenetic trend appears to be steeper in light-demanders than in shade tolerant species.  These results indicate that work on juvenile trees can detect shade tolerance differences which may have considerable bearing on forest regeneration patterns, but are not evident in seedling comparisons.