Predicting vertical PAR profiles from canopy structure in eastern forests

Background/Question/Methods

Photosynthetically Active Radiation (PAR, 400-700 nm) is a critical resource that controls the process of photosynthesis in forest canopies and is used in models of ecosystem productivity and carbon flux. The availability of PAR within the forest canopy is a major driver of competition, species composition, and seedling survival. While previous researchers have studied how understory PAR is influenced by forest structure, few have considered the effects of canopy structure on the entire vertical profile of PAR because of restrictions to canopy access. Parker (1995-2003, unpublished) suggested that a new metric of forest structure, the hypsograph, could predict average PAR profiles in an age-series of Douglas-fir/western hemlock stands in southern Washington. The hypsograph, defined as a cumulative distribution of outer canopy height, is derived from Portable Canopy LiDAR (PCL) data. This study examines two predictions: (1) forest hypsographs will mirror average vertical PAR profiles in stands of a tulip-poplar (Liriodendron tulipifera L.) association and (2) hypsographs will match average vertical PAR profiles in stands of different successional stages. To test these predictions, young, intermediate, and old growth stands were each surveyed with a research balloon or telescoping poll to obtain PAR profiles and with the PCL to quantify forest structure.

Results/Conclusions

We found that the hypsograph predicted average PAR profiles in the young stand, but over-predicted the value of average PAR at most heights in the two older stands. These results provide little general support for our hypothesis that hypsographs would predict PAR profiles across succession in eastern forests. Furthermore, this result is contrary to patterns observed in western coniferous forests where hypsographs consistently predicted the PAR profiles. One explanation for these unexpected results is differences in species composition and resulting differences in crown shape between the western and eastern forests. Differences in crown shape may result in a higher density of foliage at the top of the canopy in eastern forests because these stands have broader crowns than the conical crowned western species. Differences in crown structure may also explain why the hypsographs and PAR profiles matched in the young eastern stand, which had some dominant coniferous species, but not in the two older deciduous stands.