Tree seedling canopy responses to conflicting photosensory cues

Light with decreased red:far-red (R:FR) ratios may signal neighbor presence and trigger plant developmental responses. There is some evidence that plant canopies forage towards increased R:FR ratios, but it is unclear to what extent R:FR versus the total amount of photosynthetically active radiation (PAR) influences canopy foraging responses among forest trees. The objective of this study was to examine the relative importance of PAR and R:FR as photosensory cues leading to tree canopy foraging responses. Seedlings of Betula papyrifera Marshall (paper birch) were grown in an experimental garden. Each seedling was germinated and grown in its own shading structure and exposed to two spatially separated light environments, in a factorial design of PAR and R:FR. Plant canopy foraging was evaluated at the end of one growing season in terms of canopy displacement, canopy area, leaf number, direction of stem lean, petiole aspect, and lamina aspect with respect to experimental light treatments. Leaf number and canopy area were greater on the high PAR sides of plants, irrespective of the R:FR treatment. Seedling canopies were displaced towards the direction of high PAR, but this relationship was not significant across all treatments. Petiole aspect was random and showed no significant directedness towards any of the light treatments. Lamina aspect and the direction of stem lean were distributed towards the direction of high PAR, irrespective of the R:FR treatment. Overall, first-year B. papyrifera seedlings used PAR, rather than R:FR ratio, as a photosensory cue for canopy light foraging.