PARENT SESSION
Oral Session # 4: Forest Ecology I: Diversity, Suppression, and Release.
Presiding: B Schamp
Monday, August 4. 8:00 AM to 11:30 AM, SITCC Meeting Room 103.

If a tree bends in the forest, do we have any idea why?

Schurer, Maayke¹, Schamp, Brandon^*,1, Aarssen, Lonnie¹, ¹ Dept of Biology, Kingston, Ontario, Canada

ABSTRACT- Understory saplings of deciduous trees commonly display a ′bent′ posture, but the reasons for this are unknown. We explored three hypotheses: (1) The ′biomechanical constraints′ hypothesis suggests that bending is due to a relatively high stem length / diameter ratio resulting from strong apical control and rapid vertical growth in low light conditions; (2) The ′light harvesting′ hypothesis suggests that tree bending in the understory represents a shade-avoidance strategy since a more lateral growth form could display leaves with less overlap across a greater horizontal area, thus increasing the potential for light harvesting under low light conditions; and (3) The ′shade-avoidance′ hypothesis suggests that understory saplings exhibit directional bending toward sources of greater incident light (e.g. forest edges and canopy gaps). We collected data on tree height, stem length, stem diameter, and canopy light transmission for understory saplings of three species in woodlands of southeastern Ontario. Sugar maple (Acer saccharum), white ash (Fraxinus americana), and basswood (Tilia americana) saplings exhibiting a range of ′bendedness′ were sampled from three height classes ranging between approximately 0.5 – 3.0m. Canopy light transmission was assessed by image analysis of hemispherical canopy photographs using computer software (Gap Light Analyser). We found evidence supporting the biomechanical constraints hypothesis for medium-sized basswood saplings, but for no other size class or species. The light harvesting hypothesis was supported for large-sized sugar maple saplings only and the shade avoidance hypothesis was supported for small-sized basswood saplings. Both shade-avoidance and shade-tolerance strategies may be age/stage specific for these three tree species. It is also possible that some tree bending in the understory is due to damage from ice loading during the ice storm of 1998, or from herbivory induced activation of one or more lateral meristems in place of the terminal meristem. Further data is being collected to address these questions.

Key words: light harvesting hypothesis, biomechanical constraints hypothesis, shade avoidance hypothesis