We tested the prediction that the main stems of four common deciduous tree species grow phototropically as saplings in the forest understory in response to variable canopy structure. Our predictions were confirmed for Fraxinus americana, Acer saccharum, and Ulmus americana, which all had stems bent in the direction of greatest canopy openness. The fourth species, Tilia americana, did not demonstrate the predicted phototropic main stem bending. Our tests also enabled us to distinguish between alternative bending strategies, confirming that for three species, bending is associated with a phototropic shade avoidance response rather than a nonphototropic shade avoidance response and is not a consequence of biomechanical constraints. Additionally, we found evidence that the four species differ in the canopy conditions under which their saplings grow, suggesting niche separation along understory light environment gradients.
Stems were sampled and saplings with canopy photographs