Measurements of above and below-ground portions of woody plants ≤ m in height were made along a transect on a 31 ´ slope in tropical montane rain forest near treeline in northern Peru. Almost all stems were basally curved, and most of the belowground axes were curved and situated upslope relative to the stem base. Microscopic examination demonstrated that underground curvature, and the resulting aboveground basal stem curvature, was due to stem burial, followed by adventitious rooting. We suggest that it was litter creep and branch movements along the slope that caused this burial. A threshold model would best explain how and when these curvatures occur: plant form and growth will only be affected by soil creep and other noncatastrophic downslope forces when the plant is within critical size classes of stem diameter and rooting depth. © 1990 Taylor & Francis Group, LLC.