Rooting Characteristics of Trees in Rock: A Study of Thuja occidentalis on Cliff Faces

Sixty-two slow-growing Thuja occidentalis (eastern white cedar), 6-28 yr in age and 11-52 cm in size, were excavated from a vertical limestone cliff. Their root extension, distribution, biomass, and belowground allocation were investigated, as was the degree to which these rooting characteristics were influenced by substrate factors such as rock fracturing and the presence of soil pockets and ledges. Alson examined was the correlation between such microsite characteristics and individual tree growth rates. The results showed that the majority of trees grew in rock without soil or in very small soil pockets. Rooting was shallow, penetrating solid rock to an average of only 9 cm (maximum 30 cm). Roots were found almost exclusively in rock fissures of the softer, more weathered rock layers, penetrating harder layers only via crevices. Root competition was largely absent when trees grew in rock. The prescence of absence of soil had no effect on aboveground or belowground biomass, root: Shoot ratio, or growth rate but significantly affected the pattern of root deployment. Root: shoot ratios were within the normal range for temperature-zone trees (average 0.48). All microsite factors were poor predictors of individual plant growth rate. The likely availability of both water and nutrients near the rock surface may explain the lack of deep root penetration and a minimal need for soil. Plasticity of root deployment without a loss of uptake efficiency may be one of the characteristics that enable T. occidentalis to persist on cliff faces.