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 limesto ne cliff. Their root extension, distribution, biomass, and belowground allocation were investigated, as was the degree to which these rootin g characteristics were influenced by substrate factors such as rock fr acturing and the presence of soil pockets and ledges. Also examined wa s the correlation between such microsite characteristics and individua l tree growth rates. The results showed that the majority of trees gre w in rock without soil or in very small soil pockets. Rooting was shal low, 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 crevice s. Root competition was largely absent when trees grew in rock. The pr esence or 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 nor mal range for temperate-zone trees (average 0.48). All microsite facto rs were poor predictors of individual plant growth rate. The likely av ailability of both water and nutrients near the rock surface may expla in the lack of deep root penetration and a minimal need for soil. Plas ticity of root deployment without a loss of uptake efficiency may be o ne of the characteristics that enable T, occidentalis to persist on cl iff faces.