ROCK FRACTURES SUSTAINING VEGETATION GROWTH, A TREE’S PERSPECTIVE

The importance of water stored in unsaturated weathered bedrock for vegetation has only recently been acknowledged as a key ecohydrological process. The “classical” diagram of the Critical Zone depicts the unsaturated weathered bedrock underlying the saprolite, soil, and vegetation. However, systems like bedrock cliffs are dominated by stress fractures and bedding planes which collect and store water. In this study, we selected two tree species, Pseudotsuga menziesii (Douglas fir) and Picea engelmannii (Engelmann spruce), which commonly grow on limestone cliffs in the Canadian Rockies. We measured sap flow and stem water potential for trees growing in rock fractures and in glacial till (soil), weather variables, moisture in the sediment accumulated in the surface of fractures and on the glacial till, accumulated runoff and we collected tree cores to date the trees. Measurements show that the trees growing on cliffs have been under a prolonged water stress, some for more than 300 years. To understand how trees are surviving, we suggest a crack-tree water budget that depends on the external water input which is mostly overland flow on the cliff. The storage and water supply for the tree will depend on the size of the crack, the underlying network connectivity, the moisture of the accumulated ‘soil’, and the hydraulic strategies of the tree. We present one potential solution in which a modified Soil-Plant-Atmosphere-Continuum (SPAC), with accumulated soil and fracture networks as the substrate for the tree, can reproduce the water flow patterns measured in the trees growing here.