THE ROLE OF ROCK FRACTURES IN SUPPLYING WATER TO TREES GROWING ON CLIFFS

Trees growing at harsh sites (e.g., bedrock cliffs) are thought to exhibit growth that is sensitive to climate (i.e., controlled by precipitation and/or temperature). On cliffs, cracks (stress joints and bedding planes) allow for tree growth by providing a substrate. They collect small amounts of rock soil, and organic matter, and water. For this reason, the architecture of the cracks will play an important role in how precipitation is delivered to the trees. Depending on the crack connectivity, intensity and size, cracks may act as a reservoir from which trees can obtain water or as a channel that takes water away from the trees (i.e., the cracks are leaking). We investigate the role of fracture architecture in the growth of two species of trees, Pseudotsuga menziesii and Picea engelmannii, on a limestone cliff in the Canadian Rockies. We have found these trees to range in age from 40-400 years old. They have consistently narrow rings and sapwood throughout their lives and move small volumes of water, reflecting long term water stress. To further piece together the architecture of the cracks and their ability to supply water to trees, we use isotopic analysis of tree water, forensic analysis of rockfall scars and infiltration tests. The isotopic signals provide us with insight into the age of the source of water and shed light on the long-term storage abilities of cracks. Forensic analysis of rock scars, and infiltrations tests provide information on the connectivity and intensity of fractures.