PS 36-194 - The role of rock fractures in supplying water to trees growing on cliffs

Background/Question/Methods

Trees growing at harsh sites are thought to exhibit growth that is sensitive to climate (controlled by precipitation and/or temperature). Bedrock cliffs are an example of one of these sites. On cliffs, cracks (stress joints and bedding planes) provide a substrate for tree growth. They collect small amounts of rocks, soil, organic matter, and water. For this reason, crack characteristics will play an important role in how precipitation is collected and delivered to the trees. Depending on the connectivity, intensity, and size of cracks, they may act as reservoirs from which trees obtain water or as channels that take water away from trees. In this study, we look at the role of fracture architecture in the growth of two tree species, Douglas fir (Pseudostuga mensiesii) and Engelmann spruce (Picea engelmannii), growing on a limestone cliff in the Canadian Rockies. We obtained tree ring-width series and sapwood area from ~60 individuals. Sap flow measurements, stem water potential, and soil moisture were measured for 3 trees of each species growing in cracks. Additionally, we collected climate data, estimates of overland flow contribution to cracks, and measured hydraulic properties of the cracks. Similar measurements were taken for trees at a soil site nearby.

Results/Conclusions

Our results show that trees growing on this limestone cliff are up to 400 years old. Trees have consistently narrow rings, showing slow growth throughout their lives. The sapwood areas of trees are small, relative to the average sapwood area of trees growing at the soil site. In combination with the sap flow data, which shows very little water movement daily (between 200mL and 2 L/h), our results indicate very low water availability in this cliff habitat. These trees have likely been chronically water-stressed their whole lives. Additionally, there are large variations in the tree ring widths series between trees on the cliff. This seems indicative of differences in the cracks' abilities to store and supply water to the trees. Infiltration tests used to determine the hydraulic properties of the cracks will provide insight into the subsurface architecture of the cracks and the different crack water storage capacities. The source of water used by trees will be confirmed using isotopic tracing methods and will provide additional information on crack storage.