Print PageSoils in mountainous terrain are often relatively thin and are underlain by weathered bedrock. It is a common observation that roots of trees and shrubs grow into fractures in this bedrock to at least several meters in depth. We sought to discover the benefit that the plants accrue by accessing the rock substrate and how the rock functions as a usable medium for supporting plants. To this end, we measured the water-holding characteristics of granitic rock in different stages of weathering, analyzed the bedrock porosity relationships, measured bedrock and plant water potentials, and studied the distribution of mycorrhizae with depth.
Results/Conclusions
We conclude that the production of porosity by weathering converts biologically inert hard granitic rock into a hospitable substrate for ecosystems. Porosity allows water to flow into and through the rock. It imparts a water-holding capacity to the rock so that water can be stored for prolonged use by organisms. Organisms themselves, in the form of microbes and plant roots, invade the rock as porosity forms. In California, the weathered bedrock zone is often much thicker than the overlying soil, and is a larger reservoir for water storage. Chaparral, oak woodland, and coniferous forests have been shown to rely on bedrock-held water to survive the summer dry season. Mycorrhizae appear to play a crucial role in linking plant roots within rock fractures to the rock matrix pores where water is held.
