Woody plants obtain their water mainly from soil and use water stored in bedrock only as an emergency reserve during severe drought.

Plant-water models have traditionally put most of the usable water in soil. Bedrock was often treated as the lower boundary of the root zone, or as a reserve that deep-rooted plants might tap only when drought dried out the soil above.

Ecologists had already found plenty of exceptions. Roots can grow into fractures and weathered rock, and local studies had shown trees drawing water from below the soil. What was less clear was how common that behavior was across large regions.

A 2021 Nature study examined hydrological and ecological data across the continental United States. It found that woody plants were using water stored in bedrock across many climates and ecosystems, and not only during extreme drought. In some regions, the estimated contribution was large enough to affect ecosystem water use and carbon cycling.

Bedrock does not matter equally everywhere. The amount of available water depends on factors such as rock type, fractures, weathering, depth and whether roots can reach it. A 2024 review likewise found that some forests may have more plant-available water in rock than in the soil above, while other sites rely far less on it.

The important change was in the default assumption. Bedrock water is not merely a last-resort supply. In many woody ecosystems, it is part of the normal root-zone water budget.