The bark side of the water cycle

Woody plants are some of the tallest, largest, and longest-lived lifeforms on Earth. Their raw materials have literally framed and supported past human development, continues to do so today, and may aid humanity to combat and cope with future challenges, like climate change. All of the ecological and societal achievements of woody plants are due, in part, to a thin barrier between their internal and external worlds: bark. There is a lot of bark, too. Current estimates find that there is >40 million km2 of bark surface area, which is nearly as large as the entire Asian continent! Bark acts as both an environmental barrier (to pests and fire, for example) and an interface between woody plants and their environment. Even when bark is shed by a plant, or if a plant dies, bark persists and continues to act in the environment over years-to-decades. In forests where woody plants lose their leaves every season, bark, again, persists. Thus, bark has many opportunities to interact with the water cycle—from the top of live and dead tree canopies to the bottom of the forest floor. This Collection shines a light into this ‘bark side’ of the water cycle, examining several core concepts and new discoveries from the companion article collection in Frontiers in Forests and Global Change. There are several reasons why the ‘bark side’ of the water cycle requires our attention. First, most precipitation over land falls over forests and must pass through their leaves, bark and litter to reach the surface. At the surface, freshwater resources are getting scarcer and their management is becoming more socio-politically and economically complex. Finally, climate change is altering the patterns and timing of precipitation supplies around the world; however, the land surface and climate models used to predict and adapt to climate change only superficially include bark’s roles in the water cycle. Therefore, it is possible that an increased curiosity and awareness of this veritable ‘bark continent’ and its importance for our planet’s water cycle can improve the appreciation and conservation of forest ecosystems and related water resources.