The Water Cycle in the Forest

Have you ever thought about where rain goes when it falls from the sky? There are scientists called hydrologists who do just this. They study the water cycle as water moves through our environment, and this area of study is called hydrology. Rain is just one small part of the water cycle. The water cycle describes the movement of water on Earth and includes rain and snow, rivers and oceans, groundwater below our feet, and water that evaporates from soil, lakes, and leaves [1]. If we look closely at a forest, even more parts of the water cycle can be found. Some water lands on the leaves of trees and drops to the ground—this is called throughfall. Some water lands on leaves and flows down branches to the tree trunk and then to the soil—this is called stemflow. And some water lands on leaves and bark and is held there before being evaporated back into the atmosphere—this is called interception. Hydrologists care about how much water reaches the forest floor by throughfall and stemflow. We also care about how much water is “lost” to interception because that water cannot be used by the trees or animals in the forests, or by humans who might rely on the forests for drinking water.

What Is Bark’s Role in the Water Cycle?

Bark is the outside layer of wood that all trees have. Bark protects trees from harsh environmental conditions including weather, pests, disease, and physical damage from hungry animals. Bark is an important component of the water cycle because it acts like a sponge during storms. That means some of the rainwater is absorbed into the bark and does not make it to the forest floor. The amount of water that can be absorbed by bark depends on the tree species and its physical structure [2]. For example, if we compare the same amount (or volume) of bark among six different tree species, loblolly pine bark is about 18% solid bark and the rest of bark volume is open pore space (Figure 1A, far right). Pores inside bark are spaces that can be filled up by water during rain events. In contrast, mockernut and pignut hickory bark are about 38% solid bark and the rest is open pore space. So when it rains, loblolly pine has a lot more open pore space to absorb water!

How Do We Study Bark?

Measuring surface characteristics of bark is pretty easy. We can poke and prod the tree and measure how deep the bark is, down to the underlying wood. But sometimes, we need to take the bark off of the tree and bring it back to the laboratory to perform more experiments. Using chisels and hammers, we isolate a square patch of bark and gently pry the bark sample off of the tree. When we remove a piece of a tree to study it, we call this destructive sampling (Figure 2). Some of the experiments we might conduct on tree bark in the lab are listed in Table 1.

Some Pores Hold More Water Than Others

Just like leaves, bark is very different depending on the tree species [3]. Some trees have thick, rough bark while others have thin, smooth bark (Figure 1A). Bark consists of two layers, the outer layer and the inner layer. The outer layer is composed of dead cells while the inner layer is living. Does bark with more pore space retain more water than bark with less pore space? Not exactly!

Loblolly pine has the most pore space but, despite this, it absorbs the lowest amount of water (Figure 1B). The pore space in post oak is very similar to sweetgum and white oak, but post oak can absorb much more water than the other two species because these barks have pores of different sizes. And even though mockernut hickory bark has much less pore space than loblolly pine, mockernut hickory can absorb a lot more water. These differences tell us that water absorption into the bark is a complex process and depends on many factors. We are trying to identify these factors through our research.

Some species have larger pores and some have smaller pores, and pore size also influences how much moisture the bark can absorb directly from the atmosphere, even when it is not raining! This characteristic is call hygroscopicity (Figure 1B). When we account for hygroscopicity, the pore space that can absorb additional rainwater diminishes and the capacity of our “sponge” is smaller. Bark hygroscopicity may reach up to 60% of the total amount of water absorbed by bark.

Conclusion

Now you know that bark is a reservoir for rainwater and that different tree species can have very different bark. Managing water resources is important because forests provide more than 75% of the world’s water for drinking and growing food [4]. If we want to be good managers of water resources [5], then we need to know exactly how much rainwater will make it to the forest floor and how much might be caught by leaves and absorbed into bark. By studying these topics, hydrologists help to understand the important role that trees and forests play in the water cycle.

Funding

This work is a contribution of the Forest and Wildlife Research Center and the Mississippi Agricultural and Forestry Experiment Station, Mississippi State University. This work was supported by the National Institution of Food and Agriculture, U.S. Department of Agriculture, Mclntire Stennis capacity grant # MISZ-032100. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

Glossary

Water Cycle: ↑ Movement of water between the Earth and the atmosphere.

Hydrology: ↑ The scientific study of water and its movement on Earth.

Throughfall: ↑ Rainwater that falls through forest canopies to the forest floor.

Stemflow: ↑ Rainwater that runs down tree trunks.

Interception: ↑ Rainwater that is captured by forest canopies and evaporated back to the atmosphere.

Pore Space: ↑ Open voids that can be filled with water or air.

Destructive Sampling: ↑ Removal of sample from the object of interest.

Hygroscopicity: ↑ The ability of a substance to absorb moisture from the environment.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.