Trees Are More Than Pretty

Habitat Enhancing Land Management

“Trees are poems the earth writes against the sky."—Kahlil Gibran

Christine Middleton

What is a tree? Simple question. No simple answer. There is not one universally accepted, precise definition. Broadly, a tree is something that grows taller than most other plants. The botanical definition characterizes trees and other perennial woody plants as species that exhibit secondary growth. That means a tree or shrub grows outward as well as upward; in other words, the diameter and length of both the trunk and its branches increase each year. But that begs the question: what differentiates a tree from a shrub? Again, there is no universally accepted definition. Generally, we think of trees as very tall plants with a single woody trunk and of shrubs as somewhat shorter plants with several main woody stems growing from the base. But some trees and shrubs can be found growing both ways.

The record holders and other famous trees

Luckily, most of us don’t need a definition. We know a tree when we see one. We generalize that trees are taller and live longer than other plants. In fact, we admire the record holders! Both the tallest and the oldest tree in the world are in California: a Coastal Redwood (Hyperion) that is 380 feet tall and a Bristlecone Pine (Methuselah) that is over 4,800 years old.

Here in Texas, we can consult the Texas A&M Forest Service’s regularly updated Big Tree Registry for a beautiful slide show and a video of some of our state record holders among many Texas tree species. We find that Texas’ tallest known tree is a 139-foot Loblolly Pine (Pinus taeda) in Sabine County—like a 12-story building! A huge 1,100- year-old Southern Live Oak (Quercus virginiana) in Bosque County is possibly our oldest tree. And a Bald Cypress (Taxodium distichum) in Real County has the greatest girth; its trunk measures 475 inches (over 13 yards) around! Hays County has one of the record holders, a Box Elder (Acernegundo) that is 85 feet tall and has a circumference of 103 inches!

Our dominant oak species here in Hays County, the Escarpment Live Oak (Quercus fusiformis), reaches 20 to 60 feet tall. Bald cypresses, sycamores, pecans, cedar elms, and many of our oaks, when in deep soils with sufficient moisture and allowed to attain “old growth” size, are among our most imposing trees. Soils deepen the farther east we are in Hays, away from the soil-poor caliche and limestone terrain in western Hays County. So it’s no surprise that most of our big trees are in central and eastern Hays. Many of them have witnessed our region’s history. Four of them are listed on the Texas A&M Forest Service’s Famous Trees of Texas website: Kissing Oak, Kyle Auction Oak, Kyle Hanging Tree, and Log Cabin Oaks.

Tree structure and growth

While trees function a lot like other plants, they are different in a few notable ways. As with most plants, tree leaves combine water and carbon dioxide to produce sugar and oxygen, a process called photosynthesis. And, while we call the stem of a tree the “trunk,” its two main functions are the same: to transport water and other minerals to the leaves and to support the structure. You might think of the trunk of a tree as a special kind of stem that connects to branches rather than to leaves, fruit, or flowers. And, unlike the main stem of many forbs (soft stemmed plants), tree trunks don’t depend on water for support. Rather, it is the wood that forms, dies, and hardens each year that provides a tree’s sturdy structure.

The outer bark of a tree trunk looks very different from one tree species to another. In fact, often the nature of the bark is an important clue to a tree’s identity. But generally the bark of different kinds of trees serves the same purpose: to protect the tree’s trunk and branches from injury and disease. The bark seals out excess moisture and prevents the tree from losing too much moisture in dry conditions. It insulates the tree against cold and heat and protects it from insects and other enemies. Some trees have very thick bark that protects them from wildfires. Others have bad tasting chemicals in their bark that ward off their insect enemies. And sometimes the bark is covered with spines or thorns to discourage browsing mammals.

The inner bark is called the “phloem” (FLOW-um). It is a one-way pipeline from the leaves to other parts of the tree. Sap—water combined with dissolved sugar and nutrients—flows from the leaves down the phloem, supplying the trunk, branches, and roots with nutrition essential for life. That’s why, if a band is cut penetrating the bark and phloem around the entire trunk, the tree will eventually die from starvation. A nonchemical method for controlling invasive trees, called “girdling,” involves removal of bark down through the cambium layer. And that’s what rutting male deer sometimes do inadvertently to young, unprotected native trees here in the Hill Country.

Girdling can eradicate invasive Ligustrum growing too close to a creek to risk chemical control.

The cambium layer is the part of the tree that supports the growth of the trunk’s diameter. Every year this layer produces new wood in response to hormones called “auxins,” produced by the leaf buds on the ends of the branches as they start to grow in early spring. The growth rate of trees varies from year to year or even sometimes season to season. Factors that influence the rate of growth include rainfall, temperature, and competition from neighboring trees. Different tree species grow at different rates. In Hays County, sycamore sprouts, one of the first species sighted on the banks of the Blanco after the 2015 Memorial Day Flood, are in 2023 already young trees. The slower-growing pecan trees also washed away in that same flood are taking longer to recover.

Sycamores just south of Dripping Springs, Texas, glow in late afternoon winter light.

Trees also have a second pipeline called sapwood, which moves the water needed for photosynthesis up from the roots to the leaves. Sapwood is the new wood recently formed by the cambium layer. As newer rings of sapwood are laid down, the inner cells turn into what is called heartwood. While the heartwood is essentially dead, it does not decay as long as the outer layers of the trunk are intact. The combination of sapwood and heartwood together is called the xylem, which makes up most of the tree’s trunk and is essential in providing the support for the canopy.

Cross-section of a tree trunk

https://www.arboristnow.com/news/The-Basic-Anatomy-of-a-Tree

Tree rings

Which brings us to growth rings and the question of how they form. Tree rings result when new wood cells are generated within the cambium. As a tree emerges from dormancy and assuming conditions are favorable, these cells grow rapidly and are less dense. Later in the growing season, when heat and drought begin to exert their influence, the rate of cell growth slows down. The wood becomes less dense and darker in color. You might have noticed that in a tree’s first three to five years of life, the rings tend to be close together and hard to discern. That’s because in those first years, the tree was more focused on growing roots rather than increasing girth. After that, the tree grows rapidly, and the rings get wider. This rapid growth continues until the tree reaches the adult stage. At that point, the tree diverts carbohydrates toward flower and seed production, and the growth of the trunk generally slows.

Tree rings not only tell us how long a tree has lived, but also a lot about what conditions were like across its lifetime. Years with inadequate rain generally result in less growth and thus narrower rings. Many of the older trees here in the Hill Country undoubtedly still tell the story of the drought of the 1950s, when our area went eight years with very little rain. And sometimes when rains are scarce in the spring and return in late summer, trees like our Ashe Juniper (Juniperus ashei) develop false rings. Changes in conditions around the tree can also impact growth rates. Lost soil through erosion can reduce the moisture available, thus slowing growth. Or thinning a woodlot can spur the growth of the remaining trees. And when a tree dies, its decline is often reflected in narrower rings indicating when downturn first began, often as much as 5 years earlier.

Tree roots

The trunk is attached to the tree’s roots, which anchor the tree to the ground and provide water and minerals. Larger, more permanent roots provide for anchorage and transport. Smaller feeder roots and root hairs absorb water and nutrients. These smaller roots are more temporary, either dying or becoming part of the larger tree system after a year or two. Tree roots spread more widely and don’t generally penetrate as deeply into the soil as we sometimes imagine they do. Generally, tree root systems extend out in an irregular pattern that can be 2 to 3 times larger than the canopy. And, unless the top soil is bare or unprotected, trees concentrate their absorbing roots close to the surface because that’s where the best water, nutrients, and oxygen are usually located.

The roots of many tree species depend on soil fungi, which form root-fungus structures called mycorrhizae. This is a symbiotic—mutually beneficial—relationship. The tree supplies carbohydrates and other things the fungi need to grow. In turn, the fungus helps the tree enhance its intake of water and minerals (particularly phosphorus) by increasing the total absorption surface of the root system. Want to know more about this fascinating relationship? A good read is Suzanne Simard’s Finding the Mother Tree: Discovering the Wisdom of the Forest.

Enemies of trees

Trees have enemies too, both natural and man-made. The most destructive natural killer here in Hays County is a fungus, Bretziella fagacerum. This fungus causes oak wilt, an infectious disease that interferes with the tree’s water conducting system. Red oaks (Spanish oak, Shumard oak, blackjack oak) and live oaks are most likely to die.

Live oaks are particularly problematic because of the interconnectedness of their roots. Once one tree is infected, neighboring trees are in grave danger. White oaks (post oak, bur oak, Mexican white oak) are less commonly killed by oak wilt because of their ability to protect themselves by growing tiny balloon-like plugs called tyloses in their sapwood vessels. The tyloses prevent the fungus from moving throughout the tree’s vascular system. These plugs also make the wood of white oaks impermeable to water, explaining why white oak is favored for wine and whiskey barrels. See more on oak wilt and how to prevent its spread.

Fungi and other pathogens can affect trees in a variety of ways. On one of our HELM visits, the landowner asked us about a tree that was shedding bark at its base. We consulted the Texas A&M Forest Service. Based on the picture we sent, Woodland Ecologist Karl Flocke explained that most likely a fungus was causing the cambium layer to die, thus robbing the bark of its support. He further suggested that the large amount of organic material piled up as mulch around the base of the tree was likely the reason the bark rot had developed. The solution he suggested was to excavate around the tree down to where the trunk begins to flare into the root system. When putting mulch around a tree, we should not pile it up against the trunk.

People can be the enemies of trees, often inadvertently and sometimes with the best of intentions. Soil compaction, by livestock or by vehicle and foot traffic, can destroy soil structure by decreasing its porosity, making it harder for water to soak in, starving roots for moisture. People sometimes overwater trees, thinking they are helping them withstand a dry period. Supplemental watering of newly planted trees may be necessary until they have developed a more robust root system. Large, established trees generally don’t need to be watered unless there are signs of stress and the ground around them is extremely dry. But with climate change, our trees face a new enemy—excessive heat during times of drought. To learn more about this new threat, listen to Dr. Salwa Khan’s Mothering Earth podcast with Texas State professor, Dr. Susan Schwinning. And here’s advice from the Texas A&M Forest Service on watering trees during drought.

Why do we need trees?

Trees do a lot more for us than just look pretty. Trees are good for our health and that of the world around us. Trees provide shade—very much appreciated not only by us, but by the wildlife around us. The trees on our property provide food and homes for the birds and other wildlife we love to observe. Different birds hang out at different levels, which is just one of many reasons for encouraging diversity in our trees’ species and sizes. Trees along our waterways mitigate water temperature rise during our now more frequent heat waves, benefitting fish and other aquatic species. And trees are good for our mental and physical well-being. Who doesn’t feel better after a peaceful walk down a wooded path?

Trees do lots more for us—things that aren’t so obvious. Trees help keep our water resources clean by improving infiltration of surface water. The leaves they shed each year create a mat that acts like a sponge. This provides more time for valuable rain to soak in rather than simply running off. As the water seeps in, it is purified on its way to recharging our aquifers or as it makes its way underground before reemerging from our springs and seeps.

Trees are our partners in smart land stewardship

Trees play an important role in our fight against climate change through carbon sequestration. You may have heard about nature-based carbon credit programs that pay landowners to retain trees on their property. These credit programs are not just for landowners in the Amazon rainforest. They are for us and our neighbors. People are working hard to introduce programs right here in the Texas Hill Country.

TreeFolks is already getting carbon credits for the trees they are planting as part of their Central Texas Floodplain Reforestation Program, including the 600 seedlings they recently planted at Blue Hole Park in Wimberley, Texas.

Want to know more? Check out this talk by Rice University professor Jim Blackburn in November, 2022: TESPA and the Texas Hill Country Carbon Narrative.

So when is the best time to plant a tree? An old Chinese proverb suggests “The best time to plant a tree is twenty years ago. The second best time is now.” Here in the Texas Hill Country, the best time to plant is actually in the late fall; that provides the tree time to send out roots before it gets hot and dry again. And remember, trees like diversity, so think about something that isn’t already on your property—perhaps a bur oak because it is not as susceptible to oak wilt. Or maybe a cedar elm, pecan, or Texas ash. And think about smaller trees like redbuds and mountain laurels, whose flowers will delight pollinators come springtime. Want some more ideas? Here’s a Native Tree Growing Guide for Central Texas.

HELM (Habitat Enhancing Land Management) is now accepting requests for spring 2023 property visits. If you would like the HELM team to visit your Hays County property and provide help with your trees or other land stewardship concerns, go to https://beautifulhayscounty.org/helm/ and fill out our request form.