The Wonder of a Tree: What Makes a Tree?

Posted: September 15, 2014

We need to celebrate trees and conserve the working forest. They are essential to us and our overall well-being. To celebrate them more fully, we wanted to help expand your knowledge of just what trees do, how they do it, and how they grow and thrive as they help us out.

Trees are amazing. Here in Pennsylvania they are a ubiquitous part of our landscape. It’s hard to imagine a place in this state where you can’t see a tree somewhere in your 360° view (Try it! And inside a building does not count). Trees provide countless benefits to our health and well-being, economics, societal resources, and ecology of the world. We’ve all heard the statistics: A healthy tree can store thirteen pounds of carbon per year… filters sixty pounds of pollutants from the air… provides the equivalent net cooling of ten room-sized air conditioners operating twenty hours a day… annually, one acre of forest absorbs six tons of carbon dioxide (CO2) and puts out four tons of oxygen – enough to meet the needs of eighteen people… viewing trees outside hospital windows helps patients recover faster… trees help settle out and trap dust, pollutants, and smoke, improving air quality… a forest traps and filters significant amounts of rainwater. The public water supply for New York City is a forested watershed in the Catskills. Boston’s water supply is a forest in the north-central Massachusetts. These forests do such a good job providing clean water that the water coming out of the tap requires little or no treatment.

It’s easy to take trees and forests for granted. During our lifetime they’ve always been there. We need to celebrate trees and conserve the working forest. They are essential to us and our overall well-being. To celebrate them more fully, we wanted to help expand your knowledge of just what trees do, how they do it, and how they grow and thrive as they help us out. So what makes a tree?

A tree is a tall, long-lived, woody, branching plant.

Tree height is a function of desirable site conditions, species parameters, and competition. The goal for a tree is to develop its leaf surface area to maximize its ability to produce energy. Height allows trees to put their leaves above competitors or to maximize crown area (by layering and orienting leaves). Wood is a structural characteristic that provides both strength and function (we’ll go into this in more detail in future articles). Trees are long-lived because of the investment in structure, and lifespans can range from 30 years to 5000 years, in the case of a California bristlecone pine.

Trees are made up of roots, stem or bole, branches, and leaves. The root system anchors the tree in the soil. It also takes in nitrogen, minerals, and water. Roots are both woody and fine - woody for anchoring, fine for uptake. The stem, made of wood, provides both support and water transport from the roots to the leaves. Tree growth/development is primarily gravitropic, that is, they orient themselves in response to gravity. Branches layer and orient leaves to maximize photosynthesis (and control for environmental stresses) by maximizing the orientation to the sun without overstressing the leaves due to exposure.

Leaves are the carbohydrate production site. They take in CO2 from the air through their stomata (openings on the underside of the leaf, also the site of water transpiration), water and nutrients transported to the leaves through the stem and branches from the roots, and, with the addition of outside energy from sunlight, conduct photosynthesis, transforming CO2 and water to simple sugars and oxygen (with some waste products) (to grossly oversimplify the chemical reactions). Photosynthesis requires mineral elements (taken up through the roots) as catalysts for the reaction. The photosynthetic reactions produce organic sugars. The sugars are either converted to other organic compounds and used as building blocks for structure or function, or used as energy for plant survival and growth, resistance to insect attack and disease, transport of waste or other substances, or other biochemical pathways. At the same time photosynthesis is occurring, respiration is also occurring, which uses some of the sugars produced by photosynthesis to maintain and drive the tree’s entire system (again, grossly oversimplifying). Just as in animals and other organisms, respiration requires oxygen from the atmosphere, essential nutrients, and the sugars produced by photosynthesis.

Trees photosynthesize faster in bright light, up to full sunlight, as long as other requirements do not become limiting. Full sunlight can, at times, be associated with higher temperatures, which can decrease photosynthesis as the tree closes stomata in attempts to moderate water loss. Photosynthesis can occur at temperatures above freezing to a plateau of maximum production between 55 and 75° F. Temperatures above this level can decrease photosynthetic rates due to increased transpiration.

By contrast with optimal photosynthesis, root systems tolerate a much larger temperature range. Roots won’t grow at temperatures less than 40° F, but growth can start quickly once soil temperatures warm up in the spring. Roots can continue to grow even if the stem has stopped growing. Dry soils will also cause roots to stop growing. If there is no water for uptake, the system will shut down until conditions become more favorable. Generally root growth peaks in the early spring before the tree branches start to elongate, and in late autumn when the branches have stopped growing.

In future articles we’ll cover the woody aspects of the tree, how trees are structured and function, how trees react to changes, and reproduction. The goal of this series is to make you remember your days in biology studying plants and to keep in mind the amazing resource that are our trees and forests. We can’t take them for granted.

Prepared by Allyson B. Muth, Forest Stewardship Program Associate, Penn State Department of Ecosystem Science and Management

Contact Information

Allyson Brownlee Muth, Ed.D.
  • Forest Stewardship Program Associate
Phone: 814-865-3208