MSU Extension Forestry

The related Michigan State University Extension article, "Each spring, forests turn green from the ground up," encourages folks to take note of the natural areas in and around the neighborhood as they transform from winter to spring green. The process takes place as plants, shrubs and trees begin to flower and leaf-out in response to warming air and soil temperatures and longer days. This article summarizes some of the general reasons that explain why different species of trees begin to green-up at different times over a three-to-four-week period.

Microclimates

One factor that should be noted when explaining the timing of different tree species greening-up, or flowering and leafing out, is location. A variety of local factors can create microclimates. These are small areas that, for multiple reasons, may experience slightly different weather conditions than another location nearby. Some of those reasons include:

• A south-facing slope that receives more direct sun than its north-facing counterpart
• Small, isolated depressions that tend to collect cooler, denser air
• The air in a sunny, open field with full sun warming faster than the air in the adjacent woodlot

As a result, the trees growing within a microclimate can experience variable timing in flowering and leafing out when compared with the trees growing outside of it. For simplicity, the information in this article describes timing variations between tree species growing within the same general woodlot, under similar conditions.

Describing the sequence of spring green up in a woodlot is fairly easy to generalize for understory plants and becomes more complicated when it comes to the trees. For example, not all of the small (either young or suppressed) trees flower and leaf out first, nor is it that the tall (either older or dominant) trees green-up last. Yet, trees of the same species do tend to flower and leaf out within a few days of one another when growing in similar conditions. One of the many factors that has been found to be significant is the structure of the wood1.

Greening-up early

The tree species that are first to flower and leaf out are typically trees that have diffuse-porous wood anatomy. The diffuse porous wood anatomy means that the vessels that carry water and nutrients up and down the tree are individually small and numerous. This characteristic helps to protect these vessels from rupturing during harsh winter temperatures and allows the tree to begin conducting water and nutrients as soon as the warmer temperatures permit. A few examples of trees in this category are: maples, cherry, buckeyes, most popples, birches, willows, and alder. The diffuse porous wood structure explains why maples are tapped to obtain the early flowing sap.

Some of the trees with diffused porous wood anatomy also have an indeterminate or sustained growth pattern along each twig. This means that the tree will continue to grow leaves along a twig that is growing until the tree senses regrowth is no longer profitable and stops the growth, usually in response to dropping fall temperatures and limited day length. The ability for the tree to grow in this manner allows the tree to quickly recover from late spring frosts or insects that might damage leaves that have flushed out.

In science, there is always an exception to the rule. According to Martin Lechowicz in “Why do Temperate Deciduous Trees Leaf Out at Different Times?,” not all trees with a diffuse porous wood anatomy leaf out early. Species like blackgum, sycamore, sweetgum, basswood and beech have diffuse-porous wood and tend to leaf out later than the aforementioned species. Research suggests this is due to their historic linkages to tropical and semitropical environments that require warmer weather to stimulate the flowering and leafing out process.

Later is better for some trees

Species that tend to flower and leaf out later in the spring green-up period have ring-porous wood anatomy. Ring-porous wood has fewer, large diameter vessels that conduct the water and nutrients up and down the tree. These larger diameter vessels easily rupture in harsh winter temperatures and the trees must repair the tissues before producing flowers or leaves.

Think of each vessel as a straw and the fleshy parts of the tree (flowers and leaves) as needing to draw up water and nutrients from the roots to photosynthesize and produce starchy sugars. Cold winter temperatures can cause the straw to break, or even tear, reducing its effectiveness to transport the water and nutrients under the suction created by the flowers and leaves. So, in effect, the tree takes the time to repair these straws, or vessels, to ensure the flowers and leaves can drink from the roots. In reality, the process is much more complicated than this. The flow goes both up and down the tree, maintained by turgor pressure in the cells, which can rupture in very cold temperatures. Still, it’s a good analogy to make the point.

Most of the trees that have ring-porous wood anatomy also have a determinate growth pattern, where tiny leaves are formed within a bud at the tip of the branch. When conditions are right, the bud opens up and the leaves elongate and “flush out”. The tree can do this once or twice a season; it takes time and considerable nutrients to support consecutive flushes of leaves. Determinate growth means the tree typically cannot as easily recover from late spring frosts or insect damage to its leaves without significant costs. Examples of tree species that have both ring porous wood anatomy and determinate growth include, but are not limited to, oaks, elms, sassafras, mulberry, hickories, walnuts and ashes.

The characteristics that affect the timing of flower and leaf production also apply to trees growing outside of a forested ecosystem. Regardless of their location, most diffuse porous species will flower and leaf out earlier than ring-porous species. This will become evident by closely comparing the leafing out progress of the maple and oak trees in your yard or neighborhood. Typically, the maples will leaf out and flower at least one to two weeks before the oaks trees, sometimes longer. Keeping a mental or written journal helps to keep track of what you see.

Other articles in series:

Part one: Each spring, forests turn green from the ground up