Each spring, forests turn green from the ground up

Julie Crick<crickjul@msu.edu>, Michigan State University Extension - March 23, 2017

As temperatures warm and days get longer, forest ecosystems begin the transition from winter to spring.

One of the most notable transformations that take place during the spring months in Michigan is the conversion of our forests from winter dormancy to full summer activity. This transformation is best witnessed by looking at the same forest from a distance over the course of several weeks. At first, the green color appears at the ground level, as wildflowers begin to grow and quickly flower. Then, the shrubs and understory trees begin to flower and leaf out, giving off the illusion that the green color is creeping upwards. Finally, the trees take on a green hue, as tiny leaves expand out of the protective bud scales.

The reason for the green up can be explained because of phenology, or the study of the correlation of biological occurrences, weather conditions and climate. Phenology applies to anything from a bird migration to an insect hatching just after its host plant grows leaves. In the case of the greening of forest vegetation, phenology relates to the warming of the soil and warmer ambient air temperature, as well as days growing longer.

The coordinated process of the forest greening up in layers allows all the flowers, shrubs and trees to temporarily experience full sun conditions on their leaves. This begins the process of photosynthesis, which creates food starches that are used to develop seeds and support growth. The process of leafing out and creating flowers can be nutritionally expensive to vegetation, with most of the nutrients coming from starches stored in roots during the previous season. The warm temperatures and full sun help start-up plant biological systems and individual metabolism to produce nutrients for annual growth and seed production and to replenish the starches in the roots.

Greening from the ground up

It all begins with the spring ephemerals, which is another name for the early wildflowers that provide the first spring-green on the floor of hardwood forests. The word ephemeral means lasting for a short time; and that is exactly what these first plants do. These short-lived plants take advantage of the full sunlight that reaches their green leaves near forest floor in early spring.

At the same time the spring ephemerals leaf out and flower, the low shrubs also flower and leaf out. Then, after a few weeks, the trees begin to flower and leaf out extending the green hue into the forest canopy. Research has shown that different tree species in a forest typically flower and leaf out at different times over a two to four week period, gradually increasing the amount of green color throughout the branches in the forest. For example, a birch tree tends to leaf out much earlier than a walnut tree. To learn more, please also read a second article by Michigan State University Extension that summarizes research to explain some of the reasons for the difference in timing.

To learn more about the conditions that affect plants greening up, visit the United States Geological Service-led USA National Phenology Network (USGS NPN). The USGS NPN created the Daily Spring Index Leaf Anomaly maps to demonstrate how earlier than normal spring green-up is occurring this year in the United States. The USGS-led NPN website also features a static version of areas that have experienced green-up, or “spring” so far in 2017, and a phenology visualization tool. The maps and tools can be useful to scientists as well as citizens in the planning of seasonal and educational activities.

This year, as we all look forward to spring, keep an eye on your favorite forest and see if you can spot the greening of the forest from the ground up!