The Great Flush of 2015

Have you seen the Great Flush of 2015?  In Central Kentucky,  trees are growing very fast right now, producing new growth that is noticeable for its pale green or yellow colors.  Late flushes are complicated responses to changes in soil moisture. This is the first of several stories about the Great Flush of 2015, but also about weather patterns and, perhaps, about climate change.

What is a flush?  Think of a bud as a package of tiny leaves waiting for spring. The leaves were made late last year.  As the weather warms in spring, the little package begins to open and out come the leaves, quickly growing to capture sunlight.  As the stem grows, the distance between leaves increases. Soon, growth ends for the year. It is surprising that, in most trees, shoot growth ends by June, long before the growing season ends. A new bud sets and the leaves for next year begin to develop.  Here are couple of examples in kingnut and American beech.

Beech bud
A beech bud opening in spring
Soil Moisture Map
Change in soil moisture from June 30 to July 22 2015. Source: NOAA Climate Prediction Center

This is not the only style of shoot growth. Some fast-growing trees like yellow-poplar continue producing and growing new leaves throughout the summer until shortening days or drought bring an end to their growth.

But what happens in a year like 2015?  We are having an exceptionally wet July, setting records in many counties throughout Kentucky and adjacent states.  We are usually in mild to moderate drought at this time of the year, but right now soil is exceptionally moist.  That means that growing conditions for trees are excellent.

How do trees respond?  Instead of waiting out the year and growing again next spring, many trees produce a new flush of growth. Suddenly, it looks like spring again.  We don’t know exactly how this works, and why some trees like dogwood ignore the high moisture and snooze until spring.

For oaks, hackberries and many other trees, the Great Flush of 2015 is easy to see. The pale color of the second flush contrasts with the deep green of the first flush. Some people see these colors as an early sign of autumn, but the opposite is true: it’s a sign of a renewed spring.

The amount of growth can be astonishing.  Here is a bur oak produced a moderate first flush but an huge second flush:

Bur oak stem
Bur oak with two flushes of growth. The spring flush, between the white arrows, was 2.5 inches (6.4 cm) while the second flush (yellow arrows) is 12 inches (30.5 cm)

Why are the second flush leaves so pale? Here are two pictures that provide a simple explanation: On the left is a bur oak, on the right is a black locust. Both are continuing to produce new leaves but the locust leaves are dark green. Black locust is a nitrogen-fixing tree species, continually producing nitrogen in root nodules. The bur oak depends on nitrogen from the soil and can’t make its own. In the spring, the soil contains lots of nitrogen from decay over the winter. The tree also contains a lot of nitrogen stored in the form of protein, which it uses to make new leaves in spring. By mid-summer, both the internal reserves of nitrogen and soil nitrogen are depleted. So, the new leaves don’t have enough nitrogen to make an adequate supply of nitrogen.

Bur oak and black locust leaves
Bur oak (left) and black locust in the same soil on the same day.

In the next part of the story, we’ll talk about the trees that don’t quite growth so early. In the third part, we will talk about the weather and climate factors that great great flushes.

Scenes from the Great Flush of 2015

July flush in bur oak
July flush in bur oak with typical pale foliage
Growth flush on a bur oak
Growth flush across the whole crown of a bur oak
Hackberry trees
A stand of hackberry trees with a pale late flush.
July flush of hackberry with very pale foliage.
July flush of hackberry with very pale foliage.
Growth flush on American elm
Late flush of American elm
Kentucky coffeetree with a second flush of growth
Kentucky coffeetree with a second flush of growth