What time of year do trees grow fastest

Ring spell

Determining the age of wood on the basis of the tree rings is not always possible, even in the best state of preservation, because not all wood forms tree rings at all. But why are acacias or desert bushes unsuitable for dating? Why do annual rings arise at all, why no monthly rings? Why do the tree rings become wide when it rains and narrow when it is dry, and what exactly is wood?

Like most plants, trees do not only grow in length, but also in width in order to have sufficient stability. The longer a tree trunk becomes, the thicker it has to be so that it does not collapse under its own weight. When the tree grows in width, wood is created. So-called lignin, a vegetable hardener and preservative, is built into certain cells. The lignin prevents the cellulose cells from bursting under the high pressure.

Tree grate © H.D. Grissino-Mayer / R.K. Adams

The special wood cells in the tree trunk are made up of a layer that sits between the tree bark and the wood. This layer, the cambium, is actually a "water pipe" that enables the transport of dissolved nutrients upwards. At the same time, the cambium also produces new cells. The division of the cambium cells creates wood cells on the inside of the cambium and bast cells for the bark on the outside. As the number of cells around the tree increases, the trunk's circumference gradually increases.

If the water and nutrient situation is good, the cambium produces many wood cells, the tree grows quickly, but poor supply leads to poor growth. In the temperate climate zones, the trees also have an internal clock controlled by daylight, which regulates the development of a tree throughout the year. It determines when the trees bloom or when the leaves are shed in autumn and it leads to differentiated wood growth during the seasons. All trees in our latitudes grow fastest in spring, slower and slower until late summer, and stop growing completely over winter. This creates the typical annual rings. The light, fast-growing early wood is large-pored and therefore very soft, the dark, slow-growing late wood of the annual rings is very firm because it is smaller-pored. Where the dark latewood of one year and the light earlywood of the following year meet, the annual ring boundary can be easily recognized.

In contrast, most trees or shrubs outside of the temperate latitudes grow completely evenly throughout the year. In cold or dry periods they stop growing because the cambium stops producing wood. If it rains or it gets warmer, the trees continue to grow. Tropical tree trunks or shrubs in the Sahara therefore show recognizable growth patterns in the wood, but they are completely unsuitable for dendrochronology because these tree rings are not annual rings, but mostly "rain rings".

Acacias in the African savannah, for example, experience two rainy seasons every year and, with their two tree rings, always pretend to be twice the age. But not all native tree species are equally suitable for counting the annual rings. Softwoods such as pine or spruce usually like to reveal their age because early and late wood are very different in color. In the case of the linden tree, on the other hand, the tree rings are hardly distinguishable, which makes it difficult to clearly delimit different annual rings.

Status: 05.11.2004

November 5, 2004