Dendrochronology: How tree rings inform us about climate change

Currently, climate change leads to temperature rises and weather anomalies worldwide. But, in the past, temperature on earth has always been rising and falling, causing glacial and interglacial periods. Therefore, the question is, what is different this time? To answer this question, scientists can use an unexpected source of information: trees. Trees function as weather record keepers as every year, they record environmental cues in their growth rings, offering insights into past climates and how the climate is currently shifting. In this article, I will touch upon some basic tree physiology, how tree rings are formed, and what these tree rings can tell us about climate change.

Water transport in a tree

Trees transport water from the soil up to the leaves and branches through vessels in the wood, i.e., ‘xylem vessels’. As the water needs to be transported upwards against gravity, other forces facilitate the transport. There are several processes involved: 

Transpiration

Water molecules evaporate from the surface of the leaves into the surrounding air. This creates a negative pressure gradient, within the tree’s vascular system, pulling water upward from the roots. This process is called transpiration. 

Cohesion

Water molecules attract each other. These cohesive forces are crucial for maintaining the integrity of the water column through the tree. Within the xylem vessels, a continuous chain of water molecules within the xylem vessels is necessary at all times to allow upward water transport. If the chain is broken, the water transport has stopped. This is known as ‘cavitation’. When a xylem vessel is cavitated, the water chain is interrupted by an air bubble, and water transport is not possible anymore.

Adhesion

The third process that facilitates water transport is adhesion, which is the attraction of water molecules to the walls of the xylem vessels. Adhesive forces counteract the downward pull of gravity and ensure that water is drawn upward through the vascular system.

The formation of tree rings

Every year, a tree grows a new set of xylem vessels for water transportation, i.e., a new tree ring. Each ring represents a year of growth, with alternating layers of light and dark indicating periods of rapid growth and dormancy. These rings are formed as a result of the seasonal fluctuations in temperature and precipitation.

How the tree rings are structured depends on the weather conditions during the growing season of the ring.

Climate and weather influence tree ring growth

Tree ring width varies depending on environmental conditions. In favorable years with ample rainfall and moderate temperatures, the availability of sufficient resources results in wider rings. However, during periods of droughts, the xylem vessels should be narrower to avoid cavitation. The cohesive and adhesive forces maintain the continuity of the water column more easily in narrower rings when the water availability is lower. Therefore, during droughts or extreme weather periods, rings are narrower.

The alternating patterns of wide and narrow rings serve as a historical archive, documenting the yearly variations in tree growth. The patterns reveal not only the tree’s age, but also a record of local weather conditions throughout the tree’s lifespan. 

In dry environments, tree rings are mostly precipitation-related and typically represent wet or dry years. In cooler environments (higher latitude or elevation), tree rings are more temperature-related and therefore represent warmer or cooler years. 

Dendrochronology

Tree rings are studied using dendrochronology, the science of dating and interpreting past events through the analysis of tree rings. By examining tree rings’ width, density, and isotopic composition, scientists can reconstruct past climates and infer environmental changes over centuries or even millennia.

The International Tree-Ring Data Bank (ITRDB) contains tree ring data from forests worldwide. It also includes ring data from old buildings and even old violins. Thus, tree rings can even be used to estimate the age of human-made timber constructions.

Trees can provide us with information about the climate from many years ago. There are even tree chronologies of 3585 and 4500 years ago, developed using fossil tree remnants. 

Tree ring data has also been used to study the effect of volcanic eruptions in the past on the weather. For example, several studies used tree ring data and found lower temperatures after volcanic eruptions, probably caused by a high concentration of volcanic ashes in the air.

Conclusion

Tree ring data is strongly correlated with climate variables. Tree rings provide a crucial source of information on how climate change affects ecosystems worldwide and on large scales. Ultimately, these insights can guide our efforts to mitigate and adapt to the impacts of climate change.