Tree Rings
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Tree Rings Record History

A Record of Climate Change

Roots sprout deep into soil and rock, seeking out precious water and nutrients like a baby suckling at its mother’s breast. These subterranean tendrils tenaciously grip the fertile earth, supporting a towering trunk, which reaches for the open sky to harvest sunlight for food and energy. As trees grow, their root networks expand, accommodating the needs of the trunk and branches as they stretch upward and outward. Some years are good with water, nutrients, and sunlight in plentiful supply, but in other years, when climate challenges arise, trees scale back their growth and conserve energy. As each year passes, a tree records its impressions of the growing season.

It’s easy to think of trees as solitary lifeforms standing tall and proud on long lonely vigils in the heart of a forest. But below the ground, trees tap into mycorrhizal networks where the forest community shares water and nutrients. They also communicate through these networks with signals and messages sent via chemicals, hormones, and slow-moving electrical pulses. 

A mycorrhizal network is a symbiotic endeavor between subterranean fungi and trees, which tap into the network using thin, delicate root tips. The presence of this network, shared between trees and their neighbors, points to a sophisticated living community deeply connected with each other and the larger ecosystem. A tree’s ability to grow and add mass to its trunk each year is dependent on a combination of climate and ecosystem health.

A single tree ring encapsulates each year’s growth. Collectively, the tree rings provide a historical record of climate and ecosystem changes during the tree’s life. Trees communicate with each other via the mycorrhizal network and with humans by leaving a written record in their growth rings.

The Carolinas

Tree rings shed light on climate conditions stretching back to times long before official weather records started. Findings published in October 2021 examined 300 years of climate data from tree rings in longleaf pines across seven North and South Carolina sites.

The trees were sampled by extracting pencil-sized cores for examination under a microscope, so the trees were not damaged. Each year’s growth appears as a pair of light-colored and dark-colored wood bands, with the lighter wood representing early-season growth and the darker wood corresponding to late-season growth. The research focused on thickness variations within the dark-wood bandings. These dark bands correspond to hurricane seasons, and the thickness of dark wood varies with the amount of water available during an individual season — thicker dark-wood bands mean more rain.

By examining 300 years of tree growth, the researchers identified wet years, with excess rain, and dryer years. The tree cores provided a historical record of hurricane season rainfall, and they show a pattern of increasingly wetter conditions from the 1700s through to today.

The research conclusions identified a pattern of increasing hurricane-season rain, which was attributed to slower-moving storms. Tree rings were interpreted to indicate the forward speed of tropical cyclones is slowing, leading to increased rain and flooding as the storms stall over the Carolinas. The longleaf pines grow in sandy, well-drained soils, so only storms with intense and long-lasting rainfall provide enough rain inundation to affect the growth rings.

Tree rings speak to us about long-term changes in the coastal ecosystems of North and South Carolina. They provide direct and measurable evidence of changes wrought by Anthropocene climate change.

The American West

Tree rings detect wetter conditions, but they also provide insights into drought. The corners of Colorado, New Mexico, Arizona, and Utah come together in the dry reaches of the American Southwest. This area, known as the Four Corners region, is rich with harsh but beautiful desert landscapes.

Long before today, the area was home to the Anasazi people for more than a thousand years. Their civilization was as sophisticated as that of the Mayans. But somewhere between 1275 and 1300 A.D, the Anasazi disappeared, and the land was left empty. A single generation was all it took to end a millennium of civilization.

We glean insights into the fate of the Anasazi from humble tree rings, where dry years show up as patterns of thinner, compacted tree rings. If counted backward through time, the rings account for the last 1200 years, taking us back to the 800s when the first recorded megadrought ravaged the western USA. This long period of severe drought was a harbinger of repeated dry spells in the mid-1100s, the 1200s, and the late 1500s. The dry years of the late 1200s tie into the collapse of the Anasazi culture. As the sparse water resources of their desert homelands disappeared under the heat of a megadrought, the last of the Anasazi had no choice but to leave because, without water, there is no life.

Today we are 21 years into another megadrought, but this drought, unlike the previous ones, is exacerbated by climate change. Tree rings tell us stories worth listening to, explaining bits of long-forgotten history and warning us how the future of 40 million people is affected by the same fate that befell the Anasazi because, without water, there is no life.

Related Articles:

Water and Megadroughts: The rise and fall of civilizations (by WM House; Medium) 

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Do Trees Talk to Each Other? (By Richard Grant; Smithsonian) 

300 years of tree rings show just how badly hurricanes have soaked the Carolinas (by Kate Baggaley; Popular Science) 

About 40 million people get water from the Colorado River. Studies show it’s drying up (By Ian James; USA Today)

Riddles of the Anasazi (by David Roberts; Smithsonian) 

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.