The Value of Tree Rings: A Natural History of a Whitebark (Pinus albicaulis) Pine Cross Section
The tree rings in this whitebark pine (Pinus albicaulis) cross-section provide a view back in time. Scientists study the patterns of tree ring growth, scars, and wood coloration to determine how long the tree lived, record changes in climate, and even track past fires, insect attacks, and earthquakes!
Trees can grow for hundreds and, in some cases, even thousands of years. Each year, trees add a new layer of growth, known as a tree ring. These rings can be used to determine how long the tree lived, and they also record changes in temperature and precipitation, as well as more localized changes such as fire, insect attacks, and earthquakes. Changes in the thickness of tree rings result from seasonal and annual changes in water availability during the growing season. By analyzing patterns of tree-ring thickness, scientists have reconstructed annual variability in moisture and temperature over the last 14,000 years.
Below is an example of what tree rings can tell us about the life history of a tree, in this case a Whitebark Pine (Pinus albicaulis) from the Tobacco Root Mountains of southwest Montana and at an elevation of 8820 ft.
The life history of this whitebark pine can be interpreted by studying the patterns of tree growth through time, and physical and biological events recorded in cambial scars and wood coloration. The first six decades of life are defined by narrow ring-widths that were likely due to the short growing season experience by a small sapling tree. This is a common growth pattern in high-elevation conifers growing in snow-dominated environments due to the extended amount of time they spend below the spring snowpack.
1700s
After beginning its life in 1708, a notable early life event occurred in 1723, when the 15-year-old sapling remarkably survived a fire that burned and killed half of the cambium around the small stem. By 1780 the pine had grown large enough to access more light and a longer growing season, which increased the annual productivity of the tree resulting in larger growth rings and relatively consistent growth for the next several centuries.
1800s
The tree was again burned and damaged in a late-summer fire of 1863 during the “Civil War Drought”. This event is evident on the west-facing side of the stem where the cambium was scorched and killed, leaving a large charcoal lined scar. The pine responded by producing resins encapsulating the fire damaged area of the tree, and produced two lobes of wood around the edges of the scar over successive years of post-fire growth. Major fire scars around the base of trees are commonly referred to as a “cat face”. The exposed cat face was susceptible to moisture and rot, which penetrated the tree resulting in several inches of stained stem wood.
1900s
As the pine aged into the later half of its second century of growth (~1950 CE), tree-ring widths begin to narrow due to the rules of geometry (i.e., adding similar amounts of wood material to an increasing stem diameter), and a combination of environmental factors that increasingly stressed the tree (e.g., drought, fire and beetles). The fires of 1988 are discernable as several small scars around the stem, suggesting this drainage in the Tobacco Root Mountains had active fire activity during the extensive 88’ Yellowstone fires when large areas of the park burned. The changing climate of the northern Rockies in late-20th and early-21stcenturies also resulted in warmer drier growing seasons that reduced the growth of the nearly 300-year-old pine. In addition, fewer hard freeze events and longer warm seasons allowed for an extensive outbreak of the Mountain Pine Beetle (Dendroctonus ponderosae), which carries the blue stain fungus (Grosmannia clavigera). The fungal spores infest the phloem of the sapwood and cause the blue stain discoloration seen around the edge of the cross-section. Though it is unknown which of these factors was the ultimate cause of death, each stressor (i.e. drought, beetle, and fungus) likely contributed to the death of the tree in 2007.
Accurate Dating of Growth Years
Establishing accurate calendar-year dates on the cross-section was achieved through a technique known as crossdating, whereby common growth patterns across many trees are used to establish the exact year of growth for each tree ring. The dots on the surface of the tree allow for easy identification of specific years, decades, and centuries of growth, with specific notation defined in the key below.
• = 1 Decade (10 Years)
•• = 5 Decades (50 Years)
••• = 10 Decades (100 years)
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