We know that tree rings can reveal the age of a tree, but how do these rings form? And what else can they tell us about the life of a tree?
In temperate forests, with distinct growing and dormant seasons, trees form new wood annually in the form of growth rings. When viewed in a core or cross-section of a tree, these rings can tell a story about past disturbances and climate.
Botanically, wood is xylem tissue, a remarkable structure that conducts water and mineral nutrients to the foliage and also forms strong supporting tissue that allows trees to grow tall without collapsing. Wood is formed by the vascular cambium tissue just under the bark, splitting off new cells in a highly predictable seasonal pattern, beginning in May and ending in September. The wood that forms as new shoots expand is termed earlywood and has relatively larger passages (lumina) and thinner cell walls. As the growing season progresses and shoot growth stops, cell division changes to produce latewood, with smaller lumina and thicker cell walls. In some species, this transition is abrupt; in others, it is more gradual, but the contrast between the two wood types occurs in all temperate species and is why we see distinct annual rings.
Conifers evolved earlier than hardwoods, and their wood consists of tracheid cells with very small lumina. Hardwoods have larger vessels (or pores) that are specialized for water conduction. In ring-porous species such as oak and ash, the earlywood vessels have very large lumina, followed by dense latewood that gives these species their characteristic strength and their wood a “grainy” appearance. In diffuse-porous hardwoods such as maple and birch, large and small vessels are distributed more or less uniformly throughout the growth ring, and rings are more difficult to discern, often requiring hand lens or microscopic assessment.
Wood is synthesized from sugars produced in the foliage through photosynthesis that are translocated downward through the phloem tissue just inside the bark. Growth hormones, produced in newly growing tissue and similarly translocated, control the seasonal change from earlywood to latewood. To increase wood production of a tree, we can apply silvicultural treatments such as thinning and crop-tree release that allow the tree’s crown (branches with living foliage) to expand, increasing photosynthesis.
Annual variation in tree ring widths is strongly related to climate and can also be used to reconstruct past natural disturbances such as forest fires (from scars that heal over) and defoliating insects that reduce photosynthesis temporarily but do not kill the tree. In 2007, forest ecologist Shawn Fraver and I published an analysis of historical spruce budworm outbreaks based on a large sample of increment cores (Fig. 2) from old-growth red spruce and northern white-cedar stands in Big Reed Forest Reserve in northern Maine. By comparing radial growth (standardized) of cedar (a non-host of budworm) against spruce, we identified five distinct periods – beginning around 1976, 1914, 1808, 1762, and 1709 – where spruce growth was seriously suppressed but cedar was not, suggesting budworm outbreaks during the ensuing decades.
Some of the best historical information does not come from living trees but from timbers in historic buildings – the discipline of dendro-archaeology. In 2015, the Dakins Building in downtown Bangor was undergoing remodeling, and I managed to salvage the ends off a couple dozen large red spruce timbers. This structure was built in 1865 and was one of very few to survive the Bangor fire of 1911. The timber pictured above has a slight wane (at left), dating the outermost ring (the last year of growth) as 1864 or 1865. Counting inward from this ring dates the pith (upper right) to 1671, revealing a 194-year history that has long been gone from living spruce trees. This chronology clearly shows the 1808, 1762, and 1709 spruce budworm outbreaks, as well as other interesting growth patterns.