Trees don’t die, they become less efficient, eventually reaching the point where the physiological cost of staying alive exceeds its overall ability to carry on photosynthesis and translocate the products of photosynthesis to living cells that require energy. When this happens, tissues begin to shut down. Energy sinks within the tree are the first systems to go, and these are leaves and fine roots.
Flowers are also an energy sink, but when a tree is stressed, it quite often diverts energy to flowering and seed production. When this happens, the tree is further stressed and its total demise is imminent. No one knows why this an already stressed tree will suffer further stress for the sake of flowering, but it is believed to be a survival mechanism, attempting to assure that the tree’s DNA is passed on to a new generation.
Often, a heavier than normal flower set is the first indicator that a tree is suffering from stress. Another early indicator is crown dieback, usually from the top branches then down through the central core of the tree. Branch dieback from the bottom up is perfectly normal. It is the tree shedding foliage, usually shaded from above, that is no longer capable of paying the energy bill. Referred to as “natural pruning,” the tree is actually ridding itself of tissues that cost more energy than the branch is capable of supplying.
Of the five primary systems that make up a tree—roots, trunk, branches, flowers and leaves—it is roots that are arguably the most likely to cause trouble for the tree, but it is also the one system we can’t monitor. Even though healthy roots are essential to the overall well-being of a tree, it is often root failure due to insects, disease, environmental conditions or injury that sets the stage for crown dieback and other signs of decline.
|Photo by Thom McEvoy.|
|A root system of two sugar maple seedlings, the larger of which is 12 years old, the smaller approximately seven years old. Note the presence of decomposing leaves in the soil surrounding the fine roots. With the exception of a few coarse scaffold roots near the top of the root system, it is impossible to determine which roots belong to which seedling.|
Roots comprise about 20 to 25 percent of a tree’s total weight, which is roughly equal to the combined weight of branches, leaves and twigs. The surface area of roots, especially fine roots, is many times that of aboveground parts, which makes sense because roots rely on surface contact with soil to perform three of their five main functions: water uptake, mineral nutrient foraging and anchoring. A fourth function of roots is storage, a place for the tree to hold resources during dormancy so they are immediately available when the aboveground parts start growing again in the spring.
The fifth function of roots is hormone production. Trees respond to circumstances around them by balancing the production and export of hormones produced in roots and branches. This balancing act serves as a mechanism to allow trees to respond to changes in the environment in such a way that it appears as though the tree is thinking. For example, in most hardwood species, when the main stem is severed from the roots, the absence of hormones that suppress the development of buds in the stump or in the roots will cause those buds to sprout. Buds that develop from roots are known as suckers. Aspen is an example of a tree that will sucker when the stem is removed. Buds that form on stumps, or that emerge from the area where the stem meets the ground, are called stump sprouts.
Sprouting is a survival mechanism that allows an otherwise intact root system to quickly develop a new top since roots need energy, and trees need light to produce energy in the form of complex sugars. Sprouting is also an indicator of stress. When otherwise healthy trees start to sprout from the root crown, something has triggered the sprouting response, indicating that something is amiss.
Water and nutrient uptake are such an important function of roots that the distribution of these resources in the soil will determine the extent and pattern of rooting. For example, root elongation is usually two to three times greater in coarse, sandy soils than soils of finer texture, but total root mass and volume is often less. Why? Because in poor soils, roots must forage for resources, while in good soils, more root surface area means better access to resources and the potential for faster growth. On good sites, roots tend to be branchy and compact, but the opposite is true on poor sites, where roots are more extensive and lanky.
In virtually all circumstances, nearly 80 percent of root volume is in the top 8 inches of soil. Even for trees growing in sod, the top layer of soil is mostly humus consisting of organic debris in various stages of decay. Just below the sod is a high concentration of fine roots, also known as feeder roots. It is the feeder roots that tap into nutrients, which are byproducts of decomposition.
If you could somehow look below the soil surface with the same clarity and detail of the aboveground scene, it would look like a randomly woven tapestry in three dimensions with a lot of loose ends. About the only noteworthy pattern to this scene would be an increasing order of size for roots to the point where they join individual stems at ground level. Pick any fine root, or even any root several orders larger in size, and you would be guessing to determine which tree it belongs to. The beauty of this otherwise chaotic scene is that it doesn’t matter.
Roots graft on to one another in such a way that resources, sometimes toxins and pathogens, are shared. For example, if one could flag an essential nutrient and place it in an isolated pocket of soil, it would eventually show up in varying concentrations much further from the source than expected. This sharing, coupled with the multiple functions that roots perform, is why scientists coined the term root systems. In a wooded landscape, tree roots are everywhere below the surface and the parts that are most biologically active are also the parts that are most sensitive to disturbances like soil compaction, especially in fine-textured soils. Even the weight of human footprint is enough energy, on a wet soil in a forested landscape, to shear fine roots, compromising their ability to supply water and nutrients and leaving the roots susceptible to pathogenic organisms. Fortunately, roots are resilient and minor disturbances have little effect on an otherwise healthy tree, but when more than a third of a tree’s roots are damaged or lost due to excavations, expect to see symptoms of dieback even in the thriftiest specimens.
The author is a professor and extension forester with the Rubenstein School of Environment and Natural Resources at the University of Vermont