By Norman Helie
There is considerable decline, dieback, and death of many tree species in New England. Many professionals focus their attention on the final stages of a tree’s life, and the research usually resorts to developing new hybrids with resistance to the causal agents and treatments that protect trees from pests. Let’s consider the iconic American elm tree of New England.
For more than 40 years, the American elm tree has been treated for its preservation with a fungicide as a protectant. However, Dutch elm disease transmission is exclusively vectored by bark beetles. Trees treated with fungicides permit the colonization of the vector. Suppose there are elevated bark beetle populations in high-density elm planting. In that case, if trees are either treated or untreated with fungicides, the trees will fail because the bark beetles can kill the trees without the fungus.
Elm trees treated with fungicides masks the effects of a looming problem under the bark. Both bark beetle and fungus can inhabit the same space; this propagates disease. Beetles can then carry the disease to other trees. It is best to break this union of beetle and fungus. They should never be together. Healthy trees receiving fungicide treatments can protect the union of fungus and beetles breeding. These trees are huge threats to the elm grove. They must be removed.
The annual average loss from bark beetles is 0.01 to 2%. Bark beetles will kill a few branches or an entire tree during eruptive years. High beetle populations will kill trees with or without the fungus. Hybrid trees are also subjected to bark beetle infestations. European elm bark beetles love all elm trees. However, even hybrid tree research is focused on Dutch elm disease resistance, not bark beetle resistance. Only bark beetles can move Dutch elm disease from one location to another. But is it the cause of tree decline? Many would be surprised to hear that elm trees were already at a steady rate of decline fifty years before discovering Dutch elm disease in America (Campanella, 2003). Let’s look at tree decline and how it begins with all trees, recognizing that each tree species has its own
How Tree Decline Begins
In the early 1980s, Manion wrote that tree decline begins with environmental changes. These changes are called predisposing factors. The factors are long-term pollution, climate, and soil change. The predisposing phase subjects the tree to inciting and contributing stages towards dieback and death. These two factors can be abiotic and/or biotic. When trees are dying, inciting and contributing factors are usually at work. The majority of tree diseases are studied when the tree is already failing. Sometimes tree decline begins with inciting and or contributing factors. A good example of an inciting factor is an insect that continually eats all the foliage for several years in a row. Contributing factors can be root rot after defoliation. This paper is focused on predisposing factors. Ecological succession and predisposing factors are similar. These studies are controversial and complex. Many foresters view succession as straight-out competition, and only the strong survive. In natural forest systems, there is a harmony of companionship and competition. The anatomical and physiological growth of trees is why environmental decline studies are so complicated. Trees are very complex long-lived organisms.
Predisposing factors can also be abiotic or biotic. A pathogen can kill a healthy tree. But the environmental decline is abiotic. It is challenging to see these problems in a living growing tree. Even with all the sophisticated equipment we have today, we can’t determine the whole tree’s health. Tree health assessment today is by outward appearance only. We look at the tree bark, branches, and live crown and determine its health. When we find disease or insects, we have them cultured or identified to prove that this is the cause of the decline. The fact is much of our tree loss is due to environmental change. Researchers attempt to link tree decline to the environment but never conclude a causal relationship. Why? We know it happens, but we are unable to explain it with diagrams and narration. It is therefore assigned a category of an unexplained tree disease complex disorder. This label leads to a quagmire of causal relationships with inciting and contributing factors. The complexity of this disorder lies inside the growing host and the developing soil. Knowing “how trees grow” helps us understand the beginning stages of tree decline. Tree decline must be seen through the lenses of tree anatomy and physiology; this will allow landowners to manage and plan for the future.
Soil weathering and development are related. Since soil development begins with parent material, it must always be considered when evaluating the big picture of tree decline in natural settings (Lutz and Chandler, 1946.) Also, tree decline studies should always exclude anthropogenic interference, even if man is the primary threat to trees. Studying trees in natural settings helps us see their needs and associations.
Soil formation begins at the surface and then down progresses to the bedrock geology. As plants grow in the soil, it alters their chemistry and physics. The soil is constantly changing. It becomes a living soil, but additions of organic matter completely change the mineral faction at the surface. However, down lower in the soil profile near the bedrock, the mineral soil remains unchanged. This soil has all the original material to support plant life as it began thousands of years ago, but it cannot support plant life at that depth. From topography, steep terrain brings nourishment to valleys and floodplains. Trees grow better in these nurturing environments; thus, an actual tree decline is slower to arrive in these unique niches. Soil formation changes over time. Still, time does not change the parent material. Unexposed bedrock is static. As erosion reveals stone, it becomes subjected to time. Climate and time alter more bedrock suitable for successional plant development. The bedrock is supportive of the past plant pallet, but climate, topography, and time have influenced plant growth and soil development. The study of tree decline in various terrains can be very helpful in our understanding of the tree.
Tree species like American chestnuts are eliminated from their once-thriving environment. This tree species is endangered due to soil formation that no longer supports its anatomical and physiological development. Chestnut blight disease is only a contributing factor to its death. The Appalachian mountain range was a niche home for the American chestnut tree. It could only be found in this specific region, which encompassed various climatic zones. But further south in the mountain range, there were bigger trees because of the longer growing season.
The Appalachian range has undergone some of the most significant soil transformations ever recorded. Its peaks were once a thousand feet higher than they are today. The soil in the valley is subjected to physical and chemical weathering. In these low point areas, this is where you can find a few American chestnut survivors. The best growing American chestnut trees in my woods are on south-facing slopes at the bottom of ravines with ample water and nourishment. This nourishment flows down from exposed mineral surfaces on the hillside. This species is susceptible to changes in the mineral content of the soil.
The American Chestnut
The American chestnut tree is a very large, fast-growing tree similar to the eastern Cottonwood. Most but not all fast-growing trees are very low producers of preservatives and defense mechanisms. Preservatives help trees defend themselves against attacking organisms. The American chestnut tree, however, produces large quantities of preservatives such as tannins and lignin. This tree was both a fast grower and a prolific producer of preservatives. Certain elements are constantly needed to produce secondary metabolites like tannin and lignin, and some micronutrients are necessary on a macronutrient level for this species. All these soil minerals were once abundantly available to the American chestnut tree in its niche environment.
As time and climate influenced the soil environment, the minerals were no longer available at sufficient levels. But, an inner tree lag response can occur because trees also store nutrient resources essential for their growth and survival. Tree decline is subtle because of this internal nutrient banking. When the internal resources reach insufficient levels, they begin to compromise localized areas of growth and development. For the American chestnut tree, these areas are in the vascular (phloem) and bark meristems on the lower bole and branches. Eventually, large areas weaken and become vulnerable to infestation and invasion of parasites like chestnut blight.
The disease that killed the American chestnut tree was already throughout the entire Appalachian range in the 1800s, even though healthier trees with more preservatives looked normal. The country was in the midst of a civil war, and forest health was of little concern to the public. There were no professional plant pathologists at this time. Concerns about climate change and soil erosion were not considered until the species was gone.
The great American chestnut trees were already dying, but no one noticed until 1904 in New York. The disease swept from the north to the south giving the appearance of an epidemic. The occupation of forest pathology began here. Pathologists concluded that an introduced pathogen struck billions of trees in just 35 years. The north-to-south movement appeared to move like a wildfire, but the disease was already throughout the Appalachian mountain range. Chestnut blight is not host-specific and can survive on numerous tree species like oak. Its swift north-to-south movement can explain the bedrock geology and the chestnut trees’ ability to produce tannins.
Tannin production was enhanced by the growing season further south and by fertile soil. The soil in the north offered less nutrition than the soils in the southern Appalachian range. These unique pioneering trees were ready to die by the end of the civil war.
The history of cyclic prolonged dry weather and the great dust bowl of the 1930s were inciting events for the decline. Chestnut blight was an endemic contributing factor. This combination makes it appear to be an epidemic caused by a new strain of a pathogen. Today this species could be grown from seed if all its primeval nutrient requirements were met. It does reach maturity when it is grown in Midwest clay-based soils. These soils will provide the nutrition but eventually will wane due to nutrient deficiencies. Furthermore, the leaf litter over the soil is very important to the chestnut’s existence.
We analyze foliage, and we never find a relationship between nutrient deficiencies and tree decline. We conclude later that dieback must be due to a specific disease. Dogwood, Ash, and Maple trees have been declining for more than 70 years now. These tree species are high-base recyclers. (Tree nutrient recycling is a matter of efficiency in retranslocation and utilization in all living meristems. The soil mineral fraction may become deficient, but the demand for nutrients increases in all growing meristems.) The fall litter from these trees is constantly returning base elements like potassium, calcium, and magnesium to the forest floor. Leaf nutrient analysis is never detected below normal levels. These strong base recyclers never skip a beat. No connection to decline from the environment has ever been made. Since all wood cell walls accumulate calcium during their development, maybe it is here that we would find compromised growth. This finding would be a physiological weakness. Again, we know there is a connection, but we wait until a visible pest strikes. Then everyone gets busy with treatments to save the trees!
Tree Life History
Tree life history strategies are the study of the production of secondary metabolites and tree development. Tree life history is scarcely mentioned in our profession. First, tree life history is not a recent study or a record-keeping of a tree’s size and condition. Tree life history is internal structural strategies and the role of defenses within the whole tree (Loehle, 1987.) Loehle was able to discern the relationship between tree longevity and the volumetric heat content of the wood. He also detected a relationship between the longevity of trees and their adverse habitats.
Growth rate is a predictor of tree life history strategy. However, the slowest growing trees live longer! Increased growth is considered healthy by our industry. We often measure volumetric growth as diameter at breast height (DBH). But, there are significant tradeoffs when considering volumetric growth as healthy growth. It is always assumed that as sapwood growth increases, heartwood growth must also increase; this may not always happen. The tree transforms pre-existing sapwood to heartwood for structural and physiological support. For slow-growing species, this takes enormous energy from the canopy of photosynthetic resources. When sapwood growth is increased, the resources may not be available for this defense strategy. This exposes the tree to infections until it can catch up when there are surplus resources.
Fast-growing trees just muscle their way to defense by overcoming disease with excessive growth of sapwood. Willow and poplar trees are examples of fast-growing species. However, as the fast-growing trees mature, their lives are shortened. Rot and decay invade these fast-growing species. The American chestnut grew very fast but relied heavily upon the production of tannins and other wood preservatives. This high tannin production increased the tree’s longevity by providing a superior structural support system (Tree life history.) The entire being of this tree was tannin production. Tannins were so high in this species that fallen leaves would suppress and control organisms in the soil (Comis, 2010.) This natural control in the soil system managed growth-promoting elements like nitrogen in the organic matter—tannins returning to the soil regulated the growth of the whole tree.
Under regulated growth, even the sapwood of the American chestnut had significant natural defenses. We should remember the whole bole of the tree was highly sought after for its very high decay resistance. Un-milled lumber could be stuck in the ground without any additional preservative, and as a fence post, it would outlast any other wood product. This quality speaks volumes about chestnut’s ability to produce preservatives and resist attack by pathogens. The environmental and ecological factors hamstrung this fast-growing species’ ability to protect itself from pith to outer bark.
The diseased chestnut tree on the left is displaying inner bark delamination. This anatomical failure is caused by impaired lignification. Unique fiber bands developed tangentially in the phloem tissue of this species (Evert, 2006.) When fiber is unable to lignify the phloem, it becomes weak and is easily torn apart by the weather. Secondary metabolites produce lignin. Specific nutrition like copper for enhanced lignification is constantly needed. Copper’s natural abundance in younger soil may have improved this tree’s life history before extensive soil weathering. This element and other micro-elements should be examined more closely.
Tree decline research needs the understanding of the whole tree, its life history of self-defense. Environmental soil development must be a priority when evaluating tree decline. The forest will change due to soil development and disturbances. Still, we should save the tree species with the science of plant physiology and anatomy for future generations to witness these great trees. Tree health can’t be determined by outward appearance and general foliar analysis. Environmental and ecological studies must consider tree life history.
Campanella T. J., 2003. Republic of Shade New England and the American Elm. Yale Univ. Press. Chap.8 pp. 146-147.
Comis, D., 2010. Tannins’ Surprising Benefits for Soils, Forests, and Farms
Evert, Ray F., 2006. Esau’s Plant Anatomy. Meristems, Cells, and Tissues of the Plant Body- Their Structure, Function, and Development 3rd ed. John Wiley & Sons Inc. page 414.
Loehle, C., 1987. Tree life history strategies: the role of defenses. Can. J. For. Res. 18: 209-222. Lutz H. J. and R.F. Chandler, Jr. 1946. Forest Soils. John Wiley & Sons Inc. chap. One pp. 7-9.
Manion P. D. 1981. Tree Disease Concepts. Prentice Hall Inc. chap. 18 pp. 324-329.
Feature Image Wikimedia Nicholas A. Tonelli
About the Author
As a young boy, Norm built numerous tree forts. He would also tunnel underneath the trees, where he made many observations about how trees grow.
These simple investigations lead him into a career in arboriculture. Using scientific literature gathered from various fields, he enjoys reading about plant anatomy, nutrition, physiology and soil sciences. Taking long walks in the woods with his dog Finn helps him process this information and develop solutions to many challenges of growing trees.
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