By Rick Thomason

University of Tennessee
Johnson County Extension Director

The answer to “Why do trees die?” follows a reverse chronological sequence.  Trees die because respiration terminates.  Respiration terminates because carbohydrate production ceases and stored carbohydrates are depleted.  Carbohydrate production ceases because photosynthesis discontinues.  Photosynthesis discontinues because the factors necessary for photosynthesis are interrupted or obstructed.  Those factors include: sunlight, water, nutrients, temperature, carbon dioxide and oxygen.  Factors for photosynthesis are interrupted because of human activities or environmental changes.
To understand why or how trees die, we must first understand the processes by which they live.  These processes can be categorized under physiology, which is the branch of science dealing with the functions of living organisms and their parts.  Major physiological processes in trees include photosynthesis, respiration and translocation.  The process of photosynthesis combines carbon dioxide with water in the presence of the sunlight to produce simple sugars (known as carbohydrates) and oxygen.  This chemical reaction for photosynthesis occurs in leaves.
Photosynthesis is the most essential and basic physiological process, inasmuch as tree growth is dependent upon successful conversion of the sun’s energy into carbohydrates.  Carbohydrates are the substances by which all other organic compounds are synthesized.  They are the chief building blocks of cell walls and they form the starting point for synthesis of fats and proteins.  They are oxidized in respiration and any amount still remaining after all these processes accumulates as stored food reserves.
Carbohydrates are transported from the leaves to the stem and roots via phloem cells located inside the bark in the trunk of the tree.  The tree uses carbohydrates in respiration and other physiological processes, including growth.  Excess carbohydrates not used in growth and respiration are stored in roots, buds, stems and the cambium layer just inside the bark of the tree.
Respiration is the oxidization of carbohydrates to provide energy to keep cells alive and to fuel growth.  Respiration essentially works in reverse order of photosynthesis, whereby the synthesized carbohydrates react with oxygen to produce carbon dioxide, water and energy.  Unlike photosynthesis, which is seasonal in most climates, at least some respiration occurs at all times (even during the dormant season).
This is why the production of carbohydrates through photosynthesis must exceed the oxidation of carbohydrates through respiration.  Without a surplus of carbohydrates, tree vigor declines and eventually death occurs.  As trees age, the demand for carbohydrates increases, because the volume of respiring tissue increases while the amount of leaf surface area (photosynthesizing surface) remains fairly constant.  Less carbohydrates are made available for root and stem elongation because more is demanded for life-sustaining respiration.  Perhaps this is why younger trees, having a higher ratio of photosynthetic surface to respiring tissue and grow more rapidly than older trees.
Translocation, the third major physiological process, allows photosynthesis and respiration to function properly.  Without the “piping” system of translocation, moisture and nutrients would not reach the leaves, leaves would not produce carbohydrates, carbohydrates would not be transported to organs and respiration would cease.
Through translocation, trees allocate carbohydrates to support five different physiological processes.  These processes placed in priority order for allocation of carbohydrates are:
•         Maintenance of living tissue (respiration)
•         Production of fine roots,
•         Flower and seed production,
•         Primary growth (elongation of branches and roots)
•         Secondary/diameter growth (growth of xylem – the water-conducting cells)
When a tree is healthy and rapidly growing, each of these five processes is fueled by ample supplies of carbohydrates.  Because secondary growth is the last to receive carbohydrates, wide annual growth rings of the lower trunk indicate that the needs of the other four processes are first being met and that excesses are being used for diameter growth.

At such point, life for a tree is good.  If, however, annual growth rings (secondary growth) begin to show a narrowing, this is a first indication that tree vigor is declining and that subsequent reductions in primary growth could also soon occur.
For instance, if a tree must allocate carbohydrates to either branch and root expansion, or seed and flower production, it will choose the latter.  Likewise, production of fine roots comes before seeds and flowers and lastly, respiration is a higher priority than fine root production.
This reversal or recall of carbohydrates continues until there are essentially none left, at which point mortality occurs.  Tree mortality is not always a gradual, energy losing process.  Tree mortality can also occur rapidly through mechanical disruption. Examples include:
•         severing the cambium layer inside the bark – disrupts translocation
• compacting soil – reduces availability of water and nutrients, resulting in poor aeration (oxygen content) in the soil needed for root respiration;
•         damage to or loss of larger limbs – reduces photosynthesis and carbohydrate production
A tree growing in a suitable climate and on suitable soils will continue increasing in size until one or more factors for growth are no longer available.  More often than not, environmental factors work concurrently or sequentially to weaken trees, predisposing them to other insect, mite and disease agents, in turn leading to mortality.
So why do trees die?  Their death follows a reverse chronological sequence.  Trees die because respiration terminates.  Respiration terminates because carbohydrate production ceases and stored carbohydrates are exhausted.  Carbohydrate production ceases because photosynthesis discontinues.  Photosynthesis discontinues because the factors necessary for photosynthesis are interrupted or obstructed.  Factors for photosynthesis are interrupted because of human activities or environmental changes.