API Summer 2021



Objective evidence of biotic factors.

Changes in plant appearance as a result of abiotic and/or biotic factors.

For example: insect larvae, fungus, rhizomorphs, beetle galleries

For example: chlorosis, wilt, canopy dieback

One easy way to think of it is that biotic, which is Latin for “pertaining to life,” represents all living or formerly living components. Abiotic, which is Latin for “not living,” encompasses environ- mental factors, such as temperature, that can have long-term impacts on the survival of living organisms.

and frass (excrement), caterpillar webs, and exit/entry holes on the surface of a tree. A symptom is more subjective, such as “that tree looks pale,” which could be the result of many different fac- tors—making it difficult to diagnose the specific tree problem. A symptom is evidence of a tree’s response to stress when under attack from either biot- ic or abiotic (environmental) factors. Examples include off-color needles or leaves, wilting leaves, canopy dieback or decline, heavy cone production, and the formation of new sprouts along branch- es or trunks. Correlation is not causation. It is im- portant to note that correlation doesn’t always equal causation. Finding signs of biotic stressors may not be conclu- sive evidence that the observed pest is the agent killing the tree. A root cause, like drought, flooding, or even another pest, can weaken a tree, making it more vulnerable to subsequent attacks by a variety of things. Tree issues are often multi-faceted, caused by both biotic and abiotic factors and evidenced by both signs and symp- toms. A good arborist will diagnose the


These biotic and abiotic stressors gen- erate signs and symptoms that we can then observe and identify. Both signs and symptoms can indicate a potential issue. It can take a well-trained eye to distinguish these abnormalities. Some are obvious, like a mushroom growing on a tree trunk. Others can be more subtle, such as a slight discoloration of foliage. It is important to distinguish between signs and symptoms. A sign is objective evidence, such as “I found a mush- room on a tree trunk,” which can help diagnose a specific tree problem. A sign is an indication of the existence of a pest or disease, and typically points to biotic factors. Examples of insect-relat- ed signs include finding physical adult and/or larval insects on or in a tree, in- sect galleries beneath bark, pitch tubes

Top: Insect frass and pitch tubes are signs of insect infestation. Trees expel sap to push out insects, which forms thick pitch tubes.

Middle: Galleries beneath bark are unique fingerprints to insect species. With knowledge and training, you can positively identify insects from these intricate galleries.

Right: Canopy or branch dieback is a common symptom of abiotic stress. Reasons for this decline may be difficult to diagnose.

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