ISA- Arborist Review

abiotic - pertaining to the nonliving parts of an ecosystem, such as soil particles, bedrock,

air, water.

abscisic acid (ABA) - a plant hormone that stimulates stomatal closure and promotes seed

and bud dormancy.

abscission zone - a wall of cells at the base of a petiole, that allows the leaf to separate from

the stem.

- two functions: leaf drop and to protect the region of the stem against desiccation

acceptable risk - the amount of risk that an owner is willing to accept before tree failure.

acidic soil - having a pH less than 7.0, in contrast with alkaline which has a pH that is higher

than 7.0.

actinomycetes - microorganisms that have some characteristics of fungi and bacteria. Soil

actinomycetes are very tolerant of water stress and contribute through their actions to that

"good earth" aroma after a rain.

action threshold - the point at which a pest requires the implementation of a management

tactic or control.