I) gradually diminishing health or condition of a tree. 2) a pattern of decline conditions in a particular species not necessarily associated with a single cause. e.g.. ash decline.
Merriam-Webster Online Dictionary
decline (verb)intransitive verb
1.
archaic to turn from a straight course - stray
2.
a) to slope downward - descend
b) to bend down - droop
c) to stoop to what is unworthy
3.
a) of a celestial body to sink toward setting
b) to draw toward a close - wane the day declined
4.
to tend toward an inferior state or weaker condition - his health declined morale declined
5.
to withhold consent
6.
transitive verb
to become less in amount - prices declined
1.
to give in prescribed order the grammatical forms of (a noun, pronoun, or adjective)
2.
obsolete
a) - avert
b) - avoid
3.
to cause to bend or bow downward
4.
a) to refuse to undertake, undergo, engage in, or comply with - decline battle
b) to refuse especially courteously - decline an invitation
1.
the process of - declining
a) a gradual physical or mental sinking and wasting away
b) a change to a lower state or level - the decline of the aristocracy
2.
the period during which something is deteriorating or approaching its end - an empire in decline
3.
a downward slope
4.
a wasting disease , especially pulmonary tuberculosis deterioration
decline (Wikipedia)