The basic factors that promote plant health include sufficient water, air movement, drainage, optimal temperature and light, and nutrient availability.  Too much or too little of any of these elements can cause stress.  Tree stress is a term used to describe any condition that causes a decline in tree health.

Early symptoms of stress might include reduced growth, abnormal foliage color, vigorous water-sprouting, or premature leaf drop.  The most common causes of tree stress are related to site and/or environmental conditions.

(From the International Society of Arboriculture)