What type of environmental changes cause trees to loose their leaves?

The plant hormone, ethylene. It is involved in fruit ripening and leaf abscission.
References :
http://www.plant-hormones.info/ethylene.htm

fall!
References :

1, drought would stress trees enough for them to lose their leaves.
2. shortening days signals the tree to prepare for dormancy
3. freezing temps for many trees will make the leaves die
4. nutrient loss will also stress the tree enough to lose leaves
References :
experience

Trees generally shed their leaves in autumn and winter due to low availability of nutrients and food. Also this helps in getting rid of old and withered leaves. When spring comes, the trees come back to life with plenty of food and good weather for the new borne leaves and blossoms.

Dry weather, drought affects the tree and due to scarcity of water, rains they may start losing their leaves. Soil with poor nutrients may also impact the trees.
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