Lemon trees experience leaf loss or defoliation for a number of reasons--most of them as a result of environmental stress or problems endemic to the citrus-fruit tree genus. Growing a healthy lemon tree requires a specific climate range and environment.
Lemon trees are highly sensitive to temperature. They grow nearly year-round, do not go dormant and are unable to cope with or rebound from exposure to cold temperatures. Lemon trees will be at least partially defoliated if exposed to temperatures of 24 degrees Fahrenheit or below. Heavy frosts also cause partial defoliation. Freezing temperatures below 20 degrees kill portions of the lemon tree's woody tissue, making defoliation permanent at the damaged location.
Lemon trees grown without protection from regular winds can be defoliated and scar over time. Lemon trees are sensitive to wind damage and lose leaves when exposed to a windstorm.
Without sufficient amounts of manganese in the soil, chlorosis and leaf drop can result. The field tissue of the leaves between the veins turn yellow and begin to rot, and the leaves fall. Chlorosis also can be brought on by an overly alkaline soil that blocks the tree roots from absorbing the manganese already present in the soil.