Bonsai trees can lose leaves for several different reasons. These reasons include being shocked, not getting enough light, being kept in cold temperatures, or from over-watering or under-watering.
Bonsai trees that lose leaves from being shocked might have been left in a dark box for too long may signal that the environment has changed drastically. Returning the tree to a properly lit environment should bring the tree back to health.
Bonsai trees should not be left in direct sunlight, but too little light can cause the leaves to pale and grow in long, spindly shoots. Providing the right amount of light ensures healthy leaves. Artificial, fluorescent lighting can be used and should be provided for 10 hours per day.
Extreme cold or over-watering can cause leaves to turn black or brown on the tips. Tropical species of bonsai trees should be kept at room temperature inside the home. Sub-tropical species can withstand colder temperatures.
Watering properly is important to keep the leaves healthy. Water only when needed, not on a regular schedule or every certain number of days, to provide the best hydration for the tree and leaves.
Bonsai trees include broad-leafed and conifer species. Broad-leafed species can be either evergreen or deciduous with opposite (two leaves sprouting from the same point) or alternate leaves. Conifers are primarily evergreen, but there are a few deciduous species. The foliage on conifers includes both needle-like and scale-like varieties.