There are many reasons why an outdoor plant's leaves may turn yellow and possibly die. Sometimes this yellowing process is perfectly natural, and other times it is caused by environmental changes or pests.
Every year in the fall, deciduous perennial plant species enter a state of dormancy, where they shut down their metabolism for the winter. As winter nears, plants and trees absorb the nutrients in their leaves, then shed the leaves to the ground below. With the coming of spring, deciduous plant come out of dormancy and grow new leaves for the coming year.
Leaves may turn yellow on plants if they do not receive enough water. Dropping leaves is a defensive mechanism by the plant to conserve water because plants easily lose water through their leaves. If a plant remains without water long enough, all leaves will turn yellow and die, as will the entire plant.
Environmental variables like heat spells, frosts, drying winds and sun burn can cause outdoor plants to become stressed, causing the leaves to yellow and drop. Certain plants like ornamental ficus trees will lose it's leaves simply by moving its location in the garden. Once the environment becomes stable and the plant becomes acclimated, most plants should recover.
Disease or pest infestation may also cause a plant's leaves to turn yellow. Common pests notorious for this are spider mites and aphids. These small insects suck nutrients from leaves, stem and roots of a plant. Fungal infestations like root rot and virticillium wilt will also cause discoloration and dropping of leaves.
Nutrient deficiency will also cause yellowing of the leaves. This happens when the plant draws the nutrients the soil lacks from it's own leaves. The most common deficiency is Nitrogen deficiency, which causes all leaves to turn light yellow and growth to be stunted. Iron deficiency causes yellowing between leaf veins, and potassium deficiency causes leaf tips to yellow and die-back.