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Pachycereus is a genus of columnar cactus in the Cactaceae family. It is native to the southwestern U.S., Mexico, and Central America. One species, Pachycereus pringlei, is the tallest and heaviest cactus in the world. It is an important food source for bats and other animals. After evolving over thousands of years to survive harsh desert conditions, these cacti are threatened by human activities.
Cacti in this genus grow only 2.5 inches (6.3 centimeters) per year, but may reach a final height of 12-40 feet (4-12 meters) over the course of a potential 300-year lifespan. Protruding from the main trunk are numerous vertical branches that may extend to 5 feet (1.5 meters) across. Pachycereus thrive in U.S. Department of Agriculture Hardiness Zones 9-10, which means that the lowest temperatures tolerable temperature is 20°-30° Fahrenheit (-6.6° to -1.1° Celsius). The P. pringlei grows best in rich soil with a pH level from 6.1-7.8, i.e., mildly acidic to mildly alkaline.
These drought-tolerant cacti are designed to take advantage of sudden downpours. Pachcereus have a shallow root system and vertical ribs that expand and contract according to how much water the plant needs to store. The P. pringlei may store more than a ton of water in its trunk.
In late spring to early summer Pachycereus produces white, bell-shaped flowers towards the tops of the branches, usually on the southern exposure. These flowers open in the late afternoon and do not close until the next morning. They produce copious amounts of nectar. In the late summer, these cacti produce round, prickly fruits.
Bats and Pachycereus are interdependent. The bats feed on the cactus nectar during their southern migration. The cacti relies on the bat for cross pollination. As the bat feeds on the nectar deep within the flower, pollen sticks to its fur. Another cactus is pollinated when the bat feeds again.
During the late summer, bats return from their wintering quarters when Pachycereus is producing fruit. Bats and birds eat both the fruit and some of its 800 seeds. The cactus depends on the digestive juices of the animals to soften its outer seed coating. The softened seed coat enables germination once passed out of the animal's body. It takes about 1,000 germinated seeds to yield one surviving cactus.
Humans pose the greatest threat to Pachycereus. Clearcutting to make way for farmland and overgrazing cattle is the primary problem these cacti face. Research is underway to learn more about the mutually-beneficial relationship between this species and desert wildlife, and what the loss of the cacti would mean to the ecosystem.