James MacDonald
JSTOR
The California desert is awash in color. Following a winter of exceptional rain, desert wildflowers have bloomed en masse, carpeting the normally drab landscape with a riot of bright blues, yellows, and reds. The rare phenomenon has drawn delighted tourists from around the world (some of whom, unprepared for the heat, are passing out). The event is dubbed a “super bloom.”
What causes a super bloom? The short answer is evolution. The desert is a tough environment for a plant. Accordingly, desert plants have evolved a variety of ways to cope with heat and dryness, such as thick succulent leaves which can store water (e.g. cactus) and a special form of photosynthesis where gas exchange occurs at night to prevent water loss (also cactus).
Desert wildflowers are mostly annuals, which grow, live, produce seed, and die over the course of one year. If an annual plant is going to grow, it needs to be certain that it will be able to complete a life cycle and produce seed, or else its ability to pass on its genome—its fitness—is compromised. To make sure that it can find the right conditions for optimal growth, a desert annual seed can lie dormant for years, even decades. There is a compound in the seed coat that inhibits growth. The seed cannot germinate until the seed has been exposed to sufficient rainfall to leach the growth inhibitor out of the seed. Prime conditions for one seed will work for others, so when the right conditions come along lots of plants take advantage at once.
What counts as enough rain to really get those seeds germinating depends on temperature. In hotter deserts, more rain is required since evaporation occurs; under cooler conditions, less precipitation is necessary since evaporation is reduced. Basically, water use by plants is more efficient when it isn’t so hot. Conditions for a “good wildflower year” occur roughly every 5 to 7 years. This time frame corresponds roughly to ENSO (e.g. El Niño) years, when wetter conditions come to California. Exceptionally good years come maybe once a decade.
Since the conditions for a super bloom are local, a boom year in one area may be a dud in another. This year’s bloom is localized in Southern California, and is not connected to ENSO; these conditions are truly exceptional. Following years of punishing drought, dormant seeds were raring to go. At the same time, the tongue of moisture from the Pacific known as the Pineapple Express has caused exceptionally heavy rains and lower temperatures throughout large swaths of California. For a wildflower, the combination is like hitting the lottery. Even a brutal drought has a silver lining.