You might expect that plants hoping to thrive in California’s boom-or-bust rain cycle would choose to set down roots in a place that can store lots of water underground to last through drought years.

But some of the most successful plant communities in the state — and probably in Mediterranean climates worldwide — that are characterized by wet winters and dry summers  have taken a different approach. They’ve learned to thrive in areas with a below-ground water storage capacity barely large enough to hold the water that falls even in lean years.

Surprisingly, these plants do well in both low-water and rainy years precisely because the soil and weathered rock below ground store so little water relative to the rain delivered.

“The key point from our study is that, in many sites on the North Coast, the storage capacity is small relative to how much it rains,” said Jesse Hahm, a graduate student at the University of California, Berkeley, and one of two first authors of the study. “Because the capacity for the subsurface to store water over the wet season is small, it still rains enough, even in the dry years, to replenish the water supply. The limited below-ground storage capacity is the key mechanism that decouples the plants and how much water availability they have in the summer from big swings in winter rainfall.”

As a result, these plants are much more resilient in drought years, as evidenced by California’s relatively unscathed North Coast during recent droughts that killed hundreds of millions of trees in the Sierra Nevada.

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