By Lauren Lewis
The tree losing its leaves is arguably the most recognizable image of autumn. In temperate areas of the world, like North America, the most common reason that trees lose their leaves, a process called abscission, is to protect themselves from cold damage. The plant senses a decrease in daylight hours, and responds by withdrawing nutrients from leaves for storage (the withdrawal of green chlorophyll results in a yellow leaf), creating a layer of barrier cells between stem and leaf, and then letting the leaf detach. Here in SF you’ll see plenty of this going on pretty soon in the gingkos, stone fruits, London planes, etc.
But in plants native to Mediterranean climates like we have here in coastal California, the reason for leaf abscission isn’t cold, it’s dryness. Mediterranean climates occur in parts of Australia, central Chile, coastal California, South Africa, and of course the land around the Mediterranean Sea. These places have relatively mild temperatures year-round, dry summers, and wet winters. With mild temperatures, plants don’t have to prepare for cold by dropping leaves. Instead, the dangerous time for the plant is the dry summer, and one adaptation to this challenge is to lose leaves and go dormant during the driest months of the year. A great example is the buckeye (Aesculus californica), a staple of California’s native landscapes, whose leaves brown and drop in July. Purple sage (Salvia leucophylla) drops its large, springtime leaves during summer and replaces them with smaller, whiter leaves that reflect light and withstand heat.
This very regional pattern is made even more nuanced in San Francisco and other especially coastal parts of the Bay Area, where summers are characterized by fog and an even smaller temperature range. Plants here get some summer moisture from the fog, so they’re under less pressure to go dormant, and their dormancy can be shorter or less extreme. The first winter rain, which is possible in October, jolts dormant plants back into growth mode.
Climate change is producing changes in dormancy patterns by way of hotter summers and drier winters. A recent study of California perennial grasses noted that non-native annual grasses have recently been out-competing the once-dominant perennial species in California landscapes. The study showed that perennial species with more pronounced summer dormancy characteristics, like earlier reproduction and shallow roots, are similar to annual grasses in those characteristics, and therefore might be more competitive as droughts worsen and dormancy is made more advantageous than before. In other words, recent success of annual grasses suggests that perennial grasses that “mimic” annuals by going dormant in summer are likely to have higher survival as the climate changes. Landscape restoration efforts would therefore do well to promote summer dormant plants.