Plant Communities and Climate in Southern California

We are accustomed to thinking of Southern California climate as mild, but it is really anything but mild from the perspective of the remarkable native plants.

Lucinda McDade
RSABG Interim Executive Director

The image that is conjured by summer may be a sure way to separate native Californians from those of us who are transplants from other parts of the U.S. In the regions of California that lie within the California Floristic Province (CFP), summers are dry, hillsides turn golden and look like home, and umbrellas can be safely left in the closet for months on end (and you are guaranteed to have to look very hard to find yours come the first rains of fall). People from elsewhere—at least in the early stages of their adjustment as transplants—speak of missing thunderstorms and describe the golden hillsides as brown and unattractive.

Summers in California are, as we know, warm (to hot!) and dry (in coastal areas the aridity is mitigated by fog, at least in the early summer), whereas winters are cool and moist. In fact, the transplants have the right to claim that theirs is the more typical understanding of summer in that areas that share the Mediterranean climate regime with California are extremely limited in area worldwide. Only about 2 percent of Earth’s total land surface shares this climate regime with us and these areas occur in only four other places: southwestern (and a bit of south central) Australia, central coastal Chile, the western Cape region of South Africa and, of course, the Mediterranean proper.

We are accustomed to thinking of our climate in Southern California as mild, but it is really anything but mild from the perspective of the remarkable native plants. The climate challenges them to be physiologically active (to grow, flower, produce fruits and seeds) during the cool winter and spring months when there is moisture to support these plant processes. Just when it warms up, it also dries up and, although temperatures would otherwise be conducive to plant growth, only the most deeply rooted (or riparian) plants are able to reach the water necessary to support such growth. Most California native plants thus go dormant: they complete their growth and reproduction for the season (or forever in the case of annual plants) and our hillsides turn golden. The plants must manage to survive for five to seven months with very little moisture (and quite a bit of heat depending upon location).

Despite this challenging climate and the very limited extent of Mediterranean climate zones worldwide, these regions are all associated with remarkable plant species diversity. About 20 percent of all flowering plant species on Earth occur on just 2 percent of the Earth’s surface in these five regions. One way to think about this is in terms of species ‘density’ or number of species per unit area of land. In continental areas that are of similar latitude but that do not have a Mediterranean climate, there are 1.5-3 thousand species per million square kilometers (for reference, the state of California is 424,000 square kilometers in area). In marked contrast, Mediterranean climate zone regions have approximately 12 thousand to more than 90 thousand species per million square kilometers (the remarkable Cape region of South Africa lays claim to this highest number). Thus, for example, the flora of California houses almost a third of all plant species in the U.S. even though it accounts for only about 5 percent of the nation’s land area.

High species diversity in Mediterranean climate zone areas is accompanied by two patterns that can put these floras in peril: high levels of (1) endemism and (2) rarity. Plants (and animals) are said to be endemic when they occur only in a specified and often small area. Thus, many of the plants that occur in the California Floristic Province occur only in this area and are often further restricted in range within it. Plants with a restricted range are almost by definition rare, at least compared to those with broader distributions, but they may also be rare even within their range. You will immediately realize that these patterns can make our plants vulnerable to extinction; in fact, nearly 1/3 of the native plant taxa of California are ranked by the California Native Plant Society as rare, threatened or endangered.

Another very interesting feature of Mediterranean climate zone regions is that the plant communities are convergent: they look alike. Thus, our chaparral looks very much like the maquis of the Mediterranean, the matorral of Chile, the fynbos of South Africa, and the kwongan of western Australia. This pattern of convergence continues to the level of traits of individual plants, particularly as regards leaves, which are usually evergreen and tough no doubt as adaptations to surviving hot, dry summers (sclerophyllous is the technical term; think about our manzanitas). Interestingly, these patterns are true despite the fact that the plants of these five areas are largely unrelated taxonomically. The remarkable proteas of South Africa and Australia are not native here although the fact that many perform well here in cultivation is testament to our shared climatic regimes! Similarly, you will not find manzanitas in South Africa (except in cultivation) but you will find many many species of the genus Erica which belong to the same plant family, Ericaceae.

Those of you who have replaced your lawns with native plants may have experienced the disdain of neighbors when your landscape goes dormant in the summer. In fact, this is one of the major challenges that we face in winning acceptance for native plants in home landscapes: many do not look all that great during the hot, dry summer months. Considering that they will need very little to no water and will very rapidly return to lush beauty with the first rains of fall, I would suggest that it is asking too much of them to look good all summer too. Armed with this knowledge of the challenges that California native plants face in growing and surviving our mild-for-us but tough-for-them climate, at least you can explain what’s going on to you neighbors! These plants deserve our respect and admiration and a place in our landscapes!