Chaparral
You’ve probably heard that here in California we have a Mediterranean climate, with hot, dry summers and cool, wet winters. This climate type is somewhat uncommon around the world, and even causes us to have summer deciduous plants, like the California Buckeye, which is dormant in the summer (not enough water, instead of too cold, as it usually is for the more typical winter deciduous plants). This climate has allowed for the formation of a unique type of biome, called chaparral. Chaparral environments are composed of shrubland that is adapted to infrequent but intense fires, which are sometimes necessary for seed germination for many of the plants native to this biome. These plants tend to have sclerophyllous leaves, which are evergreen, small, hard, thick, leathery, and often spiky. The oils, low nutrient content, and toughness of the leaves discourage herbivory (being eaten) and make them very drought resistant. The rate of erosion in chaparral is very high, leaving the soil nutrient poor, and therefore unable to support large trees. This biome is considered a biodiversity hotspot, meaning it has a very large number of endemic (found nowhere else on earth) vascular plant species, and has less than 30% of its original natural vegetation.
You may have seen some native California chaparral plants on your hikes around the Bay Area.
Only a few other places around the world (below, in green) have this unique climate type and have similar biomes as a result. You’ll notice that they form along similar lines of latitude, on the west coasts of different landmasses. In fact, similar climates, and therefore, biomes, tend to form along the same lines of latitude. For example, Mediterranean climates occur around 30-44 degrees north and south of the equator. This is because latitude affects the amount of sun exposure received, with the equator receiving the most, and the poles receiving the least. This means that the equator tends to be warmer and wetter (since warm air holds more moisture) and the poles tend to be colder and drier. Air from the warm, wet equator rises, and moves away from the equator. As the air gets cooler, the water condenses and falls as precipitation. All the moisture eventually is precipitated, and this cool, dense air falls back to earth. This dry air falls around the desert latitudes. Because of the tilt of the earth, this pattern moves north in the summer and south in the winter. In the summer of the Northern Hemisphere, dry air falls further north than usual, landing on areas with a Mediterranean climate, giving us our signature hot, dry summers. In winter, when the pattern moves south again, the dry area moves back toward the deserts and prevailing winds from the west blow moisture-laden air from off the ocean and onto land, where it falls as rain, giving us our wet winters.
Because of the similarities in climate, plants from other regions with Mediterranean scrubland easily become invasive in the chaparral of California, as they thrive in these conditions, but have no natural predators.
Maquis
The analogous biome in the Mediterranean Basin is called maquis, macchia, or macchie.
Some invasive plants from this region are the European olive (Olea europaea), and yellow star-thistle (Centaurea solstitialis).
Compare the similarities between this rosemary (Salvia rosmarinus) on the left, from the Mediterranean Basin, and chamise (Adenostoma fasciculatum) on the right, from California’s chaparral. The leaves similarly stay evergreen and are all rolled up into needles almost, to mitigate water loss. Despite these similarities, they are quite unrelated, with rosemary in the mint family (Lamiaceae) and chamise in the rose family (Rosaceae).
Matorral
Similar climatic conditions in South America produce a chaparral-like biome called mattoral.
Non-native plants that can be found in this region and also in California climates include the Peruvian pepper tree (Schinus molle) and tree tobacco. I noticed, however, that these species tend to be more generalist than the European species.
Monte negro* (Heliotropium taltalense) on the left, which grows in matorral, and sticky monkey-flower (Diplacus aurantiacus) on the right, which grows in chaparral, have very similar adaptations in their leaves. They both have very resinous leaves, which help deter insects and retain water. The leaves are also very wrinkly and curl inward, helping to retain water even more. These similar adaptations have evolved despite their lack of relatedness, with monte negro in the heliotrope family (Heliotropiaceae) and sticky monkey-flower in the lopseed family (Phrymaceae).
Fynbos
The chaparral equivalent in South Africa is called fynbos. It is the most biodiverse of all these regions, with over 9,000 species, 2/3rds of which are endemic.
A number of ice plant species are invasive in coastal chaparral, many of which come from South Africa, for example, the sour fig (Carpobrotus edulis). Another invasive plant from the region is cape ivy (Delairea odorata).
The king protea (Protea cynaroides) from South African fynbos on the left, and California cudweed (Pseudognaphalium californicum) from Californian chaparral on the right, are similar in that they have dry, hardy flowers that last a long time, leaving a bigger window for pollination to occur. It is easy to see why having flowers like these would be useful in such a dry environment. These are also very unrelated, with proteas in the Proteaceae (a plant family containing banksia and macadamia), and California cudweed in the sunflower family (Asteraceae).
Kwongan
Kwongan, or Mallee, in Australia experiences similar conditions to the aforementioned plant communities.
Many acacia and eucalyptus species from Australia are invasive in California. Examples include silver wattle (Acacia dealbata) and the sugar gum (Eucalyptus cladocalyx).
The aforementioned sugar gum (Eucalyptus cladocalyx) from the kwongan on the left, and the California bay laurel (Umbellularia californica) from the chaparral on the right, both have very glandular, oily leaves with a strong fragrance, which discourage herbivory. They are also thick, hard, and leathery, further making them less yummy, and also helping to retain moisture in their often dry climates. Again, these similar looking plants are very unrelated, with eucalyptus in the myrtle family (Myrtaceae) and the California bay in the laurel family (Lauraceae).
Personal reflections
I think it's really interesting to see the different ways plants have figured out how to survive in this climate. It is incredible to see how the vegetation in each of these places looks so similar, yet so different. Looking at these landscapes from a distance, sometimes I can barely tell which continent the photo was taken on! The fact that these plants have developed such similar adaptations despite being so unrelated (none of these plants are even in the same order!) means that these characteristics are a result of convergent evolution, rather than traits passed down from a common ancestor. These plants have truly figured out the best ways to cope with the hot, dry summers and the cool, wet winters of a Mediterranean climate!
* Thank you to the top observer of Heliotropium taltalense, Chilean iNaturalist user Juan Mauricio Contreras, for helping me find the Chilean name of this interesting plant.
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