A Biogeographical Premise to Expand On

More-or-less contradictory to the “plant community” concept, it appears almost universally true that no two plant taxa have exactly the same geographic distribution (i.e. range), even though there are a great many with roughly similar ranges, a few quite striking examples being

  1. plants of the eastern half of the U.S.
  2. plants of the California Floristic Province
  3. plants of many different islands or archipelagos

In the case of California, it is therefore rather remarkable that there are no less than four tree species—common and even characteristic “indicator” species no less—that individually and in aggregate practically define as well as delimit the “quintessential California”, i.e. the central/lowland portion of the California Floristic Province: Pinus sabiniana, Quercus douglasii, Quercus lobata, Aesculus californica.

To this short list may likely be added a number of other non-tree plant species, i.e. shrubs, forbs, monocots, etc. [check ranges of California plants to compile a more-or-less exhaustive list]

It seems unlikely that the huge Mediterranean Basin has many (if ANY) such ubiquitous “indicator” plants of any kind, much less trees.