Doubtless this is often all that can be done, but there are cases in which such alien plants are probably true immigrants, recent and introduced by human agency, although their journey has been but from one neighboring zone to another. It is not impossible that historical research may sometimes give a clue to such unexpected appearance of plants away from their proper habitat. I have in mind an instance which suggests such a solution.
There is in the San Bernardino Valley a certain tract of alkaline soil, something over a mile in length, and somewhat less in width. It is moist, and the lowest part gives rise to a small stream. It is covered with Distichlis, and on it grow Suaeda torreyana, Nitrophila occidentalis and Hemizonia pungens, all of them common nitrophilous plants of the valley. But here also are found four other species of like character which belong normally in adjoining phytogeographical subdivisions. There are other alkaline tracts in the valley, apparently entirely similar to this one, and otherwise inhabited by the same plants, but these four aberrant ones are to found here only.
One of these plants, Aster carnosus, belongs well out in the desert, and has been found nowhere else in the cismontane region than here. The others are coastal plants. They are Suaeda minutiflora, Cuscuta salina, and Scirpus pacificus. The last named grows along the edges of the little stream. The Cuscuta is parasitic on Nitrophila, and has not perceptibly increased or diminished during the twenty odd years that I have observed the tract. The other two, which at first were not abundant, and grew only near the streamlet, have now spread for a mile along the public road which traverses the tract, and are plentiful.
Now it happens that by this runlet of water were once situated the home corrals of one of the nomadic bands of sheep, which in early days wandered so freely over southern California, visiting at times the coast, and even the deserts. It seems not improbable that to those sheep may be due the presence of these aberrant plants; that here they are as truely emigrants as if they came from Europe, instead of only from neighboring subdivisions of the same life area.
In three short paragraphs, Samuel Parish discusses acutely the differences between native plants and alien plants. The nuances between the two kinds of plant categories can be subtle and difficult to distinguish. It requires a knowledge of geography and history, along with botany. When I first read the article it appealed to me because it is a bit like the case surrounding the very grand California native tree known as the "Coast Redwood." So I asked myself: "What is the history of the Redwood in Los Angeles?" I know that the Redwood Tree is native only from central California to southern Oregon. However, when these trees are grown in the Los Angeles region for ornamental landscaping or are now just left-over plantings in the mountain canyons from 50 years ago or so, they cause ecological damage to southern California. These California native trees are then actually alien trees, considered as bad as weeds in any home garden, but in this case in the Santa Monica Mountains wild garden. The redwood trees take up space where true Los Angeles native elfin trees (chaparral) would otherwise prosper. Another way to look at the situation is to realize that the genuine native trees of Los Angeles would be better than Redwood trees in canyons. These native trees of Los Angeles canyons include the Arroyo Willow, California Walnut, Coast Live Oak, and White Alder. Thus, the solution is simple and it is to cut down the twenty some-odd Redwood trees in the National Park administered site of the Santa Monica Mountains National Recreation Area known Solstice Canyon Park and replace them with Arroyo Willow, California Walnut, Coast Live Oak, and White Alder. In fact, these five trees already grow adjacent to the Redwoods and would like to expand into the area of the Redwood trees. However, the Redwood trees are aggressive and will not give up their space easily to the native trees of Los Angeles. It won't be easy to remove the Redwood because the Redwood is a beautiful tree. The Redwood is a sacred tree to most Californians, and of course trees in general are seen as good things. However, we would remove only one kind of tree and then replace it with four other trees. The native birds, native mammals, and native insects that couldn't find a home in the Redwood would now be more successful with a native forest with natural integrity and wild beauty. The insects, with their eggs and young larvae, would drop off these native trees occasionally and into Solstice Creek where they might become food for the endangered Steelhead Trout. There are no insects living in the Redwoods at Solstice Canyon. Also, the leaves of Redwood trees acidify the water of Solstice Creek, whereas the native trees would not acidify the stream water and soil. Thus, the Steelhead would do better if the Redwood were gone in this way as well. In fact, there are not only no native Redwood in the Santa Monica Mountains, but there are no native conifer trees whatsoever. Any tree that is a pine, cypress, redwood, fir, or spruce that you see growing in the Santa Monica Mountains is an un-natural and non-native alien tree. I think that these are just some of the subversive and subtle ecological points that Samuel Parish was trying to make in his essay of 93 years ago.
Lastly, the sheep that Samuel Parish mentioned are no more. Los Angeles and other southern California counties are now nearly rid of sheep, also known as hooved locusts to John Muir. The few alien-evil sheep left in LA County are a few on Malibu Canyon Road in the city of Calabasas, and a few in the San Gabriel Mountains, where that horrible federal agency, the Angeles National Forest still allows them to rape our federal lands. Perhaps the National Forest could be transferred over to the National Park Service or California State Parks. At least the policy of the National Parks is to remove all sheep from its lands. If only the National Forests could get out of the "Dark Ages." Impossible I say. Therefore, we need to choose to shut down all National Forests and make them National Parks, starting first in southern California. Reading Samuel Parish's essay is so enlightening to learn that in 1908, sheep raped our coastal wetlands, interior wetlands, and beaches along the Los Angeles and Orange County Coast. Unbelievable, or almost unbelievable except for that premier Los Angeles Botanist named Samuel Parish that wrote us that exposing essay of 93 years ago. Perhaps we could enlist the help of the Sierra CLub, whose logo is the Redwood, to remove the Redwood Tree and replace them with native trees. Perhaps we could transplant the Redwood trees at Solstice Canyon back to central California, where they belong and would thrive better and for a 1000 years. I estimate that the Redwood trees at Solstice Canyon will not live too many years since this is not their natural environment. Both Samuel Parish and John Muir loved southern California's native trees, native forests, and all native plants. They would have approved of moving the Redwood trees at Solstice Canyon, so that native trees could begin to grow in their place.