Despite persistent rumors to the contrary, Portland, Oregon is not actually a land of eternal rain. Although the large number of mountain ranges in the Northwest means it is home to a number of microclimates, the climate in most of the Willamette Valley is Mediterranean. Although there are very wet winters, the summers are hot and dry. This means, that while there are many pine forests in the foothills of the mountains, and riparian forests along the waters, that most of the Willamette Valley itself, from Eugene, Oregon north to the Portland area, and continuing in places up to Vancouver, BC is an oak savanna, with drought-resistant oak trees (mostly the eponymous Oregon Oak), interspersed amongst grass and shrubs.
So, where the mystery lies is the fact that while the oak trees dominate the landscape throughout the Willamette Valley, through the Portland area, and northwards, they are missing from a three-to-five mile radius of the city of Portland itself. The only oaks commonly seen in Portland itself are ornamentals
The first answer to this riddle is that it isn't even a real question, that it is merely a sampling artefact, the fact that an amateur naturalist like myself has noticed something while tromping around doesn't make it real. If you wish to wait for a scientific sampling, you can stop reading now. However, since it is I think a real case of a plant going from predominant to missing, it seems to be more than just my imagination.
Since the basis of the oak savanna is the fact that the oaks are drought-resistant, my first two guesses about why this could be would involve water.
Portland has one major obvious microclimatic feature: The Portland Hills, sometimes called The West Hills, a range of hills about a thousand feet tall that run along the west edge of Portland. While a thousand feet is small (at least by West Coast standards), it is still large enough to have visible climactic differences: it often gets rain and snow at times when the rest of the area does not. However, there are a number of reasons why the West Hills don't make a clear reason for the missing oaks. For one thing, they mostly pick up more rain and snow during the spring and fall: times when the rest of the area is getting sufficient rainfall. It is not the amount of rain that falls during the wet season, but the ability of plants to withstand the short, but intense dry and arid season that makes them drought-resistant. In addition, at least in the basic theory, the area of more rain should be on the hills and to the west of them, not to the east of them. Also, there are many other hills of various sizes throughout the Willamette Valley, and none of them seem to have a noticeable effect on
the presence of oaks.
Another major difference between the Portland area and other areas around it is the fact that Portland is built up on river sediment, while much of the area around it is the remnant of various volcanic events, such as the Columbia River and Boring Basalts. This volcanic soil tends to be much shallower and less developed then the rich sedimentary soil around the Willamette, which goes hundreds to thousands of feet deep. Apart from any questions about the presence of trace elements in the soil, it would seem that the alluvial soil could hold much more water, and thus help plants last out the period of drought in the summer. A good theory, but also one with exceptions both way. Slightly north of Portland, at the confluence of the Columbia and Willamette Rivers, lies Sauvie Island, the largest alluvial island in the United States. The soil on the island was layed down by the two rivers, and before dykes were built (and sometimes after), the island would end up underwater. And yet, the island has large areas where oak trees predominate. On the other hand, Mount Tabor, an intrusion of basalt rocks about three miles from downtown Portland, is bare of oaks, even though it would seem that it would be full of them if thin soils were the key to their growth.
Apart from natural reasons, (which so far don't seem to bear out), another obvious answer might lie in the human impact. When European settlers came to Oregon, they were not noted for their environmental consciousness, and many trees were cut down to build with, or to clear land for farming. It would be a good guess to think that perhaps the mighty oaks that once grew around the area were all chopped up a hundred and fifty years ago, and replaced with farms and then later with ornamentals. There are two problems with this theory: first, the settlers obviously kept many trees, because there are many old trees in Portland, and it would have taken some determined (and demented) settlers to go through and kill all the oak trees. In addition, at the time Portland was settled, one hundred and fifty years ago, other cities around the area were settled. Oregon City, Oregon was an important city before Portland was, and was an early industrial city, and yet there are plenty of oaks left standing.
Of course, the human habitation of Oregon goes back much further than the European presence. The Native Americans had an impact on the landscape, and I have read that they practiced burning to keep the land open, and attractive to game. It could be that other than climate, the oak savannas were the result of controlled burns by native populations over hundreds of years. However, other than the lack of evidence about the scope of these burns, this still asks the question: why wasn't Portland included in this project?
It seems then, that the mystery is left unsolved, but open for those with a liking of geography or biology to ponder.
Since I originally wrote this, I have come to read much material that states that Native American burning of the Willamette Valley to produce a certain landscape is an established fact. So the question now becomes why the immediate Portland area was not burnt like the rest of the Willamette Valley was.