transition

Today, driving south from Ferndale to San Francisco, I watched familiar scenery out the window. I do this drive almost every month, and have come to know it quite well.

At the northern end, it's mostly conifer forest. Douglas fir, with redwood in some areas especially on terraces of the Eel River. Except for coastal terrace prairie right on the coast, grasslands are pretty much limited to certain ridge tops, although they're almost certainly smaller than they once were because of fire suppression. When oaks ring the grasslands or occur as discrete stands, which mostly happens inland a ways, they're usually Oregon oaks.

Driving down Highway 101, the land begins to change near Laytonville. Sprawling old oaks dot flat pastures just north of town. But conifers still blanket the mountains in the distance. I once spent parts of three winters on a research site west of Laytonville, and most of that preserve is old growth Douglas fir, with patches of oaks and grassland and a few unusual things like knobcone pine. Chaparral begins to become common on drier south facing slopes.

Drive another 20 minutes to Willets, and south of there the transition becomes clear. Another 20 minutes to Ukiah, and oaks probably outnumber conifers. From here south oaks and grasslands become the dominant feature of the landscape. Coast live oaks become abundant, and a little more to the south valley oak begins to appear frequently.

This transition is reflected in the animals, too. Lots of species have range boundaries in southern Mendocino County. Here, near Elk on the coast, the California red-legged frog and northern red-legged frog overlap by only about 10 km, with the range boundary trending to the northeast from there. Similarly, the California and Pacific giant salamanders are demarcated near here. Northwestern salamanders occur only north of this area. The very limited range of the red-bellied newt straddles the transition, across parts of Sonoma and Mendocino Counties. That's only the beginning of the list.

And that's todays snapshot in time. The blurry line between northern and southern community types has marched back and forth over the eons, and more recently with glacial advance and retreat. Thus the slightly messy boundary, because different species of plants and animals recolonize ground at different rates, and have different habitat preferences.

I didn't have time to stop this trip, except for lunch in Santa Rosa. On the return drive on Sunday night, I'll cover the just over 250 miles from San Francisco to Ferndale in the dark, so won't see much of the roughly 30 or 40 miles of the most rapid transition, just the shadowed outline of the closer trees. But I'll know it's there.