North Central Washington

Plant geography of the Methow Valley

by George Wooten
Originally published in the Methow Naturalist, 1999

I am convinced that He [God] does not play dice.”
- Albert Einstein

Plants are not evenly distributed along the Methow Valley. Some plants are Methow specialties which reach their zenith here: arrowleaf balsamroot, chocolate tips, swale desert parsley . . . Other plants of wide occurrence elsewhere are perplexingly absent here: vine maple, nine-leaf biscuit root, camas ... The changing floral composition is not smooth--it is abrupt and dramatic. These anomalies are part of the fascinating topic of plant geography.

Some plant families have centers of distribution which radiate outward. Examples include penstemons, which are centered near the Wallowa Mountains, desert-parsleys centered in eastern Washington, onions in eastern Oregon, loco-weeds in Nevada, and Mariposa lilies in northern California. While the Methow Valley does not appear to be a center of distribution for any plant families, we are close enough to the boundaries of other distribution centers to have specialized variations present in our flora. For example, the Chelan, or frosty penstemon, and cat's ears (related to the Mariposa lilies) are both endemic to our region, and prominent locally.

Once a species finds a new niche, its population may rapidly expand. At least one of our rare plants appears to be increasing its numbers this way. This plant, the pink dandelion, appears to be a recent hybridization event between two related native species.

In crossing the Chewuch River, one finds dramatic changes in the floral composition. Species that are dominant on one side of the river are rare on the other side, and vice-versa. For instance, four-angled mountain heather is dominant in alpine areas east of the Chewuch, while white mountain heather is dominant to the west. Many plant species exhibit population exclusion between their close relatives, and these are called allopatric species. One of these pairs is the explorer's gentian and the glaucous gentian. Despite the relative commonness of the explorer's gentian throughout North Cascades subalpine meadows, it is completely absent from the one area where glaucous gentian occurs, in the eastern Pasayten.

Certainly soils—their texture, wetness and composition—play a large part in determining the floral composition of an area. I was informed by a native American acquaintance that his tribe went to great lengths to obtain their bitterroot from the Methow Valley, even though the plant is widely distributed elsewhere east of the Cascades.

It is worth considering whether bitterroot thrives here due to the Valley's extensive sedimentary assemblages, in constrast to predominant rock types elsewhere in the state: basalts, metamorphics, and sandy glacial deposits. It is well-documented that plants can have a high fidelity to certain soils. For example, camas, which is absent from the Methow, strongly prefers clayey, basalt soils, where it is commonly found with a number of other important native American root crops.

Some of our most outstanding examples of bitterroot occur in broken shale beds, where it is tempting to attribute their presence to some factor in the shales that is absent in other soils. It is known that plants can uptake and sequester minerals from the soil and perhaps bitterroot is one of these. Geologists have even exploited this property in searching for new prospects. In our area, several plants seem to grow in greatest abundance near mineralized outcrops, including bitter cherry, northern twayblade, Kotezebue's grass-of-Parnassus and polypody, the last two of which are so rare as to make coincidence an unlikely explanation.

Plants which can tolerate harsh growing conditions are probably responding more to the lack of competition than to any preference for the harsh habitat. For example, botanists have found that serpentine endemics—plants that occur strictly on serpentine soils—do not actually prefer those soils, but their tolerance of toxic chromium frees them from crowding by other plants. Shale barrens are a harsh habitat with shallow rooting zones and rapid dessication. Without knowing whether chemical factors play a part in the growth of our bitterroot plants, it is safe to say that soil texture alone could be the most important factor in their value to native Americans. Alternatively, perhaps our plants were preferred for pragmatic reasons—the ones growing in shales being easier to find and dislodge from the ground!

Local climate can have a strong influcence on the floral makeup of an area. For instance, glacier lilies grow along the Methow River from Mazama upstream, even though there is no apparent significant difference in soil types or elevation as one goes downstream from there. However there is a strong gradient of increasing snow depth which may reach a critical point in Mazama for glacier lilies, which are fond of emerging into spring right out of melting snowbanks.

It is instructive to compare the floral make-up of the Methow with similar habitats nearby as a test plant distribution hypotheses. Sure enough, in going from Peshastin to Blewett Pass, glacier lilies suddenly occur in abundance from the bottom of the slope upward. But atop Blewett Pass, the forests are often dominated by grand fir with understories of elk sedge, while those of the Methow slopes are apparently devoid of these. Or are they? As it turns out, there are a very few places in the Methow Valley where grand fir occurs, and one of these is very near Mazama. Furthermore, there is only one place in the Methow Valley where elk sedge occurs—Driveway Butte, near Mazama. The snow depth hypothesis gains support.

A search for further parallels between these Blewett Pass and the Upper Methow leads to more surprises. Other plants that are present near Blewett Pass include western hemlock, western larch, vine maple, big-leaf maple, and trillium. Of these, two also occur rarely in the Methow—western hemlock and big-leaf maple—both near Mazama. It is as if Mazama was an isolated refuge for the same species which occur commonly many miles away on Blewett Pass.

Certainly snow depth must be an important factor in the co-occurrence of these species at two widely separated stations, but it doesn't explain why the Methow doesn't have any vine maple or trillium at all, or why western larch occurs only east of the Chewuch River, where there is less snowpack. Perhaps the limiting factors for the growth of different species could be a combination of several different environmental influences.

Peeking over Washington Pass a bit, it is notable that western hemlock, vine maple, big-leaf maple and trillium all occur in Granite Creek, and become progressively more common westward. This correlates with the known increase in precipitation in crossing the Cascades, and indicates that Mazama must be right at the edge of the tolerance of these species for available moisture. Conversely, the south face of Driveway Butte is also the Methow's furthest west extension of dryland species such as bitterbrush and arrowleaf balsamroot. So it appears likely that our floral diversity is due to a combination of soil and complex climatic factors.

But this explanation still lacks an explanation for the distribution of western larch, which grows on South summit further south than Leecher Mountain. And to put another bramble across the path, consider this: the other area where grand fir occurs in the Methow Valley is at the drier southern extremes in Black Canyon and Gold Creek, where it can be found along with another of our hypothetical moisture- and snow-pack loving species, big leaf maple. Truly, something very strange is going on here.

A possible explanation for this is illustrated by two of our rarest plants, skunk polemonium and dwarf alpine forget-me-not, both of which are quite common to the south and the north, respectively, but which only occur here on nunataks, or unglaciated peaks. If one accepts that a great ice sheet once stretched from here to Alaska, broken only by isolated nunataks, and one further assumes that all vascular plant life was impossible under this ice sheet, then the occurrence of these rare plants on nunataks suggests that they were once part of a larger, continuous population that was cut off and isolated by the glaciers. This is the Pleistocene refuge hypothesis. Other plants which prefer wet, cold conditions may have become established at the margins of the glaciers, and are now only holding out in dwindling habitats left in the wake of the glacial retreat. This is the Pleistocene residual hypothesis.

Following the retreat of the glaciers, a large mass of bare ground must have been repatriated over time by all of the plants now present. Some of the early plants to become established might have taken over an area through competitive exclusion, as is believed to have occurred soon after the glaciers melted by lodgepole pine and sagebrush. Other plants which by chance or design found themselves established in marginal habitats, nonetheless may have managed to survive at these sites over time, eventually to be found in unexpected places of the present time. This is called the founder's effect, and the characteristic pattern of such populations is one of isolated patches separated by intervening areas of unsuitable habitat, as explained in the model theory of island biogeography, generally attributed to E.O. Wilson. This is exactly the pattern of big-leaf maple and grand fir stands in the south of the Methow Valley.

One last chapter of our flora is still evolving. Late arrivals to our parts may not yet have had enough time to establish large populations, despite suitable habitats. Western larch appears to be of this ilk, as are most of our villanous companions, the noxious weeds. Both may be destined for a larger role in the Valley's future ecology.

Latin names of plants discussed in this article:
balsamroot - Balsamorhiza sagittata
big-leaf maple - Acer macrophllum
bitter cherry - Prunus emarginata
bitterbrush - Purshia tridentata
bitterroot - Lewisia rediviva
camas - Camassia quamash
cat's ears - Calochortus lyallii
Chelan, or frosty penstemon - Penstemon pruinosus
chocolate tips - Lomatium dissectum
dwarf alpine forget-me-not - Eritrichium nanum
elk sedge - Carex geyeri
glacier lily - Erythronium grandiflorum
grand fir - Abies grandis
Kotzebue's grass-of-Parnassus - Parnassia kotzebuei
lodgepole pine - Pinus contorta
nine-leaf biscuit root - Lomatium triternatum
northern twayblade - Listera borealis
pink dandelion - Agoseris lackschewitzii
polypody - Polypodium hesperium
sagebrush - Artemisia tridentata
skunk polemonium - Polemonium viscosissimum
swale desert-parsley - Lomatium ambiguum
trillium - Trillium ovatum
vine maple - Acer circinatum
western hemlock - Tsuga heterophylla
western larch - Larix occidentalis