Collecting in the West
Reprinted with permission from Arnoldia published by Arnold Arboretum of Harvard University.
During September 1981 I had the good fortune to spend several weeks on a plant- and seed-collecting expedition for the Arnold Arboretum. The trip brought me over 5000 miles of road and eight states in the American West, a terrain sculpted by wind and water and draped in a mantle of botanical wonders.
The purpose of the trip, in addition to that of expanding the Arboretum's collections, was to obtain seeds of plants that may be either endangered or of special interest to the plant sciences or nursery trades. I also wanted to collect herbarium specimens for certification and exchange, as many of the species I was looking for are poorly represented in herbaria throughout the world.
My trip began in the still air of libraries and herbaria, where I spent many hours combing herbarium sheets to gain a familiarity with the plants and sifting through the often cryptic locale data of former collectors. As my stack of notes grew, one element began to emerge and engage my fascination: the mysterious existence of plant disjunctions.
A disjunction, as the name implies, involves a discontinuity within a taxon's range. For example, the primary range of the Cascade azalea (Rhododendron albiflorum) is the Cascade Mountains, from British Columbia to Oregon, and scattered populations exist in the Rocky Mountains of Alberta, Canada, northern Idaho, and western Montana. However, an outlying population exists in central Colorado, about 500 miles from the others. A distribution pattern such as this can raise intriguing questions as to the taxon's origins and continued existence.
E.C. Pielov, in his book Biogeography, created a classification scheme that groups disjunct populations according to their origins. According to his scheme the causes of a gap in the range of an organism are as follows:
(1) Splitting of a once-continuous range because of:
(a) Geomorphological changes, i.e., an uplift of mountains
(b) Climatic changes
(c) Evolutionary differentiation and migration
(2) Establishment of new populations over long distances (jump dispersal) owing to:
(a) Natural causes
(b) Human agency
Disjunctions, therefore, are islands of botanical life, populations of plants that exist far from the mainland of a species' range. These populations may differ from primary populations - for example, they may be hardy in a greater range of habitats - and so are of interest to collectors. They also challenge the botanist to determine the cause of the split from the main range. Was it the gradual uplift of the mountains or the slow drying within the new mountain's rain shadow? Was it the movement of birds in migration or the lethargic sculpting of the glaciers?
The Rhododendron of Slavonia
The first disjunct population I encountered grew near Slavonia, an abandoned mining town in central Colorado, close to the Wyoming border. In the Routt National Forest, on the edge of the Zirkel Wilderness, grows the state's only native rhododendron species, the Cascade azalea (R. albiflorum). This species is better known as a component of the humid lowland forest as well as the subal-pine areas of the Pacific Northwest. It favors high-altitude wetlands and tends to form low clumps around lakes and streams. It has an oblong deciduous leaf and in early summer bursts into creamy white blossoms about one inch in diameter.
The species was first found in the Rocky Mountains of Canada by a Scotsman, Thomas Drummond, in the wilderness years of the early 1800s. Sir William Hooker rendered the first description in Flora Boreaii-Americana in 1840 and called it "a very beautiful and most distinct species which would be a great ornament to our gardens if it could be introduced." The latter qualifier proved prophetic, as horticulturists labored to grow it. A Dr. Graham of the Royal Botanic Garden, Edinburgh, was the first to record its flowering nonsitu and wrote: "This very distinct species was raised at the Botanic Garden from seed gathered by Mr. Drummond in British America in 1828. It does not grow freely and flowered rather sparingly in the open border for the first time in July 1837. It is to be regretted if it is found difficult of cultivation for Mr. Drummond stated it formed a very handsome shrub." The Cascade azalea has also proved difficult to grow in eastern North America, probably because of the oppressive summer heat.
I thought then that this odd Colorado population, growing so far out of range, might offer a genotypic variation that would be suitable for cultivation. One segment is centered near Slavonia and another, three miles to the northeast, near Gilpin Lake. I reached the area via a long dirt road and found no buildings or remnants of the former town, the area is now a favorite starting point for hunting trips into the Zirkel Wilderness, and on the day I arrived packs of hunters sporting state-of-the-art bows were methodically preparing for their foray into the wild. After I had prepared my own hunting gear - a few seed envelopes and some pruning shears - I crossed a stone and wire bridge and headed up an east-facing slope. It was traversed by several streamlets and springs and supported a profuse growth of mosses. Only a few hundred feet from the bridge I found my first Cascade azalea, and I soon discovered that the plant grew in profusion throughout the area. To see such an unusual plant so common in situ was indeed a rare pleasure.
A canopy of Colorado spruce (Picea Pungens) and Douglas fir (Pseudotsuga menziesii) dominated the slope but surrendered somewhat in the wetter areas. The transition zone between the forest and the open mossy area was the niche the rhododendron settled into, forming large mounded patches of yellow fall foliage. There, hidden in the cool tuck of a mountain in the wilderness, was a plant whose creamy white display in spring must stir the hearts of any who happen upon it. I searched hundreds of branches for seed but found all the capsules disappointingly green. Luckily, I was able to arrange for a later shipment from Michael Calhoun, a local resident with a special interest in the azaleas.
The background of these unique Coloradan populations is fascinating from both biogeographical and historical perspectives. The plants have had only sporadic contact with humans. I had been told at the Denver Botanical Garden that the first discoverer of the stand probably was George W. Kelly, an amateur botanist in Colorado. Now 86 and in the process of writing his 10th book, he generously answered my inquiries about the area in a warm letter. "I am just an amateur botanist but spent many wonderful days in the Slavonia area years ago. This is a real island of botany, almost identical to the coast area many miles to the west...My discovery was the first local recognition of the plant, previously all woody plants were just bushes. I was probably the first to make a herbarium collection. So far as I have heard, this group in the Slavonia area is the only one in the state." Without doubt George Kelly has done the most to make the botanical world aware of the stand, and it was his herbarium specimens that first directed me to the still-extant population.
The earliest published reference to the population is an article that named the plant as a new species, Azaleastrum warrenii. Aven Nelson of the Rocky Mountain Herbarium received a single specimen dated July 14, 1911, from Edward R. Warren, a Colorado Springs naturalist. Warren wrote to Nelson, "I found it at my camp on the lower slope of Mt. Zikel, at the head of navigation [for prairie schooners] on the 'Ute Pass Trail'. If I remember correctly, it was quite abundant. It was a low plant, perhaps not more than a foot high...I evidently did not collect much of it, for I have but a single twig left and am sending you half the flowers and leaves from it."
Nelson split the plant (as A. warrenii) from R. albiflorum on the basis of the glandular, ciliolate leaves of the former, which otherwise were glabrous. Five years later, in 1918, J. Francis Macbride transferred the species to the genus Rhododendron. Because of the paucity of pressed material, little comparative analysis with R. albiflorum was done and R. warrenii remained obscure.
In 1936 Louis O. Williams in Annals of the Missouri Botanical Garden compared three collections from the Coloradan populations with specimens of R. albiflorum from the Northwest. He believed that the two species were the same, and other botanists in the area concurred. The plant is now considered R. albiflorum.
The few isolated populations of R. albiflorum east of the Cascades probably originated by different means. Those in Alberta may be the result of jump dispersal following the Wisconsin glaciation. But the disjunct population in Colorado, which may have evaded glaciation, could have resulted either from jump dispersal or range splitting. Rhododendron seed is so small and light that it could easily have lodged itself in fur or feather. Wind is also a dispersal agent for small light seeds, and it is conceivable that seed from the Cascade populations could have been carried east in a powerful storm. Volcanoes are another possibility but a highly unlikely one. R. albiflorum is a component of the Cascade flora and inhabits the sides of both active and extinct volcanoes. The recent Mt. St. Helens eruption, which spread ash as far east as Colorado, raises the question as to whether the seeds could have been blown into the stratosphere and carried eastward. Whether the seed could survive the intense heat and poisonous gases of the blast is highly questionable, but it is an idea that gives botanists a big bang theory to call their own. Of course, it is possible that other populations will be found within the 500-mile disjunction, rendering it less dramatic.
Since George Kelly has noted the presence of other West Coast disjuncts, such as Pacific trillum (Trillium ovatum) and Lewis mimulus (Mimulus lewisii), near Slavonia, it would follow that it is a refugium rather than the end point of a jump dispersal. We might speculate that the Cascade azalea once had a wider range than it has now. Two events probably diminished the range. First, the Cascade and Sierra Nevada mountains uplifted, blocking Pacific rainstorms and creating dryer environments on their east sides. Second, the Wisconsin glaciation buried the more northerly populations under a sheet of ice. The few populations in Colorado survived probably because they were unaffected by glaciation and because the mountains on whose bases they grow held enough rainstorms to sustain them.