Charles W. Heckman, USDA Forest Service, Pacific Northwest Research Station, 3625 93rd Avenue SW, Olympia, Washington 98512, e-mail [email protected]

The Encroachment of Exotic Herbaceous into the Olympic National Forest

Abstract A floral survey in the Olympic National Fnrest and its surroundings revealed that a considerable number of introduced ruderal species have made deep inroads into the stands of native flora. Some of them, which are mainly of European and Asian origin, have been planted deliberately to stabilize the soil along roadsides and after clear cutting and burning. However, they have already established reproductive communities that are capable of spreading rapidly without additional human help. as they have already done in almost all regions of the earth subject to a temperate climate. Plotting the data from the Geographical Information System. it was shown that 12% of an area totaling 388 km~, located mainly in the Olympic National Forest, was occupied totally or in large part by this mainly ruderal flora, which accounted for at least 50% and usually nearly 100% of all plants within the area on which they occurred. This represents a major qualitative and quantitative biogeographical alteration in the regional flora of a national forest generally considered to be only slightly affected by human activity.

Introduction tion. A list of the introduced species first reported in the state from 1950 to 1996 and a discussion A phytogeographical process initiated by the of the role some of them have assumed has been wanderings of human beings has been changing provided by Toney et al. (1998). the world's flora throughout recorded history, but for the past century, this process has been accel- The purpose of this study was to determine erated at an alarming rate. It has resulted in the the structure of the biotic communities in which formation of new floral aggregations, recruited the introduced species develop, estimate their mainly from Europe and Asia and transported to present quantitative impact in the coastal forest biotic communities located on all continents ex- on the Olympic Peninsula of Washington, and cept Antarctica (Hitchcock & Cronquist 1973; provide a synopsis of the history of its develop- Weihe 1972). In many places, these species have ment and spread. It must be emphasized that these been intentionally planted, whereas in others, they communities are fundamentally different from arrived as contaminants of seed. Once established those which were formerly encountered in the local on a continent, they often spread rapidly because ruderal habitats. It is an aggregation of interna- of their great potential for massive seed dispersal tional origin rather than native. The spread of the (Baker 1974). The result has been the formation species is progressing at an extremely rapid pace, of ruderal communities throughout the temper- especially in the Pacific Northwest of North ate zones of the world from subtropical and Medi- America. For example, only 40 species of non- terranean to sub-arctic regions. The formation of native plants had been identified on the Olympic these communities has been facilitated by anthro- Peninsula prior to 1900, and 143 before 1936 (Jones pogenic alterations of the vegetation, particularly 1936). By 1995, 333 non-native species, accounting those related to housing, road building, and lei- for 25% of the flora, were found on the Olympic sure activities. These exotic species have been Peninsula (Buckingham et al. 1995). However, augmenting and even displacing endemic floral many of the same exotic ruderal species, mainly aggregations on an ever growing area of the earth's from Europe, are presently encountered in such surface (Wilgen and Richardson 1985; Kowarik, places as Japan (Kitamura et al. 1987), Australia, 1995). As pointed out by De Ferrari and Naiman New Zealand, and South Africa (Weihe 1972), (1994), even the widespread interest in this phe- as well as in North America. Previous ecological nomenon has resulted in relatively few studies studies that have provided information on the exotic on a landscape scale. However, the problem of flora of the Olympic Peninsula have focused on exotic plants in the State of Washington has been the forest succession (Fonda 1974) and those spe- drawing a considerable amount of public atten- cies penetrating the forest along the courses of

264 Northwest Science, Vol. 73, No. 4, 1999

© 1999 by the Northwest Scxentific Assocxatlon. All nghts reserved. the Dungeness and Hob Rivers (De Ferrari and the location along the Pacific Coast in northwestern Naiman 1994). There have also been studies that North America is of special interest because the concentrated mainly on communities of native native flora is typically dominated by conifers, plants in the Olympic Mountains (Fonda and Bliss ferns, mosses, lichens, and other vascular plants 1969: Henderson et el. 1989), as well as the gen- that are especially adapted to the conditions in eral flora of the region, cited above. This study, cool rainforests or moist coastal forests, Very few conducted mainly in the Sol Duc River Valley, of the native species show the adaptations simi- focuses on the importance of roads, particularly lar to those of the exotic ruderal plants, as dis- the many unpaved logging roads, as avenues of cussed below, and only a few of the ruderal spe- invasion. It is the first attempt to evaluate the cies are likely to have reached the Pacific Northwest quantitative impact of the present management without human assistance. Clear lines of demar- practices conducive to the introduction and spread cation between the invading and natural flora were of the foreign ruderal species on the landscape. observed at the start of the study, and these are described in detail below (Structure of the Rud- Location eral Aggregations). The spread of the international ruderal vegeta- Material and Methods tion was analyzed in and at the edges of the Olym- pic National Forest in western Washington, U.S.A. The cartography of the vegetation was completed The quantitative spread of these ruderal species using recent aerial photos of the Sol Duc River was determined in the northern part of the forest, Valley and parallel ridges in or adjacent to the which is drained mainly by the Sol Duc River Olympic National Forest. The regional vegeta- (Figure 1). Although such a study could have been tion was observed on the ground, and the structure made almost anywhere in the temperate zones, of the various plant communities was determined.

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Figure 1. The location of the site surveyed (black rectangle) on the Olympic Peninsula in the State of Washington.

Encroachment of Exotic Plants 265 The species present along transects at least 10 m graphing the vegetation to confirm the presence wide through the forest and along various roads of the ruderal species on these areas. and streams were determined. The locations of It was further determined that only a few ad- the species aggregations on the areas covered by ditional forested areas had been cleared of trees the maps were then plotted according to the find- since the GIS data were obtained in 1994, and ings made during the field survey. Care was taken these were still only partially colonized by plants, to visit all of the distinct zones observable in the with many areas still bare of nearly all vegeta- photos, including mature forest, replanted forests tion. The charts prepared were compared with those in various stages of development, clearcuts, lands produced to map the seral stages of the native used for agriculture or animal husbandry, road- vegetation during previous years and stored in sides, the riparian zone along streams, and the the GIS in order to provide an additional check vicinity of buildings. Surveys were also made along of the accuracy of the mapping. The data used logging roads reaching to the top of the ridges for comparison were obtained from an unpublished located within the area surveyed. Only gardens forest service report on the Sol Duc Pilot Water- around private houses were omitted form the sur- shed Analysis by Mr. David Peter. The program vey, because the amount of the area covered by ARC/INFOTM was used to calculate the area of such gardens was insignificantly small, and many the region surveyed on which the ruderal flora of the plants growing in them are not viable in occurred at the time the aerial photos were taken. natural communities. The presence of the ruderal vegetation either alone or interspersed with other plant aggregations on cleared areas was then plotted Results on the map, and the number of square meters it Structure of the ruderal aggregations covered was calculated on the horizontal plane of the maps. By this method, some underestima- While walking along the transects, it became ap- tion of the area it covered is possible because ar- parent that the exotic ruderal species generally eas on hillsides are actually greater than they ap- form aggregations that exclude native herbaceous pear on the maps. The area of all unpaved roads species along roadsides and similar disturbed ar- with a strip 1 m wide along each side as well as a eas (Table 1). In undisturbed forests dominated 5 m area on each side of Highway 101, which by the species listed in Table 2, it was usually transects the area surveyed, was added to the to- found that no exotic species were present. From tal. The paved area of the highway was not in- these observations and from photographs made cluded in the total area inhabited by ruderal veg- in the region, it became evident that the majority etation. This may also result in an underestimation of the area has either 100% or 0% coverage by of the area occupied by ruderal plants because exotic species. they usually cover much more than 5 m on each Mixtures of exotic and native species are en- side of the main roads and more than 1 m along countered along a narrow ecotone between rud- minor logging roads. The area of unpaved roads eral roadside communities and undisturbed for- themselves was included in the total because the est. These characteristically appear as grasses ruderal plants often grow in the middle of them. invading stands of forest floor vegetation. Other A total of 388.2 km 2 was surveyed using aerial mixtures of native and introduced species are photos that had been taken in 1994 and digitized present on clearcut areas. It was observed that as data in a Geographic Information System (GIS). after the trees are removed, most of the shade- The approximate percentage cover of the total area adapted plants of the forest floor quickly succumb largely dominated by herbaceous plants predomi- to dryness and strong illumination. However, thick nantly belonging to the exotic ruderal commu- stands of the sword fern [Polystichum munitum nity was calculated using the program ArcViewTM (Kaulf.) Presl.], are able to persist for long peri- by marking all bare areas and cut woodlands in ods of time among the encroaching exotic herbs, early stages of regeneration together with the which are usually intentionally seeded on the marginal strips along all roads. Field surveys were clearcuts to prevent erosion. These remain as the conducted by walking the transects during the only representatives of the native herbaceous flora, spring and summer of 1998, noting the species but they also gradually disappear with time. On present 5 m on each side of the way, and photo- the Olympic Peninsula, the woody plants that

266 Heckman TABLE 1. Species of the ruderal vegetation that colonize areas of the Olympic Peninsula disturbed by man and the geographical origins of the component species. The distribution of the individual species in ruderal habitats is influenced locally by such factors as the amount of moisture present and the amount of shading by adjacent forests. However, deliberate seeding of the roadsides and clearings cut in the forest is still a primary factor determining where the species will be encountered. Most identifications were made using Hitchcock & Cronquist (1973), but the nomenclature was up- dated to follow that of Buckingham et al. (1995). The superscript after the name refers to the first report of the species on the Olympic Peninsula, according to Jones (1936) and Buckingham et al. (1995).

Species Probable origin Pteridophyta Equisetum arvense L. North America, Europe

Dicotyledonae Achillea millefolium L. z North America, Europe, Asia Anaphalis margaritacea (L.) Bentham & Hooker ~ North America, Europe, Asia Barbarea verna (Mill.) Ascherson4 Europe Barbarea vulgaris Robert Brown4 Europe, Asia Bellis perennis L. 3 Europe Calystegia sepium (L.) Robert Brown3 Europe Capsella bursa-pastoris (L.) Medicus z Europe, Asia Chrysanthemum leucanthemum L. 3 North America, Europe, Asia Cirsium arvense (L.) Scopoli 3 Europe Cirsium edule NuttalP North America Cirsium vulgare (Savi) Tenore ~ Europe Digitalis purpurea L.:: Europe, Asia Epilobium angustifolium L.I North America, Europe, Asia Erodium cicutarium (L.) l'H6ritier~ Europe, Asia Gnaphalium uliginosum L. 4 Europe, Asia Heracleum lanatum Michaux ~ North America, Siberia Hieracium aurantiacum L. ~ Europe Hypericum perforatum L) Europe, Asia, North Africa Hypochaeris radicata L. 2 Europe Lactuca muralis (L.) Fresenius3 Europe Lapsana communis L. 3 Europe, Asia Lathyrus latifolius L. ~ Europe Lotus pedunculatus Cavanilles 4 Eurasia, Africa, Australia Lysimachia punctata L~ Europe, Asia Lythrum salicaria L~ Europe Medicago lupinula L? Europe, Asia, North Africa Melilotus officinalis (L.) Lamarck~ Europe, Asia Myosotis arvensis (L.) Hill 3 Europe, Asia, North Africa Plantago lanceolata L. ~ Europe Plantago major L. z Europe Polygonum cuspidatum Siebold & ZuccarinP Japan Polygonum sachalinense SchmidP Eastern Asia Prunella vulgaris L. vat. vulgaris~ Europe Ranunculus acris L) Europe Ranunculus repens L. ~- Europe Rumex acetosella L 2. Europe Rumex crispus L. z Europe. Asia Sagina procumbens L. ~ Europe Seneciojacobaea L. 4 Europe Solidago canadensis L.I North America Sonchus oleraceus L? Europe, Asia Taraxacum officinale EWeber ex Wiggers 2 Europe. Asia Tr~folium hybridum L. ~ Europe Table 1 continued, next page

Encroachment of Exotic Plants 267 Species Probable origin

Trillium pratense L. z Europe, Asia Tri~blium repens L.'- Europe, Asia, North Africa Vicia sativa L. z Europe, Asia, North Africa Monocotyledonae Agrostis capillaris L. 4 Europe, Asia, North Africa Agrostis stolonifera L. 4 Europe Avenafatua L. 4 Europe, Asia, North Africa Bromus hordeaceus L. 4 Europe Dactylus glomerata L. 2 Europe, Asia EIvmus glaucus Buckley ~ North America Elymus trachycaulus (Link) Gould 4 North America lanatus L. 4 Europe, Asia, North Africa Holcus mollis L. 3 Europe Juncus balticus Willdenow ~ North America, Europe Juncus bufonius L.' North America, Europe, Asia Juncus effusus L.l North America, Europe Lolium perenne L. 2 Europe Phleum pratense L.'- Europe, Asia Poa trivialis L. 4 Europe, Asia, North Africa Vulpia myuros (L.) C.C. Gmelin 4 Europe Vulpia octoflora Walter' North America

'A probable native species (Jones, 1936), "~Introduced prior to 1900, 3Not reported before 1900 but known to have been introduced prior to 1936, 4Not known to have been introduced until after 1936. develop on cleared areas are generally native, but ecotone is encountered, followed by an area on the spread of exotic woody plants, such as Scots which almost 100% of the herbaceous species are broom, [Cytisus scoparius (L.) Link], can be ex- exotic. Rarely, isolated edible thistles (Cirsium pected in the near future. edule) occur among the exotic species in the To illustrate the structure of the vegetation clearcuts, but non-native thistles are more abun- typically encountered while walking through ar- dant. In the clearcut shown, there is a seasonal eas subjected to human influence, a transect from succession involving exotic grasses, the common the middle top to middle bottom of Figure 2 can foxglove (Digitalis purpurea), scattered compos- be taken. Beginning at the bottom, a roadside ites, clovers (Trifolium spp.), and docks (Rumex aggregation is encountered in which nearly 100% spp.). In addition to the exotic herbaceous spe- of the herbaceous plants are exotic, but surviv- cies, a few native woody species and small na- ing sword fern tufts are occasionally scattered tive conifers can also be found. among the ruderal vegetation. Moving beyond the road in Figure 2, a closed In contrast to the herbaceous plants, the woody section of forest is encountered. While about 100% vegetation consists of 100% native species at most of the herbaceous plants along the roadside are locations. Surviving vine maples, Acer circinatum, exotic, those under the forest canopy are nearly and species of Rubus are particularly character- 100% native. There is a very narrow ecotone at istic of the region. Near the edge of the decidu- the edge of the forest in which there is some mix- ous trees at the bottom of the picture, a narrow ture of native and exotic species. In summary, ecotone is encountered, in which there is a mix- the transect through this photograph (Figure 2) ture of native and exotic herbaceous plants. Be- runs through unshaded areas in which nearly 100% neath the trees themselves, the native species gain of the herbaceous plants are exotic, through closed the upper hand and account for well over 50% of canopy forest in which little more than 0% of the the species present. Moving out of the trees onto herbaceous plants are exotic, and through narrow the clearcut at mid-level in the photo, another ecotones and open canopy woods in which there

268 Heckman TABLE 2. The most abundantnative plant species dominatingthe climax stage forests in the region surveyed and their native ranges. Pacific Northwest refers to the Pacific Watershed of North America and adjacent Rocky Mountains.

Species Native Range

Trees Acer macrophyllum Pursh Pacific Northwest Alnus ruhra Bongard Pacific Northwest Pseudotsuga men~,iesii IMirbel) Franco Western North America Tsuga heterophylla IRafinl Sargent Pacific Northwest Thula plicata D. Don Pacific Northwest

Woody vegetation in understory and edges of clearings Acer circinatum Pursh Pacific Northwest Oplopanax horridus {Smith ~Miquel Northern North America Rubus parviflorus Nuttall Western North America Rubus wectabilis Pursh Pacific Northwest Sambucus racemosa pubens (Michauxl House North America

Herbal understory growth Claytonia cordifolia (Watson) Pax & Hoffmann Western North America Comus unalaschkensis Ledebour Pacific Northwest Listera caurina Piper Western North America Lycopodium clavan#n L. Circumboreal to tropics Oralis oregana Nuttall Pacific Northwest (L.) Kuhn Cosmopohtan

Bryophtyes lfvloconium splendens IHedwig) Bruch,Schimper, & GiJmbel North America, Europe, Africa, New Zealand Hypnum circinale Hooker Pacific Northwest Lsothecium stoloniferum Bridel Western North America KindberRia oregana (Sullivant) Ochyra Pacific Northwest Polytrichum juniperinum Hedwig Cosmopolitan Rhizomnium glabrescens (Kindberg)Koponen Western North America Rhytidiadelphus loreus (Hedwig) Warnstorf North America, Europe Thuidium recognitum (Hedwig) Lindberg Northern Hemisphere

is some mixture of native and exotic species. The historic times, and some of these have apparently newly logged side of the hill in the upper left quarter been introduced to the Pacific Northwest from of the figure is almost bare of vegetation. The other parts of the continent during recent human native understory plants have died off since the migrations. logging, and the exotic plants have only recently begun to move onto the clearing, Spread of the exotic ruderal species The introduced ruderal plants observed dur- The field surveys during spring and summer ing the field study are shown in Table 1. A num- showed that the ruderal plants are virtually the ber of additional alien species were reported for only vegetation along most roadsides, encroach the Olympic Peninsula by De Ferrari and Naiman extensively into the clearcuts in the forest, and (1994) and Buckingham et al. (1995). The ma- share the areas with plants of subsequent seral jority of the species originated in northern Eu- stages until the conifer forests have almost com- rope and have spread throughout many parts of pletely regenerated. Along numerous transects the temperate zones (We ihe 1972). Some of them through the forests, none of the species in Table are native to eastern Asia. A few are thought to 1 were ever encountered on the floor of intact, have been present in North America since pre- mature forests or along unmodified streams, but

Encroachment of Exotic Plants 269 Figure 2. Hillsides near the site of the survey in Clallam County showing typical closed forest and clearcut areas. A transect through the center of this photo from top to bottom reveals the "all or none" nature of exotic species aggregationsin the local herbaceous flora. In contrast, the woody plants belong almost exclusively to native species. For details, see text.

they persisted on unused roads and former clear- much greater percentage of the flora is accounted ings that had been fully overshadowed by young for by this pan-temperate ruderal community be- conifers. cause the ruderal community is also promoted by A simplified map showing the Sol Duc River agriculture. and the locations occupied wholly or predomi- The ruderal community is clearly a primary nantly by the ruderal community, which consists seral stage that is gradually displaced as the sere almost exclusively of species that have become progresses. The plants are useful for stabilizing pan-temperate, is provided in Figure 3. A plani- the soil on cleared areas. For this reason, they are metric analysis of the map indicates that a total frequently planted intentionally, which has con- of 45.935 km 2 are occupied by that community. tributed to their rapid colonization of such large which equals 11.8% of the total area of the re- areas in the region. The introductions of plants, gion covered by the photos. Because the region however, has not always had benign results. Much surveyed lies mainly within a national forest and effort is currently being devoted to eradicate the is uninhabited, this percentage of land on which Scot's broom, which has become a very trouble- the native flora of herbaceous plants has been some weed since its introduction. Several intro- replaced almost entirely by species introduced from duced herbaceous species are poisonous to do- other continents is considerable. It should also mestic animals. For example, the tansy ragwort be remembered that this large area does not in- (Senecio jacobaea) produces alkaloids that are clude parts of forest and woodland in which a toxic to horses and cattle (Whitson et al. 1991), few exotic species are encroaching among the and expensive eradication programs to eliminate native forest floor species. Such encroachment it have been proposed. The meadow buttercup can be expected to occur most frequently along (Ranunculus acris) is also poisonous to cattle streams and rivers and on natural clearings. In (Parish et al. 1996). The area already colonized the inhabited parts of the Olympic Peninsula, a by the ruderal community is considerable, so

270 Heckman 411OoT,30-

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Figure 3. The area of 388.25 km z surveyed during the study with the subareas occupied wholly or to a large degree by ruderal flora shown in black. The double black lines outline the Sol Duc River, while the narrow lines indicate roads along which the ruderal plants grow. On this map, the area occupied by the herbaceous ruderal flora, which consists nearly 100% of exotic species, is at least 45.935 km 2 or 11.83% of the total. t-J -,.I judging from the fact that these species had not sula of Washington are strikingly different in struc- reached the region at all until the late 19th or early ture and ecological characteristics from those along 20th century, the present extent of their coverage the edges of the deciduous forests of Europe represents a major floral change in the Pacific (Moravec et al. 1982), where much of the rud- Northwest region of North America. eral vegetation has had its origin. Table 2 pro- vides a short list of some of the dominant native Discussion plant species of the forests on the Olympic Pen- insula, which were characterized by Franklin & History of floral change Dyrness (1973) as being mainly old growth for- The encroachment of the pan-temperate ruderal est, dominated by Sitka spruce (Picea sitchensis) community, mainly of European origin, into the and western hemlock (Tsuga heterophylla) and Pacific Northwest is typical of a process that has notable for the dense epiphytic flora. In addition been taking place throughout the temperate zones, to the abundant mosses (Schofield 1992), which mainly during this century. The pan-temperate coat much of the forest floor and the tree trunks, ruderal community has spread rapidly with the the vegetation is characterized by a particularly systems of highways and logging roads and is large variety of lichens (McCune & Geiser 1997). presently altering the flora of the temperate zones For this reason, the demarcation lines between in North and South America, Asia, and Australia, the introduced community (Figure 4) and the na- to various degrees, as demonstrated by the range tive flora (Figure 5) are recognizable at a glance. information on the individual species, provided In eastern North America, there is less distinction by Weihe (1972). The native plant aggregations between the native and non-native vegetation of the rainforest region on the Olympic Penin- because many of the introductions occurred well

Figure 4. A typical ruderal communityin the middle of a little used dirt road during the late spring. The dominant species in this aggregation are Trifolium repens and Bellis perennis.

272 Heckman Figure 5. A native forest floor communityof late spring dominated by Linnaea borealis and Comus unalaschkensis Ledeb. that occupies open ground at the edges of roads and clearings before it is overgrown by the faster-growing introduced species. over a century ago, and many species have al- to the disadvantage of any native ruderal species ready integrated themselves with the native flora present in the region. In the Pacific Northwest, (Femald 1987). Sharp ecological demarcation lines the redwood sorrel (Oxalis oregana Nutt.), wild are no longer apparent between New and Old World strawberry (Fragaria virginiana Duchesne), sweet- species. However, the encroachment of the rud- scented bedstraw (Galium triflorum Michx.), little eral plants has been no less dramatic. buttercup (Ranunculus uncinatus D. Don.), red The floral changes involving the ruderal spe- columbine (Aquilegiaformosa Fisch.), Cooley's cies are still more significant for phytogeogra- hedge-nettle (Stachys cooleyae Heller), and the phy than for economics, although this may change twinflower (Linnaea borealis L.), which is a dramatically at any time. The ruderal species are circumboreal species (Dempster 1993), are fre- considered to have a positive effect in holding quently observed in partially shaded locations at the soil in place wherever human activity has the edge of the forest along roads. However, the stripped away the natural vegetation. Usually, results of the observations along the transects con- native vegetation is able to displace the ruderal firmed that these are rapidly overgrown when- species during the course of the natural sere. ever the introduced ruderal species, particularly However, introductions of some plant species, the grasses, begin to colonize the habitat. Figure mainly those that become established in pastures, 2 shows areas being rapidly overgrown by rud- are having some severe economic effects because eral plants, and at these sites, no native ruderal of their toxicity to domestic animals (Whitson et species were ever observed. It can be assumed al. 1991; Parish et al. 1996), as mentioned above. that prior to the coming of the first European set- The rapid growth of the introduced species works tiers, the entire region provided few areas for

Encroachment of Exotic Plants 273 colonization by ruderal herbs, so the native spe- able floral change considering that during the first cies had little chance to develop varieties well half of the 19th century, the Olympic Peninsula adapted to this kind of habitat. One of the only was settled only by native Americans, whose ruderal species that has apparently been estab- hunting and fishing societies caused only mini- lished on the Olympic Peninsula since prehistoric mal change to the forest. Settlement of the re- times is the field horsetail, (Equisetum arvense), gion by persons of European descent began only which is still an abundant ruderal species is moist during the second half of the 19th century and locations (Hauke 1993). However, it is nearly pan- progressed slowly. The floral changes in the re- temperate in distribution and was apparently al- gion apparently began to accelerate considerably ready present on several continents prior to the during the first half of the 20th century, and the Quaternary Period (Hauke 1993k when the re- advance of the ruderal community with the roads treat of the glaciers permitted it to colonize the did not take place with any great speed until the region now known as western Washington (Jones second half of the century. As already discussed, 1936). Jones (1936) lbund that there had been a consid- As the sere progresses, these first stage spe- erable augmentation of the non-native flora be- cies, some of which are encountered in sunny places tween 1900 and 1936. Since that time, the intro- on the forest floor, have been overgrown by na- duction of exotic species has intensified tive species of the next seral stage, mainly woody considerably (Buckingham et al. 1995). Judging shrubs, such as the salmonberry (Rubus ~pectabilis) from the trends that have taken place during this and thimbleberry (Rubus pam'iflorus). However, century and the time required lbr introduced spe- the native species of the second seral stage are cies to become invasive (Kowarik 1995), non- being increasingly displaced by another European native species can be expected to bring about fun- introduction, the Scot's broom. This displacement damental changes in the flora of the Olympic threatens to have economic consequences because Peninsula during the next few decades. simple efforts to eradicate the rapidly spreading Along the Dungeness and Hoh Rivers to the import are proving unsuccessful, and more elabo- south, De Farrari and Naiman (1994) reported that rate methods will probably be employed, which 28% and 24%, respectively, of the plant species will be expensive but probably will not be effec- present were exotics. They discussed the role of tive, either. Unlike the ruderal herbs, the Scot's these rivers as invasion routes for the introduced broom is thought to be capable of stopping the plants, confining their discussion mainly to natural growth of young conifers. dispersion and accidental transport of seed. In the The final seral stages are still dominated by Sol Duc Valley, however, the roads seem to be native species, which eventually overshadow the the main avenues of transport, and deliberate seed- sun-loving ruderal species and eliminate them from ing of roadsides and clearcuts makes discussion the area. However, Figures 2 and 3 show that with of other dispersal mechanisms moot. increasing human activity in the forest, the stands of native climax vegetation are increasingly frag- Outlook mented into isolated stands surrounded by intro- Although there are presently few obvious dan- duced species. The roads are particularly effec- gers to the native flora due to invasive spreading tive at dissecting the native forests, and they provide of the introduced species, problems are sure to avenues of penetration that take the ruderal flo- arise in the future. Kowarik (1995) was able to ral aggregations new to the region deep into the trace the development of introduced species in forest. Brandenburg for a period of more than two cen- At the present time, the area occupied by the turies. In the case of woody plants, he noted that ruderal community, 11.83% of the total area sur- an average of 147 years elapsed between the in- veyed, is enough to cause concern. This is par- troduction of species and the time they became ticularly true because the advance of these spe- invasive. He further found that it is not possible cies along the many small roadways in the forest to predict which species will become troublesome permits this exotic vegetation to dissect the na- for the native vegetation. Even if we assume that tive forests into disjunct islands of conifers and invasiveness will manifest itself among herba- their natural understory plants. This is a remark- ceous plants more rapidly than among woody

274 Heckman species, decades may elapse before one of the essary to open the way into the forest, permitting newcomers begins to develop into a dangerous the new species to compete directly with the na- competitor of the native species. Several of the tives. It is time that attention be given to the seri- species in Table I seem to have the potential to ous consequences that can occur due to the intro- become invasive. For example, Edwards et al. duction of foreign species for erosion control or (1995) noted that the purple loosestrife (Lythrum to beautify the environment. salicaria) has a tendency to spread rapidly and form monospecific stands in other parts of North Acknowledgements America. This shows that considerable trouble with the species already introduced can be ex- Special thanks are due to colleagues in the United pected during coming decades, and new exotic States Forest Service Forest Sciences Laboratory species are still being brought to the Pacific North- at Olympia, Washington. Mr. David Thysell pro- west for landscaping purposes by both the tim- vided considerable help in identifying and con- ber industry and private groups ostensibly attempt- firming the identifications of many native and ing to manage the environment. At the present introduced plant species and for critically read- time, a clear separation of communities of exotic ing the manuscript. Mr. Rick Jordan was kind species and those of native species is maintained enough to print out the first two figures from the because the exotic species are adapted to strong GIS system and calculate the areas occupied by illumination, while the native plants are over- the roads and clearings using the program ARC- whelmingly adapted to the shade on the forest View. Funding for the study was provided by the floor. The development of shade-tolerant popu- Pacific Northwest Research Station of the U. S. lations of invasive exotic species is all that is nec- Forest Service.

Literature Cited Hauke, R. L. 1993. Equisetaceae. Horsetail family. In J. C. Hickman lEd. ), The Jepson Manual. Higher Plants of Baker, H. G. 1974. The evolution of weed~. Annual Review California. University of California, Berkeley, Cali- of Ecology and Systematics 5:1-24 fornia. Pp. 94-95. Buckingham, N. M., E. G. Schreiner, T. N. Kaye, J. E, Burger, Henderson, J. A., D. H. Peter, R. D. Lesher, and D. C. Shaw and E. L. Tisch 1995. Flora of the Olympic Penin- 1989. Forested Plant Associations of the Olympic sula. Northwest Interpretive Association, Seattle, National Forest. USDA Forest Service, Pacific North- Washington. 199 p. west Region, R6 ECOL Technical Paper 001-88, Port- De Ferrari, C. M., and R. J. Naiman 1994. A multi-scale as- land, Oregon. sessment of the occurrence of exotic plants on the Hitchcock, C. L., and A. Cronquist 1973. Flora of the Pacific Olympic Peninsula, Washington. Journal of Vegeta- Northwest. University of Washington Press, Seattle, tion Science 5:247-258. Washington. 730 p. Dempster, L. T. 1993. Caprifoliaceae. Honeysuckle family. Jones, G. N. 1936. Botanical Survey of the Olympic Penin- In J. C. Hickman lEd.), The Jepson Manual. Higher sula, Washington. University of Washington Press, Plants of California. University of California, Berke- ley, California. Pp 471-475. Seattle, Washington. 286 p. Edwards, K. R., M. S. Adams, and J. Kv6t 1995. Invasion Kitamura, S., G. Murata, and M, Hori 1987. Colored Illus- history and ecology of Lythrum salicaria in North trations of Herbaceous Plants of Japan (Sympetalae/. America. In R Pysek. K. Prach, M. Rejm~inek, and 63rd edition tin Japanese), Hoikusha, Osaka, Japan. M. Wade IEds.I, Plant Invasions. SPB Academic Pub- 297 p. l ishers, Amsterdam, The Netherlands. Pp. 161-180. Kowarik, I. 1995. Time lags in biological invasions with re- Fernald, M. L. 1987. Gray's Manual of Botany. 8th ed. gard to success and failure of alien species. In P. Pysek, Discorides Press, Portland, Oregon. 1632 p. K. Prach, M. Rejm~inek, and M. Wade IEds.), Plant Fonda, R. W. 1974. Forest succession in relation to river ter- Invasions. SPB Academic Publishers, Amsterdam, The race development in Olympic National Park, Wash- Netherlands. Pp. 15-38 ington. Ecology 55:927-942. McCune, B., and L. Geiser 1997. Macrolichens of the Pa- Fonda, R. W., and L. C. Bliss 1969. Forest vegetation of the cific Northwest. Oregon State University Press, montane and subalpine zones, Olympic Mountains, Corvallis, Oregon. 386 p. Washington. Ecological Monographs 39:271-301. Moravec, J., M. Husovti, R. Neuhtiusl, and Z. Neuh~iuslovfi- Franklin, J. F., and C. T. Dyrness 1973. Natural Vegetation of Novotnfi 1982. Die Assoziationen mesophiler und Oregon and Washington. USDA Forest Service, Pa- hygrophiler Laubwalder in der T~hechischen Sozialist- cific Northwest Forest and Range Research Station. ischen Republik.-Academia, Prague, Czechoslovakia. General Technical Report PNW-8, Portland Oregon. 292 p. 417p.

Encroachment of Exotic Plants 275 Parish, R., R. Coup6, and D. Lloyd 1996. Plants of Southern Weihe, K. von 1972. August Garcke Illustrierte Flora, 23rd Interior British Columbia. Lone Pine, Vancouver. Brit- ed. Paul Parey, Berlin, Germany. 1607 p. ish Columbia, Canada 454 p. Whitson, T. D., L. C. Burill, S. A. Dewey, D. W. Cudney, B. Schofield, W. B. 1992. Some Common Mosses of British E. Nelson, R. D. Lee, and R. Parker 199l. Weeds of Columbia. Royal British Columbia Museum, Victoria, the West. University of Wyoming, Jackson, Wyoming. British Columbia, Canada. 394 p. 630 p. Toney, J. C., P. M. Rice, and E Foncella 1998. Exotic plant Wilgen, B. W. van and D. M. Richardson 1985. The effects records in the Northwest United States 1950-1996: of alien shrub invasions on vegetation structure and fire behaviour in South African fynbos shrublands: a an ecological assessment. Northwest Science 72:198- simulation study. Journal of Applied Ecology 22:955- 209. 966. Received 2 February 1999 Accepted 16 August 1999

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