Invasion of the Exotics: the Siege of Western Washington
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PNW Pacific Northwest Research Station INSIDE The Man-Made Heritage in Forests . .2 Soil Seed Banking on the Olympic Peninsula . .3 Bad News for Small Mammals . .4 Broom and Doom in the Puget Trough? . .4 Variable-Density Thinning: Deliberate Fragments . .4 Reserves Versus True Complexity . .5 FINDINGS issue thirty-eight / october 2001 “Science affects the way we think together.” Lewis Thomas INVASION OF THE EXOTICS: THE SIEGE OF WESTERN WASHINGTON IN SUMMARY Settlement of the Pacific Northwest by immigrants from the Eastern United States changed the composition of lowland landscapes from dominance by forests and prairie to dominance by cities, suburbs, agriculture, and trans- portation infrastructure. Historical disturbance regimes imposed by English ivy is spreading throughout Holly is becoming widespread throughout nature and by indigenous people were ➢ forests in the Puget Trough, and even ➢ forest understories in the Puget Trough, large, dominant trees within the forest crowding out native, less aggressive disrupted. In the uplands, timber are not immune from their stranglehold. species. management activities replaced natu- ral disturbance regimes, often with Described by some as a form of plant and planted to beautify median strips, this well-known shrub now threatens some simplifying effects on ecosystems. biological pollution, these parts of Washington and Oregon with its People intentionally and unintention- species have been introduced— own pretty form of biological desert. sometimes by accident, but ally imported exotic species of plants It is not alone. Weedy exotics such as often with the best intentions— English ivy, Himalayan blackberry, holly, and animals. into systems where they thrive sweet cherry, and many others are spread- at the expense of native species. ing through forests at rates that are alarm- Simplification of ecosystems and intro- ing forest ecologists. duction of new species are contributing Oregon’s Living Landscape, 1998 “Initially, none of them appeared to be a to globalization of world flora, endan- big problem, but the trend is certainly he golden blaze of glory that lights alarming,” says Andrew Carey, a research gering indigenous species, and reduc- the freeway in the spring... Admit biologist with the Pacific Northwest it, you’ve admired the showy ing ecosystem function. Some perma- T Research Station in Olympia, Washington. blooming of Scot’s broom on your journey “Simplification of ecosystems and intro- nent changes have been wrought; along I-5. You might even have caught the duction of new species is helping to glob- smell if you were game to wind down the reversal of other changes and main- alize world flora, endanger indigenous window. species—especially those with narrow taining indigenous flora and fauna Yes, it lights the freeway, but it also blights habitats—and restrict ecosystem func- will require purposeful management. the forest. Introduced as an ornamental tions.” Globalization of flora is the process by which certain aggressive species tend to KEY FINDINGS dominate plant communities throughout the world, particularly where human activ- • Studies of soil seed banks in young, closed-canopy, second-growth forests ities are making conditions more con- revealed that 30 percent of all species were weedy exotics, only 24 percent ducive to invasion than they are naturally, were natives, and no tree species were present. Thinning these stands could he explains. Carey has observed that favor exotic species. where natural resources are managed in a homogeneous way, such as in plantation • Many of the small mammal communities in uniform second-growth stands are forestry or row-cropping, the propagules incomplete and differ in structure from communities in naturally young- or old- of aggressive weedy species become so growth stands. pervasive that they can move even into • Variable-density thinning has the potential to increase species richness within a suboptimal sites. short timeframe, provided it is used in a holistic approach that includes biologi- “Some permanent changes have been cal legacies, long rotations, and tree species diversity. wrought where local environmental condi- tions have favored them,” he says. “It will • Exotics may have altered the fuel environment in white oak forests sufficiently take purposeful management to reverse to pose a threat even to large-diameter oaks. Further, western gray squirrels less permanent changes and maintain have dramatically declined in these ecosystems in the last 10 years. indigenous flora and fauna.” THE MAN-MADE HERITAGE IN FORESTS n both Oregon and Washington, to rapid growth of timber and fiber. Fewer toward various late-successional goals, many low- to mid-elevation forested resources were intentionally directed to including the pivotal one of complexity of I landscapes are dominated by even- maintaining biodiversity.” understory. Many silvicultural tools are aged, young forests of a single species, now available to managers for such After decades of this approach, one of the often Douglas-fir. These landscapes are purposes: various harvest systems, reten- most apparent effects is the invasion by typically clearcut descendants. tion levels and patterns, regeneration exotics, which are attracted, opportunisti- methods, and rotation lengths. Shifts in thinking about past management cally, to areas of low competition and are coinciding with increases in human wide open spaces. They have the ability to Carey is involved with three sets of large- population, decreases in timber harvesting, respond rapidly, invade rapidly, occupy scale experiments examining the develop- and concerns about sustainability of sites for a long time, and survive in poor mental effects of exotic species on ecosys- managed forests, Carey notes. conditions. tem function in western Washington; he emphasizes that the problem is certainly “Past management often simplified forest Natives with small, particular niches not limited to his own state. The three composition and structure,” Carey beware. different studies have involved three explains. “Clearcutting, burning, planting Nonetheless, Carey believes that appropri- different forest types, and have all raised of genetically uniform trees, application of ate management intervention can help similar concerns. herbicides, and precommercial thinning these uniform second-growth forests move have diverted many ecosystem resources Purpose of PNW Science Findings To provide scientific information to people who make and influence decisions about managing land. PNW Science Findings is published monthly by: Pacific Northwest Research Station USDA Forest Service P.O. Box 3890 Portland, Oregon 97208 (503) 808-2137 Sherri Richardson Dodge, Editor [email protected] Scot’s broom, originally brought in to beautify roadsides, readily invades newly disturbed ➢ areas. Check out our web site at: United States Forest Service http://www.fs.fed.us/pnw Department of Agriculture 2 SOIL SEED BANKING ON THE OLYMPIC PENINSULA n the early stages of stand develop- ment, coniferous forests of the LAND MANAGEMENT IMPLICATIONS I coastal Pacific Northwest commonly pass through a period of dense shade and • Invasion by exotics and globalization of flora is a real and present problem, intense competition. During this phase, the requiring thoughtful landscape-level planning. abundance and diversity of understory plants decline dramatically. • Management of forests that includes a mix of tree densities at the appropriate scales may help maintain indigenous species and the natural heritage of people This closed-canopy stage represents a crit- in the Pacific Northwest. ical juncture in the development of the forest understory. Canopy closure may • Novel management techniques will be needed for restoration of extirpated result in the local extinction of some forest indigenous species to second-growth forest. species and thus shape longer term patterns of understory composition and diversity, Carey explains. DOUGLAS-FIR FOREST IN THE PUGET TROUGH A prevailing theory is that thinning dense young forests will help stimulate the 120 suppressed understory. But the mecha- A nisms by which understory species persist 100 ➢ Total plant cover in or reestablish are poorly understood. A second-growth, Douglas- 80 fir forests in the Puget cooperative study with Carey and Connie Trough: never-thinned Harrington, a research forester at the PNW 60 stands with old-growth Research Station, Charles Halpern, a legacies (Legacy 40 research associate professor at the Controls), legacy stands University of Washington, and others from Percent Cover 3 years after treatment 20 the university examined the potential with variable-density contribution of the bank of seeds retained 0 thinning (Legacy VDT), Legacy Legacy Thinned Thinned conventionally thinned in the soil. The Olympic Peninsula was Control VDT Control VDT selected for its extreme tree densities and stands (Thinned 120 Control), and thinned light exclusion during early forest regener- B stands 3 years after ation in this wet and productive climate. 100 VDT treatment (Thinned VDT). “Germination of buried viable seeds is 80 likely to be stimulated by the abrupt A = native species increases in light produced by thinning,” 60 B = exotic species Carey says. “To what degree this promotes desirable species—such as shade-tolerant, 40 Percent Cover native herbs, shrubs and trees—or encour- 20 ages the germination of exotic species, depends in part on the composition of the 0 Legacy Legacy Thinned Thinned seed