Peter W. Dunwiddie

Macrofossil and Pollen Representation of Coniferous Trees in Modern Sediments From Washington

Pollen and conifer needle macrofossils were collected from surface sediments in 30 ponds near Mount Rainier to study representation of forest trees in data from modern sediments. Percentages of major taxa calculated from these data were compared with basal area measures of forest composition within 30 m of each pond. Macrofossil percentages ofAbies amabilis, Abies lasiocarpa, Pseudotsuga menziesii, Tsuga heterophylla, and Tsuga mertensiana were significantly correlated with their basal areas in nearby forests; none was greatly over- or underrepresented. Macrofossil assemblages were similar in replicate sam- ples from surface sediments, as well as from parallel cores spanning 6000 yr. Fossil needle assemblages can therefore provide estimates of past species composition in coniferous forests in the Pacific Northwest. Also, widespread distribution of pollen was demonstrated by high pollen percentages of taxa that were absent from the flora at the sample sites, such as Alnus rubra. Pollen of Pinus and Tsuga heterophylla was overrepresented, whereas that ofAbies and Tsuga mertensiana was underrepresented. Pollen percentages of most conifer taxa were twice the values reported in another study from Mount Rainier using moss polsters. Comparisons between the macrofossil and the pollen data indicate that conifer macrofossil assemblages more accurately reflect forest composition near sample sites, and also provide greater taxonomic resolution.

A 6000-Year Record of Forest History on Mount Rainier, Washington

Sediments in three ponds between 1300 - 1500 m on the south side of Mt. Rainier were examined for plant macrofossils and pollen. Macrofossils of seral species such as Abies lasiocarpa, Pseudotsuga menziesii, Pinus monticola, Abies procera, and Pinus contorta are conspicuous from 6000 to 3400 BP. These species suggest a climate that was warmer/drier than today and favored frequent fires. Neoglacial cooling may have begun 3700-3400 BP, as species typical of higher elevations became prominent; a decline in seral species after 3400 BP suggests less frequent fires. In the last 100 yr, Tsuga heterophylla became abundant and then declined at the highest elevation site. General trends in pollen percentages are similar to the macrofossil curves. Tephra deposition from Mt. Rainier and Mt. St. Helens did not produce conspicuous changes in forest composition. Few major fires are evident from charcoal and macrofossils at these sites.

Restoring Invaded Pacific Northwest Prairies: Management Recommendations from a Region-Wide Experiment

Abstract We conducted a 5-year study at 10 sites from British Columbia to the Willamette Valley aimed at improving methods for restoring degraded prairies and oak savannas. Our manager-recommended treatment combinations were applied over 4 years and included the following components: spring and fall mowing, grass-specific and broad-spectrum herbicide, and fall burning. All treatment combinations were crossed with native seed addition. As expected, we found there was no ‘silver bullet’; while some treatment combinations led to large improvements in weed control and native diversity and abundance, the optimum combination and degree of success varied across sites. Where non-native grasses are the most pressing problem, we recommend the use of grass-specific herbicides as highly effective with minimal non-target effects on native forbs and some native grasses. Fire is a useful tool for preparing a site for seeding and can be followed closely with a broad spectrum herbicide to control rapidly resprouting weeds. Careful timing of post-fire herbicide application avoids impacting later-sprouting natives. At all sites, we recommend seed addition to enhance native diversity and abundance, as our data show even relatively high quality sites are strongly seed-limited. Repeat mowing is ineffective at reducing herbaceous weed abundance. Additionally, mowing did not increase bare soil, resulting in poor seedling establishment. If fire is not an option, we recommend testing additional treatments to increase bare soil and seeding success. At all sites, we conclude that enhancing natives and controlling invasives are likely to be most successful through repeated applications of treatment combinations.

Environmental History of a Garry Oak/Douglas-Fir Woodland on Waldron Island, Washington

Abstract Understanding a systems historical conditions is a key first step in mapping out restoration goals and strategies. We examined the age structure and stem density of a stand dominated by Garry oak (Quercus garryana) and Douglas-fir (Pseudotsuga menziesii), and related this structure to its environmental history. Our reconstruction was based on 139 increment cores and 197 stem cross-sections collected from trees in 21 plots. Individuals of both species were more than 200 years old, indicating that the stand had a mixed composition for centuries. The historical tree density was ca. one-tenth that present prior to restoration (oak release) activities. This open oak/Douglas-fir savanna was maintained for centuries by fires set by Native Americans. It began to infill with oak, and later Douglas-fir, in the 1800s, particularly following Euro-American settlement in the 1860s. Douglas-fir encroachment continued throughout the 1900s, with a very large cohort becoming established in the early 1970s. This recent wave of recruitment has occurred in many sites in the region, and may reflect interactions among climatic and environmental conditions, together with changes in land use, including the cessation of livestock grazing and logging. Oak release actions were undertaken to re-open the forest structure, and involved the removal of 55% of the trees, primarily small-diameter Douglas-fir. Comparisons with historical information suggest that the post-release stand is still much denser and biased towards Douglas-fir than the stand was historically.

The Future of Restoration and Management of Prairie-Oak Ecosystems in the Pacific Northwest

Abstract The 24 papers in this issue of Northwest Science summarize research and management presented at a 2010 meeting convened by the Cascadia Prairie-Oak Partnership, a collaboration focusing on the prairie/oak ecosystems of the Willamette Valley-Puget Trough-Georgia Basin ecoregion. We present an overview that builds on these papers to consider future threats and conservation priorities in these systems. Human population growth, encroachment by woody vegetation, the spread of invasive non-native organisms, and climatic changes all will provide future challenges. Developing and implementing techniques to abate these threats will require effective collaboration, creative research, and innovative management of natural areas. One priority will be the restoration of highly degraded habitats to increase acreage of native ecosystems, create buffers, and enhance connectivity. Other priorities will focus on detecting and eradicating newly-arrived invasives, enhancing species diversity and habitat heterogeneity, and increasing ecological resilience. Long-term commitments and investments are critical. Developing realistic restoration goals will be particularly challenging, especially when assembling new communities from the ground up, and in a world with a rapidly changing climate. To assist with goal development, we propose a system for conceptualizing restoration goals so that their relative merits can be more easily compared when deciding amongst them. We suggest evaluating goals along two continua, one related to management intensity (ecological goals) and the other to ecological impacts (cultural goals). We conclude by suggesting some specific restoration and management principles that may help to further guide conservation action, and that point toward critical information needs for future research.

Inversion of plant dominance–diversity relationships along a latitudinal stress gradient

Species interactions affect plant diversity through the net effects of competition and facilitation, with the latter more prevalent in physically stressful environments when plant cover ameliorates abiotic stress. One explanation for species loss in invader-dominated systems is a shift in the competition-facilitation balance, with competition intensifying in areas formerly structured by facilitation. We test this possibility with a 10-site prairie meta-experiment along a 500-km latitudinal stress gradient, quantifying the relationships among abiotic stress, exotic dominance, and native plant recruitment over five years. The latitudinal gradient is inversely correlated with abiotic stress, with lower latitudes more moisture- and nutrient-limited. We observed strong negative effects by invasive dominant grasses on plant establishment, but only in northern sites with lower-stress environments. At these locations, disturbance was critical for recruitment by reducing the suppressive dominant (invasive) canopy. In more stressful environments to the south, the impacts of the dominant invaders on plant establishment became facilitative, and diversity was more limited by seed availability. Disturbance prevented recruitment because seedling survival depended on a protective plant canopy, presumably because the canopy reduced temperature or moisture stress. Seed limitation was similarly prevalent in all sites. Our work confirms the importance of facilitation as an organizing process for plants in higher-stress environments, even with transformations of species composition and dominance. It also demonstrates that the mechanisms regulating diversity, including invader impacts, can vary within the same plant community depending on environmental context. Because limits on native plant recruitment are environmentally contingent, management strategies that seek to increase diversity, including invader eradication, must account for site-level variations in the balance between biotic and abiotic constraints.

Multiple Treatment Combinations and Seed Addition Increase Abundance and Diversity of Native Plants in Pacific Northwest Prairies

Invasive plants, especially non-native perennial grasses, are a critical threat to remnant prairies and oak savannas in the Pacific Northwest. Managers must control non-native plants without adversely impacting native species in fragmented prairie remnants. We describe results of a collaborative experiment replicated at 10 sites along a 500 km latitudinal gradient. Our objectives were to develop and test treatment combinations that reduce target non-native weeds with minimal nontarget impacts and increase native species diversity and abundance. By replicating experiments across the ecoregion, we tested strategies for widespread applicability. We compared four different combinations of seed addition and disturbance treatments comprising herbicide (sethoxydim and glyphosate), fire, and mowing. Each combination was created to target various factors likely to limit restoration in this system, including invasive species, litter accumulation, and limited dispersal of native species. After three years, the treatment combinations varied widely in their effectiveness. The most disturbance-intensive treatment combination (joint application of sethoxydim, burning, and postfire glyphosate) led to reduced abundance of non-native grasses and forbs without causing a decline in native species. Sethoxydim combined with fall mowing reduced non-native grasses, caused no change in non-native forbs, and increased total cover of native plants. In all cases, disturbance treatments reduced non-native cover to varying degrees but had no positive impact on native diversity except when seeds were added. Our results show that a combined treatment approach employing a variety of strategies codesigned by managers and ecologists is an efficient and effective way to improve degraded grasslands.

Fire as a Restoration Tool in Pacific Northwest Prairies and Oak Woodlands: Challenges, Successes, and Future Directions

Abstract In Pacific Northwest prairies and oak woodlands, cessation of anthropogenic burning in the mid-1800s resulted in large-scale degradation and loss of habitat due to tree and shrub encroachment. Widespread invasive species, deep thatch accumulations, and extensive moss cover now limit the ability of native plants to germinate and thrive. These changes in habitat structure and function have contributed to the decline of several plant and animal species. Over the past decade, prescribed fire has been increasingly applied throughout the Willamette Valley-Puget Trough-Georgia Basin Ecoregion and used in conjunction with other techniques (herbicide, seeding native species) to restore native habitat with variable results. This variability likely is a result of differential fire intensity, dictated by fuels, weather and application technique, all of which can be controlled for by altering fire season, fire frequency, pre-fire treatments and fire extent. In order to burn at the spatial and temporal scales necessary for effective habitat restoration, however, prescribed burn programs must overcome several socio-political, programmatic and economic challenges. This requires a collaborative approach to prescribed fire training, implementation and research. Future research on fire season, fire frequency, species-specific responses to fire and effects of fire surrogates on ecosystem structure and functioning will help to refine prescribed fire management for maximum effectiveness in prairie and oak woodland restoration.

Rethinking Conservation Practice in Light of Climate Change

Predicted changes in climate present unusual challenges to conservation planners, land managers, and restoration efforts directed toward preserving biodiversity. Successful organisms will respond to these changes by persisting in suitable microsites, adapting to novel conditions, or dispersing to new sites. We describe three general categories of strategies for restoring and managing natural systems in light of likely changes in future climate that collectively embrace many of the approaches that The Nature Conservancy is applying or considering in the state of Washington. Component redundancy suggests that in natural systems greater ecosystem resilience, despite changing climates, may be achieved by increasing species and community redundancy. Functional redundancy is the idea that different components of a system can fulfill the same functions, thereby producing the same result. Restoration projects and managers of natural systems can introduce ecologically equivalent species or novel associations of species, which may help avoid losses in biodiversity. Increased connectivity suggests that success is achieved by ensuring that suitable habitats are always within easy reach of one another. This includes conservation approaches that provide linkages, corridors, or other mechanisms to facilitate the movement of organisms as they respond to climate changes. We acknowledge that these approaches are not without risk, nor do they necessarily ensure success. However, we propose them as potential solutions among a growing suite of alternative strategies for incorporating climate change into conservation actions.

Fire History of a Douglas-Fir-Oregon White Oak Woodland, Waldron Island, Washington

Abstract The last 100 years have seen a marked decline in Oregon white oak woodlands in the Puget Sound region. Efforts to restore the woodlands cannot hope to be successful unless the role fire has played in maintaining them in the past is understood. A fire scar chronology was constructed from a Pseudotsuga menziesii—Quercus garryana community within a 155 ha study site on southeast Waldron Island, Washington. Sixty-two scars were identified on 15 crossdated Pseudotsuga samples that documented fire events between 1530 and 1908. A master tree-ring chronology was created for the period 1685 to 2004. Composite fire intervals and individual-tree fire intervals were used to characterize the fire history. Seasons of past fires were determined by analyzing fire scar position within annual ring structure. For the historical period 1700–1879, the composite mean fire return interval (FRI) was 7.4 yrs, and the mean individual-tree FRI was 18.4 yrs. The historic period mean individual-tree FRI was 18.4 yrs. In contrast, only three fires were recorded during the settlement/modern period (1880–2004), resulting in a mean individual-tree FRI of 103.8 yrs. Seasonality of past fires indicates that most fires occurred during late summer and fall. No evidence of spring or early summer burning was detected. Our study of the fire history for a site on Waldron Island shows a marked reduction in fire frequency between the historical and settlement/modern period, which we interpret as reflecting declines in Native American population size and activity and the eventual cessation of deliberate ignitions by Native Americans.