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Climate—induced forest dieback as an emergent global phenomenon: patterns, mechanisms, and projections


Recent episodes of forest stress and dieback are apparent on all forested continents of the world. In particular, substantial episodes of recent forest mortality have occurred in North America from Alaska to Mexico, affecting more than 20 million hectares and many tree species since 1997, a period of warming temperatures and significant drought in many areas. Climate change models predict substantial shifts in climatic patterns over coming decades in many regions, including warmer temperatures and increases in duration and severity of extreme drought events. Such changes increase stress on long—lived woody vegetation, directly leading to increased mortality and episodes of forest dieback. In some cases forest dieback is increased even more by climate—mediated changes in populations of insect pests, or human—altered land—use patterns and disturbances like forest fragmentation and increased fire activity. Assessing the potential for extensive climate—induced forest dieback is a key global change research topic, since woody mortality losses can occur much faster than tree growth gains, with pervasive and persistent ecological effects, including feedbacks to other disturbance processes (e.g., fire, erosion) and loss of sequestered carbon back to the atmosphere. In this session, USGS and over 20 researchers from around the world present a synthesis of climate—induced forest dieback as an emergent global phenomenon, including an international overview from ongoing research and existing literature. Collectively the papers in this organized oral session highlight global examples of forest dieback, physiological process drivers of woody plant mortality, and applications of available knowledge to regional and global scale modeling and prediction of forest dieback. Craig D. Allen, Symposium OOS 42 —— Climate—induced forest dieback as an emergent global phenomenon: patterns, mechanisms, and projections, Thursday, Aug. 9, 1:30—5:30 p.m.

Apparent climatically induced increase of tree mortality rates in a temperate forest: After tracking the fates of more than 20,000 trees in a network of old—growth forest plots in the Sierra Nevada of California for over two decades (1983 — 2004), USGS scientists found that tree death rates have increased significantly over the past 20 years. Death rates increased not only for all trees combined, but also across most elevational zones and for the two dominant groups of conifers, firs and pines. The rising death rate for trees was paralleled by increasing summer drought due to warming temperatures. These findings suggest that Sierran forests, and potentially other forests of dry climates, may be sensitive to temperature—driven increases in drought, making them vulnerable to extensive die—back during otherwise normal periods of reduced precipitation. Phil van Mantgem, COS—142 —— Climate change: physiological and population response, Friday, Aug. 10, 10:10 a.m.

Multi—species community response to extirpation of a top carnivore on San Miguel Island, Channel Islands, California: The island fox was the sole terrestrial carnivore in the simplified ecosystem on San Miguel Island, part of the southern California Channel Islands. The loss of the fox from the island in the last 10 years has been followed by many dramatic community changes. The population of deer mice —— the only native small mammal on the island —— now exhibits cyclic multi—year fluctuations, and changes in deer mouse population dynamics may have in turn affected aspects of vegetation and plant species ecology. The number of avian predators on the island has increased as well, particularly the northern harrier. Previously, harriers were uncommon winter visitors to San Miguel, but now many of these ground—nesting raptors breed on the island every year. Non—native black rats were formerly restricted to a few isolated areas around the periphery of the island, and have now spread more widely. Likewise, western gulls were formerly restricted to offshore rocks, and now nest on the main island of San Miguel. USGS researcher Charles Drost will discuss these myriad changes in relation to management of San Miguel Island, with a specific focus on the rats and gulls. The changes on San Miguel have direct implications for other islands in the Channel Islands group, and probably for other island systems as well. Charles Drost, OOS 47 —— The California Channel Islands: conservation and restoration in dynamic ecosystems, Friday, Aug. 10, 10:50 a.m.

Patterns of rarity among the listed plants of Santa Cruz Island: using the raw materials for recovery: More than a century of intense grazing has changed and degraded the landscape within which the native plants of the Channel Islands evolved. Now, after the removal of non—native feral pigs in 2005—2007, Santa Cruz Island is poised for potential rapid rebound, but rapid change brings special challenges for rare plants. On Santa Cruz Island, there are 45 plant species found only in the Channel Islands, 9 of which are federally listed as threatened or endangered. These nine species are quite different in the kinds of habitats they prefer and span a range of life histories from tiny annuals seen only intermittently to long—lived shrubs. All, however came through the intense ranching period with relatively few, widely separated populations that now provide the raw materials for population recovery. USGS surveys show that most of the populations that were known in the mid—1900s persist today but at variably low numbers, some of which are already showing increases since feral sheep were removed about 20 years ago. USGS research reveals that each of these plant species faces unique challenges for recovery, resulting from such diverse factors as loss of pollinators, altered habitats, and instability caused by weather patterns and altered microclimate regimes. Katherine Chess, OOS 47 —— The California Channel Islands: conservation and restoration in dynamic ecosystems, Friday, Aug. 10, 2007, 8:20 a.m.

Impacts, effects, and food webs: integrating science and conservation perspectives into non—native species management on the Channel Islands: Unanticipated outcomes from non—native species management programs provide compelling evidence that “restoration” will often not be a useful concept to base these programs on. Management of non—native species, especially in systems with many invaders that have been present for long periods, is likely to send systems on unpredictable trajectories. This may lead to alternative ecosystem states and transitions, some of which may or may not meet “restoration” or “recovery” goals. Non—native species management programs would benefit greatly by: conducting studies that are not focused exclusively on documenting “impacts,” but identifying the roles non—native species play within ecosystems; using multi—trophic level conceptual models as a management program framework; identifying the phase of the invasion process target species are in and linking this explicitly to likely management options; planning and evaluating program outcomes from both conservation and ecological perspectives; and, implementing monitoring programs several years before, during, and after management actions. This framework is especially important in the Channel Islands, where most management actions are, in one way or another, related to reducing or eliminating effects from non—native plants and animals. Rob Klinger, OOS 47 —— The California Channel Islands: conservation and restoration in dynamic ecosystems, Friday, Aug. 10, 9:00 a.m.

Linking response of aquatic biota to road restoration activities: Between 1973 and 1975, USGS collected aquatic biological data in streams in the Redwood Creek watershed, in northern California. At that time, the basin had extensive areas of timber harvest, unstable hillslopes from road construction, and eroding stream channels. Since then, about 300 kilometers of abandoned logging roads have been removed, and previously logged areas have revegetated. In 2004 and 2005, USGS researcher Mary Ann Madej resampled the previously studied streams to compare aquatic health in restored basins. She found that over the last 30 years, sediment transport rates in Redwood Creek have decreased, pools are more frequent and deeper, streamside canopy cover has increased, and summer water temperatures are now adequate for salmon.. Likewise, the number of filtering—collector aquatic insects, which are expected to increase in response to disturbance, were higher in 1974 than in 2004. Insects with life cycles of 2 or more years, indicative of more stable channel conditions, were more abundant in 2004—2005 than in 1974, and insect diversity was also higher, although the density and biomass of tailed frogs in restored basins were still lower than in pristine streams. Based on a small sample size of captured fish, steelhead condition also improved in streams from 1974 to 2005. Habitat improved both in basins with road restoration and basins in which logging has ceased. Mary Ann Madej, OOS 8 —— Restoring physical and ecological connections in roaded landscapes, Tuesday, Aug. 7, 8:20 a.m.

Does post—fire seeding in a Mojave Desert shrubland rehabilitate habitat for the desert tortoise? Wildfires in the Mojave Desert alter habitat for the threatened desert tortoise by removing shrub structure and enhancing invasive annual grasses. In June 2005, lightning—ignited wildfires burned almost a half million acres of desert tortoise habitat in southern Nevada. In December 2005, burned habitat was seeded with native species representative of intact tortoise habitat to accelerate the establishment of shrubs and herbaceous forbs. USGS researcher Lesley DeFalco and her colleagues evaluated the success of seeding and determined whether tortoises used seeded burned areas. In the first year after seeding, plant establishment was low, and seedling densities were not significantly different between seeded and unseeded treatments. However, this result reflects rainfall that was over 40 percent below average during the 6 months following seeding, and the potential suppression by invasive grasses. Tortoise activity — as determined by the presence of live tortoises, burrows and fresh scats — was not enhanced by the seeding. However, less tortoise activity in the burns compared with unburned areas suggests that tortoises moved away from burned habitat one year after the fire. Summer movements of tortoises along the burn margin indicate that tortoises are foraging in burned areas, where annual plant production is abundant, while remaining close to unburned remnants that provide shrub cover to escape thermal extremes. Lesley DeFalco, COS—123 —— Ecology of arid and semiarid habitats I, Thursday, Aug. 9, 2:30 p.m.

How does vegetation mediate mercury cycling in tidal wetlands? The roles of wetland plant physiological and decay processes: Experimental and comparative studies in the mercury—contaminated San Francisco Bay—Delta have demonstrated the significant influence of plant structure and growth, rhizosphere activity, and decomposition on mercury availability and the production of methylmercury. Methylmercury is a neurotoxin of concern for human and wildlife health. USGS researcher Lisamarie Windham—Myers will review the relative influence of emergent and aquatic plant biomass on mercury cycling in tidal marshes, both as living and decomposing tissues. In addition, Windham—Myers and her USGS colleagues synthesized data from similar studies in three tidal marsh regions where mercury bioaccumulation is also of concern: New Jersey’s Hackensack Meadowlands, Louisiana’s Barataria Basin, and the San Francisco Bay—Delta region. The researchers found that the rates of methylmercury production are a function of both microbial activity and inorganic mercury availability. In wetland sediments where methylmercury production rates are limited by microbial activity, processes associated with the plant rhizosphere will have a greater influence on methylmercury rates? than will decay processes. Conversely, in wetland sediments where rates of methylmercury production are limited by inorganic mercury availability, the influence of vegetation biomass decay will dominate. Lisamarie Windham—Myers, COS 75 —— Rhizosphere: root function and root interactions, Wednesday, Aug. 8, 2007, 8:40 a.m.

Leaf size, litter flammability, and restoration fire: changes in stand composition influence pattern of burning: Over the last century, fire exclusion in the forests of the Sierra Nevada has allowed surface fuels to accumulate and has led to increased tree density. The kinds of trees in an area have also changed as shade—tolerant tree species crowd out shade—intolerant species. To restore forest structure and reduce surface fuels, managers have increasingly used prescription burning. Yet prescribed burns are constrained because they may not be able to simulate natural burning patterns, and could therefore lessen the extent to which this management practice can return forested ecosystems to a more naturally a naturally functioning state. USGS researcher Dylan Schwilk examined the roles that changes in the kinds of trees in a stand may play in influencing burning patterns. Evidence from both prescribed fire and wildfire at Sequoia National Park suggests that even leaf size, by changing how dense the litter bulk is, significantly influences the pattern and severity of burns. Shorter—leaved shade—tolerant species, which have increased since fire exclusion, may play a negative feedback role by reducing fire severity. Dylan Schwilk, Symposium 8 —— Plant functional traits as tools for ecological restoration, Tuesday, Aug. 7, 2007, 4:20 p.m.

Does the most popular consumer strategy, parasitism, matter to food webs? Parasitism is the most common animal lifestyle, yet food webs rarely include parasites. The few earlier studies have indicated that including parasites leads to obvious increases in species richness, number of links, and food chain length. A less obvious result was that adding parasites slightly reduced connectance, a key metric considered to affect food web stability. However, reported reductions in connectance following the addition of parasites resulted from an inappropriate calculation. According to USGS researcher Kevin Lafferty and colleagues, two alternative corrective approaches applied to four published studies yields an opposite result —— parasites increase connectance, sometimes dramatically. In addition, this study finds that parasites can greatly affect other food web statistics such as nestedness, chain length, and linkage density. Further, while most food webs find that the top levels are least vulnerable to natural enemies, the inclusion of parasites revealed that mid, not low, trophic levels suffered the highest vulnerability to natural enemies. These results show that food webs are very incomplete without parasites. Most notably, recognition of parasite links may have important consequences for ecosystem stability as they can increase connectance and nestedness. Kevin Lafferty, SYMP 10 —— The assembly and disassembly of ecological networks: Restoration and conservation at multiple trophic levels, Wednesday, Aug. 8, 10:10 a.m.

A simple framework for an invasive species early warning system for counties: The number of non—native plant and animal species establishing themselves in the United States continues to rise. After prevention fails at the national level and a species becomes established, reproduces, continues to spread, and becomes invasive, the most successful action at a local level is early detection followed by eradication. Consequently, it is increasingly important to have tools and technologies to predict where invasive species may show up next. USGS researchers and their partners at Colorado State University and the Biota of North America Program have developed a simple early warning modeling tool for non—native plants. This tool uses readily available species distribution and environmental data that allows better predictive capabilities when more detailed species abundance and distribution data are lacking. During this presentation, USGS researcher Catherine Jarnevich will demonstrate the usefulness of this predictive tool for ten invasive species. Catherine Jarnevich, COS 128, Invasion: management approaches and assessment, Thursday, Aug. 9, 4 p.m.

The butterflies and moths of North America: A database for research, education, and conservation: Butterflies and moths are pollinators that play a crucial role in sustaining biodiversity and agricultural production worldwide by enabling plants to reproduce. Yet many species of pollinators have been declining, some dramatically, partly due to losses of traditional migratory and nectar corridors. All pollinators are potentially threatened by climate change. Butterfly and moth management and conservation efforts are under way, but limited by information access. Distribution data and information about habitat requirements of even the most common species are scattered in published literature or limited to generalized distribution maps in field guides. USGS researcher Elizabeth Sellers will demonstrate that Butterflies and Moths of North America (BAMONA) is a user—friendly database that seeks to close the information gap by providing access to valuable butterfly and moth distribution data for the region. Collaborators in the BAMONA project are the Big Sky Institute at Montana State University, the U.S. Geological Survey Northern Prairie Wildlife Research Center, and the USGS National Biological Information Infrastructure — Mountain Prairie Information Node. More than 215,000 individual records for the United States and Mexico, and nearly 2,800 species accounts, are now accessible via the BAMONA website through dynamic distribution maps, regional checklists, and species accounts updated daily. The site is successfully communicating with a wide variety of users including professional lepidopterists, teachers, students, and the public. Elizabeth Sellers, poster, PS 72, Friday, Aug. 10: 8:30—10:30 a.m.

Development and conservation in South Florida: Integrating ecological and economic values for decision support: Intense development pressures exist in the agricultural lands outside of the Urban Development Boundary in Miami—Dade County, Florida. Decisions whether to allow development, to preserve current lands uses, or to restore historical habitats have significant effects on both local and regional ecological values, as well as future local and regional economic values. The U.S. Geological Survey, partnering with the U.S. Park Service and researchers at the University of Florida and the University of Pennsylvania, has developed a prototype for a web—enabled geospatial information tool that will help decision—makers better assess the ecological and economic values at stake in upcoming decisions. The South Florida Ecosystem Portfolio Model, or EPM, integrates ecological information with information relevant to economic land value and development patterns. Even as researchers continue to refine the underlying models, users can still explore different land use/land cover scenarios, including restoration and development scenarios, and test assumptions and explore tradeoffs among ecological and development priorities. USGS scientist William B. Labiosa will also discuss how an important potential future use of the tool is for communicating DOI interests and priorities in ecologically sensitive areas that face development pressure. William B. Labiosa, COS 4 —— Conservation ecology and ecosystem management: planning and policy, Monday, Aug. 6, 4 p.m.

Wetland restoration and birds: Case studies from San Francisco Bay, Chesapeake Bay, and the Florida Everglades: A plethora of wetland restoration projects are under way across the North American landscape, ranging from small, community—based projects of less than 1 hectare, to thousands of hectares. The goals of small projects are generally focused on replanting and sustaining native wetland vegetation, while larger projects often incorporate wildlife components as part of the criteria for “success.” USGS researcher Michael Erwin uses examples from larger restoration projects from Florida, San Francisco Bay, and Chesapeake Bay, to illustrate major challenges in planning and implementing those parts of the projects that include waterbirds. Such projects often provide an ideal setting for the application of adaptive management, but long—term data management and oversight are required to ensure that project “success” (or failure) is not short—term only. Erwin offers specific examples of pitfalls to avoid and how to avoid them to help ensure success over the long haul. Michael Erwin, OOS—12, Reducing uncertainty: Adaptive management in the South Bay Salt Pond Restoration Project, Tuesday, Aug. 7, 9:50 a.m.


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