Topic:

Vegetation and Water Resources in the 21st Century: Simulating the Potential Impacts of Global Warming

Issue:

Emissions of greenhouse gases from fossil-fuel combustion and tropical deforestation could warm the Earth by 1.5-3.5^o C, or more within the next century, a rate of warming unprecedented within the past several thousand years. The potential impacts of such rapid global warming are sufficiently serious to have produced an international effort of ca. 160 countries under the United Nations' Framework Convention on Climate Change (FCCC) to slow global warming, through emissions control and other land-use policies. The FCCC has commissioned the Intergovernmental Panel on Climate Change (IPCC) to conduct assessments at regional to global scales of the potential impacts of global warming and their possible feedbacks to climate change.

Findings:

· A Landscape to global vegetation distribution model, MAPSS (Mapped Atmosphere- Plant-Soil System), has proven capable of simulating the changes in vegetation distribution and runoff under altered climate and CO₂ concentration. The MAPSS model is process- based, allowing incorporation of the physiological effects of elevated CO₂ on plant water- use-efficiency and productivity. The model fully couples the vegetation with site hydrology allowing joint assessments of changes in vegetation distribution and water resources. MAPSS simulates both the type of vegetation (e.g., conifer or broadleaf) and it's density and is thus able to accurately simulate the characteristics of all upland vegetation from deserts to wet forests.

· The IPCC has made extensive use of the MAPSS model output in regional and global assessments of climate change impacts on vegetation, especially Forest Resources. The IPCC is commissioned to produce assessments every five years, the second of which was produced in 1995 (Climate Change 1995, Cambridge University Press, 1996). MAPSS output was utilized in the analysis of impacts on the forest sector (Volume II, Impacts Adaptations, and Mitigation of Climate Change) and in the analysis of biosphere- atmosphere feedbacks (Volume I, The Science of Climate Change).

· The IPCC produced a special report to assist in the recent Kyoto negotiations of the FCCC on controlling the emissions of CO₂. The new report, The Regional Impacts of Climate Change, Cambridge University Press, 1998, focused on multi-sector assessments within ten large regions of the globe, one of which was North America North of Mexico. The MAPSS team leader, Ron Neilson, was appointed by the Office of Science and Technology Policy of the White House as the Lead Author of the forest sector for the North American region. Dr. Neilson produced new MAPSS simulations for the special report, based on the latest future climate projections and in so doing was able to provide assessment products to all ten regions for the overall assessment of vegetation and water resources. The global assessment results were presented in a special Annex to the report.

· Under future global warming, forest boundaries that are limited by cold temperature will shift toward the poles and upward in elevation. However, boundaries that are limited by water, for example, transitions between closed forest and open savanna, could shift toward either drier or wetter conditions, depending on several factors. Increased temperatures will tend to cause drought-induced declines; regional increases in precipitation could mitigate the negative effects of increased temperatures; and, elevated CO₂ concentration could increase the water-use-efficiency of trees and also mitigate the negative impacts of elevated temperatures.

· In the early stages of global warming, benefits from elevated CO₂ concentrations could dominate the negative effects of increased temperatures, resulting in a broadscale increase in productivity and density of most forests worldwide. However, the benefits from elevated CO₂ could possibly saturate within a few decades; while, temperatures continue to rise. Eventually, perhaps toward the end of the 21st century, the negative effects from elevated temperature would become dominant and forests could begin a very large-scale drought-induced decline, thus negating the earlier gains.

· Temperate Conifer Forests in the Northwest will expand in importance in Alaska and in Southwest Canada. Tundra and Taiga/Tundra regions of Alaska will be much reduced in area with expansions of both Boreal and Temperate Conifer forests into those regions. Forests in Washington and Oregon could initially expand in area and density over the next few decades. However, if warming continues, early gains could be more than offset by later losses in both area and density of forests. Drier forest types, such as those in the Klamath region, could expand further north in the coast range and fires could become more prevalent west of the Cascades over the mid to latter parts of the 21st century.

· East of the Cascades, fires could increase whether the region gets greener or drier. If the region initially gets greener, more fine fuels would accumulate, potentially exacerbating the current high fuel conditions, resulting in more and larger fires under periodic shortterm droughts. Under drying conditions, later in the 21st century, high fuel loads would become dry and larger fires could again become likely.

· If forests in the Northwest initially respond with increased productivity and density, they would sequester more carbon, acting as a negative feedback to further global warming. However, increased forest densities would result in more water use by trees and less water could become available for irrigation and domestic uses. Under continued warming, if forests begin to decline, they would use less water and in combination with less snow formation and earlier melt could result in increased winter flows with possible flooding.

· The MAPSS models are being continually enhanced. MAPSS was originally a steady- state biogeography model, being able to simulate a map of potential natural vegetation under a long-term average climate. Emerging technology couples the MAPSS vegetation distribution model with two different ecosystem nutrient cycling models and a process-based fire model in order to simulate the spatially explicit dynamics of vegetation at landscape to global scales under both stable and changing climates. At the landscape scale, the dynamic vegetation model is also being coupled to a state-of-the-art watershed hydrology model. These new models will be useful for exploring management options at all scales from landscape to regional, national and global.

Management Implications:

· Traditionally, forest management agencies, worldwide, have been under the mission of managing forests for timber and wood products production. This mission is already being redefined in the United States and elsewhere to include other issues, such as water conservation, erosion, biodiversity, stream integrity and recreation. With global warming and the possibility of international greenhouse gas emissions control treaties, the forest management mission could expand further to include carbon sequestration. One school of thought would manage forests for 'Old growth' conditions, since old growth forests tend to hold more carbon than younger forests, as well as being favorable for biodiversity. Another school of thought would manage forests for younger stands that with frequent harvesting could be used to rapidly 'pump' carbon into long-lived wood products.

· Shifting distributions and changing productivity of forests would alter regional forest markets within the global forest marketplace. National and regional economies could be altered. Management agencies will be challenged to balance workforce needs in a shifting geography of supply and demand.

· Long-term forest management plans have been constructed under the assumption of a stable climate. Notable examples in the Northwest include FEMAT and the emerging Interior Columbia Basin Management Plan. If forests respond initially with expansion in area and density, to be followed by contractions in area and density, then the expectations of future growth trajectories in the current plans would need significant modification. The potential long-wave pattern of increased productivity followed by declines carries repercussions with respect to water resources, fires, carbon sequestration and biodiversity. Early gains in carbon sequestration could be lost if the climate continues to warm. However, if forests in the Northwest are allowed to accrue significant carbon, then streamflows, necessary for fish, irrigation and domestic uses, could be reduced. Balancing the management of these issues will be even more difficult than it is today.

Lead Scientist:

Ronald P. Neilson, Bioclimatologist, PNW

Collaborators:

Dominique Bachelet, Oregon State University

Chris Daly, Oregon State University

Jim Lenihan, Oregon State University

Ray Drapek, Oregon State University

Phil Sollins, Oregon State University

Susan Stafford, Oregon State University

Sue Ferguson, USDA Forest Service

Ruby Leung, Battelle, Pacific Northwest Laboratory

Mark Wigmosta, Pacific Northwest Laboratory

Steve Running, University of Montana

Dennis Ojima, Colorado State University

Bill Parton, Colorado State University

Tim Kittel, National Center for Atmospheric Research

Linda Mearns, National Center for Atmospheric Research

The MAPSS team collaborators have benefited from the collaboration by producing numerous peer-reviewed publications and further developing their own science careers. The MAPSS team is continually investigating and developing new theories of ecosystem water, energy and nutrient processes and disturbance regime processes and their interactions with vegetation community processes. Thus, the team contributes to both basic and applied science. The collaborations have also supported graduate students, which have contributed to the MAPSS science. The collaborators have also benefited from a joint approach to quantitative sciences and the management of large and complex databases. All of the collaborators are benefiting from the creation of generic ecosystem tools that can be used for investigation of basic ecosystem processes as well as for exploration of ecosystem management options in an integrated, multi-resource context.

Priority: (in rank order)

6. Natural Disturbance Regime Management

4. Framework for Integrated Management

1. Terrestrial Ecological Processes