Upland Forest Change

Accordion Container

Overview

Today the Willamette River Basin’s vegetation is predominately a mix of grasslands and croplands in the valley floor and coniferous forest in the uplands. This vegetation mix is expected to change as rising temperatures create a less-favorable climate for existing vegetation and as forest fires increase in frequency and intensity. WW2100 upland forest modeling simulated how climate change is likely to affect forest composition, forest area burned by wildfires, and the resulting impact on timber harvest and evapotranspiration. Our results suggest that climate change will become an increasing influence on forest management decisions throughout the 21st century. In our simulations, low snowpack and hotter, drier summers lead to a two- to nine-times increase in land area burned by forest wildfires. The fires open up lands to transition to new forest types better suited to the changing climate. At high elevations, cool conifer forests replace subalpine forests. At mid-elevations, Douglas-fir and western hemlock forest types shift to mixed forest types. Increases in wildfire reduce the availability of forestland for timber harvest and affect hydrology.

Methods

Forest Modeling in Brief

The forest modeling team developed component models for Willamette Envision that simulate how upland forests will age and change through time, given forest type, climate conditions, and disturbance by wildfire and harvest. Here we provide a brief explanation of forest modeling in Willamette Envision. For full details on methods and results from WW2100 forest modeling studies, refer to Turner et al. (2015, 2016).

On an annual basis, Willamette Envision determines the forest type and age in each modeling polygon based on information from models of forest growth and succession (called forest state-and-transition models, STMs). The STMs determine the sequence of forest types that occur over time. In the simplest case, the forest progresses from new growth following a disturbance, through different successional stages, and ultimately to an old growth forest state. Depending on the type and timing of disturbances, forest growth and succession can follow alternate pathways specified by different STMs. When a disturbance occurs, the forest can also “reset” to a new forest type better suited to the current climate conditions. These new “potential” vegetation types were determined for the three WW2100 climate scenarios using offline runs of a dynamic global vegetation model called MC2. MC2 simulates wildfire occurrence and simulates the type of vegetation best suited to grow at a location based on climate, soil, elevation, and latitude.

Willamette Envision simulates forest harvest on the landscape, according to criteria defined for each scenario. For example, a scenario can specify a harvest rate (the total area of forest harvested each year) for specific forest age classes and land ownership categories (e.g., private lands with forests older than 40 years). Over the simulation, Willamette Envision randomly selects modeling polygons that meet the criteria for harvest. Users can also prescribe the extent of wildfire (the forest area burned per year), and Envision places fires randomly on the landscape. In WW2100, the extent of wildfire was determined from historical observations and the offline runs of MC2 for the three WW2100 climate scenarios. MC2 takes into account factors such as air temperature, relative humidity, and ensuing forest moisture conditions, and determines the area of forests that burn each year. Hotter, drier conditions lead to more extensive wildfires.

Additional Modeling Details:

The initial condition of the landscape, which classifies different species of vegetation, and state and transition models (STMs) were based on work from the Integrated Landscape Assessment Project (ILAP) (Halofsky et al., 2014; INR, 2013). Boundaries for land ownership and protection status were from the US Geological Survey (GAP, 2014).
Harvest rates in the Reference Case scenario were based on the observed harvest rate from 1986-2010 in the Willamette River Basin (Kennedy et al., 2010; Kennedy et al., 2012). These Landsat-based observations suggested a harvest rate of 1.3% per year across all private forestland, equivalent to ~11,740 hectares per year (29,000 acres) and 0.5% per year on public lands, equivalent to ~3,240 hectares per year (8,006 acres). Harvest on public lands was limited to unreserved stands with ages between 40-80 years, since older forests are largely conserved for wildlife on public lands.
The initial probability of fire in the Reference Case scenario were based on observations of the Landsat record (Kennedy et al., 2012), and we did not stratify by ownership class. Over the 1986-2010 period, 0.2% per year of forestland area was burned in the Willamette River Basin. The future extent of fire was based on the MC2 results, thus capturing the increasing incidence of fire associated with a warming climate. Annual area burned was input to Willamette Envision, and fires were placed randomly on the landscape. Fire size was 22,500 ha except when only a fraction of that was needed to reach the prescribed total area burned.
We assigned values for Leaf Area Index (LAI), a measure of forest canopy cover, for each forest type and stand age class based on off-line runs of the Biome-BGC productivity model (Thornton et al., 2002). LAI is used to estimate forest evapotranspiration in hydrologic modeling.

Results

Select Findings from Upland Forest Analysis

From the valley floor east to the crest of the Cascade Range, air temperatures decrease and precipitation increases as elevation rises (Fig. 1). These gradients drive a change from maritime conifer to cool needleleaf forest and ultimately to subalpine conifer forest. This vegetation mix is expected to change as rising temperatures create a less-favorable climate for existing vegetation and as forest fires increase in frequency and intensity. Here we highlight some of the key findings from WW2100 forest modeling. For more detailed analysis, refer to Turner et al. (2015, 2016).

Figure 1. The Willamette River Basin study domain: a) Vegetation cover and conifer age class, b) Elevation.

Figure 1. The Willamette River Basin study domain: a) Vegetation cover and conifer age class, b) Elevation. (Figure from Turner, 2015)

Wildfire

Under the various climate scenarios, frequency of wildfires relative to the historical period increases as temperature increases.
Under the Low Climate Change (LowClim) scenario, the area burned per year is only slightly below the historical rate.
Under the Reference Case (Reference) scenario, the simulated forest area burned per decade in the 2010-2100 period is 0.6% per year (vs. 0.2% per year in recent decades). Fire tends to be concentrated in particular years, with as much as 25% of the forested area burning in a high fire year late in the 21st century (Fig. 2).
The High Climate Change (HighClim) scenario (warmest temperatures) induces the largest areas of fire per year, with average area burned per year increasing by a factor of nine relative to the historical period.
In the Reference and HighClim scenarios, the proportion of the Willamette River Basin that is recently burned and relatively open increases. (Note: WW2100’s simulations did not include pest and pathogen disturbances, which are also likely to increase with climate warming.) More relatively open areas result in lower mean leaf area. These decreases in leaf area lead to reduced growing season evapotranspiration, despite higher evaporative demand due to higher temperatures.
Several of the WW2100 alternative scenarios influence the forest uplands. In the case of the Upland Wildfire Suppression (FireSuppress) scenario, the incidence of fire is maintained at the contemporary rate (0.2% of area per year). This assumption results in an increase in the proportion of the forest area with a difference (disequilibrium) between climate and potential vegetation type. In the Extreme scenario, the average area burned rises to 0.8% per year. Results in terms of vegetation change are similar to the Reference scenario. In the Managed scenario, the assumed fire rate is low relative to the Reference scenario and rotation age is reduced. These assumptions mean that the landscape is able to sustain contemporary rates of harvest on public and private lands.

Figure 2. Area burned per year in the three climate scenarios.

Figure 2. Total area burned in the Willamette Basin: a) LowClim, b) Reference, c) HighClim.

Harvest

Under climate change, the increased prevalence and power of forest fires is expected to affect how and if trees, such as the commercially important Douglas-fir, are harvested. The WW2100 forest team found that as temperatures increase from 2010-2100 (note: all three representative scenarios show some degree of warming), forest fires increase and mature forests available for harvest correspondingly decrease. (Worth noting in these results: Today’s forest age class distribution in the Willamette River Basin differs substantially between public and privately owned forestland. A significantly larger proportion of the forestland in older age classes are on public land, which also tends to be at higher elevations.)
Under the LowClim and Reference scenarios, the harvest rate on private land is stable at about 1.5% of the area per year, and the harvest rate on public lands is stable at a rate of about 0.5% per year (Fig. 3).
Under the more severe fire regimes in the HighClim scenario, harvestable forest area decreases, such that the harvest rate begins declining towards the end of the century on both private and public forestland (Fig. 3).

Figure 3. Area harvested per year (public and private): a) LowClim, b) Reference, c) HighClim.

Figure 3. Area harvested per year (public and private) in three Willamette Water 2100 modeling scenarios: a) LowClim, b) Reference, c) HighClim. (Figure from Turner, 2015)

Vegetation Shifts

Plant species in the Northern Hemisphere are moving to higher latitudes and elevations in response to climate change. These climate-induced shifts in vegetation are due primarily to rising temperatures, as plant species migrate to areas with temperatures ranges they are adapted to. This shift in vegetation, already observed in the Northwest, is expected to continue under climate change. WW2100’s forest team produced the following findings concerning vegetation shifts and climate change for the Willamette River Basin:

The climate-induced shifts in potential vegetation cover type for the Willamette River Basin under the three climate scenarios are proportional to the magnitude of the climate change (Fig. 4). Under the LowClim scenario (least warming), there is little change in potential vegetation type, whereas with the HighClim scenario (highest warming) the potential vegetation cover type changes over the entire Willamette River Basin by the end of the 21st century.
Existing forest vegetation types tend to be replaced by other types of forest.
Subalpine forests, now dominated by Subalpine fir, are replaced by cool conifer forests, such as the Pacific silver fir.
At the mid-elevations, the maritime conifer forest, generally associated with Douglas-fir and western hemlock, shift to more mixed forest types with hardwood species such as the Big Leaf Maple, and conifer species such as the Grand fir increasing in dominance.
Potential vegetation cover type in the relatively low elevation Willamette Valley periphery change from maritime conifer to subtropical mixed forest in the Reference and HighClim scenarios, here driven by the coldest month temperature. The most likely broadleaf species to replace the existing Willamette Valley maritime conifer species are the Pacific madrone and tanoak. Both species currently dominate at mid-elevations in the Sierra Nevada Mountains 800 km (500 miles) to the south. These evergreen broadleaf species would benefit from the projected warmer winters and can tolerate drier summers.
The change in the actual vegetation type following a disturbance lags the change in potential vegetation type, meaning what vegetation type the climate could support based on the MC2 model runs, in all scenarios. By 2100 in the LowClim scenario, the area of upland vegetation in disequilibrium is 22% of the total forest area. However, that proportion rises in the two warmer climate scenarios (53% in the Reference and 56% in HighClim scenarios) by the end of the century. Much of the speckling in the actual vegetation cover by 2100 (Fig. 5b) is where stand replacing disturbances, either harvests or fire, induces a change in vegetation cover type to the underlying potential vegetation cover type (Fig. 5a).

Figure 4. Time series for potential vegetation cover type proportions of the Willamette River Basin uplands: a) LowClim, b) Reference, c) HighClim.

Figure 4. Time series for potential vegetation cover type proportions of the Willamette River Basin uplands: a) LowClim, b) Reference, c) HighClim. (Figure from Turner, 2015)

Figure 5. Vegetation distribution in 2100 for the Reference Case scenario: a) Potential Vegetation Cover type (from MC2), a) Actual Vegetation Cover type (from Envision).

Figure 5. Vegetation distribution in 2100 for the Reference scenario: a) Potential Vegetation Cover type (from MC2), a) Actual Vegetation Cover type (from Envision). (Figure from Turner, 2015)

Conclusions

The recent climate in the western U.S. is already warmer than in previous decades, and increases in tree mortality have been linked to climate change. Spatially explicit landscape simulation of potential and actual vegetation could be particularly effective in adaptation efforts. Using climate observations, stands in different locations could be regularly assessed for the degree to which the vegetation type is out of equilibrium with the local climate, and hence at risk for attack by pests and pathogens. The most vulnerable stands could be prioritized for thinning or harvest.

Dynamic global vegetation models (DGVMs) driven by the latest downscaled climate data could provide resource managers with guidance on what type of vegetation to replant after a disturbance. Our results support the conclusion that climate change will become an increasing influence on forest management decisions throughout the 21st century. The projected increase in the risk of fire points to investments in fire management.

More Information

Contributors to WW2100 Forest Research

David Turner, OSU Forest Ecosystems & Society (lead)
David Conklin, Oregon Freshwater Simulations
John Bolte, OSU Biological & Ecological Engineering

References

GAP. (2014). US Geological Survey, Gap Analysis Program (GAP). National Land Cover, Version 2. http://gapanalysis.usgs.gov/gaplandcover/data/

Halofsky, J. E., Creutzburg, M. K., & Hemstrom, M. A. (2014). Integrating social, economic, and ecological values across large landscapes (General Technical Report PNW-GTR-896). Corvallis, Oregon: USDA Pacific Northwest Research Station.

Halofsky, J. E., Hemstrom, M. A., Conklin, D. R., Halofsky, J. S., Kerns, B. K., & Bachelet, D. (2013). Assessing potential climate change effects on vegetation using a linked model approach. Ecological Modelling, 266, 131-143. http://dx.doi.org/10.1016/j.ecolmodel.2013.07.003

INR. (2013). Integrated Landscape Assessment Project. Retrieved October 15, 2015, from http://inr.oregonstate.edu/ilap

Kennedy, R. E., Yang, Z., & Cohen, W. B. (2010). Detecting trends in forest disturbance and recovery using yearly Landsat time series: 1. LandTrendr—Temporal segmentation algorithms. Remote Sensing of Environment, 114(12), 2897-2910. http://dx.doi.org/10.1016/j.rse.2010.07.008

Kennedy, R. E., Yang, Z., Cohen, W. B., Pfaff, E., Braaten, J., & Nelson, P. (2012). Spatial and temporal patterns of forest disturbance and regrowth within the area of the Northwest Forest Plan. Remote Sensing of Environment, 122, 117-133. http://dx.doi.org/10.1016/j.rse.2011.09.024

Path Landscape Model. (2015). Retrieved October 15, 2015, from http://essa.com/tools/path-landscape-model/

Rogers, B. M., Neilson, R. P., Drapek, R., Lenihan, J. M., Wells, J. R., Bachelet, D., & Law, B. E. (2011). Impacts of climate change on fire regimes and carbon stocks of the US Pacific Northwest. Journal of Geophysical Research: Biogeosciences, 116(G3). http://dx.doi.org/10.1029/2011JG001695

Thornton, P. E., Law, B. E., Gholz, H. L., Clark, K. L., Falge, E., Ellsworth, D. S., … Sparks, J. P. (2002). Modeling and measuring the effects of disturbance history and climate on carbon and water budgets in evergreen needleleaf forests. Agricultural and Forest Meteorology, 113, 185–222.

Turner, D. P., Conklin, D. R., Vache, K. B., Schwartz, C., Nolin, A. W., Chang, H., ... & Bolte, J. P. (2016). Assessing Mechanisms of Climate Change Impact on the Upland Forest Water Balance of the Willamette River Basin, Oregon. Ecohydrology. http://dx.doi.org/10.1002/eco.1776

Turner, D. P., Conklin, D. R., & Bolte, J. P. (2015). Projected climate change impacts on forest land cover and land use over the Willamette River Basin, Oregon, USA. Climatic Change, 133(2), 335-348. http://dx.doi.org/10.1007/s10584-015-1465-4

Web page authors: D. Turner, N. Gilles
Page last updated: September 2016

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About Willamette Envision

Willamette Envision is a whole watershed model, i.e. it attempts to represent all the significant processes related to the supply and fate of water in the entire basin. These processes are both natural (e.g. precip, snow dynamics, infiltration, runoff, evapotranspiration) and human (e.g. reservoir operations, irrigation, municipal water use, crop choice). The model takes exogenous projections of climate, population, and income as its drivers. It operates by simulating the processes across the entire basin for one time step, recording the effects on the landscape, and then advancing to the next timestep. Some processes are modeled at a daily timestep, for example streamflow and evapotranspiration, while others are modeled at an annual timestep, such as population growth and urban expansion. For more information about Willamette Envision, refer to the model introduction page.

Water cycle diagram. Image modified from NASA, https://pmm.nasa.gov/education/water-cycle.

About WW2100 Modeling Scenarios

The Willamette Water 2100 project modeled 22 scenarios, a reference or base case, and a suite of alternative scenarios. The Reference Case scenario represents future conditions in the Willamette River Basin, under expected trends in population and income growth, existing policies and institutions, and a mid-range climate change projection. A suite of 18 alternative scenarios explore the influence of a single model driver or policy setting at a time. They each vary a single element or assumption from the Reference Case. Three alternative scenarios vary multiple scenario elements from the Reference Case and represent plausible thematic narratives such as “Extreme” or “Worst Case.” Refer to the scenarios page for a detailed description of the WW2100 modeling scenarios, their purpose, and their assumptions.

Willamette Water 2100

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