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Examples of Scaling: Long-Term Vegetation Plots

Examples of Scaling: Long-Term Vegetation Plots


This is updated from the original regionalization document by Kari Bisbee.

Steve Acker and Art McKee
Spatial and temporal extent and resolution
History of project
Basic approach
Key findings
Future directions
Related publications/a>


To examine patterns and rates of forest succession, and to measure tree growth and mortality, we maintain a network of long- term vegetation plots across western Oregon and Washington.

Spatial and temporal extent and resolution

The network extends through much of western Oregon and Washington, with sites representing most of the major forest zones and successional stages in the region. The duration of records ranges from temporary plots measured a single time, to permanent plots with up to 90 years of observations.


Long-term observations are the only direct means to document and understand the intrinsically slow changes of our forests which are dominated by long-lived trees species (individuals commonly live 400 to 1000 years). Long-term observations can quantify levels and dynamics of natural forest structures to be used as guides in ecosystem management. Long-term observations provide the definitive measure of long-term productivity and, thus, sustainable harvest levels. Long-term observations also provide a basis for monitoring effects of global climatic change as well as other changes.

History of Project

Most of the permanent plots were established during two intervals: 1910 to 1948, and 1970 to 1989. The earlier plots were established by U.S. Forest Service researchers to quantify timber growth in young stands of important commercial species and to help answer other applied forestry questions. The more recent period of plot establishment began under the Coniferous Forest Biome program of the International Biological Program during the 1970s, and continued under the Long-term Ecological Research program. A broader set of objectives motivated plot establishment since 1970, especially quantification of composition, structure, and population and ecosystem dynamics of natural forests.

Basic Approach

The plots are best considered as point samples with the objective of understanding temporal and spatial gradients of controls. Plots have one of three spatial arrangements: (1) contiguous rectangles subjectively placed within an area of homogeneous forest; (2) circular plots subjectively placed within an area of homogeneous forest; and (3) circular plots systematically located on long transects to cover an entire watershed, ridge, or reserve. Rectangular study areas are mostly 1.0 ha or 0.4 ha (1.0 ac) in size. Circular plots are 0.1 ha (0.2 ac). The tree stratum is the focus of work in closed-forest study areas. All trees larger than a minimum diameter (5 cm for most areas) are permanently tagged. Tree diameters are remeasured, and mortality recorded, every 5 or 6 years.

The network also includes plots in early successional forest on three small (10-100 ha) watersheds within the Andrews Experimental Forest that were experimentally harvested in the 1960s and 1970s. Plots are distributed throughout the cutover areas of the watersheds, either regularly spaced along transects, or randomly placed within plant-community-type strata. Seedlings, saplings, and trees; and cover, biomass, and species composition of vascular understory plants have been measured on these plots beginning prior to harvest. Measurements were recorded annually for five to 10 years after harvest and are now taken every 3 to 4 years.

In several of the closed-forest study areas, natural disturbances have created early successional patches, increasing the diversity of environments contained within the permanent plot network. The disturbances include windthrow in coastal western hemlock- Sitka spruce forest, flooding in mid-elevation riparian old-growth at the Andrews Experimental Forest, and wildfire in mountain hemlock forest in the High Cascades.

Key findings

Acker et al. (2002) used permanent plot observations in young (10-35 yr), mature (100-120 yr), and old (450+ yr) Douglas-fir and western hemlock forests to investigate the relative contributions of reduced NPP and mortality to the decline in stand biomass accumulation with age. Bole biomass accumulation rate increased over time in the young watershed, remained constant over time in the mature watershed, and varied between positive and negative in the old watershed. They concluded that both decreasing bole NPP and increasing tree mortality contribute about equally to the decline in biomass accumulation with stand age.

Bible (2001) used plots from our permanent forest plot network to examine temporal patterns of mortality rates, tree population structure, biomass accumulation, input of coarse woody debris, and causes of mortality of Douglas-fir and western hemlock in the Cascade Mountains of western Washington and Oregon. Temporal patterns of stand structure and biomass differed between Douglas-fir and western hemlock. High mortality rates for Douglas-fir occurred in young to mature stands due largely to suppression, whereas in old growth, mortality rates were substantially lower and causes were from density-independent agents. As growth of old-growth Douglas-fir declined, death of even a few trees removed more biomass than was produced, resulting in a sharp decline in biomass stocks in old-growth forests. Rates of western hemlock mortality were low in young and mature stands, but in old-growth, rates were greater due to a combination of suppression, snow loading and physical damage from falling trees.

Future Directions

We plan to continue the measurement program, tap more of the potential of the plot network for addressing basic and applied ecological questions, and strengthen collaboration with other institutions and agencies. We are particularly interested in strengthening our ability to assess changes in carbon storage and our ability to establish the effects of climate change on forest succession. To achieve these goals will require collaboration with complementary long-term plot programs such as Continuing Vegetation Survey and Forest Inventory and Analysis of the Forest Service, and researchers studying forest dynamics with other tools, such as practitioners of remote sensing, paleoecology, and simulation modeling. We hope that future collaborative efforts will include regional syntheses and cross-biome research and monitoring.


The long-term vegetation plot network has benefited greatly from the foresight early in the 20th century of silvicultural researchers such as T.T. Munger of the Pacific Northwest Forest and Range Experiment Station (PNW) of the U.S. Forest Service. Researchers from the Olympia (especially Robert O. Curtis) and Corvallis (Jerry Franklin, Fred Swanson and colleagues) Laboratories of PNW have been instrumental in maintaining interest in the permanent plots and safeguarding the data. The plot network has been supported by grants DEB-76-11978, DEB-79-25939, DEB-80-12162, BSR-83-00370, BSR-85-14325, and BSR-90-11663 from the National Science Foundation to the H.J. Andrews Experimental Forest Long- Term Ecological Research Program, and cooperative agreements between Oregon State University and PNW.


Acker, S. A., C. B. Halpern, M. E. Harmon, and C. T. Dyrness. 2002. Trends in bole biomass accumulation, net primary production, and tree mortality in Pseudotsuga menziesii forests of contrasting age. Tree Physiology, 22: 213-217.

Bible, K. 2001. Long-term patterns of Douglas-fir and western hemlock mortality in the western Cascade Mountains of Washington and Oregon. Ph.D. dissertation. University of Washington.

Related publications

Acker, Steven A.; McKee, W. Arthur; Harmon, Mark E.; Franklin, Jerry F. 1998. Long-term research on forest dynamics in the Pacific Northwest: a network of permanent forest plots. In: Dallmeier, F.; Comiskey, J. A., eds. Forest biodiversity in North, Central, and South America and the Caribbean: Research and Monitoring; 1995 May 23-25; Washington, DC. New York, NY: The Parthenon Publishing Group, Inc.: 93-106. (Jeffers, J. N. R., ed; Man and the Biosphere Series; 21).

Acker, S. A.; Sabin, T. E.; Ganio, L. M.; McKee, W. A. 1998. Development of old-growth structure and timber volume growth trends in maturing Douglas-fir stands. Forest Ecology and Management. 104: 265-280.Acker, S. A.; Sabin, T. E.; Ganio, L. M.; McKee, W. A. 1998. Development of old-growth structure and timber volume growth trends in maturing Douglas-fir stands. Forest Ecology and Management. 104: 265-280.

Acker, S.A., P.A. Harcombe, S.E. Greene, and M.E. Harmon. 2000. Biomass accumulation over the first 150 years in coastal Oregon spruce?hemlock forest. Journal of Vegetation Science 11: 725-738.

Franklin, J. F., and D. S. DeBell. 1988. Thirty-six years of tree population change in an old-growth Pseudotsuga-Tsuga forest Canadian Journal of Forest Research 18: 633-639. Halpern, C. B., and T. A. Spies. 1995. Plant species diversity in natural and managed forest of the Pacific Northwest. Ecological Applications 5: 913-934.

Greene, S. E.; Harcombe, P. A.; Harmon, M. E.; Spycher, G. 1992. Patterns of growth mortality and biomass change in a coastal Picea sitchensis - Tsuga heterophylla forest. Journal of Vegetation Science. 3: 697-706.

Harcombe, P.A., M.E. Harmon, and S.E. Greene. 1990. Changes in biomass and production over 53 years in a coastal Picea sitchensis-Tsuga heterophylla forest approaching maturity. Canadian Journal of Forest Research 20: 1602-1610

Munger, T. T. 1946. Watching a Douglas-fir forest for thirty-five years. Journal of Forestry 44 (10): 705-708.

Van Pelt, R. and J. F. Franklin. 2000. Influence of canopy structure on the understory environment in tall, old-growth, conifer forests. Can. J. For. Res. 30: 1231-1245.