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Background
Information Page

 

Data Analysis Methods Used in DecAID

Descriptions of most of the data analysis methods used in DecAID can be found in the background papers.

The statistical basis for DecAID can be found here.

Some additional methods not described in the background papers include:

- conversion of down wood (log) data to percent cover
- source of inventory summaries of dead wood
- the old-growth study as a source of data on down wood
 

Conversion of Down Wood (Log) Data to Percent Cover

The literature is highly variable in the way that log data is reported. In order to compare data among studies, log data was converted into a common measure of percent cover.

Inventory data (analyzed by Janet Ohmann and Karen Waddell) were used to report log data for combinations of wildlife habitat type and structural stage. The following measures from the same data set were calculated:

Regressions were then run to calculate the relationships among the 4 measures (see Table 1, below).

Correlations between percent cover, volume, and linear meters/ha were strong.  The relationship between percent cover and logs/ha was weak, though still statistically significant.  Based on the analysis, when log data were reported in the literature in terms of volume or linear meters/ha, we converted it to percent cover using the regression equations.  However, logs/ha data were not converted.  Conversions were specific to wildlife habitat type and structural stage using the regression equations (e.g. percent cover = volume/Xvar).
 

Table 1. Summary of linear regressions: log volume, logs/ha, and linear meters on % cover of logs
 
 
  Seral    Volume m3 Logs/ ha Linear  meters
Wildlife Habitat Type Stage n r2 X var r2 X var r2 X var
Eastside Mixed Conifer Forest mid & late 3325 0.7715 24.438    
  early 732 0.7864 17.887 0.3761 138.204 0.9304 574.032
  mid 3009 0.7622 22.586 0.4239 125.19 0.8929 480.51
  late 316 0.8834 28.313 0.4637 90.84 0.8774 373.62
Eastside Ponderosa Pine Forest          
  early 494 0.8009 20.258 0.4977 205.83 0.9199 535.48
  mid 1180 0.7733 25.359 0.2618 178.91 0.8555 462.94
  late 168 0.8213 24.844 0.5576 262.18 0.8606 455.01
Lodgepole Pine Forest          
  early 338 0.9146 14.16 0.1331 118.33 0.9653 637.53
  mid 355 0.8635 15.358 0.618 104.57 0.9568 612.47
  late 9 0.7323 13.769 *0.1226 123.01 0.9586 671.44
Montane Mixed Conifer Forest mid & late 1984 0.8142 26.121    
  early 355 0.8229 34.356 0.3513 85.292 0.7582 340.11
  mid 1560 0.8676 22.842 0.5877 96.486 0.9165 440.59
  late 424 0.9138 38.75 0.517 70.469 0.8778 268.63
SW Oregon Mixed Conifer Forest mid & late 807 0.7738 32.53    
  early 132 0.8328 34 0.5098 127.58 0.8331 371.87
  mid 522 0.75 30.326 0.5629 121.19 0.8421 398.9
  late 285 0.8479 35.53 0.3867 91.497 0.8207 319.1
Westside Lowland Conifer/Hardwood 
Forest		 mid & late 1951 0.8661 43.973    
  early 286 0.773 29.3 0.6886 143.15 0.8512 395.13
  mid 1171 0.8612 38.436 0.572 83.3 0.8069 316.23
  late 780 0.8602 44.075 0.4348 71.369 0.7981 267.78
Westside Douglas-fir and White Oak Woodland          
  early 40 0.7474 21.295 0.7124 159.04 0.945 513.93
  mid 207 0.7841 30.337 0.5102 111.76 0.8028 395.8
  late 64 0.8701 32.146 0.405 78.772 0.8819 360.45

 

Source of Inventory Summaries of Dead Wood

(summarized from Ohmann and Waddell, in press)

The dead wood summaries were derived from data collected on field plots as part of ongoing, strategic-level inventories conducted by the USDA Forest Service and the USDI Bureau of Land Management (BLM). The inventories provided data on stand characteristics, live trees, snags, and down wood collected on 16,867 plots from 1984-1997 on forest land across Oregon and Washington. Field plots consisted of a cluster of up to five subplots and included a series of fixed- and variable-radius circular plots. The FIA plots were confined to single, homogenous forest conditions by moving subplots according to a predetermined pattern. In contrast, the CVS and NRI subplots were installed in fixed positions and the plot could encompass multiple forest conditions. Live trees and snags were sampled on circular plots and down wood was sampled along transect lines established within plot boundaries. The species, diameter at breast height (DBH), and decay class were recorded for each snag tallied. Down wood measurements included the species, diameter at the point of intersection and at large end, piece length, decay class, and evidence of use by wildlife. Detailed information about inventory sample designs, field procedures, and compilation methods are available from the individual agencies that conducted the inventories. (Also see Max et al. 1996, USDA Forest Service 1992, CVS website http://www.fs.fed.us/r6/survey, and FIA website http://www.fs.fed.us/pnw/fia/ .)

Per-hectare estimates of snag density and down wood density (number of pieces), volume, and percent cover were calculated for each plot. CVS and NRI plots were split into multiple forest conditions for analysis if the plot encompassed more than one land class (forest or nonforest), or more than one vegetation series (defined by the tree species that would dominate the site in the absence of disturbance) within forest land. Each plot was classified into a wildlife habitat type and structural condition. Wildlife habitat type was determined by evaluating a plot’s potential vegetation (series) and ecoregion, and alliance was determined by a combination of potential and current vegetation variables.

Relative frequency distributions and nonparametric (distribution-free) tolerance limits of snag and down wood densities were calculated from the inventory database after adjusting the plot data for unequal sampling intensity (see The Statistical Basis for DecAID for more information). The characteristics of snags and down wood were summarized only for wildlife habitat types sampled by at least 10 plots. The dead wood summaries include trees of the following characteristics


References Cited

Max, Timothy A.; Schreuder, Hans T.; Hazard, JohnW.; Oswald, Daniel D.; Teply, John; Alegria, Jim. 1996. The Pacific Northwest Region vegetation and inventory monitoring system. Res. Pap. PNW-RP-493. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station; 22 p.

U.S. Department of Agriculture Forest Service. 1992. Forest Service resource inventories: an overview.  Washington, DC: U.S. Department of Agriculture, Forest Service, Forest Inventory, Economics, and Recreation Research; 39 p.

Summary: the Old Growth Study

The following information is from Spies et al. (1988). See Spies et al. (1988) for more detail about sample design and methods, and for a map of study sites.

The Old Growth Study was conducted in Douglas-fir-dominated forests in Washington and Oregon in three physiographic provinces: southern Washington Cascade Range, western Cascade Range of Oregon, and southern half of the Oregon Coast Range. The study area encompassed the western hemlock vegetation zone, the lower elevational portion of the Pacific silver fir zone, the northern margin of the mixed-conifer zone in southern Oregon, and the eastern margin of the Sitka spruce zone in the Oregon Coast Range. A total of 196 stands representing different ages (40 to 900 years) and site conditions were sampled in 1983 and 1984. All stands originated after wildfires that killed all or nearly all of the overstory trees. The stands had not been disturbed by roads, harvesting, or other forest management activities. Sampled stands were classified as young (<80 yr), mature (80-199 yr), or old growth (>200 yr).

Stand size ranged from about 4 to 20 ha. Within each stand, five nested circular plots were established systematically at 100 or 150 m apart. Each set of nested plots consisted of a 0.05-ha plot for down wood, a 0.1-ha plot for all snags, and a 0.2-ha plot for snags >50 cm dbh and >15 m tall.

All down wood >10 cm diameter (large end) that projected into the plot were measured. Data recorded included length (within the plot), horizontal diameter at both ends, species, and decay class. All snags >0.1 m tall with upper diameters >10 cm (10 cm dbh for snags >1.4 m tall) were measured in the 0.1-ha plot and recorded by species, dbh, height, and decay class.

The volume and number of logs and snags and the projected area of logs were computed by species and decay class for each plot in the stand. Volume was computed using the formula for a cone. Stand values were then calculated from the means of the plot values.

For comparing to the DecAID wildlife and inventory summaries, we considered all Old Growth Study plots to be in westside conifer hardwood forest. The physiographic provinces used in the Old Growth Study were the same as the subregions used in DecAID, except no Old Growth Study data were available for the Washington Coast Range. We assumed that the “young” stands of the Old Growth Study fell within the “mid-successional” category used in DecAID, and that both “mature” and “old growth” stands fell within “late-successional.” For DecAID, we used the estimates of down wood cover published in Spies et al. (1988, Table 3, p. 1694). For estimates of snag density, we obtained the original snag data from Tom Spies and recompiled them using the same minimum dbh of 25.4 cm as used for the inventory data.

From Spies et al.'s (1988) work, we present descriptive statistics because tolerance limits were unavailable.
 

Reference:

Spies, Thomas A.; Franklin, Jerry F.; Thomas, Ted B. 1988. Coarse woody debris in Douglas-fir forests of western Oregon and Washington. Ecology 69:1689-1702.
 


 
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