Snag Dynamics in Western Oregon and Washington
INTRODUCTION
To achieve desired amounts and characteristics of snags and down wood, managers require analytical tools for projecting changes in dead wood over time, and for comparing those changes to management objectives such as providing dead wood for wildlife and ecosystem processes.
A quantitative analysis of dead wood dynamics through time can be accomplished using the USDA Forest Service’s Fire and Fuels Extension (FFE) of the Forest Vegetation Simulator (FVS). However, the following additional information on rates of snag recruitment and fall from across forests of Oregon and Washington, which are not incorporated into FVS-FFE may also be useful in planning for future levels of dead wood. Two separate analyses, using 2 different sets of data are presented below.
- The first analysis is based on remeasurement of individual snags from the Continuous Vegetation Survey (CVS) data. This analysis uses data from Forest Service (FS) lands only. Fall rates of snags are adjusted based on tree species, size, and site characteristics using a combination of data and expert knowledge incorporated into Bayesian Network (BN) models. These data are from across Oregon and Washington.
- The second analysis is based on remeasurement of individual snags on non-federal lands from the Forest Inventory and Analysis (FIA) program. This analysis includes snag recruitment rates, fall rates by cause of death, size and species, and compares rates from disturbed (harvested) and undisturbed sites. These data are from western Oregon and Washington only.
OREGON AND WASHINGTON
(Coarse Wood Dynamics Model) CWDM Update
Snag Fall Rates
Kim Mellen-McLeanAugust 2016
The Coarse Wood Dynamics Model (CWDM) (Mellen and Ager 2002) provides information on the dynamics of snags and down logs in forested ecosystems across Oregon and Washington. The fall and decay rates in CWDM have been updated, and the updated rates have been incorporated into Forest Vegetation Simulator-Fire and Fuels Extension (FVS-FFE), however funding and support has not been available to update the stand- alone model. The updated snag fall rates are displayed and discussed here.
METHODS
Snag fall and height loss rates were derived from Continuous Vegetation Survey (CVS) remeasurement data from Forest Service lands in Oregon and Washington. Time between remeasurements varied from 2 to 11 years. On the initial visit to each plot, all live and standing dead trees were tallied and species, height, diameter at breast height (DBH), and decay class were recorded. At the second visit, trees previously tallied as snags were noted as still standing, fallen, or harvested. If snags were still standing, their current DBH, height, and decay class were recorded. Fall rates were prorated to a per year basis due to differences in remeasurement periods.
Only snags with which fell due to natural causes were used to calculate snag fall rates. A total of 42,736 snags were used in the snag fall calculations (Table 1). Fall rates for white fir and grand fir were combined as “true fir” to increase sample size and because fall rates are similar for the 2 species. Fall rates were much faster east of the Cascade crest as compared to west of the crest for Douglas-fir, true firs and Lodgepole pine, thus fall rates are separated geographically for these species.
Reliable data were not available from the CVS plots to incorporate factors that may affect snag fall rates. For that reason, the base fall rates calculated from CVS data were adjusted in relation to site characteristics using a Bayesian Network (BN) which combined data and expert knowledge. (See Marcot et al. 2001 and Marcot 2006 for a discussion of the use of BNs). Factors in the BN included: soil moisture, soil depth, slope position, and presence of insect and/or disease. Conditional probabilities in the BN, reflecting the effect of each factor, were assigned by expert panel. Fall rates determined by adjusting the base fall rate by 0.875- to 2-times depending on factors present. For details see CWDM Documentation.
Assumptions and Limitations
It is assumed that the fall rates calculated from CVS data are unbiased estimates of average fall rates across conditions effecting fall rates and not skewed toward extreme conditions that significantly increase or decrease fall rates. Based on the statistically rigorous design of the CVS sampling protocol, this should be a reasonable assumption.
- Root rot causes snags to fall more rapidly than any other insect or pathogen, or site condition.
- Ridgetop locations and shallow soils increase snag fall rates.
- Soil moisture is secondary or tertiary to other site influences. Wet soils will decrease fall rate because roots are saturated and thus don’t rot as fast.
- Lag times between time of death and time snags begin to fall range from 0 to 23 years depending on species and size of snag (Harmon et al. 1986, Dunn and Bailey 2012, and Mark Harmon personal communication)
Fire is not incorporated in to the projection of snag fall through time. Most fire return intervals are longer than the longest 11-year remeasurement period from the CVS data, especially when considering fire suppression and exclusion. Fire may partially consume snags and/or increase fall rates. This should be kept in mind when looking at the life span of snags, especially in areas with frequent fire return intervals due to wild or prescribed fire. In some areas snags may be consumed before they decay to the point of providing soft snags. Fire also creates snags, and snag densities are likely to increase immediately after fire, however, individual snags are unlikely to reach the “maximum ages” displayed in Table 4 and Table 5 in areas with frequent fire-return intervals.
Snag Fall Rates
Rates of snag fall are influenced by a variety of conditions and variables, including species, size, site conditions, presence of insects and pathogens.
Key Findings from Analysis of CVS Data
Species has the most influence on fall rates with cedar standing the longest and red alder falling the fastest (Table 4 and Table 5). It takes cedar 8-times as long for half of a cohort of snags to fall than for half a cohort of red alder snags to fall. Other major influences on snag fall include:
- Large snags stand from 2- to 5-times longer than small snags of the same species.
- Douglas-fir on the west side of the Cascade crest fall slower than Douglas-fir on the eastside of the Cascade crest.
Species specific average fall rates are displayed in Table 2 and Table 3 by physiographic province. The tables display the percent of snags expected to be standing by time since death in 5-year increments. The percent standing at any given time can also be viewed as the likelihood that a given snag will still be standing. “Maximum age” is based on the time at which > 95% of snags in cohort have fallen (≤ 5% still standing).
In all provinces, cedar, white pine and Douglas-fir remain standing the longest, while Lodgepole pine, alder and poplar species (cottonwood and aspen) fall the fastest (Table 5). However, live alder and cottonwood often contain pockets of heartrot, thus these trees may provide long-term nesting habitat for cavity nesting birds
Model Adjusted Fall Rates
Table 4 displays adjusted fall rates, in terms of the percent of snags that have fallen after 10 years, for three snag size classes and various tree species. The highest rates are adjustments for decayed snags, on shallow soils, on ridgetops, and with root rot. The lowest rates are adjustments for sound snags, on deep soils, away from ridgetops with no insect or disease effects. Note that the snag fall lag time is not used for the highest fall rates. See CWDM Documentation
MANAGEMENT IMPLICATIONS
These findings have several implications for planning for desired future conditions of snags. The high fall rate (almost half) of recent mortality trees needs to be considered when planning for future recruitment of snags and down wood. Fall rates predicted by CWDM support the FVS analysis that, for most species and especially smaller snags, a majority of snags will fall within the first 10 years (Table 2). Trees that fall soon after death provide snag habitat only for very short periods of time or not at all, but do contribute down wood habitat. In fact, these trees are a desirable source of down wood as they will often begin as mostly undecayed wood and, if left on the forest floor, will proceed through the entire wood decay cycle with its associated ecological organisms and processes that are beneficial to soil conditions and site productivity (see section on Ecosystem Processes Related to Wood Decay). Because many existing snag dynamics models assume that all mortality trees are recruited as snags, a major implication of these findings is that these models will substantially overestimate future snag abundance and underestimate amounts of down wood.
The cause of tree death also needs to be considered in planning for snags and down wood. Trees killed by insects, animals, suppression, and diseases other than root disease, are most likely to remain standing as snags. The quality of snag habitat also varies with mortality agent and should be considered as well -- see insect and disease discussions for more information.
Because snag retention is so strongly affected by harvest activities, dead wood should be planned for separately for disturbed and undisturbed stands. Although 62% of snags on disturbed plots in the FIA study were cut down, it's likely that more snags could be retained in harvest units than our findings indicate, by applying creative approaches to snag placement that also address safety and operational concerns (see Neitro et al. 1985). Our findings suggest that snag size (DBH) and species should be considered when identifying particular snags to retain in harvest units. The larger the snag diameter, the more likely it is to survive harvest operations and remain standing in future years. Species of cedar can be expected to stand the longest (although cedar tends to have low overall wildlife value, whereas hardwoods, Sitka spruce, and true firs will be shortest-lived (but often have higher wildlife value (see the Ancillary Data section in summary narratives)). Again, both the quality and the longevity of snag habitat needs to be considered (see the Considerations for Stand Dynamics section in summary narratives).
Forest Vegetation Simulators and Dead Wood Dynamics Models
Dead wood dynamics models can be used to better project the recruitment, fall, and decay of snags and down wood on a site to determine amounts and characteristics of dead wood over time. Dead wood dynamics models can guide efforts at green tree retention and snag and down wood creation. Estimates of tree mortality can be obtained from forest simulation models such as the Forest Vegetation Simulator (FVS), Organon, or Zelig, and entered into the dead wood dynamics models as recruited snags and down wood. Existing dead wood dynamics models do not yet incorporate all of the recently available information on tree dynamics from remeasurement of permanent plots such as presented in this paper. In the future, forest simulators and dead wood dynamics models will be updated and improved to include the best available information.
Several existing dead wood dynamics models are available for use, which are described in the Coarse Wood Dynamics Model (CWDM) (Mellen and Ager 2002), the Snag Recruitment Simulator (SRS) (Marcot), and the Snag Dynamics Projection Model (SDPM) (McComb and Ohmann 1995). In addition, the Forest Vegetation Simulator (FVS) now has a Fire and Fuels Extension (FFE) that tracks and simulates decay and fall of standing dead trees, and decay of down wood (i.e., 'surface fuels'). The fall and decay rates from the CWDM update, discussed above, have now been incorporated in to the FFE for all variants in Oregon and Washington
WESTERN OREGON AND WASHINGTON
Snag Fall and Recruitment Rates from Remeasured FIA Plots
Janet L. Ohmann
July 26, 2002
The summaries presented here are based on remeasurement data from permanent periodic Forest Inventory and Analysis (FIA) plots on nonfederal lands. The plots sample several DecAID vegetation conditions on the west side of the crest of the Cascade Mountains in Washington and Oregon ("westside"), but are predominantly from the Westside Lowland Conifer-Hardwood wildlife habitat type. Similar tree remeasurement data are available for eastern Washington and Oregon, but have not yet been compiled. Repeat measurements on permanent plots to determine tree death and fall are very reliable, whereas estimates of change in snag height and decay class are less accurate and are not presented here. Also, because repeatable measurements of down wood along transects are problematic, data on down wood dynamics at the individual log level are unlikely to be provided by regional forest inventory plots. The information presented here therefore focuses on live tree mortality, snag creation, and snag fall.
This project was funded in part by the Forest Health Monitoring Program, USDA Forest Service. A poster version of this information is at https://www.fs.fed.us/foresthealth/fhm/posters/posters02/snag.pdf. Cartography is by Matt Gregory.
Methods
Rates of snag recruitment, decay, and fall were summarized from repeat measurement data for FIA plots in western Oregon and western Washington (Table 6, Figure 1). Each plot and tree was measured twice over a 10-year period. On the initial visit to each plot, all live and standing dead trees were tallied and species, height, diameter at breast height (DBH), and decay class were recorded. At the second visit, trees previously tallied as snags were noted as still standing, fallen, or harvested. If snags were still standing, their current DBH, height, and decay class were recorded. Trees that died since the first measurement (mortality) were noted as still standing (recruited snags) or fallen, and cause of death was recorded.
Because snag dynamics are strongly influenced by logging and forest management activities, data were analyzed separately for disturbed and undisturbed plots. 'Disturbed' plots were those where any kind of tree cutting or silvicultural treatment (clearcut, partial harvest, precommercial thin, commercial thin, incidental harvest) was recorded for the 10-year remeasurement period. Disturbances that occurred prior to the remeasurement period are not accounted for in the analysis. Snag recruitment (fall rate of mortality trees) and snag fall also are summarized by State and by tree size class.
Key Findings
Snag Recruitment
- Rates of mortality tree recruitment as snags varied by cause of death (Table 7). Trees killed by insects, animals, and suppression were most likely to remain standing as snags over the 10-year remeasurement period. Not surprisingly, trees killed by weather (including windthrow) and root disease were most likely to fall down soon after death.
- Nearly half of all mortality trees, dying from all causes combined, fell within 10 years of death and were not recruited as snags (Table 7).
Rates of Snag Fall and Harvest
- In undisturbed stands, 30% of all snags ≥ 25.4 cm (10.0 in) DBH fell down over the 10-year period (Table 8).
- Snag fall rates in undisturbed stands were substantially lower for the largest snags: most (93%) snags >100 cm (39 in) DBH remained standing over the remeasurement period (Table 8). Snags in the two smaller DBH classes fell at about the same rate (30% and 33%).
- Snag retention was strongly impacted by harvest activities. Only 15% of snags ≥ 25 cm (10 in) DBH remained standing after disturbance (Table 9). Most snags (62%) were cut down and either left on site or removed. A smaller percentage (16%) fell down naturally. Smaller snags were more likely to be cut down than larger snags.
- In undisturbed stands, western redcedar and 'other conifer' (mostly cedar) snags stood the longest (Table 10.). Hardwoods, Sitka spruce, and true firs fell at the greatest rates.
Literature Cited
Dunn, Christopher J., and John D. Bailey. 2012. Temporal dynamics and decay of coarse wood in early seral habitats of dry-mixed conifer forests in Oregon’s Eastern Cascades. Forest Ecology and Management 276:71-81.
Harmon, M. E., J. F. Franklin, F. J. Swanson, P. Sollins, S. V. Gregory, J. D. Lattin, N. H. Anderson, S. P. Cline, N. G. Aumen, J. R. Sedell, G. W. Lienkaemper, K. Cromack, Jr, and K. W. Cummins. 1986. Ecology of coarse woody debris in temperate ecosystems. Advances in Ecological Research 15:133-302.
Marcot, Bruce G., Richard S. Holthausen, Martin G. Raphael, Mary M. Rowland, Michael J. Wisdom. 2001. Using Bayesian belief networks to evaluate fish and wildlife population viability under land management alternatives from an environmental impact statement. Forest Ecology and Management 153:29-42.
Marcot, Bruce G. 2006. Habitat modeling for biodiversity conservation. Northwestern Naturalist 87:56-65.
McComb, W.C.; Ohmann, J.L. 1995. Snag Dynamics Projection Model (SDPM): a model of snag recruitment, decay, and fall for Pacific Northwest forests. Unpublished.
Mellen, K., and A. Ager. 2002. A coarse wood dynamics model for the western Cascades. pp 503-516. In: Laudenslayer, William F., Jr.; Valentine, Brad; Weatherspoon, C. Philip; Lisle, Thomas E., technical coordinators. Proceedings of the symposium on the ecology and management of dead wood in western forests. 1999 November 2-4; Reno, NV. Gen. Tech. Rep. PSW-GTR-181. Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture. http://www.fs.fed.us/psw/publications/documents/gtr-181/040_MellAger.pdf
Rose, C. L., B. G. Marcot, T. K. Mellen, J. L. Ohmann, K. L. Waddell, D.L. Lindley, and B. Schreiber. 2001. Decaying wood in Pacific Northwest forests: concepts and tools for habitat management. Pp. 580-623 in: D.H. Johnson and T. A. O'Neil, ed. Wildlife-habitat relationships in Oregon and Washington. Oregon State University Press, Corvallis OR.
Table 1. Number of remeasured snags, by species, from CVS data from Oregon and Washington.
| Species | Number of remeasured snags |
|---|---|
| Silver fir | 2292 |
| White fir | 1785 |
| Grand fir | 4139 |
| Subalpine fir | 2353 |
| Red alder | 346 |
| Western larch | 1985 |
| White-bark pine | 405 |
| Lodgepole pine | 6919 |
| Engleman spruce | 905 |
| White pine | 834 |
| Ponderosa pine | 2663 |
| Douglas-fir | 13417 |
| Western redcedar | 707 |
| Western hemlock | 2683 |
| Mountain hemlock | 1207 |
| Poplars | 96 |
| Total – all species | 42736 |
Table 2. Snag fall rates for medium sized snags by physiographic province and species. Medium size snags are 25-75 cm dbh (10-29 in)a or 20 to 50 cm dbh (8-20 in)b depending on species. Columns below species reflect percent of trees still standing at 5 year increments. Unadjusted base fall rates and snag fall lag times were used in the calculations of percent snags still standing. Maximum age is year at which 95% of snags have fallen.
| years since death | Cedara | Douglas-fira | True fira | Western hemlocka | Mountain hemlockb | White pineb | Lodgepole pineb | Silver fira | Sprucea | Poplarsb | Alderb |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 |
| 5 | 100 | 100 | 85 | 84 | 83 | 100 | 83 | 80 | 75 | 62 | 60 |
| 10 | 100 | 100 | 71 | 71 | 69 | 100 | 68 | 63 | 56 | 38 | 36 |
| 15 | 91 | 91 | 60 | 59 | 57 | 83 | 56 | 50 | 42 | 24 | 22 |
| 20 | 83 | 82 | 51 | 50 | 47 | 68 | 46 | 40 | 32 | 15 | 13 |
| 25 | 75 | 74 | 43 | 42 | 39 | 56 | 38 | 32 | 24 | 9 | 8 |
| 30 | 69 | 67 | 36 | 35 | 33 | 46 | 32 | 25 | 18 | 6 | 5 |
| 35 | 62 | 61 | 31 | 30 | 27 | 38 | 26 | 20 | 13 | 4 | 3 |
| 40 | 57 | 55 | 26 | 25 | 23 | 32 | 21 | 16 | 10 | 2 | 2 |
| 45 | 52 | 50 | 22 | 21 | 19 | 26 | 18 | 13 | 8 | 1 | 1 |
| 50 | 47 | 45 | 19 | 17 | 16 | 21 | 15 | 10 | 6 | 1 | 1 |
| 55 | 43 | 41 | 16 | 15 | 13 | 18 | 12 | 8 | 4 | 1 | 0 |
| 60 | 39 | 37 | 13 | 12 | 11 | 15 | 10 | 6 | 3 | 0 | 0 |
| 65 | 35 | 33 | 11 | 10 | 9 | 12 | 8 | 5 | 2 | 0 | 0 |
| 70 | 32 | 30 | 9 | 9 | 7 | 10 | 7 | 4 | 2 | 0 | 0 |
| 75 | 29 | 27 | 8 | 7 | 6 | 8 | 6 | 3 | 1 | 0 | 0 |
| 80 | 27 | 25 | 7 | 6 | 5 | 7 | 5 | 3 | 1 | 0 | 0 |
| 85 | 24 | 22 | 6 | 5 | 4 | 6 | 4 | 2 | 1 | 0 | 0 |
| 90 | 22 | 20 | 5 | 4 | 3 | 5 | 3 | 2 | 1 | 0 | 0 |
| 95 | 20 | 18 | 4 | 4 | 3 | 4 | 3 | 1 | 0 | 0 | 0 |
| 100 | 18 | 17 | 3 | 3 | 2 | 3 | 2 | 1 | 0 | 0 | 0 |
| Max age | 170 | 165 | 90 | 85 | 80 | 90 | 80 | 65 | 55 | 35 | 30 |
| Fall rate/yr | 0.018 | 0.019 | 0.031 | 0.032 | 0.034 | 0.035 | 0.035 | 0.041 | 0.05 | 0.076 | 0.08 |
| years since death | Cedara | Western larcha | White pinea | Western hemlocka | Mountain hemlockb | Douglas-fira | True fira | Sprucea | Ponderosa pinea | Lodgepole pineb | Poplarsb |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 |
| 5 | 100 | 100 | 100 | 84 | 83 | 100 | 80 | 75 | 100 | 64 | 62 |
| 10 | 100 | 100 | 100 | 71 | 69 | 100 | 63 | 56 | 69 | 41 | 38 |
| 15 | 91 | 89 | 86 | 59 | 57 | 81 | 50 | 42 | 47 | 26 | 24 |
| 20 | 83 | 79 | 73 | 50 | 47 | 66 | 40 | 32 | 32 | 17 | 15 |
| 25 | 75 | 70 | 63 | 42 | 39 | 53 | 32 | 24 | 22 | 11 | 9 |
| 30 | 69 | 63 | 53 | 35 | 33 | 43 | 25 | 18 | 15 | 7 | 6 |
| 35 | 62 | 56 | 46 | 30 | 27 | 35 | 20 | 13 | 10 | 4 | 4 |
| 40 | 57 | 50 | 39 | 25 | 23 | 28 | 16 | 10 | 7 | 3 | 2 |
| 45 | 52 | 44 | 33 | 21 | 19 | 23 | 13 | 8 | 5 | 2 | 1 |
| 50 | 47 | 39 | 29 | 17 | 16 | 19 | 10 | 6 | 3 | 1 | 1 |
| 55 | 43 | 35 | 24 | 15 | 13 | 15 | 8 | 4 | 2 | 1 | 1 |
| 60 | 39 | 31 | 21 | 12 | 11 | 12 | 6 | 3 | 2 | 0 | 0 |
| 65 | 35 | 28 | 18 | 10 | 9 | 10 | 5 | 2 | 1 | 0 | 0 |
| 70 | 32 | 25 | 15 | 9 | 7 | 8 | 4 | 2 | 1 | 0 | 0 |
| 75 | 29 | 22 | 13 | 7 | 6 | 6 | 3 | 1 | 1 | 0 | 0 |
| 80 | 27 | 20 | 11 | 6 | 5 | 5 | 3 | 1 | 0 | 0 | 0 |
| 85 | 24 | 17 | 10 | 5 | 4 | 4 | 2 | 1 | 0 | 0 | 0 |
| 90 | 22 | 15 | 8 | 4 | 3 | 3 | 2 | 1 | 0 | 0 | 0 |
| 95 | 20 | 14 | 7 | 4 | 3 | 3 | 1 | 0 | 0 | 0 | 0 |
| 100 | 18 | 12 | 6 | 3 | 2 | 2 | 1 | 0 | 0 | 0 | 0 |
| Max age | 170 | 140 | 105 | 85 | 80 | 80 | 65 | 55 | 45 | 35 | 35 |
| Fall rate/yr | 0.018 | 0.022 | 0.029 | 0.032 | 0.034 | 0.038 | 0.041 | 0.05 | 0.063 | 0.072 | 0.076 |
Table 3. Snag fall rates for large sized snags by physiographic province and species. Large size snags are > 75 cm dbh (> 29 in)a or > 50 cm dbh (> 20 in)b depending on species. Columns below species reflect percent of trees still standing at 5 year increments. Unadjusted base fall rates and snag fall lag times were used in the calculations of percent snags still standing. Maximum age is year at which 95% of snags have fallen.
| years since death | Douglas-fira | Cedara | White pinea | Western hemlocka | Mountain pineb | Silver fira | True fira | Sprucea | Alderb |
|---|---|---|---|---|---|---|---|---|---|
| 0 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 |
| 5 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 70 |
| 10 | 100 | 100 | 100 | 92 | 92 | 91 | 89 | 80 | 49 |
| 15 | 100 | 100 | 100 | 85 | 85 | 83 | 78 | 63 | 34 |
| 20 | 97 | 95 | 95 | 78 | 78 | 75 | 69 | 50 | 24 |
| 25 | 93 | 89 | 89 | 72 | 72 | 69 | 61 | 40 | 17 |
| 30 | 90 | 84 | 84 | 66 | 66 | 62 | 54 | 32 | 12 |
| 35 | 87 | 80 | 80 | 61 | 61 | 57 | 48 | 25 | 8 |
| 40 | 84 | 75 | 75 | 56 | 56 | 52 | 43 | 20 | 6 |
| 45 | 81 | 71 | 71 | 51 | 51 | 47 | 38 | 16 | 4 |
| 50 | 78 | 67 | 67 | 47 | 47 | 43 | 33 | 13 | 3 |
| 55 | 75 | 64 | 64 | 43 | 43 | 39 | 29 | 10 | 2 |
| 60 | 73 | 60 | 60 | 40 | 40 | 35 | 26 | 8 | 1 |
| 65 | 70 | 57 | 57 | 37 | 37 | 32 | 23 | 6 | 1 |
| 70 | 68 | 54 | 54 | 34 | 34 | 29 | 20 | 5 | 1 |
| 75 | 65 | 51 | 51 | 31 | 31 | 27 | 18 | 4 | 0 |
| 80 | 63 | 48 | 48 | 29 | 29 | 24 | 16 | 3 | 0 |
| 85 | 61 | 45 | 45 | 26 | 26 | 22 | 14 | 3 | 0 |
| 90 | 59 | 43 | 43 | 24 | 24 | 20 | 13 | 2 | 0 |
| 95 | 57 | 40 | 40 | 22 | 22 | 18 | 11 | 2 | 0 |
| 100 | 55 | 38 | 38 | 21 | 21 | 17 | 10 | 1 | 0 |
| Max age | 425 | 285 | 285 | 190 | 190 | 165 | 130 | 70 | 45 |
| Fall rate/yr | 0.007 | 0.011 | 0.011 | 0.016 | 0.016 | 0.018 | 0.023 | 0.041 | 0.06 |
| years since death | Cedara | White pinea | Douglas-fira | Western larcha | Western hemlocka | Mountain hemlockb | True fira | Ponderosa pinea | Sprucea | Lodgepole pineb |
|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 |
| 5 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 78 |
| 10 | 100 | 100 | 100 | 100 | 92 | 100 | 89 | 85 | 80 | 60 |
| 15 | 100 | 100 | 100 | 100 | 85 | 92 | 78 | 71 | 63 | 47 |
| 20 | 95 | 95 | 92 | 92 | 78 | 85 | 69 | 60 | 50 | 36 |
| 25 | 89 | 89 | 85 | 85 | 72 | 78 | 61 | 51 | 40 | 28 |
| 30 | 84 | 84 | 78 | 78 | 66 | 72 | 54 | 43 | 32 | 22 |
| 35 | 80 | 80 | 72 | 72 | 61 | 66 | 48 | 36 | 25 | 17 |
| 40 | 75 | 75 | 66 | 66 | 56 | 61 | 43 | 31 | 20 | 13 |
| 45 | 71 | 71 | 61 | 61 | 51 | 56 | 38 | 26 | 16 | 10 |
| 50 | 67 | 67 | 56 | 56 | 47 | 51 | 33 | 22 | 13 | 8 |
| 55 | 64 | 64 | 51 | 51 | 43 | 47 | 29 | 19 | 10 | 6 |
| 60 | 60 | 60 | 47 | 47 | 40 | 43 | 26 | 16 | 8 | 5 |
| 65 | 57 | 57 | 43 | 43 | 37 | 40 | 23 | 13 | 6 | 4 |
| 70 | 54 | 54 | 40 | 40 | 34 | 37 | 20 | 11 | 5 | 3 |
| 75 | 51 | 51 | 37 | 37 | 31 | 34 | 18 | 9 | 4 | 2 |
| 80 | 48 | 48 | 34 | 34 | 29 | 31 | 16 | 8 | 3 | 2 |
| 85 | 45 | 45 | 31 | 31 | 26 | 29 | 14 | 7 | 3 | 1 |
| 90 | 43 | 43 | 29 | 29 | 24 | 26 | 13 | 6 | 2 | 1 |
| 95 | 40 | 40 | 26 | 26 | 22 | 24 | 11 | 5 | 2 | 1 |
| 100 | 38 | 38 | 24 | 24 | 21 | 22 | 10 | 4 | 1 | 1 |
| Max age | 285 | 285 | 200 | 200 | 190 | 195 | 130 | 95 | 70 | 60 |
| Fall rate/yr | 0.011 | 0.011 | 0.016 | 0.016 | 0.016 | 0.016 | 0.023 | 0.031 | 0.041 | 0.045 |
Table 4. Fate of snags within the first 10-year period based on fall rates from CWDM (2-11 year remeasurement period). Percent fallen based on high rate does not include a lag time; percent fallen based on low rate includes the unadjusted lag time.
| Species | Percent fallen based on high rate | Percent fallen based on low rate | Lag time |
|---|---|---|---|
| Silver fira | 70% | 42% | 0 |
| Grand fira | 89% | 60% | 0 |
| Red alderb | 96% | 73% | 0 |
| Lodgepole pineb | 85% | 55% | 0 |
| Cedara | 62% | 18% | 5 |
| Douglas-fir – westa | 64% | 18% | 5 |
| Douglas-fir – easta | 81% | 30% | 5 |
| Ponderosa pinea | 97% | 61% | 3 |
| Western hemlocka | 73% | 46% | 0 |
| Species | Percent fallen based on high rate | Percent fallen based on low rate | Lag time |
|---|---|---|---|
| Silver fira | 51% | 27% | 0 |
| Grand fira | 53% | 27% | 0 |
| Red alderb | 82% | 51% | 0 |
| Lodgepole pineb | 82% | 51% | 0 |
| Cedara | 19% | 0% | 10 |
| Douglas-fir – westa | 35% | 0% | 10 |
| Douglas-fir – easta | 46% | 0% | 10 |
| Ponderosa pinea | 78% | 36% | 3 |
| Western hemlocka | 54% | 30% | 0 |
| Species | Percent fallen based on high rate | Percent fallen based on low rate | Lag time |
|---|---|---|---|
| Silver fira | 18% | 3% | 5 |
| Grand fira | 31% | 8% | 5 |
| Red alderb | - | - | - |
| Lodgepole pineb | - | - | - |
| Cedara | 15% | 0% | 15 |
| Douglas-fir – westa | 15% | 0% | 15 |
| Douglas-fir – easta | 31% | 0% | 15 |
| Ponderosa pinea | 46% | 12% | 5 |
| Western hemlocka | 33% | 9% | 5 |
Table 5. Number of years for 50% of various species of snags to fall under average (unadjusted) site conditions.
| Species | Years for large snags (> 29”1 or > 20”2 dbh) | Years for medium snags (10-29”1 or 8-20”2 dbh) |
|---|---|---|
| Cedars 1 | 95 years | 75 years |
| Douglas-fir on the west side 1 | 90 years | 45 years |
| Douglas-fir on the east side1 | 45 years | 32 years |
| Ponderosa pine1 | 30 years | 13 years |
| Hemlocks1 | 30 years | 15 years |
| True firs1 | 35 years | 17 years |
| Lodgepole pine2 | 7 years | |
| Alder 2 | 10 years | 7 years |
| Populus2 | 8 years | 7 years |
Table 6. Forest Inventory and Analysis plot data in western Oregon and Washington.
| Western Oregon | Western Washington | |
|---|---|---|
| Number of plots | 338 | 669 |
| % of plots undisturbed* over remeasurement period | 68 | 72 |
| Inventory dates | Mid-1980s, mid-1990s |
Late 1970s, late 1980s |
| Number of mortality trees | -- | 499 |
| Number of remeasured snags | 1,128 | 2,076 |
* No tree cutting or silvicultural treatment.
Table 7. Fate of mortality trees of all species and > 25.4 cm (10.0 in) DBH by cause of death over a 10-year period, western Washington.
| Cause of death | Percent still standing | Percent fell down |
|---|---|---|
| Insects | 96 | 4 |
| Root disease | 52 | 48 |
| Other rots | 63 | 37 |
| Animals | 100 | 0 |
| Weather | 28 | 72 |
| Suppression | 79 | 21 |
| Other / unknown | 61 | 39 |
| All causes | 56 | 44 |
* No tree cutting or silvicultural treatment.
Table 8. Fate of remeasured snags over a 10-year period by diameter class (DBH) in undisturbed* stands, western Oregon and Washington.
| Snag fate | < 25.4-50.0 cm (10-20 in) | 50.1-100.0 cm (20-39 in) | > 100.0 cm (39 in) | All sizes |
|---|---|---|---|---|
| Percent still standing | 61 | 59 | 93 | 62 |
| Percent fallen | 30 | 33 | 4 | 30 |
| Percent shrank to <25.4 cm (10.0 in) DBH or <2 m (7 ft) tall | 9 | 8 | 3 | 8 |
* No tree cutting or silvicultural treatment over the 10-year remeasurement period.
Table 9. Fate of remeasured snags by snag size over a 10-year period by diameter class (DBH) in disturbed* stands, western Oregon and Washington
| Snag fate | < 25.4-50.0 cm (10-20 in) | 50.1-100.0 cm (20-39 in) | > 100.0 cm (39 in) | All sizes |
|---|---|---|---|---|
| Percent still standing | 8 | 15 | 42 | 15 |
| Percent fallen | 17 | 17 | 3 | 16 |
| Percent shrank to <25.4 cm (10.0 in) DBH or <2 m(7 ft) tall | 3 | 9 | 8 | 7 |
| Percent cut down | 72 | 59 | 47 | 62 |
* Tree cutting or silvicultural treatment occurred over the 10-year remeasurement period.
Table 10. Fate of remeasured snags over a 10-year period by species in undisturbed* stands in western Oregon and Washington
| Snag fate | Douglas-fir | Western hemlock | Western redcedar | Sitka spruce | True fir | Other conifers** | Hard-woods |
|---|---|---|---|---|---|---|---|
| Percent still standing | 70 | 67 | 76 | 30 | 37 | 78 | 29 |
| Percent fallen | 23 | 20 | 12 | 67 | 40 | 22 | 65 |
| Percent shrank to <25.4 cm (10.0 in) DBH or <2 m (7 ft) tall | 7 | 13 | 12 | 3 | 23 | 0 | 5 |
* No tree cutting or silvicultural treatment over the 10-year remeasurement period.
** Incense cedar, Alaska yellow-cedar, Port-Orford cedar, redwood, Pacific yew, and mountain hemlock.
Figure 1. Plot locations and topography
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