Snag Size Research Articles

Level and pattern of overstory retention shape the abundance and long-term dynamics of natural and created snags

Standing dead trees, or snags, serve myriad functions in natural forests, but are often scarce in forests managed for timber production. Variable retention (VR), the retention of live and dead trees through harvest, has been adopted globally as a less intensive form of regeneration harvest. In this study, we explore how two key elements of VR systems — level (amount) and spatial pattern of live-tree retention — affect the carryover and post-harvest dynamics of natural and artificially created snags. We present nearly two decades of data from the DEMO Study, a regional-scale experiment in VR harvests of Douglas-fir-dominated forests in the Pacific Northwest. Snag losses to harvest were greater at 15 than at 40% retention (67 vs. 47% declines in density) and greater in dispersed than in aggregated treatments (64 vs. 50% declines). Densities of hard and tall (≥5 m) snags were particularly sensitive to low-level dispersed retention, declining by 76 and 81%, respectively. Despite these losses, post-harvest densities correlated with pre-harvest densities for most snag size and decay classes. In contrast to initial harvest effects, snag densities changed minimally over the post-harvest period (years 1 to 18 or 19), with low rates of recruitment offsetting low rates of loss. Post-harvest survival of snags was greater at 15 than at 40% retention (79 vs. 69%), as were rates of decay (68 vs. 52% of hard snags transitioned to soft). However, pattern had no effect on either process. Snag recruitment did not vary with retention level or pattern at the scale of the 13-ha harvest unit, but was several-fold greater in the 1-ha aggregates (14.3–27.8 snags ha−1) than in the corresponding dispersed treatments (4.2–5.3 snags ha−1). Snag size (diameter) distributions showed greater change in dispersed than in aggregated treatments, reflecting greater loss of smaller snags and recruitment biased toward larger snags. Created snags showed uniformly high survival (97%), irrespective of treatment, but rates of decay were greater at lower retention. If a goal of VR is to sustain snag abundance and diversity through harvest, emphasis should be placed on minimizing initial losses, either by reducing the intensity of felling in areas of dispersed retention or locating forest aggregates in areas of greater initial snag density, diversity, or incipient decay.

Influence of Forest Management and Sylvicultural Treatments on Abundance of Snags and Tree Cavities in Mountain Mixed Beech Forests

In this study the influence of forest management on the characteristics of snags and tree cavities in the Hyrcanian forests of Iran was investigated. In particular, the effect of two sylvicultural treatments (shelter wood and single-tree selection) was assessed in comparison to protected stands. The abundance, diameter, height, and degree of decay of snag species, and the characteristics of birds excavated and natural cavities (number and height from the ground level) were measured by systematic plots in each stand. The results showed that the abundance, species diversity, and size of snags in both of the sylvicultural treatments were significantly lower than the protected stands. The number of birds excavated and natural cavities on thicker snags with moderate decay was significantly higher than other decay classes. Abundance, species diversity, size of snags, and number of tree cavities in the single-tree selection stands were significantly higher than sheltered-wood stands.

Survival dynamics of mechanically topped Douglas-fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla) snags in Douglas-fir plantations, Oregon, USA

Structural enrichment in commercial tree plantations is a potential tool to increase snag numbers but relatively little information is available about how species, size, and spatial distribution of created snags are associated with longevity of these structures. We created 1197 snags in 31 harvest units from 1997 to 1999 in the Cascade and Coast Ranges, Oregon, USA, by topping live trees with harvesting equipment. We used an experimental design to distribute created snags at three densities and as either single or clumped created snags. We fit Weibull and log-logistic Accelerated Failure Time models and found that the median failure time was insensitive to the choice of distribution. We found a small positive effect of diameter at breast height (DBH) and a slight negative effect of increasing distance between created snags on survival. Assuming Weibull and log-logistic distributions at mean observed values of DBH and distance between snags, median survival times for Douglas-fir (Pseudotsuga menziesii) were 21.0 (95% confidence interval: 19.3, 22.8) years and 21.2 (19.7, 22.8) years, respectively. For western hemlock (Tsuga heterophylla), median survival times were 13.0 (11.9, 14.2) years and 12.5 (11.4, 13.7) years, respectively. Although the two failure distributions had similar median failure times, the log-logistic implies a higher survival probability over time for snags that remained standing at the end of the study period. Created snags can be a useful supplement for harvest units rotated at ∼45 years and Douglas-fir will be available for longer as standing structures. For example, under the log-logistic model, a predicted 5% of Douglas-fir snags are retained to rotation age, so that 40 snags per hectare would be required at harvest to maintain 2 snags per hectare through stand rotation. Snags created from western hemlock will provide an early rotation pulse but are unlikely to last longer than 20 years. Our results suggest that longevity can be increased by maximizing the snag size within the safety constraints of harvesting equipment. Scattering of snags may have a slightly negative effect on snag survival but this outcome should be weighed against potential ecological benefits of variation in snag distributions.

Dwarf Mistletoe and Breeding Bird Abundance in Ponderosa Pine Forests

Southwestern dwarf mistletoe (Arceuthobium vaginatum subsp. cryptopodum) parasitizes ponderosa pine (Pinus ponderosa). It can kill severely infected trees and induce the growth of dense masses of branches that can affect foraging and nesting habitat for wildlife. We tested the hypothesis that higher densities of breeding birds would be correlated with higher levels of southwestern dwarf mistletoe in ponderosa pine forests of Arizona. We estimated densities of 15 species of breeding birds and measured 26 habitat elements in 19 stands. Average dwarf mistletoe ratings (DMR) ranged from 0.0 (uninfested) to 3.7 (severely infested). Although we observed higher densities with higher infestation of dwarf mistletoe for 2 bird species and lower densities for 3 bird species, the effect of dwarf mistletoe on these was minor. Instead, higher snag density, an indirect measure of past mistletoe infestation, was a more important predictor of bird density. With greater snag size or density, the species richness and densities of 3 bird species increased. Because dwarf mistletoe infection can create snags, retaining groups of dwarf mistletoe-infected trees in ponderosa pine stands will provide a continued source of snags, which are important habitat for many wildlife species.

Recommendations for snag retention in southwestern mixed‐conifer and ponderosa pine forests: History and current status

ABSTRACTSnags provide habitat for numerous species of wildlife. Several authors have provided recommendations for snag retention in southwestern mixed‐conifer and ponderosa pine (Pinus ponderosa) forests. Most recommendations were presented in terms of minimum snag density and/or size. I summarized the history of recommendations for snag retention in these forest types, and used data from a current study of snag populations (conducted within Coconino and Kaibab National Forests, north‐central AZ, USA, 1997 through 2012) to assess congruence between existing snag populations and various recommendations. Most recommendations were based on studies of cavity‐nesting birds; therefore, this analysis emphasized characteristics of snags containing excavated nest cavities. Proportions of plots that met minimum management targets varied among recommendations, ranging from 34% to 100% in mixed‐conifer forest and from 7% to 95% in ponderosa pine forest. Failure to meet density targets often was caused by a shortage of snags that met minimum size criteria rather than by a shortage of snags. Many snags containing excavated nest cavities did not meet the minimum size criteria in some recommendations. It may be possible to reduce those minimum size criteria while still providing substrates for cavity‐nesting birds. Studies explicitly linking snag size and density to demography of cavity‐nesting birds are badly needed, however, as are studies documenting ecologically sustainable snag densities. Until such data are available, managers should continue to emphasize snag recruitment and retention, with the focus on larger snags, and to ensure that snags are well‐distributed, but not uniformly distributed, across the landscape. © 2016 The Wildlife Society.

Individual snag detection using neighborhood attribute filtered airborne lidar data

The ability to estimate and monitor standing dead trees (snags) has been difficult due to their irregular and sparse distribution, often requiring intensive sampling methods to obtain statistically significant estimates. This study presents a new method for estimating and monitoring snags using neighborhood attribute filtered airborne discrete-return lidar data. The method first develops and then applies an automated filtering algorithm that utilizes three dimensional neighborhood lidar point-based intensity and density statistics to remove lidar points associated with live trees and retain lidar points associated with snags. A traditional airborne lidar individual-tree detection procedure is then applied to the snag-filtered lidar point cloud, resulting in stem map of identified snags with height estimates. The filtering algorithm was developed using training datasets comprised of four different forest types in wide range of stand conditions, and then applied to independent data to determine successful snag detection rates. Detection rates ranged from 43 to 100%, increasing as the size of snags increased. The overall detection rate for snags with DBH≥25cm was 56% (±2.9%) with low commission error rates. The method provides the ability to estimate snag density and stem map a large proportion of snags across the landscape. The resulting information can be used to analyze the spatial distribution of snags, provide a better understanding of wildlife snag use dynamics, assess achievement of stocking standard requirements, and bring more clarity to snag stocking standards.

Long-term dead wood changes in a Sierra Nevada mixed conifer forest: Habitat and fire hazard implications

Dead trees play an important role in forests, with snags and coarse woody debris (CWD) used by many bird and mammal species for nesting, resting, or foraging. However, too much dead wood can also contribute to extreme fire behavior. This tension between dead wood as habitat and dead wood as fuel has raised questions about appropriate quantities in fire-dependent forested ecosystems. Three plots installed in mixed conifer forest of the central Sierra Nevada in 1929 illustrate how amounts and sizes of dead wood have changed through time as a result of logging and fire exclusion. Diameter of snags was measured and CWD was mapped in the old-growth condition, prior to logging. Snags were re-measured in 2007 or 2008, and CWD was re-mapped in 2012. Snag density increased from 15.2ha−1 in 1929 to 140.0ha−1 in 2007/2008. However, average snag size declined, with 72% and 22% of snags classified as medium or large in 1929 and 2007/2008, respectively. Mechanisms of tree mortality also appear to have changed with greater mortality in smaller size classes, possibly as a result of higher live tree density. CWD volume, mass, and cover did not differ significantly between 1929 and 2012, with increased tree mortality and lack of periodic consumption by fire apparently compensating for the loss of inputs of large wood due to past logging. However, number of logs increased from 28ha−1 to 76ha−1 and average size declined substantially. Because larger-sized dead wood is preferred by many wildlife species, the current condition of more, smaller, and more decayed woody pieces may have a lower ratio of habitat value relative to potential fire hazard than it once did. Size, density, and stage of decomposition are therefore potentially better metrics for managing dead wood than mass and/or volume alone. To restore dead wood to conditions more like those found historically will require growing larger trees and reducing the inputs of dead wood from small and intermediate-sized trees. Fire, which preferentially consumes smaller and more rotten wood, would also help shift the balance to larger and less decayed pieces.

Density and Size of Snags, Tree Cavities, and Spruce Rust Brooms in Alaska Boreal Forest

Abstract To forecast the potential effects of forest management on wildlife habitat, I surveyed late-seral features in boreal forest near Fairbanks, Alaska. I sampled 75 randomly selected plots stratified among nine stand types to count and recorded physical characteristics of snags, cavitytrees, and Picea spp. trees with rust brooms. Snag density differed among some stand types (range, x = 10–72/ha) and increasedwith mean age of stand type, whereas cavity density (2–17/ha) and broom density (3–46/ha) showed no trend with stand age. Only 15% of 199 cavity openings were large enough (>50 cm2) and had a shape (width:height ratio, 0.5–1.5) that made them likely to besuitable for use by larger birds or arboreal mammals. The oldest and most valuable stand type for timber harvest (Picea glauca >23 cm dbh) often had trees with larger cavity openings and larger broom volumes than treesin other types. I recommend retention of rare specimens of late-seral features, considerations for feature recruitment in managed forests, and further documentation of wildlife use and associated fitness.

Snag longevity of Douglas-fir, western hemlock, and western redcedar from permanent sample plots in coastal British Columbia

Snags are important both as structural components and as animal habitat in forests, but abundance is often low and their dynamics poorly understood in young, managed stands. Using a large data set of 19,622 snags from permanent plots in second-growth forests of coastal British Columbia, we modeled snag longevity (time from tree mortality to snag fall) for three species: Douglas-fir (Pseudotsuga menziesii), western hemlock (Tsuga heterophylla), and western redcedar (Thuja plicata). Snag longevity was strongly related to species and snag size (diameter): the median snag longevity was 16 years for Douglas-fir, 11 years for hemlock and 5 years for redcedar. Western redcedar was predominantly in the subcanopy and its rapid fall rate was related to the small size of its snags. In addition to diameter, other attributes (height to diameter ratio, height, and live crown ratio before death) contributed significantly to models for one or two of the species. However, site level variables did not contribute significantly to any of the models. Snags greater than 50cm diameter, especially Douglas-fir snags, have the potential for persistence well beyond 20 years in these second-growth forests, and could be important for wildlife.

Snag longevity in relation to wildfire and postfire salvage logging

Snags create nesting, foraging, and roosting habitat for a variety of wildlife species. Removal of snags through postfire salvage logging reduces the densities and size classes of snags remaining after wildfire. We determined important variables associated with annual persistence rates (the probability a snag remains standing from 1 year to the next) of large conifer snags (≥23 cm diameter breast height (dbh)). Our study sites were located within two wildfires in western Idaho (Foothills fire of 1992 and Star Gulch fire of 1994). Study sites in the Foothills were partially salvage-logged (one-half of standing snags ≥23 cm dbh removed), and sites in the Star Gulch burn were unlogged. Snags were monitored within 0.04 ha plots for 8–9 years beginning in 1994 in Foothills Burn and 1995 in the Star Gulch Burn. A total of 1131 ponderosa pine ( Pinus ponderosa) and Douglas-fir snags ( Pseudotsuga menziesii) were monitored during the study period. Data were collected on snag species, height, decay class, and diameter at breast height. We also collected stand-level data (e.g., slope, aspect) and quantified information on remotely sensed data (e.g., pre-fire crown closure, burn severity) at the pixel-level (30 m × 30 m) and within 1 km of the plot centers (landscape-level). We modeled annual snag persistence as a function of data on all three scales using non-linear mixed-effects models. Additionally, we expected that variables on multiple scales, including information on burn severity and pre-fire crown closure, would be influential in determining snag persistence. The best models of snag persistence were selected using an information theoretic approach (i.e., AICc). Small-scale variables (age, height, dbh, tree species, decay, and snag density) were best predictors for both wildfire areas. In contrast to our expectations that large-scale variables would influence annual persistence rates, these factors did not appear in top models. Persistence was shorter for ponderosa pine than Douglas-fir snags. Additionally, smaller snags in plots with fewer snags fell sooner than did larger snags in more dense stands. Age of snag was also an important variable predicting snag persistence (older snags are more likely to fall). Snag longevity (the total amount of time the snag remained standing) also varied between the two sites. The predicted half-life of a ponderosa pine snag was 7–8 years in salvage-logged plots and 9–10 years in unlogged plots. The predicted half-life of Douglas-fir snags was longer than ponderosa pine, at 12–13 years in the salvage logged burn versus 15–16 years in the unlogged burn. On the partially logged sites, the primary effects of salvage logging on snags appeared to be the reduction of the average snag size (diameter and height) and density, which in turn reduced the subsequent longevity of individual snags. We concluded that the main effects of the partial-salvage logging appeared to be a reduction in overall snag density and average snag size which in turn reduces average persistence time. Management practices that preserve dense stands of snags will promote the longer-term persistence of suitable snags as nesting habitat for cavity-nesting birds.

Coarse woody debris in a Carpathian subalpine spruce forest

Number, volume and size of snags, broken and uprooted trees as well as advancement of their decomposition and arrangement were analysed in a subalpine spruce forest in the Babia Gora massif (West Carpathians). The most important results are: 1. The amount of coarse woody debris noted on Babia Gora was among the highest in subalpine spruce forests, however it was lower than in unmanaged boreal spruce forests, and much lower than in the mountain and boreal forests of North America. 2. Most trees died standing, whereas uprooting was the rarest cause of mortality. The probability that a tree would die standing was greatest among thinnest and thickest individuals. The greater the diameter of spruce the higher was the probability of breakage. Uprooting was most common among trees of moderate diameter. 3. Snags and broken stems were randomly distributed, whereas windthrows were clumped. 4. Decomposition of spruce logs was a very slow process as these logs remain visible on the soil surface for about 150 years after tree death. The process is slower than in central Sweden, but much quicker than in boreal forests of northern Sweden. 5. The diameter of dead trees and the spatial pattern of stand breakdown changed in recent years as a result of stand development and changes of causal factors. 6. In recent years intensity of stand breakdown was nearly two times higher than stand growth when the volume of the trees was taken into account. This indicates the probability of the stand to enter the breakdown stadium.

Standing-dead tree component of the boreal forest in central Saskatchewan

The standing-dead tree component was assessed in part of the upland forests in the mixedwood boreal forest in central Saskatchewan. Comparisons of the average number of dead trees per hectare were made between softwood stands, softwood-dominant mixedwood stands, hardwood-dominant mixedwood stands and hardwood stands. General trends in snag size and density were evaluated by comparing similar stand types of various height classes. The majority of the snags within the four forest associations were of small diameter (2–10 cm), comprising from 0.6 to 18.6% of the stems in these stands with the number of small-diameter dead trees per hectare decreasing in the older, taller stands. The density of small-diameter snags ranged from 440 ha −1 in 5 m softwood stands to 55 ha −1 in 25 m hardwood stands. The density of large diameter snags (>21 cm dbh) rarely comprised more than 5% of a stand.