Snag Height Research Articles

Forest structural heterogeneity positively affects bird richness and acoustic diversity in a temperate, central European forest

IntroductionForests managed for timber production can also be managed for biodiversity conservation by retaining structures typical of old-growth forests, which provide heterogenous structures for forest-dwelling species, including birds. Ecoacoustic monitoring of forest birds is now a well-studied field, however the extent to which acoustic indices can reflect bird assemblage responses to stands of lower or higher structural heterogeneity is disputed. MethodIn this study, we acoustically surveyed 75 plots over two years in mature, previously managed forests and computed acoustic indices from the recordings. We first identified an acoustic index that significantly correlated with bird richness over more than one spring season. Next, we tested the response of bird richness to individual forest structural elements using linear regressions. We then repeated this analysis but combined the individual structural elements into one structural composition variable using an NMDS and gam smooth overlay, to compare the effect of individual forest structures versus overall forest heterogeneity on bird richness. We then repeated this analysis using our selected acoustic index, the Normalized Difference Soundscape Index (NDSI), to see if it followed the same patterns as bird richness in response to individual and collective forest structural elements.ResultsOur results showed that plots with high bird richness were also associated with high NDSI values, and high values of both variables occurred in plots with high structural heterogeneity (tree species richness, tree size variability, and snag height variability) and low amounts of standing and lying deadwood. DiscussionOur findings suggest that once an acoustic index can be identified as a robust correlate of bird richness, it can serve as a proxy for the response of birds to differences in forest structural heterogeneity within a managed forest context. It is therefore possible for forest managers interested in conserving or increasing bird richness in their production forests to identify a robust acoustic index to track the response of bird richness to management decisions over time.

Prescribed fire and thinning influence snag density and size in the southern Appalachian Mountains

Snags, or standing dead trees, are an important structural component of forest ecosystems. Many animals, including endangered species, depend on snags for foraging, protection, or raising young. Climate change, habitat loss, and modification of natural disturbance regimes contribute to changes in the availability and characteristics of snags in forests. Therefore, understanding what natural and artificial processes promote snags with the characteristics necessary for wildlife is a significant conservation concern. We examined how low-severity prescribed fire affected the density and characteristics of snags at 80 sites in the Talladega National Forest, Alabama. We sampled sites within 4 prescribed fire intervals, including 1–3 (previously thinned 4–23 years ago), > 3–8, > 8–12, and > 12 years. At each site, we measured snags across transects on 3 different slope positions, including the ridge, mid-slope, and valley, to account for slope-influenced fire behavior and stressors. The average diameter of snags increased in stands with the shortest prescribed fire interval, but snag height, decay class, and percentage of bark remaining were similar across all fire intervals. Snag density was lowest in the shortest fire interval due to fewer small- and medium-sized hardwood snags. A higher density of large snags was found in the shortest fire interval compared to the longest fire interval. Ridges had a greater density of snags compared to mid-slope and valley positions due to more small- and medium-sized pine snags. Although thinning followed by frequent, low-severity prescribed fire reduces snag density, the increase in density of large-diameter snags provides high-quality habitat for snag-dependent birds and bats. More intense fire and other stressors on ridges likely promote higher densities of snags. Our research indicates forest managers can use prescribed fire and thinning to accomplish multiple management goals while continuing to produce valuable snags for wildlife.

Factors influencing the diversity of deadwood, a crucial microhabitat for many rare and endangered saproxylic organisms

Deadwood (DW) is a crucial component of habitats, with both its volume and diversity considered critical for saproxylic organisms. Deadwood position, thickness, snag height, species composition, and decay stage were studied in the context of eight factors characterizing habitats, stands, and management types. A multivariate multiple regression model was used to analyze data from 29,098 sample plots.Deadwood diversity (a 5-dimensional dependent variable) was found to strongly depend on DW volume in a given area and on the species composition of the stand. A less pronounced effect was exerted by site fertility and moisture, the age and DBH structure of the stand, and terrain type. The model also revealed that the location of the stand in managed or unmanaged areas did not have a direct impact on deadwood diversity, with stand features being more important.Analysis of individual qualitative characteristics showed that DW thickness was affected by the same seven factors as DW diversity. DW position and decay stage were influenced by four factors: site fertility and moisture as well as stand age and species composition. In addition to the species composition of stands, DW species composition was positively influenced by more fertile and moist sites and a more varied DBH structure of stands. A significant negative effect was identified for high DW volume, which indicates that the deadwood accumulated in a stand tends to be derived from the tree species that was most vulnerable to mortality at a given time. Snag height variation was influenced by such stand characteristics as age, DBH structure, and species composition; the other significant factors were DW volume and terrain type. The diversity levels of individual DW characteristics were significantly and positively correlated, which means that all of them often revealed high or low diversity at the same time.Our study showed that forests with low DW volume additionally face the problem of DW quality. Thus, efforts to increase DW volume should be focused on those DW characteristics that are lacking (large trees, species diversity, and height variation of snags). DW diversity is lower in younger stands, with little DBH variation, and in forests which are more easily accessible, located mostly in the lowlands, and growing on less fertile and dry or mesic sites.

Downed wood dynamics in the riparian and littoral zone of small lakes in tolerant hardwood forests

Large wood (LW) is an important structural feature in forested lake ecosystems, but little is known about the production of LW in riparian forests and its movement across the forest–lake ecotone, both of which we describe here for eight small (<30 ha) lakes in Ontario, Canada. The creation of snags through live-tree death varied by species, ranging from 0.24% year−1 for eastern white cedar to 5.60% year−1 for balsam fir. Snag breakage was best described as a function of snag height and snag fall described as a function of tree species, decay stage, and diameter. Long LW pieces and pieces generated close to the lake were more likely to be deposited at the shoreline. The density of LW at the shoreline was 125–550 pieces·km−1, and density and volume of LW were positively related to riparian slope. LW was a stable resource at the shoreline, with a recruitment rate about equal to the loss rate and average movement along the shoreline of 2.89 cm·year−1. LW volumes at the shoreline and in the littoral zone of the lake were positively related to lakebed slope. LW could reside in the littoral zone for centuries, providing physical structure for multiple generations of aquatic organisms in these small lakes. Our results indicate that critical habitat for fish that need aquatic LW extends at least 5 m into riparian forest and that removal of trees within 5 m of the forest edge would reduce LW input to the littoral zone.

Dynamics of beetle-killed snags following mountain pine beetle outbreaks in lodgepole pine forests

Snags (standing dead trees) are important components of forest ecosystems that, among other benefits, provide critical habitat for many species of wildlife, but also represent important safety concerns to firefighters, forest workers, and the public. We identified factors that influence the fall rates of lodgepole pines, Pinus contorta Dougl. ex Loud., killed by bark beetles during a severe regional-scale outbreak of mountain pine beetle, Dendroctonus ponderosae Hopkins, that occurred in 2004–2012. Data were obtained during annual assessments (2010–2019) of a network of 107 0.081-ha circular plots installed in P. contorta forests in Colorado, Idaho, Montana, Utah and Wyoming, U.S. A total of 3789 P. contorta snags were monitored, and in each case we recorded, among other variables, the year the snag was created and the year the snag fell to the forest floor. Among snag age classes (i.e., number of years since tree death), the highest number of snags (1046) were 12 years since death (YSD), and those 13 and 14 YSD exhibited the lowest fall rates to date (<10%) despite being the oldest in our study. Snags 13 and 14 YSD were among the largest in diameter. Snag fall occurred in every snag age class from 1 to 14 YSD, with the greatest proportion of snag fall events occurring 4–8 YSD. By 2019, 24.7% (937) of snags fell to the forest floor. We modeled snag fall using a Cox’s proportional-hazards model that included six covariates of interest: elevation (m), slope aspect (categorical, cardinal direction), slope (%), canopy cover (%), snag height (m), and snag height:dbh (m, diameter at 1.37 m in height). Slope aspect had the strongest influence on fall rates. Northern aspects, increased canopy cover, and taller snag heights decreased the probability of snag fall. Conversely, southern aspects and increased height:dbh ratios (i.e., taller, skinner snags) increased the probability of snag fall. The predicted half-life (i.e., the amount of time since death required for 50% of the snag population to fall to the forest floor) was ~16 YSD after which the function predicted a linear, ~0.04/year decline in snag survival probability 15–30 YSD. The observed longevity of P. contorta snags confers important ecological benefits for some wildlife, and may offer opportunities for extended periods of salvage. However, checking (cracks) rapidly occurred, and many snags 6 YSD and older had at least one check at 1.37 m and 5.5 m in height, which can negatively impact lumber recovery. The relatively slow snag fall rates observed in our study also lengthen concerns regarding hazard trees, human safety, and protection of critical infrastructure. The implications of these and other results to management of P. contorta forests are discussed.

Fire and distance from unburned forest influence bird assemblages in Southern Andean Yungas of Northwest Argentina: a case study

BackgroundWildfires affect vegetation structure, functions, and other attributes of forest ecosystems. Among these attributes, bird assemblages may be influenced by the distance from undisturbed to fire-disturbed forests. Information about this influence is essential for designing management plans aimed at conserving birds’ diversity in undisturbed forests, which contributes to their sustainability. In Northwest Argentina, timber extraction and man-caused fires threaten the sustainability of Southern Andean Yungas forests. In this region, we evaluated, in relation to a reference unburned forest, the effects of close and distant fire-disturbed patches on bird assemblages, exploring also relationships between burned and unburned forest structures on bird assemblages. On each site, we determined forest structural variables, and twice per year, from 2015 to 2017, we recorded birds seen or heard on every site, from dawn to 1000 hours, at 30 0.5 ha counting points.ResultsAbundance and richness of bird species were lower in the unburned reference site than in close- and distant-burned sites; the farther the distance of burned sites to the unburned site, the less similarity in bird assemblages. Bird assemblage abundance appeared to be associated with snag height and basal area. However, bird species in mature forests were present at all sites and outnumbered those typically found in forest edges, or secondary or disturbed forests.ConclusionsConnected areas between unburned and burned forest patches provide habitat for birds living in both environments. Preventing forest fragmentation by reducing the number and size of wildfires, promoting selective timber logging, and banning post-fire snag removal will help promote suitable habitat for different bird assemblages and contribute to Yungas forests’ sustainability. This study is the first step toward understanding how fire-patch distances to unburned forests may affect bird assemblages in subtropical Southern Andean Yungas forests in South America and provide a basis for comparison with other subtropical ecosystems around the world.

What factors influence colonization of lichens, liverworts, mosses and vascular plants on snags?

The dead standing trees i.e. snags are known as habitat for epiphytic and epixylic species including first of all lichens and bryophytes. The vascular plants are much rarer on this type of coarse woody debris (CWD). The eighty snags (CWD elements higher than 1.5 m) of Norway spruce Picea abies and beech Fagus sylvatica in the Karkonosze Mts. were examined for the presence of lichens, liverworts, mosses and vascular plants. The height of snags, their decomposition stage, cover of bark, diameter at breast height (DBH) as well as site conditions (elevation, slope and aspect, presence in forest community) were measured and noted. The percent cover of plants and lichens were estimated on each snag. Totally 99 taxa were recorded. There lichen species were dominant (44), followed by mosses (34), liverworts (13) and there were only 8 vascular plants. The total species richness varied from 1 to 22 taxa. The species composition growing on snags was subjected to canonical correspondence analysis and statistical analyses. They revealed that the species identity of snag is one of the most important factors influencing species composition. The number of species is positively correlated with DBH whereas decomposition stage, presence of bark, snag height are not significant factors. The species richness increases also with altitude what is connected with higher abundance of spruce snags. The occurrence of snags in this area is mainly associated with forest management practices in the past. Despite of some observed patterns in colonization of snags they are important habitat especially for lichens.

Mark–recapture estimation of snag standing rates in Northern Arizona mixed‐conifer and ponderosa pine forests

ABSTRACTSnags (standing dead trees) are important components of forests that provide resources for numerous species of wildlife and contribute to decay dynamics and other ecological processes. Managers charged with managing populations of snags need information about standing rates of snags and factors influencing those rates, yet such data are limited for ponderosa pine (Pinus ponderosa) and especially mixed‐conifer forests in the southwestern United States. We monitored standing rates of snags in 1‐ha plots in Arizona mixed‐conifer (n = 53 plots) and ponderosa pine (n = 60 plots) forests from 1997 through 2012. We used the Burnham live–dead, mark–resight model in Program MARK and multimodel inference to estimate standing rates during 5‐year intervals while accounting for imperfect detection. Because snag standing rates may be influenced by plot characteristics, we used plots rather than snags as sampling units and conducted bootstrap analyses (500 iterations per model) to resample plots and estimate standing rates and associated parameters. We modeled standing rates in 3 discrete steps. First, we selected a parsimonious base model from a set of models including snag species, and then we evaluated models created by adding snag and plot covariates to the base model in steps 2 and 3, respectively. Snag standing rates differed among snag species and 5‐year sampling intervals. Standing rates were positively related to snag diameter, negatively related to snag height, and were lower for snags with intact tops than for broken‐topped snags. Standing rates also were positively related to topographic roughness, elevation, tree density, and an index of northness, and negatively related to slope and relative topographic exposure. Our results provide comparative data on standing rates of multiple species of snags based on a large and spatially extensive sample and rigorous analysis, and quantify the relative importance of several snag and plot characteristics on those rates. They indicate that modeling snag dynamics is complicated by both spatial and temporal variation in standing rates and identify areas where further work is needed to facilitate such modeling. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.

Snag characteristics and dynamics following natural and artificially induced mortality in a managed loblolly pine forest

A 14-year study of snag characteristics was established in 41- to 44-year-old loblolly pine (Pinus taeda L.) stands in southeastern USA. During the initial 5.5 years, no stand manipulation or unusually high-mortality events occurred. Afterwards, three treatments were applied consisting of trees thinned and removed, trees felled and not removed, and artificial creation of snags produced by girdling and herbicide injection. The thinned treatments were designed to maintain the same live canopy density as the snag-created treatment, disregarding snags that remained standing. We monitored snag height, diameter, density, volume, and bark percentage; the number of cavities was monitored in natural snags only. During the first 5.5 years, recruitment and loss rates were stable, resulting in a stable snag population. Large snags (≥25 cm diameter) were common, but subcanopy small snags (10 to &lt;25 cm diameter) dominated numerically. Large natural snags survived (90% quantile) significantly longer (6.0–9.4 years) than smaller snags (4.4–6.9 years). Large artificial snags persisted the longest (11.8 years). Cavities in natural snags developed within 3 years following tree death. The mean number of cavities per snag was five times greater in large versus small snags and large snags were more likely to have multiple cavities, emphasizing the importance of mature pine stands for cavity-dependent wildlife species.

Snag Longevity and Decay Class Development in a Recent Jack Pine Clearcut in Michigan

Abstract To better understand the factors that influence the longevity and decay class development of natural and girdled snags in jack pine (Pinus banksiana Lamb.) plantations managed for Kirtland's warbler (Dendroica kirtlandii Baird) breeding habitat in the northern Lower Peninsula of Michigan, we followed the fate of 335 jack pine and oak (Quercus spp.) snags. After 2.5 years, 41% of snags snapped or uprooted, with most snapping or uprooting occurring within the first year. Jack pine snags experienced higher rates of snapping or uprooting than oak or all other snags combined, regardless of whether natural or girdled. Girdling by itself or as an interaction term had no significant effect on snapping or uprooting for either jack pine or oak, but both diameter (P = 0.03) and height (P = 0.01) influenced snapping and uprooting in oak. Thirty months after treatment, the percentage of snags among decay classes differed between species of snag and snag types (natural-girdled), with snag height inversely related to snag decay class development. These results suggest that snag development will occur rapidly in recently clearcut jack pine stands and that higher densities of snags may be needed to be retained or created if management goals are to emulate more natural conditions.

Analysis of Nest Sites of the Resplendent Quetzal (Pharomachrus mocinno): Relationship between Nest and Snag Heights

Abstract The Resplendent Quetzal (Pharomachrus mocinno) is of particular conservation concern because of its iconic status in Central American culture. This species is a secondary cavity nester and modifies abandoned woodpecker nest sites in dead tree trunks (i.e., snags). We used 11 historical nest sites, reported in 1969, from Atitlan, Guatemala and 10 recent nest sites from San Gerardo de Dota, Costa Rica to examine if a relationship exists between nest and snag height. There were significant differences between Costa Rica and Guatemala in both nest height (6.3 vs. 10 m, respectively; t-test14 = −2.49, P = 0.042) and snag height (8.1 vs. 14.0 m, respectively; t-test13 = −2.39, P = 0.033). There was no difference in nest heights relative to snag heights for Costa Rica (0.76) and Guatemala (0.77; t-test17 = −0.20, P = 0.84). One aspect of conservation efforts for this species has been placement of nest boxes to provide nesting sites for additional pairs. Our results provide a better understan...

Snag density and use by cavity-nesting birds in managed stands of the Black Hills National Forest

We examined whether cavity-nesting bird abundance was related to the density of snags in managed ponderosa pine stands ( Pinus ponderosa Laws.) on the Black Hills National Forest. We also examined whether snag variables were related to bird use of snags as nest sites and for foraging. Study plots ( n = 144 plots) were established throughout the forest in managed ponderosa pine stands and data on the density, size, and condition of 2886 snags were collected. We searched snags for cavities and signs of foraging, and surveyed plots for cavity-nesting birds ( n = 272 counts). Nine species of cavity-nesting birds were detected, with red-breasted nuthatch ( Sitta canadensis), black-capped chickadee ( Poecile atricapillus), and hairy woodpecker ( Picoides villosus) occurring most frequently. The mean number of cavity-nesting birds at a plot was independent of snag density or other plot variables. Larger DBH and greater snag height were positively associated with the presence of a cavity, and advanced stages of decay and the presence of a broken top were negatively associated with the presence of a cavity. Snags in larger DBH size classes had more evidence of foraging than expected based on abundance. Combining the data on the presence of a cavity and evidence of foraging, snags with large DBH were used most by cavity-nesting birds. Our study found no relationship between the number of cavity-nesting birds and snag density across the range of snag densities, snag sizes, and snag conditions measured. However, the densities of large snags may have been too low to influence the abundance of cavity-nesting birds, limiting our ability to detect such an effect.

Selection of Day Roosts by Female Long-Legged Myotis in the Central Oregon Cascade Range

We radiotracked 16 female long-legged myotis (Myotis volans) in the central Oregon Cascade Range that used a total of 41 day roosts. Large Douglas-fir (Pseudotsuga menziesii) snags averaging 97 ± 7 (SE) cm diameter at breast height (dbh) and 38 ± 3 m in height were the most commonly used roost structures (88%). The odds that a snag was used as a day roost increased as snag height increased (P < 0.001); after snag height was accounted for, the odds of use decreased as stand height within 20 m of the snag increased (P = 0.024). The frequency of occurrence of roosts between young and late seral stands did not differ from that by chance in these 2 stand conditions (P = 0.76). Day roosts generally occurred in upland habitats associated with streams that contained night roosts. Management of large diameter, tall snags that extend above the canopy will provide 1 component of day-roost habitat for long-legged myotis in managed landscapes.

Snag Availability and Communities of Cavity Nesting Birds in Tropical Versus Temperate Forests

Standing dead trees (snags) ate critical foraging and nesting sites for animals, notably birds, yet little is known about snag dynamics or communities of cavity nesting birds in low latitude forests. In a comparison among 10 New World forests, we observed that snags in tropical and subtropical forests (N = 5 sites) occurred at lower densities (3.5-20.5 snags/ha versus 21.2-49.3 snags/ha) and had larger average diameters (27.6-34.2 cm versus 11.2-31.2 cm) than snags in temperate and boreal forests (N = 5 sites). No latitudinal trends were evident in average density of live stems or average snag height. An analysis of avifaunal lists from these sites indicated that low latitude forests supported 2.5 times as many cavity nesting species and approximately equal numbers of primary cavity excavating species as high latitude forests

The relationship between cavity-nesting birds and snags on clearcuts in western Oregon

Relationships between cavity-nesting birds (CNB) and density and characteristics of snags were investigated on 13 clearcuts in central coastal Oregon. Species richness and density of CNB were positively ( P<0.05) related to snag density and were still increasing at the maximum snag density evaluated. Cavity-nesting birds selected ( P<0.05) snags taller than 6.4 m and greater than 78–102 cm in diameter, and avoided ( P<0.05) snags less than 28 cm in diameter. Snags of intermediate decay stages were used for nesting more ( P<0.05) than snags of early and advanced stages of decay. Cavity-nesting birds selected snags with more ( P<0.05) bark cover (greater than 11%) than the average cover found on available snags. Individual CNB species exhibited significantly different ( P<0.05) selections for snag height, diameter, hardness and bark cover. To optimize density and richness of CNB, forest managers should provide ≥ 14 snags ha −1 between 28 and 128 cm diameter at breast height (dbh), between 6.4 and 25 m tall, with at least 10% bark cover, and with a majority in hardness stages 3 and 4.