Snag density varies with intensity of timber harvest and human access

Many species of vertebrates depend on snags (standing dead trees) for persistence, and limited research suggests that snag density is lower in areas of intensive timber harvest and increased human access. While intensive timber harvest is one source of potential snag loss, ease of human access to forest stands may also facilitate loss via firewood cutting of snags. Accordingly, we hypothesized that density of snags (number of snags/ha) would decline in forest stands with increasing intensity of timber harvest and increasing ease of human access. We tested our hypothesis by sampling stands under varying levels of timber harvest and access on National Forest land in the northwestern United States. Stands with no history of timber harvest had 3 times the density of snags as stands selectively harvested, and 19 times the density as stands having undergone complete harvest. Stands not adjacent to roads had almost 3 times the density of snags as stands adjacent to roads. Unharvested stands adjacent to non-federal lands and closer to towns had lower snag density, as did stands with flat terrain in relation to nearest road. Our findings demonstrate that timber harvest and human access can have substantial effects on snag density. Meeting snag objectives for wildlife will require careful planning and effective mitigations as part of management of timber harvest and human access.     

Snag degradation pathways of four North American boreal tree species

Many studies have highlighted the importance of deadwood, whether standing (snags) or fallen, in boreal ecosystems dynamics. However, a dearth of literature exists regarding the persistence and degradation pathways of these structures in northeastern American boreal species.     

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 50 cm 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.     

Temporal dynamics of snags and development of snag habitats in wet spruce–fir stands in east-central British Columbia

Patterns of tree mortality, rates and type of tree and snag fall, and relationships between snag characteristics and potential wildlife habitat value were estimated for hybrid spruce (Picea glauca (Moench) Voss × engelmannii Parry ex Engelm.) and subalpine fir (Abies lasiocarpa (Hook.) Nutt.) in east-central British Columbia in order to provide important parameters for deadwood modelling. We sampled 172 snags (52 spruce and 120 fir) for species, size, morphological, and habitat attributes, and used dendroecological techniques on a subsample of these (n = 158) to estimate year of death. Input of snags appeared to occur at a nearly constant rate in these stands. Estimated annual tree fall, including live trees and snags, was 5.3% for hybrid spruce and 6.1% for subalpine fir and stem breakage was more frequent than uprooting. Long-term annual snag fall rates were 4.6% for hybrid spruce and 2.9% for subalpine fir. Discriminant analysis based on time since death correctly classified snags into three decay classes for 85% and 72% of spruce and fir, respectively. Snags that potentially could provide important functions for wildlife habitat were more prevalent in fresh and intermediate classes for hybrid spruce and in intermediate and old classes for subalpine fir. The results provide valuable parameters for further development of deadwood models, which are an important tool for development of best practices for deadwood management.     

Dead wood threshold values for the three-toed woodpecker presence in boreal and sub-Alpine forest

Predicting species' responses to habitat loss is a significant challenge facing conservation biologists. We examined the response of both European three-toed woodpecker subspecies Picoides tridactylus tridactylus and P. tr. alpinus to different amounts of dead wood in a boreal and a sub-Alpine coniferous forest landscape in central Sweden and Switzerland, respectively. Habitat variables were measured by fieldwork in forests with breeding woodpeckers (n=10+12) and in control forests without breeding woodpeckers (n=10+12) in the same landscape. Logistic regression analyses revealed steep thresholds for the amount of dead standing trees and the probability of three-toed woodpecker presence in both Sweden and Switzerland. The probability of the presence of three-toed woodpeckers increased from 0.10 to 0.95 when snag basal area increased from 0.6 to 1.3 m² ha⁻¹ in Switzerland and from 0.3 to 0.5 m² ha⁻¹ in central Sweden. In Switzerland, a high road network density was negatively correlated to the presence of woodpeckers (r=−0.65, p=0.0007). The higher volumes of dead wood in Switzerland, where population trends are more positive, than in central Sweden, where the population is declining, would suggest that the volumes of dead wood in managed forests in Sweden are too low to sustain three-toed woodpeckers in the long-term. In terms of management implications, we suggest a quantitative target of at least 5% of standing trees in older forests being dead over at least 100 ha large forest areas. This corresponds about to ?1.3 m² ha⁻¹ (basal area) or ?15 m³ ha⁻¹ (volume), still depending on site productivity.     

Changes in quantitative patterns of dead wood in maritime pine plantations over time

Although the volume of dead wood is commonly acknowledged as an indicator of biodiversity in sustainable forest management schemes, only few data are available for plantation forests. To evaluate the volume and qualify the diversity of woody debris along a chronosequence of maritime pine plantations, we sampled downed woody debris, snags and stumps in 143 stands of different ages. To test the hypothesis that the pattern of dead wood accumulation mainly results from silvicultural operations, we developed a predictive model. It combined an empirical growth model evaluating the amount of dead wood produced by successive thinnings with a decay function that estimated the loss of dead wood with time.     

Amphibian and reptile community response to coarse woody debris manipulations in upland loblolly pine (Pinus taeda) forests

Coarse woody debris (CWD) has been identified as a key microhabitat component for groups that are moisture and temperature sensitive such as amphibians and reptiles. However, few experimental manipulations have quantitatively assessed amphibian and reptile response to varying CWD volumes within forested environments. We assessed amphibian and reptile response to large-scale, CWD manipulation within managed loblolly pine stands in the southeastern Coastal Plain of the United States from 1998 to 2005. Our study consisted of two treatment phases: Phase I treatments included downed CWD removal (removal of all downed CWD), all CWD removal (removal of all downed and standing CWD), pre-treatment snag, and control; Phase II treatments included downed CWD addition (downed CWD volume increased 5-fold), snag addition (standing CWD volume increased 10-fold), all CWD removal (all CWD removed), and control. Amphibian and anuran capture rates were greater in control than all CWD removal plots during study Phase I. In Phase II, reptile diversity and richness were greater in downed CWD addition and all CWD removal than snag addition treatments. Capture rate of Rana sphenocephala was greater in all CWD removal treatment than downed CWD addition treatment. The dominant amphibian and snake species captured are adapted to burrowing in sandy soil or taking refuge under leaf litter. Amphibian and reptile species endemic to upland southeastern Coastal Plain pine forests may not have evolved to rely on CWD because the humid climate and short fire return interval have resulted in historically low volumes of CWD.     

Snag dynamics in post-harvest landscapes of western Newfoundland balsam fir-dominated boreal forests

We examined changes in standing dead tree (snag) density and biomass with time following harvest across a chronosequence of balsam fir (Abies balsamea) dominated boreal forests in western Newfoundland, Canada. Current snag management practices in Newfoundland recommend a minimum of 10 snags/ha on the post-harvest landscape. Snags declined significantly in the first two decades of the chronosequence. The rapid rate of decline in snag density which occurred immediately post harvest was likely attributable to windfall and domestic harvest for firewood. A second rapid rate of snag density decline occurred 10–15 years post harvest which potentially reflected the average lifespan of snags in western Newfoundland. Average snag densities approximated the minimum management goal during the period from 15 to 60 years since harvest (YSH). However, 53–60% of sites sampled in the 15–60 YSH period contained <10 snags/ha. Snag densities then increased with forest age, again reaching high levels 81–100 YSH which were comparable to the density at the beginning of the chronosequence. Achieving the goal of 10 snags/ha on all post-harvest sites in western Newfoundland, especially between 15 and 60 YSH, will require changes to current forest management practices.