Live-tree Retention 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.

What does FSC forest certification contribute to biodiversity conservation in relation to national legislation?

Forest certification has emerged as a voluntary, market-driven tool for sustainable forest management (SFM). Its legitimacy depends on its ability to achieve its objectives and to retain the support of stakeholders such as NGOs and the companies that adopt it. This study presents a novel approach for assessing the contributions of forest certification to biodiversity conservation, based on Forest Stewardship Council (FSC) certification in four northern European countries (Finland, Sweden, Estonia, Latvia). In each case, national FSC certification requirements related to specific biodiversity targets were compared with requirements in national legislation. Nearly 80% of the assessed certification requirements were more prescriptive than the national legislation. One-third of these requirements (3–8 per country) were assessed to have a positive contribution to biodiversity conservation, whereas four requirements (up to 2 per country) were assessed to have a low positive contribution. FSC requirements to protect Woodland Key Habitats were identified as having a positive contribution in all four countries, whereas requirements regarding live tree retention in harvests and preserving dead wood had a positive contribution in three countries each. Despite often prescribing similar measures, the other requirements with positive contributions varied between countries depending on the national legislative baseline. The remaining requirements could not be assessed through expert evaluation, indicating the need for additional empirical research to evaluate how the normative requirements translate to impacts in the field, and how the national context may affect their implementation. The approach is globally applicable, repeatable, and provides a basis for designing systematic empirical assessments of the certification impact.

Seven decades of change in forest structure and composition in Pinus resinosa forests in northern Minnesota, USA: Comparing managed and unmanaged conditions

An understanding of long-term patterns of forest structural and compositional development is critical for anticipating management outcomes and developing appropriate silvicultural strategies for restoring complex forest conditions. In most cases, this information comes from stand-level assessments; however, the impacts and outcomes of management and other disturbances on forest development occur over multiple spatial scales across a landscape. We compared historical (1941) and contemporary (2012–2014) forest structure and composition on 301 plots distributed across managed and unmanaged, late seral red pine (Pinus resinosa)-dominated forests in a 1230ha landscape in north-central Minnesota, USA. Discriminate factor analysis was used to determine which compositional and structural attributes best described the forest conditions between two sampling periods (1941, 2012–2014) and management histories (managed and unmanaged). Plot basal area, average diameter of live trees, richness of tree size classes, and the basal area of standing deadwood were the four most important variables in discriminating between the managed and unmanaged plots in 1941 and 2013. In some cases, structural conditions between managed and unmanaged plots converged, including contemporary BA, trees per hectare, size inequality, and structural complexity indices. In contrast, several attributes, including standing deadwood basal area and percent hardwood basal area, were significantly greater in unmanaged plots after 72years and highlight the lasting influence of land use on these structural and compositional conditions. The broad ranges of structural and compositional conditions observed across the landscape highlight the importance of having spatially varying desired future conditions across managed stands to approximate this range in live and dead-tree attributes in unmanaged forests. In addition, the lower basal area of standing dead trees documented in this and other comparisons of unmanaged and managed P. resinosa systems argues for an increased emphasis on deliberate deadwood and live-tree retention to recruit these features if the restoration of late-successional forest conditions is included as a management objective.

Slash harvesting does not undermine beetle diversity on small clear-cuts containing sufficient legacies

Logging residues (slash) constitute an increasingly important source of renewable energy in forested countries, but their intensive extraction can reduce biodiversity. Important research issues include the mitigation potential of reduced logging and slash extraction levels, and their feasibility in production forestry. We performed a comparative study on beetle assemblages in relation to slash extraction practices in Estonia, where silviculture retains higher deadwood amounts than in intensive forestry systems, slash extraction level is ca. 50 %, and final fellings retain at least 5 % of the standing stock. We sampled beetles using flight-interception traps on six pairs of retention cut sites (one conventional and one slash-harvested; average size ca. 2 ha; in three forest site-types). The material comprised 11,948 beetle specimens identified to species level (500 species). Species of conservation concern occurred regularly in all sites, while a total of 18 putative pest species comprised only ca. 3 % of individuals in both treatments. There were no clear influences of slash extraction on species richness, abundance or assemblage composition. The main gradients in the assemblage composition were trapping season and site type; the latter co-varying with tree-species composition and deadwood volumes. We conclude that slash harvesting that retains (depending on forest type) 5–25 m3 ha−1 of coarse deadwood and 10–35 m3 ha−1 of fine deadwood neither reduces beetle diversity (at least of highly mobile species in the short term) nor affects pest outbreaks in our study system. To sustain beetle diversity, sufficient deadwood legacies should be supplemented with live-tree retention for a continuous deadwood supply during post-harvest succession.

Level and pattern of overstory retention influence rates and forms of tree mortality in mature, coniferous forests of the Pacific Northwest, USA

Mortality of retained trees can compromise the ecological objectives of variable-retention harvest. We used a large-scale experiment replicated at six locations in western Washington and Oregon to examine the influences of retention level (40% vs. 15% of original basal area) and its spatial pattern (aggregated vs. dispersed) on the rate and form of tree mortality for 11–12years following harvest. Cumulative mortality of conifers was greater at lower levels of retention and in dispersed treatments—a result common to most seral groups and canopy strata. The greatest losses, averaging 18% of stems, occurred at low levels of dispersed retention. Mortality peaked in the first year in dispersed treatments and at low levels of aggregated retention, then rates declined to levels comparable to the controls (∼0.8% of stems/year). Harvest-related bole damage—common in the dispersed treatments—did not increase risk of mortality. Standing dead accounted for most mortality at greater levels of retention, but uprooting was nearly as common at lower retention—particularly in dispersed treatments. Forest aggregates (1ha in size) did not exhibit greater overall mortality than controls. In fact, at 40% retention, mortality rates were reduced in the aggregates due to greater survival of late-seral and suppressed stems, presumably a response to edge creation and increased resource supply. Although cumulative mortality was no higher at 15% retention, larger harvest-unit openings and greater exposure of aggregates resulted in greater uprooting among dead trees. Current standards and guidelines for retention harvests on federal lands in the Pacific Northwest require a minimum of 15% retention within each harvest unit, with 70% of this distributed in aggregates of ⩾0.2ha. Our results suggest that these minimum standards lead to greater risk of tree mortality in dispersed settings and to elevated rates of uprooting regardless of pattern, potentially compromising the objectives of live-tree retention. However, with larger (1-ha) aggregates and moderate levels of retention, managers have considerable flexibility in implementing variable retention without risk of excessive mortality.

The dying legacy of green-tree retention: Different habitat values for polypores and wood-inhabiting lichens

The expanding practice of live-tree retention on clear-cuts represents a model case for conservation planning, because the high mortality of those trees requires balancing conflicting management goals in unpredictable conditions over long time frames. We explored the habitat provisioning function of dying retention trees for polypore and epixylic lichen assemblages. We sampled fallen trunks and snags created by known retention-tree deaths during the first post-harvest decade on 46 cut areas in Estonia, hemiboreal Europe. Those trees (particularly large fallen aspens) hosted a species-rich polypore assemblage that included several species of conservation concern. Lichen colonization of wood was slower and most species were found on pine snags. At the tree scale, the total species richness was highest on the trees that had died by trunk breakage creating both a snag and a log. To represent all species at the cutover scale, equal retention of different tree species and dead-wood types was sufficient for common species, but selective retention appeared necessary for species of conservation concern. The most frequent polypore species of conservation concern were either characteristic of specific substrates (notably aspen trunks) or early-successional stand conditions. We conclude that (i) dying retention trees are unlikely to provide dead-wood continuity at the stand scale and this function should be assessed at the landscape scale; (ii) effective habitat provisioning should include predicting and affecting the causes of tree death and linked management decisions based on the diversity, size, and longevity of the trees in specific landscape contexts.

Impacts of post-harvest slash and live-tree retention on biomass and nutrient stocks in Populus tremuloides Michx.-dominated forests, northern Minnesota, USA

Globally, there is widespread interest in using forest-derived biomass as a source of bioenergy. While conventional timber harvesting generally removes only merchantable tree boles, harvesting biomass feedstock can remove all forms of woody biomass (i.e., live and dead standing woody vegetation, downed woody debris, and stumps) resulting in a greater loss of biomass and nutrients as well as more severe habitat alteration. To investigate the potential impacts of this practice, this study examined the initial impacts (pre- and post-harvest) of various levels of slash and live-tree retention on biomass and nutrient stocks, including carbon (C), nitrogen (N), calcium (Ca), potassium (K), and phosphorus (P), in Populus tremuloides Michx.-dominated forests of northern Minnesota, USA. Treatments examined included three levels of slash retention, whole-tree harvest (WTH), 20% slash retention (20SR), and stem-only harvest (SOH), factored with three levels of green-tree retention, no trees retained (NONE), dispersed retention (DISP), and aggregate retention (AGR).Slash retention was the primary factor affecting post-harvest biomass and nutrient stocks, including woody debris pools. Compared to the unharvested control, stocks of biomass, carbon, and nutrients, including N, Ca, K, and P, in woody debris were higher in all treatments. Stem-only harvests typically contained greater biomass and nutrient stocks than WTH, although biomass and nutrients within 20SR, a level recommended by biomass harvesting guidelines in the US and worldwide, generally did not differ from WTH or SOH. Biomass in smaller-diameter slash material (typically 2.5–22.5cm in diameter) dominated the woody debris pool following harvest regardless of slash retention level. Trends among treatments in this diameter range were generally similar to those in the total woody debris pool. Specifically, SOH contained significantly greater amounts of biomass than WTH while 20SR was not different from either WTH or SOH.Within P. tremuloides systems, we observed high stocks of smaller diameter slash material for all prescribed slash retention treatments. Most notably, WTH retains much more material than anticipated, up to 50% of available slash. These results reflect the high levels of breakage during winter harvest operations in these stands and, consequently, warrant consideration when anticipating the impacts of biomass harvesting on woody debris pools. Further investigation is necessary to understand how deliberate slash retention levels and season-of-harvest impact woody debris in other forest systems.

Regeneration and stand development following a spruce budworm outbreak, spruce budworm inspired harvest, and salvage harvest

Spruce budworm ( Choristoneura fumiferana (Clemens)) (SBW) outbreaks are a major disturbance that influence stand dynamics and succession in balsam fir ( Abies balsamea (L.) Mill.) dominated forests of eastern Canada. We used stand and regeneration data collected from five plots in each of one immature and five mature stands before, during, and 30 years following the last major SBW outbreak in the Cape Breton Highlands to examine the role of stand and regeneration characteristics in shaping future stand development. Comparisons were also made between regeneration in four SBW outbreak stands versus two SBW salvage stands, with and without a subsequent precommercial thinning, and with 25 plots that underwent SBW emulation harvest. In mature unharvested balsam fir stands, species composition 30 years following the SBW outbreak was closely related to predisturbance species composition, and in immature fir stands, hardwood composition increased from 0% to 4%–27%. Species composition in harvested stands varied depending on whether intolerant hardwoods had been precommercially thinned, where thinned stands had 30% less hardwood 30 years postdisturbance than unthinned stands. Seedling density decreased by 17%–85% from 1979 to 1989 in all SBW outbreak stands, but average seedling height increased by 17%–500% as live canopy cover decreased from an average of 50% to 4%. Results suggest that advanced regeneration should be protected during harvest of balsam fir dominated stands, post-outbreak precommercial thinning will increase tree growth, and live tree retention can help develop late-seral structural characteristics in second-growth stands.

Canopy microclimate response to pattern and density of thinning in a Sierra Nevada forest

Restoring Sierra Nevada mixed-conifer forests after a century of fire suppression has become an important management priority as fuel reduction thinning has been mandated by the Healthy Forests Restoration Act. However, in mechanically thinned stands there is little information on the effects of different patterns and densities of live-tree retention on forest canopy microclimate. This study compared gradients of air temperature and vapor pressure deficit (VPD) through the vertical forest profile among an overstory-thin, an understory-thin, an un-thinned control, and a riparian environment in a Sierra Nevada mixed-conifer forest. Temperature and humidity were recorded for a year by 60 data loggers arrayed in 12 trees at 5, 15, 25, 35, and 45 m above the forest floor. Both thinning treatments had significantly more extreme summer daily ranges of temperature and VPD than the control across heights. The overstory-thin resulted in the greatest maximum temperatures, VPDs, and VPD range among all sensors at 5 m, and significantly higher summer maximum temperatures and VPDs than the control in lower strata (≤15 m). The understory-thin also had significantly higher summer maximum temperatures than the control (≤15 m), but these too were significantly less than in the overstory-thin nearest the surface at 5 m. Understory thinning did not alter the mean or range of microclimate as much as overstory thinning. Riparian microclimate had significantly lower minimums and means, and greater daily ranges of temperatures and VPDs than the control. Results suggest that thinning canopy cover significantly increases the extremes and variability of understory microclimate compared to thinning from below and no-thin treatments.

Understorey protection harvest expedites recolonisation of boreal forest stands by North American red squirrels

Clearcut harvesting removes old-forest structure from stands, rendering these habitats unsuitable for tree-dependent mammal species such as North American red squirrels ( Tamiasciurus hudsonicus). Unsuitability of boreal mixedwood forest clearcuts may persist for almost a century, longer than most harvest rotations in this region. In these systems, one alternative to clearcutting is mixedwood understorey protection (MUP) harvesting. MUP harvesting selectively removes mature deciduous canopy trees, retaining some mature and immature trees in the overstorey and understorey, and promoting release of understorey conifers. Live tree retention may also serve to promote early recolonisation by tree squirrels. We compared red squirrel abundance and demography before and after MUP harvest, and between different stand types, in the boreal forest of northern Alberta, Canada. Red squirrels were mark-recaptured in MUP, mixedwood reserve, deciduous, and conifer stands June–July 2001–2003. Red squirrel abundance and masses differed between stand types within years. Abundance and masses did not differ between pre- and post-harvest mixedwood stands across years. Across all stand types, red squirrel abundance was predicted by abundance of spruce trees and fungi. Our results indicate that MUP harvesting retains forest structure required by red squirrels, thus allowing persistence of red squirrel populations; it thus holds promise as a component of the natural disturbance model for maintaining arboreal sciurid populations in managed landscapes.

Disturbance and post-harvest ground conditions in a structural retention experiment

Federal guidelines now specify retention of live trees on cutting units within the range of the northern spotted owl, but to date there have been no formal tests of the ecological or silvicultural implications of these guidelines, nor of possible tradeoffs associated with different spatial patterns of retention. We examined disturbance patterns and post-harvest ground conditions in mature forests of western Oregon and Washington in a factorial experiment that compared both level (15 vs. 40%) and pattern (dispersed vs. aggregated) of live-tree retention. Ground cover associated with logging disturbance (slash, disturbed soil, and intact forest floor), slash depth, and the difference (pre- vs. post-logging) in coarse woody debris (decay class I) were quantified on four treatment units (two factors, two levels) at six locations (blocks). Significantly greater disturbance was evident at lower levels of retention (15%) and on aggregated cutting units, but even greater differences were evident among blocks, reflecting the predominant influence of initial site differences and yarding methods (ground-based, suspension cable, or helicopter). Short-term responses of vegetation to varying levels and patterns of structural retention are likely to be shaped by levels of ground disturbance. Thus, we expect that block- and treatment-level effects will have important consequences for tree regeneration and understory composition in these forests. Conventional strategies used to minimize soil disturbance and to manage slash accumulation during clearcut logging should be applicable to variable retention systems.

Nest predation in and adjacent to cutblocks with variable tree retention

Partial harvesting, or the retention of live trees in cutblocks, is thought to reduce the impacts of clearcutting on wildlife biodiversity. An unintended biproduct of this practice may be an increase in nest predation in and adjacent to these cutblocks. We examined patterns of predation on artificial ground and shrub nests in and adjacent to stands where varying amounts of trees were left after harvesting in Alberta’s boreal mixed-wood forest. We placed nests in old, aspen-dominated stands adjacent to 3-year-old cutblocks with high (86–133 trees/ha), intermediate (48–55 trees/ha), and low (10–36 trees/ha) residual tree retention, and in forested controls (246–414 trees/ha). In a second experiment, we placed nests in residual tree clumps in high and intermediate retention cutblocks and in forested controls. Each nest contained one painted plasticine and one Chinese Painted Quail ( Coturnix chinensis) egg and predators were identified based on marks left in plasticine eggs. Vegetation structure and composition and overhead and lateral cover were measured around each nest. Data were analysed with a nested logistic regression approach, using generalized linear models with quasi-likelihood functions to analyse nest predation with respect to treatment and vegetation characteristics around the nest. In both experiments the identified nest predators were murid rodents, red squirrels ( Tamiascurus hudsonicus) and birds. Nests closer to the ground were more likely to be depredated. Nest predation was not related to the level of residual tree retention in either experiment nor to the size of residual tree clumps in the second experiment. Vegetative cover at the nest did not enter explanatory models, nor did most measures of vegetation structure and composition around the nest. The few vegetation factors that did enter models were not consistent between experiments and generally the models had low predictive power. We conclude that, within 3 years postharvest, elevated nest predation did not occur in tree-retention cutblocks when compared to unharvested forests, nor in high (compared with low and intermediate) tree-retention cutblocks.

Distribution of epiphytic macrolichens in relation to remnant trees in a multiple-age Douglas-fir forest

Alternatives to clear-cutting are being implemented to increase biodiversity of managed forests in the Pacific Northwest. Lichens are an integral component of old growth, but lichen biomass develops slowly in forests. We evaluated the long-term potential of live tree retention for lichen conservation in Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests. We sampled lichen litterfall in a 2-ha stand that contained 200- to 600-year-old remnant trees scattered in a forest composed mostly of 100-year-old trees that established following fire. We used association, principal components, and regression analyses to relate lichen litterfall biomass to the proximity of remnant trees. Two epiphytic lichens were strongly associated with remnant trees: the foliose cyanolichen Lobaria oregana (Tuck.) Müll. Arg. and the fruticose green algal lichen Sphaerophorus globosus (Hudson) Vainio. Biomass of both species was highest near remnant trees, and biomass was slightly higher within groves of remnant trees than it was at the edges of these groves or near isolated trees. Lichens appear to have persisted on remnant trees through the last fire and are slowly recolonizing younger trees from this source of propagules. Retention of live trees, maintenance of hardwoods, and longer rotation periods have great potential to maintain old-growth-associated lichens in at least some managed forests.