Second-growth Stands Research Articles

Biomass accumulation in trees and downed wood in northern hardwood forests: Repeated measures of a successional chronosequence in New Hampshire, USA

Successional, second-growth forests dominate much of eastern North America; thus, patterns of biomass accumulation in standing trees and downed wood are of great interest for forest management and carbon accounting. The timing and magnitude of biomass accumulation in later stages of forest development are not fully understood. We applied a “chronosequence with resampling” approach to characterize live and dead biomass accumulation in 16 northern hardwood stands in the White Mountains of New Hampshire. Live aboveground biomass increased rapidly and leveled off at about 350 Mg/ha by 145 years. Downed wood biomass fluctuated between 10 and 35 Mg/ha depending on disturbances. The species composition of downed wood varied predictably with overstory succession, and total mass of downed wood increased with stand age and the concomitant production of larger material. Fine woody debris peaked at 30–50 years during the self-thinning of early successional species, notably pin cherry. Our data support a model of northern hardwood forest development wherein live tree biomass accumulates asymptotically and begins to level off at ∼140–150 years. Still, 145-year-old second-growth stands differed from old-growth forests in their live ( p = 0.09) and downed tree diameter distributions ( p = 0.06). These patterns of forest biomass accumulation would be difficult to detect without a time series of repeated measurements of stands of different ages.

Aboveground carbon accumulation by second‐growth forests after deforestation in Hawai'i

Successional processes ultimately determine and define carbon accumulations in forested ecosystems. Although primary succession on wholly new substrate occurs across the globe, secondary succession, often following storm events or anthropogenic disturbance, is more common and is capable of globally significant accumulations of carbon (C) at a time when offsets to anthropogenic carbon dioxide (CO2 ) emissions are critically needed. In Hawai'i, prior studies have investigated ecosystem development during primary succession on lava flows, including estimates of C mass accumulation. Yet relatively little is known regarding secondary succession of Hawaii's native forests, particularly regarding C mass accumulation. Here we documented aboveground C mass accumulation by native- and nonnative-dominated second-growth forests following deforestation of mature native lowland rainforests in the Puna District of Hawai'i Island. We characterized species composition and stand structure of three distinct successional forest stand types: those dominated by the native tree, Metrosideros polymorpha ('Ōhi'a), and those dominated by invasive nonnative trees, Falcataria moluccana (albizia) and Psidium cattleianum (strawberry guava). We compared M. polymorpha-dominated and F. moluccana-dominated second-growth forests to adjacent mature M. polymorpha-dominated forests as well as young M. polymorpha-dominated forests undergoing initial stages of primary succession on 36-years-old lava fields. Aboveground carbon density (ACD) values of mature primary forest stands (171 Mg/ha) were comparable to those of mature continental tropical forests. M. polymorpha-dominated second-growth stands attained nearly 50% of ACD values of mature primary forests after less than 30 years of post-disturbance succession and exhibited aboveground carbon accumulation rates of ~3 Mg C·ha-1 ·year-1 . Such rates were comparable to those of second-growth forests in continental tropics. Rates of ACD accumulation by second-growth forests dominated by nonnative F. moluccana stands were similar, or slightly greater than, those of M. polymorpha-dominated stands. However, M. polymorpha individuals were virtually absent from stands dominated by either P. cattleianum or F. moluccana. Taken together, results demonstrated that re-establishment and rapid accumulation of C mass by M. polymorpha stands during secondary succession is certainly possible, but only where populations of nonnative species have not already colonized areas during early stages of secondary succession.

Effects of variable-density thinning on non-native understory plants in coniferous forests of the Pacific Northwest

Old-growth forests serve as critical habitat for many sensitive species, but management practices have diminished their prevalence, and former regions of old-growth are now dominated by second-growth stands lacking the structural heterogeneity, diversity, and species richness that these older forests possess.In western Washington state in the Pacific Northwest of the United States, the Olympic Habitat Development Study was designed to address this issue and hasten the development of specific old-growth features in second-growth stands using variable-density thinning. One concern with such methods, however, is the potential to introduce non-native plants, which can have negative ecological and economic impacts. Here, we examine how variable-density thinning influences desirable forest characteristics, such as increased plant species diversity, versus the less desirable effects of non-native plant species introduction. We test two hypotheses regarding plant invasions. First, thinning would promote establishment of non-native, shade-intolerant species, but their abundance would gradually decline over time. Second, thinning disturbance and increased heterogeneity of canopy cover would initially promote understory richness of all species, although richness would decline over time with canopy closure and increased cover of shrubs and regenerating trees. We found that the number and cover of non-native species initially increased after thinning, peaking at 16 species present in variable-density thinned treatments in year three. By year 17, 11 species remained throughout the seven 6.5 ha treatment plots sampled, and cover was negligible. As predicted, species richness increased following thinning, however, native species richness remained elevated through year 17, contrary to our hypothesis. Furthermore, native species diversity also increased following thinning and remained higher in thinned treatments than controls through year 17. Our results show that variable-density thinning in temperate coniferous forests can enhance native, but not exotic, plant richness and diversity in the long term.

Second-growth redwood forest responses to restoration treatments

Following 20th century logging, much of the natural coast redwood (Sequoia sempervirens [D.Don.] Endl.) range consists of dense second-growth stands with slow tree growth and low biodiversity. There is a landscape-scale effort in much of coastal northern California to increase tree growth rates and ecosystem biodiversity via thinning treatments, thereby hopefully accelerating the development of old-growth forest characteristics. Redwood National Park has been experimenting with thinning in these forest types since the 1970s. This study investigated short- (1 year post-thinning) and long-term (40 years post-thinning) responses of redwood physiology (water potential and stomatal conductance), redwood growth (via tree-rings), and forest biodiversity (understory plants, birds, and mammals) to restoration thinning treatments. We found that thinning second-growth redwood forests with 40% basal area reduction treatments 1) did not meaningfully influence tree water potential, 2) increased tree stomatal conductance in the short-term, 3) increased tree growth in the long-term, 4) increased understory plant diversity, and 5) did not affect bird or mammal diversity. Collectively, these findings demonstrate that thinning second-growth redwood forests has the potential to accelerate the development of old-growth characteristics. This verification of the efficacy of restoration treatments is important information for land managers, as plans are currently underway to apply these treatments at the landscape-scale. This study can provide useful baseline data to aid future assessments of long-term forest responses to contemporary restoration efforts.

Habitat disturbance can alter forest understory bird activity patterns: A regional-scale assessment with camera-traps

Despite forests are a biodiversity reservoir and a wellbeing source for the humankind, they have been largely disturbed in the past decades. Disturbed and naturally-regenerated forests and forest plantations represent 75% of the world forested area, stressing the need of managing non-pristine forests. While we know that habitat disturbance alters forest structure, reducing species richness and abundances, its effects on animal behavior remain little explored. We used camera traps to test if daily activity patterns of forest birds change along a 450-km disturbance gradient in the Valdivian temperate rainforests of southern Chile. We contrasted bird activity at four conditions: old-growth and second-growth native forests, native forests subject to selective logging, and abandoned exotic plantations with native vegetation. We contrasted habitat structure among conditions and estimated activity kernels to compare bird activity. We also made activity comparisons examining bird foraging guilds (nectarivores, frugivores, and insectivores) and habitat preference (interior vs edge). Habitat structure was different among conditions, being the old-growth native stands the most contrasting habitat. Bird daily activity patterns differed among conditions as well. We observed that bird activity reaches a peak early in the morning at the old-growth native forests, at noon in the plantations, and in the afternoon at the logged forests, while it was more variable in secondary forests. We obtained similar results among bird foraging guilds, but insectivorous birds presented the most contrasting differences. Likewise, we found a similar pattern between forest interior and edge species, which was more evident for forest interior birds. Logged forests presented the largest activity differences, despite being structurally similar to second-growth stands. Forest disturbance not only alters bird species richness and abundance, but also their behavior. Bird daily activity patterns may be directly affected by daily variations in forest microclimate conditions and indirectly affected by chantes in resource availability, which may be affecting their ecological interactions. Therefore, forest management and restoration strategies should take animal behavior into account.

Variable-density thinning promotes differential recruitment and development of shade tolerant conifer species after 17 years

Promoting patchy recruitment of shade tolerant tree species into the midstory is an important step in developing structural diversity in second-growth stands. Variable-density thinning (VDT) has been proposed as a strategy for accelerating structural diversity, as its combination of within-stand treatments (harvest gaps, thinning, and non-harvested skips) should create variable overstory and understory conditions. Here we report on western hemlock (Tsuga heterophylla (Raf.) Sarg.) seedling and sapling densities in five mixed-conifer stands and Sitka spruce (Picea sitchensis (Bong.) Carr.) seedling and sapling densities in two stands in western Washington at 3,7, 10, and 16–17 years after VDT. Additionally, we report on western hemlock advance regeneration growth and survival in two stands over 14 years. Western hemlock seedling density was highest in the thinned treatment but only significantly so in Year 10. In contrast, the gaps contained significantly more western hemlock saplings in Years 7 and 10 and significantly greater growth of western hemlock advance regeneration through Year 10. Skips embedded within the VDT did not differ significantly from unharvested reserves in terms of seedling or sapling densities of either species. Sitka spruce seedling density was highest in the gap and thinned treatments, but saplings were uncommon in all treatments. Collectively, these results indicate that our variant of VDT promoted patchy, midstory recruitment of western hemlock but failed to recruit Sitka spruce saplings in either stand where it established. Consequently, more intensive variants of VDT may be required to promote midstory recruitment of species less tolerant of shade than western hemlock.

Ecological issues related to second-growth boreal forest management in eastern Quebec, Canada: Expert perspectives from a Delphi process

The Eastern boreal forests of Quebec, Canada, have been extensively harvested over the past decades. Second-growth stands originating from sites harvested between 1920 and 1950 will soon reach the stage allowing for a second harvest. In order to guide the decision-makers responsible for ecosystem-based management of these forests, their specific management issues must be identified, based on the best knowledge available. In this context, we used the Delphi method and asked experts to identify and prioritize the main ecological issues related to the management of second-growth forests. Fourteen experts participated in at least one round of the process, out of an initial population of 30known experts. After three rounds of questions, experts identified the maintenance of old-growth forests as the most important issue related to second-growth forest management in this region. The protection of woodland caribou and its habitat, and land fragmentation by forest roads were the second and third most important issues identified by the Delphi survey participants. These issues are not unique to second-growth forests, but should be given priority in considering management strategies associated with second-growth stands.

Development of mature second-growth Sequoia sempervirens forests

Mature second-growth coast redwood (Sequoia sempervirens) forests—logged over 100 yr ago—are an important resource in the redwood region, but development of regenerating forests beyond rotation age (~50 yr) is not well understood. Continuous long-term data are especially lacking, considering that the maximum possible age of second-growth stands is over 160 yr. Here we examine accumulation of tree biomass, leaf area, and canopy structure in three mature second-growth forests in California (Arcata, Big River, Oakland) that range in age from 133 to 159 yr since clearcut logging. Four fixed-area plots form the basis of this examination, including three 1-acre plots established in 1923 that together permit examination of Sequoia forest development over nearly a century. The four plots held 963 to 1476 Mg ha−1 of aboveground tree biomass and 9.0 to 13.7 of tree leaf area index. From 1923 to 2017, the Big River plot exhibited rapid biomass accumulation (up to 23 Mg ha−1 yr−1) as a densely stocked nearly pure Sequoia stand on an alluvial terrace. The two low-elevation plots near Arcata exhibited slower (up to 13 Mg ha−1 yr−1) but consistent growth with an increasing dominance of Sequoia, as co-occurring conifers (Abies grandis, Picea sitchensis, Pseudotsuga menziesii) steadily declined in number despite rapid early growth. Increasing Sequoia dominance, substantial density-independent mortality in recent decades, and shifts in tree size distributions illustrate structural maturation as these forests approach an old-growth condition. The Oakland plot, which occurred ~ 300 m higher in elevation and received less than two-thirds as much annual rainfall, was a nearly pure Sequoia stand with canopy structure similar to, but shorter than, the Big River plot. Despite differences in elevation and rainfall, aboveground biomass of individual large trees in all four plots exceeded 15 Mg with Sequoia biomass increments averaging > 200 kg yr−1 during the 21st century. Characteristics and developmental trajectories of these plots provide realistic benchmarks for management of alluvial, upland, and inland Sequoia forests.

Dynamics of the diameter distribution after selection cutting in uneven- and even-aged northern hardwood stands: a long-term evaluation

Selection cutting is defined as a tool for uneven-aged silviculture. Dependence on diameter distribution by forestry practitioners for identifying stand conditions has led to misuse of selection-like cuttings in even-aged northern hardwood stands. Our study used several long-term data sets to investigate the temporal stability in numbers of trees per diameter class in uneven-aged northern hardwood stands treated with single-tree selection and in 45-year-old second-growth stands treated with selection-like cuttings. We analyzed data from New York, Michigan, and Wisconsin to determine changes through time in number of trees across 2.5 cm diameter classes, shifts in the shape and scale of the three-parameter Weibull function used to describe the diameter distributions, and dynamics of associated stand attributes. Findings showed that single-tree selection cutting created and sustained stable diameter distributions and uniformity of conditions through consecutive entries in uneven-aged stands. By contrast, these characteristics varied through time in the second-growth stands that had been treated with selection-like cuttings. Analysis also showed that the Weibull shape and scale parameters for stands under selection system migrated towards those of the recommended target diameter distribution in the uneven-aged stands. These parameters diverged from the target with repeated use of selection-like cuttings in the second-growth even-aged stands.

Legacy forest structures in irregular shelterwoods differentially affect regeneration in a temperate hardwood forest

Foresters are faced with managing forests for a broad variety of societal demands such as wildlife habitat, aesthetics and site stability while still providing monetary value to landowners through timber harvest. Regeneration methods need to be developed that offer alternatives to traditional management for timber to help meet these goals. Regeneration responses in third-growth forest stands, or those following harvest of second-growth stands are not well-studied. Such forests are now widespread across temperate regions and may exhibit growth patterns different than those reported previously for regenerating second growth that originated on land abandoned from agriculture. There is a need for research that can lead to new methods or modification of existing practice. Our study examines legacy trees and the associated regeneration across a 25-year chronosequence of 34 irregular shelterwoods regenerated from second-growth forest in southern New England. We compared regeneration of the three most common regenerating tree species: red maple (Acer rubrum), black birch (Betula lenta), and red oak (Quercus rubra). We used regression analyses to examine changes through time in height growth of the tallest regeneration for each species as well as changes in total abundance. Similarly, regressions for annual height growth of regeneration in relation to legacy overstory basal area were compared for each species. Over time, self-thinning occurred in the regeneration of all focus species, but at different rates. Black birch thinned most dramatically through time, and the saplings that survived retained a high position in the canopy. Red oak self-thinned most slowly, and its tallest stems retained competitive height, although did not surpass black birch. As legacy overstory basal area increased, annual growth of red oak regeneration slowed. Stand dynamics patterns are therefore significantly different from those of second growth forests where oak is a more vigorous competitor in both number and growth as compared black birch and red maple. Our results suggest that resource managers need to both recognize these differences in stand development and consider the tradeoff between increasing legacy trees and decreases in growth rate of oak regeneration, as well as long-term effects of increased structure post-timber harvest.

Non-native plant species show a legacy of agricultural history in second-growth forests of southeastern Ohio

In second-growth forest the abundance and diversity of non-native plant species often differ considerably with stand age and the history of disturbance. To understand the role of agricultural history in shaping non-native forest floras, we compared successional trajectories between second-growth stands that were formerly cultivated and pastured, two historically common land uses differing in soil disturbance and vegetation structure. Successional change was inferred from a replicated chronosequence consisting of 40 second-growth stands in southeastern Ohio, USA. The non-native flora was described in terms of community composition and individual species abundance and compared across biotic and abiotic gradients. Abundance of non-native species generally declined through the chronosequence in both land-use categories but was significantly greater in formerly cultivated sites. Non-native richness in the oldest post-agricultural sites was significantly greater than in undisturbed forest, implying that the nonnative flora has not stabilized after 80 years. Species dispersing seeds by ingestion were more abundant in formerly pastured sites; clonal species were more common in cultivated stands. Thus, the non-native forest flora shows a long-lived legacy of variation between stands of different agricultural histories. The distribution of non-native species suggests colonization of established forest as well as relictual survival from the open, agricultural stage. Although non-native abundance declines with successional time, a subset of shade tolerant species should be considered a long-term component of the second-growth flora.

Seed longevity and dormancy state in an invasive tree species: Ailanthus altissima (Simaroubaceae)1

Successional tree species often form soil seedbanks, suggesting that extended seed dormancy may also facilitate regeneration in nonnative species. We examined seed dormancy in Ailanthus altissima, which is currently spreading into long-established forests of Europe and eastern North America. Ailanthus seeds were sewn into replicated nylon mesh bags and buried in 10 second-growth stands in southeast Ohio, USA. North- and south-facing slopes were included as a coarse-grained test for environmental control. Bags were recovered at 2-mo intervals and seeds were tested for viability and germination. Fifty percent of Ailanthus seeds survived the full length of the study (24 mo) after an initial phase of partial germination (51–79% of viable seeds). Remaining seeds returned to a state of induced dormancy, allowing a second flush of germination (3–28%) after 18 mo. After the first 5 mo no further decline in seed number was observed, suggesting persistence in the soil well beyond the 2-yr length of this study. No slope-aspect effect was observed. Long-term seed dormancy potentially facilitates Ailanthus invasion by buffering against extended canopy closure or allowing passive dispersal with movement of soil. Complete eradication of a population will depend on elimination of the soil seed bank, which may require many years. Management should avoid creating canopy openings near existing populations with the goal of suppressing sexual reproduction and seedling establishment.

Forest micro-environment develops through time: Changes in physical and structural heterogeneity follow abandonment from two forms of agriculture

Agriculture alters and homogenizes the physical environment, and persistent changes potentially affect community development in post-agricultural forests. We ask whether physical and structural development differs between second-growth stands of contrasting ages and agricultural histories. Formerly cultivated and pastured sites were expected to follow separate trajectories of succcessional development but eventually to converge as natural processes obscured the environmental legacies of agriculture. We used a replicated chronosequence to examine microenvironment development in successional stands over 80years following canopy closure and compared successional stands with a control group of long-established stands. Physical, edaphic, and structural variables were measured in each site. Microtopography increased through successional time, while canopy openness and soil bulk density declined. Formerly cultivated sites showed less microtopographic variation and higher bulk density than pastured sites, consistent with a history of plowing. Formerly cultivated sites showed spatial homogenization in microtopography, soil organic matter, and herb cover. Stands in the 61–80year age class did not differ significantly from long-established forest in any of the environmental variables or the spatial distribution of such variation. Our results suggest that 80years of forest development is sufficient for most environmental legacies of agriculture to fade, reducing their potential to influence plant community composition. However, contrasting land-use history is still evident for several decades and may contribute to community structure over that period.

Tree ring δ15N as validation of space-for-time substitution in disturbance studies of forest nitrogen status

Forest ecosystem nitrogen (N) response to disturbance has often been examined by space-for-time substitution, but there are few objective tests of the possible variation in disturbance type and intensity across chronosequence sites. We hypothesized that tree ring δ15N, as a record of ecosystem N status, could validate chronosequence assumptions and provide isotopic evidence to corroborate N trends. To test this we measured soil N availability, soil δ15N, and foliar N attributes of overstory Douglas-fir (Pseudotsuga menziesii) and understory western hemlock (Tsuga heterophylla) across three old-growth stands and nine second-growth plantations on southeast Vancouver Island, British Columbia (Canada). Increment cores for wood δ15N were retrieved from three co-dominant Douglas-fir per plot. Bulk soil δ15N was well aligned with both foliar and recent wood δ15N, demonstrating the utility of wood δ15N in monitoring ecosystem N status. Strongly contrasting trends in tree ring δ15N were evident among second-growth stands, with most trees from plantations older than 50 years exhibiting steep declines (3–4‰) in δ15N but with no temporal trends detected for younger plantations. The discrepancy in tree ring δ15N suggests disturbance history varied considerably among second-growth sites, likely because of greater slash loads and hotter broadcast burns on older cutblocks. As a consequence, the pattern of increased soil N availability and foliar N concentration with time since disturbance derived from the chronosequence could not be validated. Tree ring δ15N may provide insights into disturbance intensity, especially fire, and correlations with foliar N concentration could inform the extent of changes in stand nutrition.

Tree squirrel abundance and demography in managed coniferous forests of British Columbia are within the range of natural fluctuations of old-growth stands

The American red squirrel (Tamiasciurus hudsonicus Exrleben) and northern flying squirrel (Glaucomys sabrinus Shaw) are important mammal species in coniferous forests that are widely distributed across temperate and boreal ecological zones. Although T. hudsonicus and G. sabrinus apparently prefer late-successional forests, their population dynamics show no clear pattern in young second-growth and old-growth conifer forests. We used a compilation of study results that had standardized methodology and sampling effort to compare tree squirrel responses across a range of forest conditions. We tested the hypotheses (H) that abundance, reproduction, recruitment, and survival would be higher in (H1) old-growth than second-growth stands, (H2) unthinned than thinned second-growth stands, and (H3) lightly than heavily thinned stands. Tree squirrel populations in old-growth stands were considered the “standard” treatment to which other treatments in second-growth stands (unthinned and variously thinned) were compared. Thinned stands were grouped into low, medium, and high densities of trees. Datasets from seven published studies included 2804 Tamiasciurus and 837 G. sabrinus individuals, 25 study years, and 158 trapping periods. Mean abundance of Tamiasciurus was similar among treatment stands, ranging from 0.62 to 1.29·ha−1. Mean numbers of G. sabrinus were highest in old-growth stands at 0.65·ha−1, followed by the low- and medium-density stands at 0.50·ha−1, the high-density stands at 0.25·ha−1, and finally the unthinned second-growth stands at a low of 0.12·ha−1. In terms of effect size, mean proportional change in abundance, relative to that within old-growth stands, of both squirrel species in the three thinned stands were within the range of natural fluctuations in old-growth stands. Populations of Tamiasciurus in the unthinned stands were not within the range of fluctuations in the old-growth stands. Mean number of successful pregnancies and total recruits were similar among stands for Tamiasciurus, with variability in female breeding and winter survival. Mean number of recruits of G. sabrinus was highest in old-growth stands with no other differences in demographic attributes among stands. The biological significance of these differences in demographic variables was small in terms of effect sizes. Tamiasciurus and G. sabrinus seem to persist in a relatively broad range of young managed forest habitats, as well as in old-growth stands.

Agricultural history drives structure and tree species composition of second growth forest over 100 years in southeastern Ohio, USA

AbstractQuestionMost modern forest in eastern North America has recolonized after abandonment from agriculture. Cultivation and pasturing, historically the dominant forms of agriculture, differ in their environmental legacies and potentially influence stand development following abandonment. We ask how the legacy of agriculture plays out in development of second‐growth stands, and whether tree community composition and structure differ between sites with contrasting land‐use histories.LocationSoutheast Ohio, USA.MethodsThirty‐five second‐growth stands were sorted into a replicated chronosequence of formerly cultivated and pastured sites spanning 80 yr. Stand age and land‐use history were determined from historical aerial photographs and from site characteristics including microtopography and soil profiles. In addition, a control group was selected consisting of long‐established forest stands showing no signs of agricultural disturbance. At each site stand composition and structure were described using the point‐centred quarter method.ResultsStand density declined and basal area increased through the chronosequence but neither pastured nor cultivated sites reached levels observed in the control group. Density decreased most dramatically between the 41–60‐ and 61–80‐yr age classes, consistent with competitive stand thinning. Tree community composition changed through time, reflecting a shift away from light‐demanding successional species and toward long‐lived, shade‐tolerant species characteristic of long‐established forests. Composition and structure did not differ significantly between cultivated and pastured sites, but individual species did show significant differences, with the greatest contrast evident shortly after abandonment.ConclusionsStand development following agriculture appears to be a process of convergence in species composition and density. Because most deciduous forest in eastern North America is <80‐yr‐old, these results suggest that most forest is still accruing biomass and has yet to reach a stable density and composition.

Old-growth characteristics 20 years after thinning and repeated fertilization of lodgepole pine forest: Tree growth, structural attributes, and red-backed voles

Restoration of old forests by developing late seral structural attributes in young forests may utilize pre-commercial thinning (PCT) and fertilization to accelerate stand growth and development. These silvicultural interventions are very likely required to develop complex stand structure in young forests, both vertically and horizontally, and late-seral structural features over decades, rather than centuries. We evaluated hypotheses (H) that large-scale PCT of young lodgepole pine (Pinus contorta) stands to a wide range of densities, and repeated fertilization, at 20-years after initiation of treatments would enhance (H1) size (diameter and height growth), structural features, and merchantable volume of crop trees; (H2) development of old-growth structural attributes; and (H3) abundance, reproduction, and survival of the southern red-backed vole (Myodes gapperi). Study areas were located near Summerland and Kelowna in south-central British Columbia, Canada. Each study area had ten treatments: four pairs of stands thinned to densities of ∼250 (very low), ∼500 (low), ∼1000 (medium), and ∼2000 (high) stems/ha with one stand of each pair fertilized five times at 2-year intervals, an unthinned stand, and an old-growth stand.Mean 20-year diameter of lodgepole pine crop trees was similar (19–20cm) in the heavily thinned (≤1000 stems/ha) fertilized stands with that (19–22cm) in the old-growth stands. Mean height of all managed stands (9–12m) was lower than that (18–20m) of the old-growth stands. Mean crown volume of crop trees was highest in the very low-, low, and fertilized medium-density stands (3.5–5.9 times higher, on average) than those in the unthinned and old-growth stands. Large tree diameters and crowns occurred in the heavily thinned and fertilized stands, but mean stand merchantable volume was much reduced, primarily because of the lower densities of trees, and hence provided partial support for H1. Mean species diversity of conifers was highest in the very low-density stands and old-growth stands. Mean structural diversity of conifers was generally similar among stands. Old-growth structural attributes: large dominant trees with substantial crowns, multi-layered canopies of conifers, some canopy openings, and understory abundance and diversity of herbs and shrubs appeared in the heavily thinned and fertilized stands, and hence partially supported H2. M. gapperi populations in 5 of 8 managed stands at Summerland were at the same or higher abundance (particularly the very low-density) than in the old-growth stand, with resident breeding animals and recruitment of both adult and juvenile voles, and hence H3 seemed to be supported. Heavy thinning to ≤1000 stems/ha combined with fertilization seemed necessary to achieve these structural developments in our 33-year-old second-growth stands.

Acceleration of old-growth structural attributes in lodgepole pine forest: Tree growth and stand structure 25 years after thinning

Acceleration of forest succession to develop late-seral structural characteristics in younger stands may be achieved by silvicultural practices such as pre-commercial thinning (PCT). Young second-growth stands of lodgepole pine (Pinus contorta) range across several million ha of the inland Pacific Northwest of North America and respond positively to various stand treatments. Objectives of this project were that large-scale stand thinning to a wide range of densities, at a 25-year period after PCT, would enhance: (1) productivity and structural features (diameter and height growth, and crown volume and dimensions of crop trees; (2) merchantable volume of crop trees; and (3) development of old-growth structural attributes. Three stands thinned to densities of ∼500 (low), ∼1000 (medium), and ∼2000 (high) stems/ha at age 17years, with an unthinned young pine and old-growth pine stand for comparison, were located near Penticton in south-central British Columbia.Lodgepole pine grew faster in mean diameter (cm) in the low- (13.89) than either of the medium- (10.2) or high-density (9.2) stands, but mean height growth increment (8.8–9.7m) was similar over the 25-year period since PCT. Mean diameter was 2.2 times greater in the low-density stand than unthinned stand. The medium- and high-density stands also tended to have large diameters (1.7–1.9 times that of the unthinned stand) and crowns, but still with mean heights 5–6m less than trees in the pine component of the old-growth stand. Mean merchantable stand volume (m3/ha) was 1.7–1.9 times higher in the medium- (231) and high-density (257) stands than low-density (137) stand, and comparable to the old-growth pine (225). Mean crown volume (m3) of crop trees was substantially greater in the low- (88) than in the medium- (27) and high-density (31), unthinned (4), or old-growth stands (4 pine only). Mean structural diversity of five layers of coniferous trees was highest in the low- and medium-density stands, with declining diversity from the high-density to unthinned to old-growth stands. In terms of old-growth structural attributes, large dominant trees with substantial crowns, multi-layered canopies of conifers, some canopy gaps, and understory patchiness of herbs and shrubs appeared in the heavily thinned (⩽1000stems/ha) stands at 25years post-thinning. These stands were 42years old at this re-measurement, and hence were not considered “old-growth” but the heavily thinned stands seemed to have some old-growth structural attributes. Silvicultural trade-off of stand volume gain vs. old-growth attributes may be necessary for low-density stands.

Repeated spring precipitation shortage alters individual growth patterns in Scots pine forests in the Western Alps

In the inner-Alpine Aosta Valley, severe growth reductions occurred in Scots pine stands during 1987–1993 and 2003–2005. Trees more sensitive to May precipitation exhibited stronger growth decline during these periods. Over the last decades, Scots pine (Pinus sylvestris L.) decline has involved large areas in the European Alps. Although the species is supposed to be drought resistant, increased temperatures and droughts are often indicated as predisposing causes of the decline. Nevertheless, the exact climate conditions that initiate the decline, and the reasons why they differentially affect individual trees, are largely unknown. Our aims were to identify climate constraints on Scots pine growth, and elucidate the effect of individual characteristics, such as tree age, size, crown condition, mistletoe occurrence, competition, and sensitivity to climate, on tree growth decline and recovery after repeated climatic stress. We analysed 232 trees in four second-growth stands located at 985–1350 m a.s.l. in the inner-Alpine Aosta Valley, NW Italy. Multi-year growth declines, occurred in 1987–1993 and 2003–2005, were related to repeated May precipitation shortage, while temperatures and summer precipitations played a minor role. Growth decline was stronger in the stand at lower elevation and with higher competition intensity. At the individual scale, trees more sensitive to May precipitation exhibited lower growth rates during both dry periods, but not a faster recovery. In the lower elevation stand, tree growth decline was significantly related to both crown transparency and mistletoe abundance, which was almost absent in the other stands. We conclude that future variations in the spring precipitation regime could threaten Scots pine more than warming per se, in inner-Alpine valleys around 1000–1400 m a.s.l. Still, different individual sensitivity to precipitation will likely result in patchy patterns of healthy and declining trees within the same stand.

Thinning treatments had minimal effect on soil compaction in mixed-conifer plantations

If biomass utilization results in soil compaction and reduced forest productivity, the potential benefits may be considered to be not worth the long-term impacts. We analyzed soil strength, an indicator of soil compaction, prior to and following commercial thins (sawlog and biomass harvest) and mastication treatments in 24- to 30-year-old mixed-conifer plantations in the central Sierra Nevada. Soil strength in mature, untreated second-growth stands was also measured as a reference. Neither the commercial thins nor the mastication treatments resulted in statistically detectable increases in compaction. Most of the existing compaction came from the original regeneration harvest that established the plantations several decades earlier. It will be important to monitor repeat treatments and long-term effects, but this study suggests that managers should not expect large impacts from thinning treatments on soil compaction in forests such as the one studied here as long as best practices are used.