Exposed Mineral Soil Research Articles

Natural regeneration and early development of Scots pine seedlings after gap cutting in northern Finland

ABSTRACT In the northern boreal region, tree growth, timber yields and economic returns are low, meaning low regeneration costs are the basis of profitable forestry. If natural regeneration is successful, it may be favoured over expensive planting or seeding. We studied the regeneration success and early growth of Scots pine in gaps in terms of seedling density and height 10 years after gap cuttings in central Finnish Lapland. Three gap sizes (diameters of 20, 40, and 80 m) were studied on patch scarified xeric and sub-xeric sites in six random blocks and a total of 18 replicates of each. The number of pine seedlings was high across the gap sizes. The proportion of empty regeneration plots (size 5 m2) was ∼ 2%. Site preparation substantially increased the number of seedlings. The growth of seedlings was faster in larger gaps, but a high proportion of exposed mineral soil decreased it. The results suggest that all studied gap sizes regenerated naturally well, and that soil scarification exposing 10–20% of the surface area or even less can be enough to achieve regeneration goals. Gaps of a diameter of 40 m or more are required to achieve an optimal balance between seedling density and growth.

Direct and indirect effects of fire on germination of shortleaf pine seeds

BackgroundShortleaf pine is a fire-adapted tree species, and prescribed fire is commonly used to increase its regeneration success, improve wildlife habitat, and reach conservation objectives associated with open forest ecosystems. We studied direct effects of heat and smoke on shortleaf pine germination in a greenhouse study and effects of season of burning on the number of new germinants in a field study. Improved understanding of fire effects on shortleaf pine seed and regeneration success can help refine burn prescriptions to better meet specific management objectives.ResultsTemperatures ≥ 120 °C eliminated germination of shortleaf pine seeds in a greenhouse trial, and exposure of seeds to 60 °C resulted in no reduction in germination compared to the unheated control regardless of duration of exposure. At 80 °C, duration of heat exposure mattered, with exposure for 10 min reducing germination compared to unheated controls. Smoke exposure had no effect on germination. A field experiment showed that fall burns (prior to seedfall) resulted in greater initial germinant counts than early spring burns (after seedfall but before germination) or unburned controls, which both resulted in greater initial germinant counts than late spring burns (after germination).ConclusionsSeason of prescribed burning can affect the success of shortleaf pine germination. Late spring burning resulted in high mortality of young germinants. Burning in early spring likely resulted in direct damage to some seeds due to heating but may have also had indirect benefit by exposing mineral soil. Fall burning, before the dispersal of shortleaf pine seed, yielded the highest germinant count and is recommended if improving natural regeneration from seed is the primary objective.

Forest resilience and regeneration dynamics following wildfire disturbance

AbstractSpecies that are adapted to specific disturbance regimes may be sensitive to changes such as severity, frequency, and extent of ecosystem perturbation. Along range margins, these sensitivities may be exacerbated by concurrent changes in climate resulting in loss of resilience and shifts to alternative ecosystem states. Pinus banksiana (jack pine) is a serotinous species dependent on fire disturbance to open cones, release seed, reduce understory competition, and expose mineral soil. The Altona Flat Rock in northern New York is a globally rare sandstone pine barrens, at the southern edge of jack pine's range. In the summer of 2018, a wildfire burned approximately 200 ha of the Flat Rock barrens, providing the opportunity to study post‐disturbance jack pine and understory resilience. We aimed to establish if jack pine regeneration would occur, and if so, would it be influenced by fire severity, pre‐fire stand characteristics, and competition. We predicted that higher seedling densities would be associated with higher burn severity, higher pre‐fire jack pine stand density, and lesser understory competition. We collected seedling density and understory composition data annually from 2019 to 2022 across 45 plots spanning the disturbed and adjacent undisturbed barrens. In 2021 and 2022, seedling heights were also collected. Seedling densities following the disturbance ranged from 275,385 to 390,513 ha−1; = 357,821 ha−1. Jack pine seedling density was positively associated with fire severity and moss abundance for all years. Pre‐fire stand density along with ericaceous shrub, grass, and duff abundance were also positively associated with seedling density in some years. Seedling height was positively associated with fire severity along with grass and blueberry abundance. These factors, for both establishment/survival and growth, point to the importance of seed supply, microsite conditions, and facilitation in jack pine regeneration success. At this point, it does not appear that the interaction of disturbance with changing climate has shifted enough to prevent the recovery of this ecosystem to its pre‐disturbance jack pine‐dominated state. However, ongoing climate change may still have an impact on seedlings/saplings as this forest continues to mature.

Differences in regeneration niche mediate how disturbance severity and microclimate affect forest species composition

Climate change is altering forest composition through species-specific responses to fire and drought. Future forest composition will depend on how the different regeneration niches of co-occurring species align with current environmental conditions, especially after fire, which can promote germination by exposing mineral soil. Few studies, however, have examined the effects of disturbance severity and microclimate on post-fire regeneration to define and compare the regeneration niches of co-occurring tree species. We used seven years of annual demography and microenvironment data from a 25.6-ha fully censused, stem-mapped forest dynamics plot in California, USA, to examine how disturbance severity, snow duration, and temperature extremes affect the survival of Abies concolor and Pinus lambertiana seedlings that germinated naturally after a low- to moderate-severity fire. We defined disturbance severity at the microsite level, based on characteristics of the substrate, and at the neighborhood level, based on tree mortality. Both disturbance severity and snow duration had species-specific effects on seedling survival, but these differed by life stage. During the germination year, later snow disappearance was associated with a 0.5 increase in survival probability for A. concolor but hardly affected P. lambertiana; in contrast, higher neighborhood disturbance severity increased survival of both species. After the germination year, higher substrate burn severity was associated with a 0.8 increase in survival probability for A. concolor but hardly affected P. lambertiana; higher neighborhood disturbance severity and later snow disappearance increased annual survival of both species, but maximum summer temperature had minimal effect. Overall, available seed, higher substrate burn severity, higher neighborhood disturbance severity, and later snow disappearance promoted natural regeneration. However, lower substrate burn severity and earlier snow disappearance in the germination year disadvantaged A. concolor seedlings, increasing the relative abundance of P. lambertiana seedlings compared to the local tree population. Our results indicate that natural post-fire compositional shifts toward drought-tolerant Pinus species–and away from less drought-tolerant Abies species–are possible in the Sierra Nevada, with potential benefits for forest persistence under climate change. Broadly, we show that species differences in regeneration niches shape how disturbance severity and microclimate affect forest species composition.

Manipulating overstory density and mineral soil exposure for optimal natural regeneration of Scots pine

In northern boreal region the growth of forests is slow, and yield and profit are low, which is why low reforestation costs are important for profitable forestry. If natural regeneration is successful, expensive artificial forest regeneration (planting or direct seeding) can be avoided. In this study, we look at the impact of overstory density and site preparation on natural regeneration and seedling growth of Scots pine. Study stands were established in different parts of Northern Finland and in each stand following treatments: 50, 150 and 250 trees ha−1 or unthinned control, where the stand density was ≥ 250 trees ha−1, were randomly allocated to experimental plots. In addition, site preparation (disc trenching, 4000–5000 m ha−1) was carried out on two experimental plots in which tree density was either 50 or 150 trees ha−1. In the experimental stands seedling number, age and growth were monitored for 11 years. Monitoring revealed that the number of seedlings increased with decreasing tree density. Average seedling height growth was very low or even non-existent in the unthinned control and in the densest (250 trees ha−1) treatment, but increased when the density of trees decreased. The highest seedling number and the highest growth were achieved when the tree density was 50 trees ha−1 and the soil was prepared to expose mineral soil. Achieving e.g. 2000 seedlings ha−1, would need about 40% exposition of mineral soil. The required low tree density implies that not only seed supply from seed trees and site preparation is important for regeneration success in northern boreal Scots pine forests but also the reduction of competition by mature trees.

Natural regeneration potential and dynamics in boreal lichen woodlands of eastern Canada following soil scarification

Boreal lichen woodlands (LWs) are stable low tree-density zones of the Canadian boreal forest whose afforestation has been proposed as a way to create new C sinks and thus mitigate climate change. Planting operations in these remote areas are however costly and time-consuming, and may not be necessary when soil scarification is followed by dense natural regeneration. In the present study, we assessed the natural regeneration potential and dynamics in six boreal LWs of Québec, Canada, 11 years after soil scarification. The number, size (height and stem diameter) and age of seedlings were measured in 2-4 sampling plots per site (18 plots in total). Our data show that scarification operations produced on average 1,400 m2 ha–1 of exposed mineral soil (scarification intensity of 14%) with, however, a large within-site variability. The natural regeneration was mainly composed of black spruce seedlings (> 95%), averaged ∼12,000 seedlings ha–1 across the six sites and significantly varied among sites, mostly due to the variation in scarification intensity. Seedling density averaged ∼9 seedlings m–2 of exposed mineral soil and increased with seed tree mean diameter at breast height (DBH) (R2 = 0.51; P < 0.05) but not with the density of seed trees, revealing the importance of old and large seed trees in natural regeneration success. Together, scarification intensity and the DBH of remaining seed trees explained ∼60% of the variation in natural regeneration density across the 18 sampled plots. The rate of establishment of seedlings was generally high – with on average 60% of the carrying capacity of the substrate being reached within three years following scarification – and increased with seed tree mean DBH (R2 = 0.77; P < 0.05). However, the growth rate of seedlings was very low. Eleven years after scarification, 60% of the seedlings were < 15 cm and the height of 10-yr-old seedlings averaged 27.5 cm. Thus, even though seedling establishment was successful, the biomass accumulated by the natural regeneration was negligible in the span of a decade. Therefore, the implementation of afforestation following scarification appears to be necessary to create significant C sinks in the midterm.

Seedbed not rescue effect buffer the role of extreme precipitation on temperate forest regeneration.

Alterations in global climate via extreme precipitation will have broadscale implications on ecosystem functioning. The increased frequency of drought, coupled with heavy, episodic rainfall are likely to generate impacts on biotic and abiotic processes across aquatic and terrestrial ecosystems. Despite the demonstrated shifts in global precipitation, less is known how extreme precipitation interacts with biophysical factors to control future demographic processes, especially those sensitive to climate extremes such as organismal recruitment and survival. We utilized a field-based precipitation manipulation experiment in 0.1 ha forest canopy openings to test future climate scenarios characterized by extreme precipitation on temperate tree seedling survival. The effects of planting seedbeds (undisturbed leaf litter/organic material vs. scarified, exposed mineral soils), seedling ontogeny, species, and functional traits were examined against four statistically defined precipitation scenarios. Results indicated that seedlings grown within precipitation treatments characterized by heavy, episodic rainfall preceded by prolonged drying responded similarly to drought treatments lacking episodic inputs. Moreover, among all treatment conditions tested, scarified seedbeds most strongly affected seedling survivorship (odds ratio 6.9). Compared with any precipitation treatment, the effect size (predicted probabilities) of the seedbed was more than twice as important in controlling seedling survivorship. However, the interaction between precipitation and seedbed resulted in a 27.9% improvement in survivorship for moisture-sensitive species. Seedling sensitivity to moisture was variable among species, and most closely linked with functional traits such as seed mass. For instance, under dry moisture regimes, survivorship increased linearly with seed mass (log transformed; adjusted R2 = 0.72, p < 0.001), yet no relationship was apparent under wet moisture regimes. Although precipitation influenced survival, extreme rainfall events were not enough to offset moisture deficits nor provide a rescue effect under drought conditions. The relationships reported here highlight the importance of plant seedbeds and species (e.g., functional traits) as edaphic and biotic controls that modify the influence of extreme future precipitation on seedling survival in temperate forests. Finally, we demonstrated the biophysical factors that were most influential to early forest development and that may override the negative effects of increasingly variable precipitation. This work contributes to refinements of species distribution models and can inform reforestation strategies intended to maintain biodiversity and ecosystem function under increasing climate extremes.

Short-Term Vegetation Response to Bulldozed Fire Lines in Northern Great Plains Grasslands

A bulldozed fire line is a fire-suppression technique that limits fire movement by altering fuel continuity through vegetation removal and mineral soil exposure. The ecological impacts of a bulldozed fire line may exceed the effects of the fire itself through lasting changes in the soil and vegetation properties; however, little research has been performed to quantify these impacts in grassland systems. In this study, we compared vegetation properties among burned, unburned, and bulldozed fire line conditions on two August 2012 grassland wildfires in Montana. Standing biomass, by growth form, was quantified in 2013 and 2014, and compared using a generalized linear model. Perennial grass production was significantly reduced, while annual grass and annual forb biomass increased in response to the fire line treatment. Shrub and total vegetation standing crop were reduced in response to the fire line in 2013; however, the treatment effects were diminished by 2014. The burned and unburned treatments were generally similar within two years post-fire. The loss of perennial grasses and invasion of competitive annual grasses such as cheatgrass (Bromus tectorum L.) may limit the vegetation recovery of the fire line and promote further invasion of annual grasses into these systems. The marginal impact of the fires on these plant communities suggests the need to limit the use of ad hoc bulldozed fire lines as a suppression activity. If a bulldozed fire line is constructed, we suggest limiting soil disturbance by restricting blade depth to remove only surface vegetation and restricting bulldozer use to flat slopes, even if working with the contour, and incorporating re-seeding as part of or immediately after fire line construction.

Monitoring Campsite Soil Erosion by Structure-from-Motion Photogrammetry: A Case Study of Kuro-dake Campsites in Daisetsuzan National Park, Japan

Camping in the mountain regions has several negative impacts, including soil erosion in the campsites. In many national parks, efforts to monitor and manage campsite soil erosion are insufficient or even lacking. Owing to the lack of implementation of formal management practices, the current and former Kuro-dake campsites in Daisetsuzan National Park (DNP) have suffered serious soil erosion. To gain more insights into campsite soil erosion, we investigated these two campsites by short-term monitoring through structure-from-motion (SfM) photogrammetry surveys with ground control points (GCPs). These surveys were conducted three times consecutively in the current (2017, 2018, and 2019) and former Kuro-dake campsites (2018, 2019, and 2020). Two sets of digital elevation models (DEMs) of the current and former campsites were produced with the resolutions of 0.015 and 0.025 m, respectively. We detected that the elevation changes exceeded by 0.03–0.04 m, using the DEMs of difference (DoDs) maps. Soil loss around the gullies was observed at each campsite. In the current campsite, soil losses of 2.20 and 0.30 m3 were identified in the periods of 2017–2018 and 2018–2019, respectively. In the former campsite, soil losses of 1.55 and 22.27 m3 were identified during 2018–2019 and 2019–2020, respectively. In addition, a large amount of exposed mineral soil with exposed rocks was observed at each campsite. Various erosion rates obtained in the short-term study indicated the importance of continuous monitoring. High-resolution and high-accuracy topographic products generated from the SfM photogrammetry survey enabled the detection of a small amount of campsite soil erosion and provided a new method to monitor campsite degradation. This study demonstrated that a simple closure of the former campsite in 1992 failed to reduce the impact of soil erosion. In the case of the current and former Kuro-dake campsites, burying the gullies with boulders and setting up boulders or wood fences at the termini of the gullies would be a potential solution to stop or mitigate further erosion. Thus, formal management including monitoring and maintenance of site conditions should be introduced to Kuro-dake as well as other unmanaged campsites in DNP.

Regeneration Dynamics on Treefall Mounds and Pits for 10 Years after a Windfall in a Natural Mixed Forest

The treefall mounds and pits resulting from uprooting caused by strong winds is an indispensable microtopography for the regeneration of many tree species through improved light conditions and mineral soil exposure. These microtopographies are expected to become more important because global warming is predicted to increase windstorm frequency. This study aimed to clarify the characteristics of mounds and pits that contribute to the early establishment of major tree species in a natural mixed forest of northern Japan. The 39 tip-up mounds caused simultaneously by a strong typhoon in September 2004 were selected. In 2006, light intensity and soil moisture contents were measured in each mound and pit. Seedlings of all tree species were counted, and in 2014, the height of saplings was measured. The initial seedling density, regardless of tree species, was significantly higher in the pits than on the mound, but the density at the 10-th growing season decreased significantly in the pits and was comparable between the two locations. Intense light conditions favor regeneration of Betula sp. (Betula platyphylla and B. ermanii). In contrast, for Abies sachalinensis and Picea glehnii, the light intensity had a negative effect, so it was concluded that regeneration of conifers would be limited under the condition where strong winds form large gaps.

Precipitation declines influence the understory patterns in Nothofagus pumilio old-growth forests in northwestern Patagonia

Forest understories are essential to plant diversity and ecosystem functioning. However, studies about changes in understory patterns as affected by varying precipitation are scarce. Pure Nothofagus pumilio (common name: lenga) forests dominate the eastern side of the Andes mountains in Patagonia across an ample range of precipitation (~1500–500 mm). By studying the same forest type, in the same developmental stage (old-growth), we aimed to isolate the effects of precipitation upon these N. pumilio ecosystems, particularly for the understory. Three sites were selected with annual average precipitations of ~1000 mm (humid), ~800 mm (mesic), and 600 mm (dry), with a distance of 30 km between the humid and the dry sites, and only 18 km between the mesic and the dry sites. In each site, we established three 40 × 40 m plots in 4 blocks, and 30 1 m2 regeneration subplots within each plot. In each subplot we measured vascular plant cover, richness and diversity (alpha and beta), litter cover and coarse woody debris, plus several abiotic variables. We analyzed the data with mixed analysis of variance, differences of understory plant communities through blocked distance-based multivariate analysis of variance, and visualized the groups (sites) with non-metric multidimensional scaling. Indicator species at each site were identified through blocked species indicator analysis. The dry site differed significantly compared to the humid and mesic sites, with the lowest understory cover (4 vs. 82–78%), plant richness (15 vs. 25–26 species), and Simpson diversity index (0.05 vs. 0.66–0.64) . Beta turnover diversity was higher between the dry site with either the humid and the mesic sites (βt = 0.613 and 0.561, respectively), which in turn had more species in common (βt = 0.115). An increase in exposed mineral soil, soil water content, and leaf area index occurred from dry to humid sites, and vice versa for transmitted radiation and litter cover. All sites had different indicator species, but with indicator values increasing from dry to humid sites. The dramatic impoverishment of the plant community once precipitation drops within the range of 800 and 600 mm per year in Northern Patagonia sets a warning to the potential effects of climate change upon N. pumilio-dominated forest ecosystems and their plant diversity. Some forest management and potential adaptation strategies are proposed.

Logging Mats and Logging Residue as Ground Protection during Forwarder Traffic along Till Hillslopes

Forest soils in Northern Europe are generally trafficked by forest machinery on several occasions during a forest rotation. This may create ruts (wheel tracks), which could increase sediment transport to nearby surface water, reduce recreational value, and affect tree growth. It is therefore important to reduce soil disturbance during off-road forest transportation. In this study, rut depth was measured following forwarder traffic on study plots located along four harvested till hillslopes in Northern Sweden with drier soil conditions uphill and wet conditions downhill. The treatments included driving 1) using no ground protection, 2) on logging residue (on average, 38–50 kg m–2) and 3) on logging mats measuring 5×1×0.2 m. The hillslopes contain areas with a high content of boulders, stones, and gravel as well as areas with a significant content of silt. Six passes with a laden forwarder with four bogie tracks were performed. On the plots with ground protection, the application of logging residue and the application and removal of logging mats necessitated additional passes. Rut depth was measured using two methods: 1) as the difference in elevation between the interpolated original soil surface and the surface of the rut using GNSS positioning (Global Navigation Satellite Systems), and 2) manually with a folding rule from an aluminium profile, placed across the rut, to the bottom of the rut. The two methods generally gave similar results. Driving without ground protection in the upper part of the hillslopes generated ruts with depths &amp;lt;0.2 m. Here, the rut depth was probably modified by the high content of boulders and stones in the upper soil and drier soil conditions. In the lower part of the hillslopes, the mean rut depth ranged from 0.21 to 0.34 m. With a few exceptions, driving on logging residue or logging mats prevented exposure of mineral soil along the entire hillslope. Soil disturbance can thus be reduced by acknowledging the onsite variability in ground conditions and considering the need for ground protection when planning forest operations.

Prescribed fire effects on Pinus palustris woodland development after catastrophic wind disturbance and salvage logging

Scientifically informed strategies to manage naturally disturbed forests are critical to support the sustained provisioning of ecosystem goods and services. In fire-adapted ecosystems, catastrophic canopy removal can disrupt surface fuel continuity and challenge the continued use of low-intensity prescribed fire. Although salvage logging is used globally after natural disturbance events, little information is available on how salvage logging interacts with subsequent use of prescribed fire. This study investigated the impacts of operational-scale prescribed fire on Pinus palustris (longleaf pine) stand development in areas differentially impacted by an April 2011 EF3 tornado and a subsequent salvage logging operation. Twenty 0.04-ha nested plots were systematically established in mature, wind-disturbed, and salvage-logged sites (n = 60) to measure seedlings, saplings, woody fuels, organic litter, and mineral soil before and after prescribed fire. Prescribed fire-induced fine fuel consumption, mineral soil exposure, and substantial sapling density reductions were observed throughout the treatment area. Prescribed fire effects were not apparently impacted by salvage logging, which did not alter the amount of fine fuels available for prescribed fire consumption. Despite overall sapling density reductions, fire-resistant P. palustris saplings exhibited increased densities on wind-disturbed and salvage-logged sites. Pinus palustris seedlings, however, exhibited marked post-fire reductions, which contrasted with a strong resprouting response observed among top-killed hardwood species. Concerning woody plant recovery, this study indicated that salvage logging was not detrimental to P. palustris stand development and that prescribed fire effectively enhanced recovery in unlogged and logged wind-disturbed sites.

Effects of spot mounding and inverting on growth of conifers, exposed mineral soil and natural birch regeneration

In Nordic forests, consistent evidence about better seedling survival rate and increased growth due to site preparation have been obtained in numerous studies. Proper site preparation method can reduce costs of the whole regeneration chain through its effects on survival of planted seedlings, abundance of natural regeneration and competition in early stand development. This study compared the natural regeneration of birches (silver birch ( Roth) and downy birch ( Ehrh.)), amount of exposed mineral soil, and growth of planted seedlings between spot mounding and inverting site preparation methods. Present study was conducted in eight forest stands established in 2012 or 2015. Even though difference was not statistically significant, inverting exposed less mineral soil than spot mounding and thus reduced the natural regeneration of birch seedlings by 6135 seedlings ha compared to spot mounding. However, the variation between regeneration areas was remarkable. There was no difference in seedling mortality or growth between the site preparation methods. In order to achieve high growth of conifers, moderate amount of exposed mineral soil and thus less naturally regenerated birch, inverting should be favored over spot mounding.Betula pendulaB. pubescens–1

Recruitment dynamics and population structure of willows in tundra disturbed by retrogressive thaw slump thermokarst on Alaska’s North slope

Retrogressive thaw slumps (RTS) are large (> 1 ha) depressions of exposed mineral soil on hillslopes caused by the thaw and displacement of ice-rich permafrost soils in high-latitude regions; since the 1980s the number of RTS observed on Alaska’s North Slope has increased by two-thirds. Some RTS in the Toolik Lake area are filled with tall (≥ 0.5 m) willow thickets, likely within decades after disturbance. Tall shrub thickets are different in structure and function from mixed dwarf tundra communities and may have different long-term impacts on ecosystems and wildlife. Currently it is unknown to what degree seedlings versus clonal recruitment contribute to shrub thickets. We assessed size and distribution of clones (modular stems of individuals) using eight microsatellite (SSR) markers to genotype leaf tissue of 223 willow ramets (stems) at two sites: an RTS aged 11–30 years since disturbance, and nearby undisturbed moist acidic tussock (MAT) tundra. Genotypes of known clones from excavated ramets were used to determine the mutation rate of clones. Spatial arrangement of ramets within clones was assessed in 18 × 18 m sampling grids nested at 2 m (“far clones”), 1 m (intermediate), and 0.25 m (“near clones”) between ramets. We identified 121 genotypes including 10 clonal genotypes in the RTS, and 63 genotypes including 11 clonal genotypes in the undisturbed MAT. Percent distinguishable was greater than 76% at both sites. Mean spatial distance between clonal ramets was not different at either site but among far clones, ramets were separated 7–16 m downslope in the RTS. Salix pulchra was the dominant willow at both sites; a third of willow genotypes in the RTS were identified as S. glauca, a disturbance colonizer. Rarer and hybrid species comprised 4–20% of all genotypes but were more abundant in the RTS. At both sites, within-species expected heterozygosity (HT) ranged from 0.49 - 0.85. Our results suggest: 1) sexual recruitment and low clonal expansion likely explain genetic diversity of willows in disturbed and late-successional sites; and 2) downslope separation of far clones in the RTS suggests disturbance effects; and 3) species richness was higher in the disturbed site.

Large site prep tractors for fire line establishment in southern pine stands in Alabama

ABSTRACTTrials of fire line establishment with a mulcher were conducted with a 261 kW Caterpillar 586C site prep tractor and Cat HM825 mulcher head (2.5 m cutting path). The fire lines were intended for the application of prescribed fire. The trials were on three sites and in two stand types (cutover and thinned pine), and at three production speeds (0.6, 0.8, and 1.2 km h−1). The slowest speed (0.6) was a two-pass application at 1.2 km h−1. Treatments were applied in 16 blocks to 48 segments which ranged from 100 to 300 m in length. Mineral soil exposure ranged from 50% to 80% of the treatment width for all but one block. Slower speeds produced significantly greater mineral soil exposure with some significant interactions of site and treatment. Productive delays were caused by maneuvering and retreatment of skips, due to sudden changes in terrain or obstacles. Productive delays were 9% and 27% of the total time in cutover and thinned stands, respectively. Even with productive delays, most design speeds could be achieved. At the higher speeds, mulched fire line costs ($ km−1) were near current costs for bladed trails. However, mulcher productivity (km h−1) could be 2 to 3 times greater than dozers in open woodlands or young stands where the mulcher can easily navigate. Opportunities for using a wheeled mulcher to create fire lines might include locations that can take advantage of on-road travel between sites or locations that require spot treatments of high fuel loads in high-risk locations.

Soil bacterial and fungal response to wildfires in the Canadian boreal forest across a burn severity gradient

Global fire regimes are changing, with increases in wildfire frequency and severity expected for many North American forests over the next 100 years. Fires can result in dramatic changes to carbon (C) stocks and can restructure plant and microbial communities, with long-lasting effects on ecosystem functions. We investigated wildfire effects on soil microbial communities (bacteria and fungi) in an extreme fire season in the northwestern Canadian boreal forest, using field surveys, remote sensing, and high-throughput amplicon sequencing in upland and wetland sites. We hypothesized that vegetation community and soil pH would be the most important determinants of microbial community composition, while the effect of fire might not be significant, and found that fire occurrence, along with vegetation community, moisture regime, pH, total carbon, and soil texture are all significant predictors of soil microbial community composition. Burned communities become increasingly dissimilar to unburned communities with increasingly severe burns, and the burn severity index (an index of the fractional area of consumed organic soils and exposed mineral soils) best predicted total bacterial community composition, while whether a site was burned or not was the best predictor for fungi. Globally abundant taxa were identified as significant positive fire responders in this system, including the bacteria Massilia sp. (64 × more abundant with fire) and Arthrobacter sp. (35 × ), and the fungi Penicillium sp. (22 × ) and Fusicladium sp. (12 × ). Bacterial and fungal co-occurrence network modules were characterized by fire responsiveness as well as pH and moisture regime. Building on the efforts of previous studies, our results consider a particularly wide range of soils, vegetation, and burn severities, and we identify specific fire-responsive microbial taxa and suggest that accounting for burn severity improves our understanding of microbial response to fires.

Regeneration niches in Nothofagus-dominated old-growth forests after partial disturbance: Insights to overcome arrested succession

Arrested succession may lead to losses of productivity and biodiversity in forests after disturbance. In this condition, recalcitrant understory vegetation often dominates over long periods of time and delays forest recovery and succession. Arrested succession has been defined as a type of forest degradation because ecosystem processes and functions that underlie successional dynamics are dramatically reduced. We used the Hutchinson’ niche concept to understand the relative importance of environmental variables that define niche axes of species leading to arrested succession (recalcitrant understory vegetation) and to identify a shift along niche axes that promote the successional progression of long-lived early- and late-seral tree regeneration after partial disturbance due to restoration treatments. We studied Nothofagus old-growth forests in the Chilean Andes, where after selective harvests (i.e., a partial overstory disturbance) the understory is commonly dominated by dense thickets of native bamboo (Chusquea spp.), which has been shown to arrest succession by preventing tree regeneration. To overcome this, sites are typically partially scarified (24 to 45% of the area) following harvest to control bamboo and to encourage early-seral tree regeneration. The partial overstory and ground disturbance leaves a wide array of conditions (environmental gradient) that allowed us to investigate the factors under which a given plant group may regenerate, grow, and dominate the forest. Ordination and niche modeling were used to investigate the niche breadths for bamboo and regenerating tree species, as well as the environmental niche axes at this early successional stage, i.e., eight years after disturbance. The results showed different trends in the shift of centroid and extent of realized niches among three plant groups. For instance, performance of the early-seral group was related to litter cover (–), exposed mineral soil (+) and soil water content (+), which resulted in a niche expansion. In contrast, soil water content affected bamboo (–) and may be responsible for the reduction in realized niche space. Light had less influence on defining niches for tree regeneration during early successional stages. Linking the niche concept to succession provided a simple display of multifaceted issues and thus a better understanding of underlying processes and mechanisms. It provides an example of how application of this concept can help to determine situations in which such regeneration can (or cannot) be established successfully and promote the progression of succession following disturbance. This knowledge provides important insights for designing management and restoration practices based on strong ecological foundations.

Mounds facilitate regeneration of light-seeded and browse-sensitive tree species after moderate-severity wind disturbance

Using natural disturbance as a guide to management is an approach to develop resilience, maintain or restore natural processes, and sustain ecosystem goods and service. Here, we compare features of mounds resulting from tree uprooting, tree structure and composition, and browsing in recent 10–12-year-old moderate-severity wind disturbance events and reference stands in old hemlock– (Tsuga canadensis–) hardwood forests in Wisconsin, USA. Compared to reference stands, recent, partial blowdown stands had reduced overstory tree density and canopy heights, and more large gaps, coarse woody debris, exposed mineral soil, and newly-created mounds. Regeneration of light-seeded species was greater in blowdown stands relative to reference stands. We found an association between old mounds and overstory eastern hemlock and yellow birch, indicating mounds provide a long-term competitive advantage for these light-seeded species relative to pits and undisturbed areas. Notably, we found that the distance to the canopy was shortened by 30% for trees regenerating on young mounds in blowdown stands. In addition, light-seeded species, such as yellow birch, grew above browse height (2 m) in 10+ years on young mounds. In contrast, on flat and pit microsites, saplings were short (<2 m) and more likely browsed. Maintaining and/or protecting naturally created mounds may facilitate regeneration of light-seeded, browse-sensitive species in mixedwood forests that evolved under infrequent, moderate-severity wind storms. Therefore, reserving a subset of uprooted trees from salvage operations could provide suitable substrate for germination and long-term development advantages, especially for light-seeded and browse-sensitive species and stands managed for high conservation values.

One-time application of biosolids to ungrazed semiarid rangelands: 14 yr soil responses

Biosolids have been shown to improve forage production and soil quality on semiarid rangelands in the short term, but less is known about longer-term impacts of one-time biosolids applications. The objective of this study was to determine the effects of a single, surface biosolids application (at 20 dry Mg ha−1) on stability of soil aggregates, bulk density, total carbon (C) and nitrogen (N), permanganate-oxidizable carbon (POXC), polysaccharides, pH, nutrient availability, and soil water content (all at 0–7.5 cm depth) 14 yr following application to ungrazed rangelands in the Central interior of British Columbia. Fourteen years following the biosolids application, aboveground plant biomass was almost two times greater with biosolids application than on control, while exposed mineral soil and microbiotic crust significantly decreased in biosolids plots. Despite differences in aboveground biomass, there was no difference in total soil C and N, POXC, and polysaccharides between biosolids and control plots. Biosolids-amended soil did exhibit significantly greater mean weight diameter of water-stable aggregates, lower pH, increased spring soil water content, and increased availability of Fe3+, Zn2+, Cu2+, and phosphate ions. These findings indicate that the long-term improvements to soil on ungrazed rangeland are possible even from a single biosolids application.