Does wood mulch trigger microbially mediated positive plant-soil feedback in degraded boreal forest sites? A post hoc study.

Introduction: Reforestation of degraded lands in the boreal forest is challenging and depends on the direction and strength of the plant-soil feedback (PSF). Methods: Using a gradient in tree productivity (null, low and high) from a long-term, spatially replicated reforestation experiment of borrow pits in the boreal forest, we investigated the interplay between microbial communities and soil and tree nutrient stocks and concentrations in relation to a positive PSF induced by wood mulch amendment. Results: Three levels of mulch amendment underlie the observed gradient in tree productivity, and plots that had been amended with a continuous layer of mulch 17 years earlier showed a positive PSF with trees up to 6 m tall, a closed canopy, and a developing humus layer. The average taxonomic and functional composition of the bacterial and fungal communities differed markedly betweenlow- and high-productivity plots. Trees in high-productivity plots recruited a specialized soil microbiome that was more efficient at nutrient mobilization and acquisition. These plots showed increases in carbon (C), calcium (Ca), nitrogen (N), potassium (K), and phosphorus (P) stocks and as well as bacterial and fungal biomass. The soil microbiome was dominated by taxa from the fungal genus Cortinarius and the bacterial family Chitinophagaceae, and a complex microbial network with higher connectivity and more keystone species supported tree productivity in reforested plots compared to unproductive plots. Discussion: Therefore, mulching of plots resulted in a microbially mediated PSF that enhances mineral weathering and non-symbiotic N fixation, and in turn helps transform unproductive plots into productive plots to ensure rapid restoration of the forest ecosystem in a harsh boreal environment.

A longer wood growing season does not lead to higher carbon sequestration.

A reliable assessment of forest carbon sequestration depends on our understanding of wood ecophysiology. Within a forest, trees exhibit different timings and rates of growth during wood formation. However, their relationships with wood anatomical traits remain partially unresolved. This study evaluated the intra-annual individual variability in growth traits in balsam fir [Abies balsamea (L.) Mill.]. We collected wood microcores weekly from April to October 2018 from 27 individuals in Quebec (Canada) and prepared anatomical sections to assess wood formation dynamics and their relationships with the anatomical traits of the wood cells. Xylem developed in a time window ranging from 44 to 118 days, producing between 8 and 79 cells. Trees with larger cell production experienced a longer growing season, with an earlier onset and later ending of wood formation. On average, each additional xylem cell lengthened the growing season by 1 day. Earlywood production explained 95% of the variability in xylem production. More productive individuals generated a higher proportion of earlywood and cells with larger sizes. Trees with a longer growing season produced more cells but not more biomass in the wood. Lengthening the growing season driven by climate change may not lead to enhanced carbon sequestration from wood production.

Datasets of productivity and vegetation composition of boreal stands from an experiment comparing silviculture scenarios of increasing intensity after 20 years.

This data article describes datasets of plant community composition, dendrometric measurements, quantity and quality of snags of humid boreal stands (Quebec, Canada) from an experiment comparing silviculture scenarios of increasing intensity: (i) careful logging around advance growth (CLAAG); (ii) CLAAG followed by pre-commercial thinning; (iii) plantation followed by mechanical release; and (iv) plantation followed by chemical release and within five naturally disturbed sites. These data enable researchers to examine vegetation biodiversity recovery, ecosystem variables such as dead wood, and boreal stand productivity 20 years following the start of increasing-intensity silviculture scenarios. As a result, these data can be used to investigate the trade-off between keeping important ecosystem aspects of natural forests and maintaining and/or growing merchantable wood production at the stand level. This trade-off is the paradigm of forest ecosystem-based management, which aims to reduce the ecological distance between natural and managed forests in order to balance ecological challenges with the provision of socioeconomic services.

High exposure of global tree diversity to human pressure.

Safeguarding Earth's tree diversity is a conservation priority due to the importance of trees for biodiversity and ecosystem functions and services such as carbon sequestration. Here, we improve the foundation for effective conservation of global tree diversity by analyzing a recently developed database of tree species covering 46,752 species. We quantify range protection and anthropogenic pressures for each species and develop conservation priorities across taxonomic, phylogenetic, and functional diversity dimensions. We also assess the effectiveness of several influential proposed conservation prioritization frameworks to protect the top 17% and top 50% of tree priority areas. We find that an average of 50.2% of a tree species' range occurs in 110-km grid cells without any protected areas (PAs), with 6,377 small-range tree species fully unprotected, and that 83% of tree species experience nonnegligible human pressure across their range on average. Protecting high-priority areas for the top 17% and 50% priority thresholds would increase the average protected proportion of each tree species' range to 65.5% and 82.6%, respectively, leaving many fewer species (2,151 and 2,010) completely unprotected. The priority areas identified for trees match well to the Global 200 Ecoregions framework, revealing that priority areas for trees would in large part also optimize protection for terrestrial biodiversity overall. Based on range estimates for >46,000 tree species, our findings show that a large proportion of tree species receive limited protection by current PAs and are under substantial human pressure. Improved protection of biodiversity overall would also strongly benefit global tree diversity.

Detecting Compensatory Growth in Silviculture Trials: Empirical Evidence From Three Case Studies Across Canada.

Compensatory growth (CG) appears common in biology and is defined as accelerated growth after experiencing a period of unfavorable conditions. It usually leads to an increase in biomass that may eventually equal or even surpass that of sites not experiencing disturbance. In forestry, with sufficient time the stand volume lost in a disturbance such as a thinning operation could match or even exceed those from undisturbed sites, respectively called exact and overcompensation. The forest sector could benefit from enhanced productivity and associated ecosystem services such as carbon storage through overcompensation. Therefore, detection of CG in different types of forests becomes important for taking advantage of it in forest management. However, compensatory growth has not been reported widely in forestry, partially due to the paucity of long-term observations and lack of proper indicators. Legacy forest projects can provide a suitable data source, though they may be originally designed for other purposes. Three case studies representing different data structures of silviculture trials are investigated to evaluate if compensatory growth is common in forest stands. Our results showed that compensatory growth occurred in all three cases, and thus suggested that the compensatory growth might indeed be common in forest stands. We found that the relative growth (RG) can serve as a universal indicator to examine stand-level compensatory growth in historical long-term silviculture datasets. When individual tree-based measurements are available, both volume and value-based indicators can be used in detecting compensatory growth, and lumber value-based indicators could be more sensitive in detecting overcompensation.

Nine years of in situ soil warming and topography impact the temperature sensitivity and basal respiration rate of the forest floor in a Canadian boreal forest.

The forest floor of boreal forest stores large amounts of organic C that may react to a warming climate and increased N deposition. It is therefore crucial to assess the impact of these factors on the temperature sensitivity of this C pool to help predict future soil CO2 emissions from boreal forest soils to the atmosphere. In this study, soil warming (+2-4°C) and canopy N addition (CNA; +0.30-0.35 kg·N·ha-1·yr-1) were replicated along a topographic gradient (upper, back and lower slope) in a boreal forest in Quebec, Canada. After nine years of treatment, the forest floor was collected in each plot, and its organic C composition was characterized through solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. Forest floor samples were incubated at four temperatures (16, 24, 32 and 40°C) and respiration rates (RR) measured to assess the temperature sensitivity of forest floor RR (Q10 = e10k) and basal RR (B). Both soil warming and CNA had no significant effect on forest floor chemistry (e.g., C, N, Ca and Mg content, amount of soil organic matter, pH, chemical functional groups). The NMR analyses did not show evidence of significant changes in the forest floor organic C quality. Nonetheless, a significant effect of soil warming on both the Q10 of RR and B was observed. On average, B was 72% lower and Q10 45% higher in the warmed, versus the control plots. This result implies that forest floor respiration will more strongly react to changes in soil temperature in a future warmer climate. CNA had no significant effect on the measured soil and respiration parameters, and no interaction effects with warming. In contrast, slope position had a significant effect on forest floor organic C quality. Upper slope plots had higher soil alkyl C:O-alkyl C ratios and lower B values than those in the lower slope, across all different treatments. This result likely resulted from a relative decrease in the labile C fraction in the upper slope, characterized by lower moisture levels. Our results point towards higher temperature sensitivity of RR under warmer conditions, accompanied by an overall down-regulation of RR at low temperatures (lower B). Since soil C quantity and quality were unaffected by the nine years of warming, the observed patterns could result from microbial adaptations to warming.

Influence of Root System Characteristics on Black Spruce Seedling Responses to Limiting Conditions.

Roots directly affect planted seedling adaptation to new growing conditions at reforestation sites. To test the influence of root characteristics on the short-term response of seedlings to limiting resources (water, nutrient, or oxygen), we conducted two experiments. We compared (1) the growth and physiology of three types of four-year-old black spruce (Picea mariana (Mill.) BSP) seedlings (Containerized, highly developed initial roots restricted to a plug; bareroot, less developed but unrestricted initial roots; deeply-planted containerized, restricted initial and adventitious roots) to different combinations of irrigation and fertilization. We also investigated (2) the cellular plasticity of adventitious and initial roots to three irrigation regimes including flooding. Bareroot seedlings had better relative growth rates in height than containerized seedlings, probably due to their larger initial size. On the other hand, containerized seedlings took better advantage of fertilization, as shown by a higher relative growth rate in diameter compared to bareroot seedlings and were less affected by water limitation, possibly due to the root plug acting as an additional water reserve capacity. For containerized seedlings, the presence of adventitious roots was beneficial to height growth and physiological performances compared to seedlings with initial roots only. Adventitious roots showed great cell plasticity, particularly under flooding conditions.

Contribution of adventitious vs initial roots to growth and physiology of black spruce seedlings.

Black spruce (Picea mariana [Mill.] BSP) is a boreal tree species characterized by the formation of an adventitious root system. Unlike initial roots from seed germination, adventitious roots gradually appear above the root collar, until they constitute most of mature black spruce root system. Little is known about the physiological role they play and their influence on tree growth relative to initial roots. We hypothesized that adventitious roots present an advantage over initial roots in acquiring water and nutrients. To test this hypothesis, the absorptive capacities of the two root systems were explored in a controlled environment during one growing season. Black spruce seedlings were placed in a double-pot system allowing irrigation (25 and 100% water container capacity) and fertilization (with or without fertilizer) inputs independent to initial and adventitious roots. After 14 weeks, growth parameters (height, diameter, biomass), physiology (net photosynthetic rate, stomatal conductance, shoot water potential) and nutrient content (N, P, K, Ca and Mg foliar content) were compared. Most measured parameters showed no difference for the same treatment on adventitious or initial roots, except for root biomass. Indeed, fertilized black spruce seedlings invested heavily in adventitious root production, twice as much as initial roots. This was also the case when adventitious roots alone were irrigated, while seedlings with adventitious roots subjected to low irrigation produced initial root biomass equivalent to that of adventitious roots. We conclude that black spruce seedlings perform equally well through adventitious and initial roots, but if resources are abundant, they strongly promote development of adventitious roots.

Conifer Regeneration After Experimental Shelterwood and Seed-Tree Treatments in Boreal Forests: Finding Silvicultural Alternatives.

Forest regeneration is a key element in achieving sustainable forest management. Partial harvest methods have been used extensively in temperate broadleaf and mixedwood ecosystems to promote regeneration on poorly stocked sites and to maintain forest composition and productivity. However, their effectiveness in promoting conifer establishment has yet to be demonstrated in unmanaged boreal forests, especially those dominated by black spruce (Picea mariana (Mill.) BSP) where constraints for regeneration differ from those found in more meridional regions. We aimed to evaluate conifer seedling density and dimensions, 10 years after the onset of a gradient of silvicultural treatments varying in harvesting intensities, and to identify the critical factors driving the regeneration process. Study blocks of even-aged black spruce stands in the eastern Canadian boreal forest were submitted to three variants of shelterwood harvesting: a seed-tree harvest, a clear-cut and an untreated control. Shelterwood and seed-tree harvesting were combined with spot scarification to promote regeneration. Shelterwood and seed-tree harvesting produced a density of conifer regeneration sufficient to maintain forest productivity, but they did not promote seedling growth. Black spruce was the predominant species in terms of regeneration density, with proportions 3-5× higher than that for balsam fir (Abies balsamea (L.) Mill.). Ten years after treatment, seed-origin black spruce seedlings were abundant in skidding trails, while layers dominated the residual strips. Balsam fir density was not influenced by treatment nor by tree position relative to skidding trails. Balsam fir and black spruce had different responses to treatment in terms of height and diameter, the former exhibiting a better growth performance and larger diameter in the residual strips. Spot scarification created micro-sites that had a significant impact on the regeneration process. Overall, our results support that shelterwood and seed-tree harvesting combined with scarification enable adequate regeneration in black spruce stands, confirming these treatments as viable silvicultural alternatives to clear-cutting when required by sustainable forest management objectives.

Towards the development of multifunctional molecular indicators combining soil biogeochemical and microbiological variables to predict the ecological integrity of silvicultural practices.

The impact of mechanical site preparation (MSP) on soil biogeochemical structure in young larch plantations was investigated. Soil samples were collected in replicated plots comprising simple trenching, double trenching, mounding and inverting site preparation. Unlogged natural mixed forest areas were used as a reference. Analysis of soil nutrients, abundance of bacteria and gas exchanges unveiled no significant difference among the plots. However, inverting site preparation resulted in higher variations of gas exchanges when compared with trenching, mounding and unlogged natural forest. A combination of the biological and physicochemical variables was used to define a multifunctional classification of the soil samples into four distinct groups categorized as a function of their deviation from baseline ecological conditions. According to this classification model, simple trenching was the approach that represented the lowest ecological risk potential at the microsite level. No relationship was observed between MSP method and soil bacterial community structure as assessed by high-throughput sequencing of bacterial 16S rRNA gene; however, indicator genotypes were identified for each multifunctional soil class. This is the first identification of multifunctional molecular indicators for baseline and disturbed ecological conditions in soil, demonstrating the potential of applied microbial ecology to guide silvicultural practices and ecological risk assessment.

Bluejoint Is an Effective Bio-Barrier Species on Mine Covers.

Covers with capillary barrier effects (CCBE) are used to prevent acid mine drainage from mine wastes in the short term. However, the long-term efficiency of CCBE can be affected by trees because their roots may reduce the ability of covers to limit oxygen migration and also physically damage the CCBE. Two plant species that are native to boreal Canada, bluejoint () and sheep laurel (, were selected as bio-barrier species (BBS) to test if they reduce the growth and root system architecture of trees established on mine covers (balsam poplar [], willow [ spp], and black spruce []). The experiment was established in 2008 on a mine tailings impoundment located in northwestern Quebec, Canada. Trees were measured for height, diameter, and biomass. Coarse roots were excavated from the plots and digitized in three dimensions. Compared with the control (no BBS), bluejoint strongly decreased tree height and diameter increment, biomass, maximum root depth and radial extension, total root length and volume, and number of second- and third-order tree roots. Height and diameter increment, biomass, maximum root depth and volume, and number of second-order roots of balsam poplar increased with sheep laurel compared with control conditions, whereas willow showed no response to this treatment. Most characteristics of black spruce (except root-to-shoot ratio and number of second-order roots) improved in the presence of sheep laurel compared with the control. Thus, bluejoint was a more efficient BBS than sheep laurel. Bio-barriers comprised of bluejoint can be used as a countermeasure for controlling tree invasion of CCBE.

Ecological gradients driving the distribution of four Ericaceae in boreal Quebec, Canada.

Understory species play a significant role in forest ecosystem dynamics. As such, species of the Ericaceae family have a major effect on the regeneration of tree species in boreal ecosystems. It is thus imperative to understand the ecological gradients controlling their distribution and abundance, so that their impacts can be taken into account in sustainable forest management. Using innovative analytical techniques from landscape ecology, we aimed to position, along ecological gradients, four Ericaceae found in the boreal forest of Quebec (Canada) (Rhododendron groenlandicum, Kalmia angustifolia, Chamaedaphne calyculata, and Vaccinium spp), to regionalize these species into landscape units relevant to forest management, and to estimate the relative importance of several ecological drivers (climate, disturbances, stand attributes, and physical environment) that control the species distribution and abundance. We conducted our study in boreal Quebec, over a study area covering 535,355 km(2). We used data from 15,339 ecological survey plots and forest maps to characterize 1422 ecological districts covering the study region. We evaluated the relative proportion of each ericaceous species and explanatory variables at the district level. Vegetation and explanatory variables matrices were used to conduct redundancy, cluster, and variation partitioning analyses. We observed that ericaceous species are mainly distributed in the western part of the study area and each species has a distinct latitudinal and longitudinal gradient distribution. On the basis of these gradients, we delimited 10 homogeneous landscape units distinct in terms of ericaceous species abundance and environmental drivers. The distribution of the ericaceous species along ecological gradients is closely related to the overlaps between the four sets of explanatory variables considered. We conclude that the studied Ericaceae occupy specific positions along ecological gradients and possess a specific abundance and distribution controlled by the integration of multiple explanatory variables.