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Simple Summary: We conducted this study in the boreal forest of Québec, Canada, to compare insect damage on white spruce (Picea glauca) trees within open-canopy, multispecies plantations, and trees naturally regenerated under mature mixed-wood canopies. Our plantation sites were established post-clearcutting and compared with naturally regenerated post-fire sites dominated by trembling aspen. Over two years, observations were made on young trees from ten sites in each environment, focusing on overall rates of herbivory, galls, spruce budworm damage, and sawfly defoliation. Despite a minor increase in spruce budworm damage in under-canopy forests from 2020 to 2021, overall, damage levels remained low, suggesting minimal impact on tree growth or mortality at this early outbreak stage. Contrary to initial assumptions, we found that insect damage was comparably low in both plantation and naturally regenerated environments, casting doubt on the idea that the enhanced sunlight and accelerated growth observed in plantations necessarily increase their susceptibility to pest attacks. In the context of global forest restoration efforts and Canada's pledge to plant 2 billion trees by 2050, the resilience of white spruce plantations to insect damage is of paramount importance. Amidst concerns over increasing pest damage due to climate change, the observed resilience of multispecies plantations underpins the effectiveness of ecosystem-based management in maintaining low insect damage levels without compromising growth. This balance highlights the potential of mixed-species plantations to mimic natural forest conditions, contributing to biodiversity conservation and sustainable forestry practices. This research illuminates the intricate dynamics between tree growth, environmental conditions, and pest vulnerability, offering valuable insights for future forest management and conservation strategies. In managed boreal forests, both plantations and natural regeneration are used to re-establish a cohort of conifer trees following harvest or disturbance. Young trees in open plantations generally grow more rapidly than under forest canopies, but more rapid growth could be compromised by greater insect damage. We compared insect damage on white spruce (Picea glauca (Moench) Voss, Pinaceae) growing in plantations with naturally regenerated trees under mature forest canopies in boreal forests (Québec, Canada). We selected ten sites in the naturally regenerated forest and in small, multispecies plantations and sampled ten young trees of 2.5–3 m (per site) in late summer 2020 and again in early and late summer 2021. We compared overall rates of herbivory, galls (adelgids), damage by the spruce budworm (Choristoneura fumiferana, Clemens), and defoliation from sawflies. Overall, insect herbivory damage remained at similarly low levels in both habitats; an average of 9.3% of expanding shoots were damaged on forest trees and 7.7% in plantation trees. Spruce budworm damage increased from 2020 to 2021 and remained higher in under-canopy trees, but damage rates were negligible at this early stage of the outbreak (1.5% in forest vs. 0.78% of buds damaged on plantation trees). While damage due to galls was higher in plantations, the overall low level of damage likely does not pose a significant impact on the growth or mortality of young trees. [ABSTRACT FROM AUTHOR]

Commercial production of high-quality lumber for Nordic conifers is negatively impacted by long rotation age and adverse negative correlations between growth and wood quality traits. A prospective solution to ensure sufficient fibre quality from future plantations is to identify key wood traits for desired applications and to consider them in tree breeding programs. In this study, we used the widespread and largely reforested white spruce (Picea glauca [Moench] Voss) in Canada to investigate the genetic control of wood flexural properties such as stiffness, i.e. modulus of elasticity (MOE), and strength, i.e. modulus of rupture (MOR). We also looked at their phenotypic and genetic correlations with other wood quality and growth traits to assess the efficiency of indirect methods of selection to improve wood flexural properties in the context of multi-trait selection in tree breeding programs. To achieve this, standardized solid wood samples, growth records and standing tree wood quality traits were collected from 289 trees belonging to 38 white spruce families from a polycross genetic trial established on two different sites in the province of Quebec, Canada. Flexural stiffness and strength, height, diameter at breast height (DBH) and wood density showed moderate to high heritability. Flexural stiffness was also positively correlated at the genetic level with flexural strength, average wood density and acoustic velocity as an indirect measure of dynamic MOE (⁠|${r}_{\mathrm{G}}$|  = 0.99, |${r}_{\mathrm{G}}$|  = 0.78 and |${r}_{\mathrm{G}}$| = 0.78, respectively). When selecting the top 5 per cent of the trees, the expected genetic gains varied from 3.6 per cent for acoustic velocity to 16.5 per cent for MOE. Selection based on wood density and acoustic velocity would result in considerable genetic gains in flexural stiffness. Several multi-trait selection scenarios were tested to investigate the genetic gains obtained from selecting with different combinations of growth and wood quality traits. The results showed that indirect selection for wood flexural properties by means of acoustic velocity and wood density are efficient methods that can be combined in operational white spruce breeding programs to increase simultaneously genetic gains for growth and wood flexural properties. [ABSTRACT FROM AUTHOR]

Global warming is predicted to extend the growing season of trees and plants, and advance spring phenology. However, intensification of extreme climate events in midlatitude forests, from weakening of the jet stream and atmospheric blockings, may expose trees to increased risk associated with more frequent late-spring frosts. Still, little is known regarding the intraspecific variation in frost tolerance and how it may be shaped by local adaptation to the climate of seed origin. As part of an assisted migration trial located in different bioclimatic zones in the province of Quebec, Canada, and following an extensive late-spring frost that occurred at the end of May 2021, we evaluated the frost damages on various white spruce (Picea glauca) seed sources tested on three sites (south, central, and north). The severity of frost damages was assessed on 5,376 trees after the cold spell and an early spring warming which advanced bud flush by approximately 10 days on average. The frost damage rate was similar among sites and seed sources and averaged 99.8%. Frost damage severity was unrelated to the latitude of seed origin but was variable among sites. The proportion of severely damaged trees was higher in the northern site, followed by central and southern sites. The proportion of severely damaged trees was linearly and inversely related to tree height before the frost event. Apical growth cancelation was not significantly different among seed sources including local ones, and averaged 74, 46, and 22%, respectively, in central, northern, and southern plantation sites. This study provides recommendations to limit the loss of plantation productivity associated with such a succession of spring climate anomalies. Implications for seed transfer models in the context of climate change and productivity of spruce plantations are discussed in the light of lack of local adaptation to such pronounced climate instability and ensuing large-scale maladaptation [ABSTRACT FROM AUTHOR]

An agreement between the provincial government of Québec, Canada and the forest industry executing forest management activities on public lands has been established regarding non-utilized woody material (NUWM) left on the cutting area. Problems linked to this agreement are compounded by labor shortages, which have an impact on the precision of the mandatory inventories. The objectives of this study were to: (1) reconstruct and estimate the merchantable NUWM volume beyond the last processed log of balsam fir and white spruce with the use of harvester on-board computer (OBC) data, (2) design a software tool to estimate and spatialize merchantable NUWM, and (3) perform an explorative comparison between the OBC method and conventional field inventory. In total, five sites were harvested to develop the volume algorithms. Each site was harvested by a single-grip harvester operating a different OBC system (OPTI4G, Log Mate 500, and Log Mate 510). Results suggest that, with Varjo's model and linear regression, estimation of NUWM volume using OBC data is possible. The spatialization tool positioned NUWM within the harvest area for StanForD and StanForD 2010. The explorative comparison highlighted a possible cost reduction of approx. 36.8 $/ha and an increase of precision for the OBC method. [ABSTRACT FROM AUTHOR]

Precise delineation of individual tree crowns is critical for accurate forest biophysical parameter estimation, species classification, and ecosystem modelling. Multispectral optical remote sensors mounted on low-flying unmanned aerial vehicles (UAVs) can rapidly collect very-high-resolution (VHR) photogrammetric optical data that contain the spectral, spatial, and structural information of trees. State-of-the-art tree crown delineation approaches rely mostly on spectral information and underexploit the spatial-contextual and structural information in VHR photogrammetric multispectral data, resulting in crown delineation errors. Here, we propose the spectral, spatial-contextual, and structural information-based individual tree crown delineation (S3-ITD) method, which accurately delineates individual spruce crowns by minimizing the undesirable effects due to intracrown spectral variance and nonuniform illumination/shadowing in VHR multispectral data. We evaluate the performance of the S3-ITD crown delineation method over a white spruce forest in Quebec, Canada. The highest mean intersection over union (IoU) index of 0.83, and the lowest mean crown-area difference (CAD) of 0.14 m 2 , proves the superior crown delineation performance of the S3-ITD method over state-of-the-art methods. The reduction in error by 2.4 cm and 1.0 cm for the allometrically derived diameter at breast height (DBH) estimates compared with those from the WS-ITD and the BF-ITD approaches, respectively, demonstrates the effectiveness of the S3-ITD method to accurately estimate biophysical parameters. [ABSTRACT FROM AUTHOR]

Conifer breeding programmes are increasingly selecting for wood quality in addition to growth so to ensure a sufficient flow of high-quality fibre from future forest plantations. As wood density is one of the most frequently used indicators of wood quality, there is a growing interest to consider this trait in selection, and thus enhance the properties of end-use products from planted trees. However, wood density varies at different scales within trees, with pith-to-bark patterns and year-to-year fluctuations representing two important sources of variation. From both physiological and end-use points of view, it is desirable to produce stems with limited pith-to-bark and year-to-year wood density variation. In the present study, we assessed patterns of pith-to-bark and year-to-year variation in 2196 wood density patterns and evaluated the genetic control of traits characterizing this variation. The experimental data came from a 15-year-old white spruce genetic trial representing 93 full-sib families replicated in two contrasting environments in Quebec, Canada. To separate pith-to-bark from year-to-year variation, non-linear models were developed to describe pith-to-bark patterns of variation in the mean ring density (MRD) of individual trees as well as for latewood density (LWD) and latewood proportion. We observed that pith-to-bark variation was more under genetic control than year-to-year variation, for which only LWD and proportion of latewood width to overall ring width reached moderate genetic control. Little genotype-by-environment interaction was observed although wood density patterns differed significantly between sites. The present approach could help identify trees or families that tend to have limited pith-to-bark and year-to-year variation in wood density as part of tree genetic improvement programmes to provide future trees with more uniform and desirable wood attributes. [ABSTRACT FROM AUTHOR]

Increasing air temperatures and changing precipitation patterns due to climate change can affect tree growth in boreal forests. Periodic insect outbreaks affect the growth trajectory of trees, making it difficult to quantify the climate signal in growth dynamics at scales longer than a year. We studied climate‐driven growth trends and the influence of spruce budworm (Choristoneura fumiferana Clem.) outbreaks on these trends by analyzing the basal area increment (BAI) of 2058 trees of Abies balsamea (L.) Mill., Picea glauca (Moench) Voss, Thuja occidentalis L., Populus tremuloides Michx., and Betula papyrifera Marsh, which co‐occurs in the boreal mixedwood forests of western Quebec. We used a generalized additive mixed model (GAMM) to analyze species‐specific trends in BAI dynamics from 1967 to 1991. The model relied on tree size, cambial age, degree of spruce budworm defoliation, and seasonal climatic variables. Overall, we observed a decreasing growth rate of the spruce budworm host species, A. balsamea and P. glauca between 1967 and 1991, and an increasing growth rate for the non‐host, P. tremuloides, B. papyrifera, and T. occidentalis. Our results suggest that insect outbreaks may offset growth increases resulting from a warmer climate. The observation warrants the inclusion of the spruce budworm defoliation into models predicting future forest productivity. [ABSTRACT FROM AUTHOR]

The use of wood ash as a soil amendment in afforestation and reforestation efforts is increasing. While most studies suggest benefits or neutral results on tree growth and survival, a few studies indicate adverse effects. Hybrid larch, jack pine and white spruce were studied at three northwestern Quebec plantation sites after they received wood ash at two application rates. Soil chemical properties, foliar nutrients and seedling growth and mortality were monitored over a period of eight years. The response of soil to ash application was mostly observed in the forest floor and was more pronounced in year 3 than year 8, likely due to the acidifying nature of the boreal soils studied. Jack pine growth increased linearly with wood ash application rates, white spruce growth showed an inconsistent and delayed positive response under the higher application rate, and hybrid larch growth and survival were either increased or decreased under the lower application rate depending of site but decreased at all sites under the higher application rate. The divergence in growth response between tree species underlines a trade-off between species with rapid acquisition of resources (e.g., pine, larch) to species that use more conservative strategies and store nutrients in their tissues for longer periods (e.g., spruce). In the case of hybrid larch, it accumulated larger amounts of Mn in its needles under the higher application rate and thus, the high bioavailabil - ity of Mn appears to have been detrimental to its survival and growth. Its higher sensitivity to Mn addition from ash is likely due to its highly acquisitive (nutrients) nature compared to other coniferous species as well as the initial levels of available Mn levels in the soil. The contrasted growth responses reported here under similar growing conditions highlight the importance of identifying suitable species, sites and application rates to maximize the benefits of wood ash amendments for future tree plantations in the boreal forest. [ABSTRACT FROM AUTHOR]

Key Message: A model describing species composition, density and diameter distribution of saplings was developed from operational inventory data. It could be used as an input into growth models calibrated exclusively with merchantable trees to correct some recruitment bias. Important differences in distributions were found between plantations and naturally regenerated stands. Longer-term monitoring would be required to observe the effects of thinning treatments on saplings. Context: Saplings play important ecological and structural roles in forest stands. They also constitute the pool of candidate trees that are responsible for recruitment of merchantable sized trees. However, sapling information is often absent from regular inventory measurements (e.g. where no saplings are measured) even though they could be used as inputs in predicting recruitment in merchantable trees. Aims: The objectives were to develop models describing density and diameter distribution of saplings from operational inventories, e.g. having only merchantable tree inventory, and to evaluate how stand type (naturally regenerated stands and plantations) and various thinning treatments influence these distributions. Methods: Using data from both white spruce (Picea glauca [Moench] Voss) plantations and naturally regenerated balsam fir (Abies balsamea (L.) Mill.) stands having been commercially thinned, a zero-inflated poisson regression was used to model the stand density and a gamma regression to predict the two parameters of the Weibull used to characterize the diameter distribution. Results: Despite the fact that the operational inventory data is often limited (e.g. species, dbh, height), the accuracy of the models was good and unbiased. It could be integrated into growth models to complete missing sapling distributions and help to correct some recruitment bias. Important differences in species composition, density and diameter distribution were found between plantations and naturally regenerated stands, but only a moderate response in diameter distribution was observed with thinning treatments. Conclusion: These models will enable managers to estimate saplings in intermediate aged softwood forests of eastern Quebec using harvesting inventories or National Forest Inventory. Characterization of differences between plantations and naturally regenerated stands will be useful for integrating intensive plantation silviculture with ecosystem-based management. Longer-term follow-up would be needed to better evaluate the effects of thinning treatments. [ABSTRACT FROM AUTHOR]

Copyright of Forestry Chronicle is the property of Canadian Institute of Forestry and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Intensive plantation forestry is a potent strategy for forest managers to increase wood production on a smaller forest land acreage, especially with the use of genetically improved reforestation stock. The main drawback with conventional conifer improvement is the several decades it takes before stock deployment, which is particularly acute in the context of climate change and evolving wood markets. Use of genomic selection allows to drastically shorten breeding cycles, resulting in more flexibility and potentially increasing benefits. This study compares the financial performance of five white spruce (Picea glauca) breeding and deployment scenarios, from conventional breeding to genomic selection in conjunction with top-grafting or the use of somatic embryogenesis, in the context of plantations established by the Quebec government on public lands with banned herbicide use. We estimated the land expectation value (LEV) for the five scenarios applied to eight site productivity indices, and considered costs and revenues from breeding, plantation establishment, silviculture, and harvest. LEVs at 4% discount rate were positive for all scenarios on all site indices, and varied from $197 to $2015 ha−1 assuming mechanical brushing of the plantations. The scenarios integrating genomic selection resulted in the highest LEVs, which increased with site index. We also conducted sensitivity analyses with 3% and 5% discount rates, with a range of costs and benefits, and with herbicide control of competing vegetation. These results should help orientate public investment decisions regarding the integration of genomic selection at the operational level in tree breeding and reforestation programs on public lands. [ABSTRACT FROM AUTHOR]

Copyright of Forestry Chronicle is the property of Canadian Institute of Forestry and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

• Introducing spruce in plantations promoted the establishment of bryophytes. • Poplar plantations provided habitat for lichens living on deciduous tree trunks. • Mixed plantations were composed of terrestrial bryophytes and epiphytic lichens. • Vascular plant composition was sensitive to the land-use history. Although monocultures are important for timber production, they are often associated with lower biological diversity than mixtures. Thus, mixed plantations have been suggested as a way to enhance biodiversity because of their inherent compositional diversity. However, the effects of monocultures versus mixtures on understory diversity and composition can vary in different ecosystems. The objective of this study was to assess how monocultures and mixed plantations influence understory vegetation diversity and composition in the boreal forest region of southern Quebec. We sampled plantations established with deciduous Populus trichocarpa Torrey & A. Gray × balsamifera L. and P. maximowiczii Henry × balsamifera L. and coniferous Picea abies (L.) Karst. and Picea glauca (Moench) species planted in monocultures and in mixed plantations on abandoned farmlands and a forest site. We assessed understory vegetation diversity and composition in each canopy type (coniferous, deciduous, mixed) and in each plantation type. We evaluated bryophyte and lichen diversity and composition specifically in tree microhabitats: soil, tree bases, and tree trunks. We found that vascular plant and lichen species richness was similar in all plantation types, while bryophyte species richness was higher in spruce monocultures and in mixed plantations compared to poplar monocultures. Our results also highlight how land-use history influenced vascular plant composition as abandoned farmland sites were composed of more ruderal vascular plants, while the previously forested site was composed of species found in natural forests. In the context of reforestation and plantations, our study suggests mixing spruce with poplars to maximize understory vegetation diversity as the addition of spruce in mixed plantations promoted the establishment of terrestrial bryophytes, while poplars favored the establishment of epiphytic lichens. [ABSTRACT FROM AUTHOR]

Vast areas of hardwood and mixedwood forests of eastern North America have been high-graded in the past and need silvicultural treatments to increase their value and productivity. To rehabilitate a high-graded temperate mixedwood stand, in Quebec, Canada, we used a split–split-split plot design with three replicates to assess different seed-tree and strip cutting systems, in combination with scarification and planting. The experiment consisted of three regeneration cuts in main plots: 10 seed-trees/ha, 40 seed-trees/ha and a 40-m wide strip clearcut (0 seed-tree/ha) with 60 seed-trees/ha in leave strips, thereby resulting in four levels of tree retention, and all included understory brushing. We applied two types of scarification (patch scarification or disk-trenching) to subplots, two regeneration modes (natural regeneration or planting with white spruce [Picea glauca]) to sub-subplots and two mechanical release treatments (softwood or mixedwood production) to planted sub–sub-subplots. Density of seed-trees did not affect the natural regeneration dynamics after 5 years, but disk-trenching was more efficient for the establishment of yellow birch (Betula alleghaniensis) and sugar maple (Acer saccharum). Few seed-trees of desirable white spruce were present and most died standing, confirming the importance of supplemental planting. Height growth of planted seedlings was 15% higher in the 0 and 10 (26–27 cm/year) than in the 40 and 60 (23 cm/year) seed-trees/ha treatments, and release doubled mean height growth (33.1 vs. 16.6 cm/year). Despite intensive site preparation, pre-established beaked hazel (Corylus cornuta) and mountain maple (Acer spicatum) were present at high densities in the regeneration stratum. Controlling this recalcitrant layer might be the greatest challenge for rehabilitating degraded stands of the mixedwood forest, especially since the use of herbicides is prohibited on Quebec's public lands. [ABSTRACT FROM AUTHOR]

• We exposed seedlings of conifer species to three realistic simulated future climates. • Seedlings of conifer species experienced high mortality (75%) induced by drought. • Conifer species are highly vulnerable at the seedling stage to future drought. • Hydraulic safety margins are good indicators of mortality risk for conifer species. The future climate of northern temperate forests is projected to be drier and warmer by the end of this century. As a result, more drought-induced forest dieback events are anticipated in northeastern North America, and assessing the vulnerability of dominant tree species to drought is critical for understanding the future composition of these forests. In a greenhouse experiment, we exposed two-year-old seedlings of Picea glauca (Moench) Voss, Picea mariana (Mill.) B.S.P. and Pinus strobus L. to three future climate treatments for southern Quebec, Canada, and evaluated their mortality, growth, and foliage water status responses to soil water availability and atmospheric drought. Using a unique approach, climate treatments emulated droughts of different frequencies, durations, and intensities. Treatments closely simulated one growing season, with changes in air temperature and relative humidity every six hours and daily adjustment in the amount of water delivered to the seedlings. The three species experienced high mortality (75%) in all water-limited treatments compared to a control treatment that provided non-limiting soil moisture (0% mortality). The biomass of the seedlings that survived was 40% lower than that of control seedlings. Our results confirmed that the hydraulic safety margins, defined as the difference between seasonal minimum water potential and xylem water potential leading to 12, 50 and 88% of hydraulic conductivity loss, were good predictors of probability of tree mortality. Therefore, hydraulic safety margins are useful functional traits that can be used to compare the vulnerability of various species to drought and then provide crucial information to practitioners and policymakers to adjust forest management to climate change. We showed that three dominant conifer species of northern temperate forests were highly vulnerable to drought in future climates. Because drought is projected to be a significant threat to forests, understanding potentially adaptive physiological responses to drought, such as hydraulic safety margins of tree seedlings, is important for predicting the response of forest regeneration and composition in warmer and drier climates. [ABSTRACT FROM AUTHOR]

We present a broad class of semi-parametric models for time series of random sums of positive variables. Our methodology allows the number of terms inside the sum to be time-varying and is therefore well suited to many examples encountered in the natural sciences. We study the stability properties of the models and provide a valid statistical inference procedure to estimate the model parameters. It is shown that the proposed quasi-maximum likelihood estimator is consistent and asymptotically Gaussian distributed. This work is complemented by simulation results and applied to time series representing growth rates of white spruce (Picea glauca) trees from a few dozen sites in Québec (Canada). This time series spans 41 years, including one major spruce budworm (Choristoneura fumiferana) outbreak between 1968 and 1991. We found significant growth reductions related to budworm-induced defoliation up to two years post-outbreak. Our results also revealed the positive effects of maximum summer temperature, precipitation, and the climate moisture index on white spruce growth. We also identified the negative effects of the climate moisture index in the spring and the maximum temperature of the previous summer. However, the model's performance on this data set was not improved when the interactions between climate and defoliation on growth were considered. This study represents a major advance in our understanding of budworm–climate–tree interactions and provides a useful tool to project the combined effects of climate and insect defoliation on tree growth in a context of greater frequency and severity of outbreaks coupled with the anticipated increases in temperature. • We present a broad class of semi-parametric models for time series of random sums of positive variables. • We study the stability properties of the models and provide a valid statistical inference procedure to estimate the model parameters. • This work is applied to the time series representing growth rates of white spruce from a few dozen sites in Québec (Canada). This time series spans 41 years, including one major spruce budworm outbreak between 1968 and 1991. [ABSTRACT FROM AUTHOR]

Research Highlights: The spatial distribution of trees results from several ecological processes that can be difficult to measure. We applied a point process modelling approach that uses the diameter and species of neighbouring trees to represent inter-tree interactions through repulsive and attractive processes. Thinning treatments slightly influence the tree spatial distribution of trees in white spruce plantations. Integrating this "spatialiser" into growth models could help improve stand simulations following various thinning treatments over larger areas and longer periods. It could also allow for the use of spatially explicit models when tree position is not available. Background and Objectives: Tree spatial patterns result from several ecological processes and have important implications in forest ecology and management. The use of spatial information can significantly improve our understanding of forest structures. However, this implies intensive field work that is rarely integrated into forest inventories. The aims of this study were to develop a spatial distribution simulator of trees in white spruce plantations and to evaluate the influence of thinning treatments. Materials and Methods: A point process modelling approach was used to represent inter-tree interactions through repulsive and attractive process in white spruce (Picea glauca (Moench) Voss) plantations in eastern Quebec, Canada, that had been commercially thinned five years ago. Balsam fir (Abies balsamea (L.) Mill.) and hardwoods together can represent 30–40% of the basal area of these plantations. Results: The diameter and species of each tree's two closest neighbours were found to be the most important predictors in explaining the observed distances between trees. Despite the short period since thinning treatments, results showed that the treatment had slight significant effects on tree interactions. However, their impact on the global spatial distribution of stands is quite limited. Conclusions: Using only a few readily-available variables (species and diameter of trees), this "spatialiser" will make it possible to assign spatial coordinates to trees and generate realistic stand spatial structures even after various silvicultural treatments. [ABSTRACT FROM AUTHOR]

• White spruce is more severely damaged by growing-season frosts than black spruce. • Tree height was supressed even 13 years after planting. • Microtopography impacted tree height, thus mound planting could favor growth. • Mixed plantations of white and black spruce would increase plantation success. Reforestation in the boreal forest is challenging; trees must survive to large daily temperature variations and to the cold environment. Even if local tree species are adapted to withstand these harsh environmental conditions, spruce plantation failure after artificial regeneration occurs frequently, with important impacts on sustainable forest management. We hypothesized that this regeneration problem is caused by recurrent frost events occurring during the growing season. These events would freeze the terminal bud and the newly formed needles of the planted trees, thus limiting photosynthesis capacity and height growth. Our goal was to identify key permanent physical attributes of the landscape (elevation, slope shape and angular slope) and of microsite conditions (hole vs. mound) that best predict tree height and frost damage to foliage. In summer 2016 and 2017, we sampled tree height of 2,943 white spruce (Picea glauca [Moench] Voss) and black spruce (Picea mariana [Mill.] B.S.P.) trees in 66 monoculture plantations aged between 6 and 13 years distributed in the Clay Belt region of Quebec (Canada), and environment prone to frequent growing-season frosts. Using linear and binomial mixed regression models, we analyzed the effects of the physical attributes of the landscape and of microsite conditions on tree height (linear) and on frost damage (binomial). Tree height increased with increasing elevation and when seedlings were planted on mounds compared to planted in holes. The impact of microsite conditions on tree height increased as plantations aged, but the importance of elevation on tree height decreased with age. The probability of frost damage to foliage decreased for trees planted on mounds compared to trees planted in holes and from concave to convex slopes. These relations were most important in young plantations, but trees showing growth problems were still shorter by 2 m, even 13 years after planting. We also observed differences between species: white spruce was significantly more damaged by frost and was smaller compared to black spruce. Therefore, growing-season frosts can cause growth suppression problems in white spruce plantations established in the boreal mixedwood region. Since microsite conditions also play a key role in driving plantation success, mechanical site preparation techniques should not only focus on reducing the competition between the planted trees and the competing vegetation but should also focus on limiting frost damage by planting trees on elevated microsites. Our results will support forestry practices limiting plantation failure in boreal mixedwoods. [ABSTRACT FROM AUTHOR]

• Recovery in absence of treatments showed the resilient nature of mixedwood stands. • Understory conifers largely contributed to recovery of stand basal area and vigor. • Active rehabilitation could speed improvement of composition, quality and structure. • Shelterwood combined to scarification during good seed year is recommended. • Prescriptions should include provision of conifers in lower and upper strata. In northeastern North America, mixed hardwood-conifer forests have commonly been harvested with selective practices such as diameter-limit cutting. By removing trees of highest commercial value, these cuts often left stands with highly variable density and reduced volume, less species diversity, and lower wood quality. The lack of care to the regenerating layer also resulted in regeneration deficiencies, especially on productive sites supporting yellow birch (Betula alleghaniensis Britton)-conifer stands. We assessed the 15-yr effects of four rehabilitation scenarios on stand growth, quality, vigor and regeneration in an experiment established in Quebec, Canada. The trial compared untreated experimental units (e.g. control), uniform shelterwood method (50 overstory trees/ha), strip clearcutting (20-m strips), and seed-tree method (10 overstory trees/ha), all combined with 3 site preparation treatments (no scarification, mechanical raking, and spot scarification). After 15 years, the ongoing recovery of growth, vigor and regeneration in the control and untreated part of the strip clearcut plot reflected the resilient nature of these mixedwood stands, with conifer (Abies balsamea [L.] Mill. , Picea rubens Sarg., Picea glauca [Moench] Voss) saplings and poles contributing to the recovery of total BA and that of vigorous trees (i.e. acceptable growing stock [AGS]). The control and strip clearcut treatments, however, had high BA of unacceptable growing stock. Shelterwood plots recovered more slowly in BA and AGS, but had greater BA of high-quality trees than the control. Seed-tree method was less optimal because of its slow recovery and high understory competition. The lack of synchronization with a good seed year limited the efficiency of scarification, and increased the abundance of non-commercial species. An active rehabilitation strategy combining shelterwood system and ground disturbance (e.g. raking) during a good seed year should help to improve stand quality and regeneration. Yet, our results indicate that retaining conifers in the understory and overstory would ensure a seed source and accelerate recovery. Where seed sources and conifer advance regeneration are lacking, enrichment planting may be necessary to maintain a mixedwood composition over time. [ABSTRACT FROM AUTHOR]

• Gap model ZELIG-CFS is evaluated using long term inventory and disturbance data. • ZELIG-CFS makes better predictions of common trees when disturbances are simulated. • Deciduous species responsible for observed compositional change are underestimated. • Suggested improvements: integration of regeneration strategies and disturbance risk. The lack of long term independent historical data constitute a major impediment to the further improvement and validation of forest simulation models. In this study, we evaluate the degree to which the gap model ZELIG-CFS realistically predicts the long term stand development of mixed forests in southeastern Quebec when disturbance history is considered. An early 1930 forest inventory and disturbance data were used to evaluate model simulation performance over the 1930–2013 period. ZELIG-CFS well predicted the basal area of balsam fir in simulations with and without disturbances. When disturbances were simulated, the predictions for white spruce and white birch were more realistic than in the simulation without disturbances. At the same time, deciduous species that contributed the most to the compositional changes observed during the 20th century in mixed forests (red maple, sugar maple, and trembling aspen) were systematically underestimated by the model. Several improvement avenues are suggested: (1) A better integration of regeneration strategies of tree species, especially the integration of a probability of vegetative reproduction for sprouting species like red maple, sugar maple, trembling aspen and northern white cedar, (2) Coupling gap models with a stand risk model to better simulate natural and human disturbance probability as a source of exogenous mortality. [ABSTRACT FROM AUTHOR]