Boreal Forest Growth Research Articles

Permafrost melting enhances growth but leads to less intra-annual density fluctuations in boreal Larix gmelinii forests at its southernmost limit in northeast China

Permafrost melting due to climate warming in recent decades has produced significant effects on forest ecosystems, especially the boreal biome at its southernmost limit in Asia. How this warming affects wood formation of trees at intra-annual resolution is unclear, yet is crucial for assessing the impact of permafrost melting on boreal forest growth. In this study, we compared the radial growth and intra-annual wood density fluctuations (IADFs) of Dahurian larch (Larix gmelinii Rupr.) at a permafrost (PF) and a non-permafrost (NPF) site at the southernmost permafrost limit in northeast China and quantified their relationships with climate factors. Drought in early summer was the main factor limiting growth of Dahurian larch. The basal area increment (BAI) of trees at both sites increased initially and then decreased in the 1980s, probably in response to warm-dry climate conditions. Earlywood IADFs (IADF-E) occurred in 14.0% and 9.3% of dated rings at the NPF and PF sites, while the frequency of latewood IADFs (IADF-L) was 6.8% and 2.7% at these two sites. The frequency of IADF-E in trees at both sites was positively and negatively related to June temperatures (and vapor pressure deficit) and precipitation, respectively, suggesting drought stress in June triggered the formation of IADF-E. The IADF-Ls were probably formed in response to warm temperatures in the late growing season. A higher BAI and a lower frequency of IADF-Es of trees at the PF site than at the NPF site indicated that permafrost melting could alleviate drought stress in early summer and promote radial growth of Dahurian larch. This greatly improved forest carbon sequestration and wood quality of some northeastern Asian boreal forests may offset to some extent the adverse effects of warming-drying climates at some sites of northeast Asia. Larch IADF-Es recorded extreme droughts in early summer, giving us a new sight for reconstructing high-frequency extreme climate events. If climate warming continues, the benefits of permafrost melting will gradually disappear and even turn negative due to warmer-dryer climate conditions. Our findings provide valuable information for boreal forest management and conservation under future global warming.

Contrasting water-use strategies to climate warming in white birch and larch in a boreal permafrost region.

The effects of rising atmospheric CO2 concentrations (Ca) with climate warming on intrinsic water-use efficiency and radial growth in boreal forests are still poorly understood. We measured tree-ring cellulose δ13C, δ18O, and tree-ring width in Larix dahurica (larch) and Betula platyphylla (white birch), and analyzed their relationships with climate variables in a boreal permafrost region of northeast China over past 68years covering a pre-warming period (1951-1984; base period) and a warm period (1985-2018; warm period). We found that white birch but not larch significantly increased their radial growth over the warm period. The increased intrinsic water-use efficiency in both species was mainly driven by elevated Ca but not climate warming. White birch but not larch showed significantly positive correlations between tree-ring δ13C, δ18O and summer maximum temperature as well as vapor pressure deficit in the warm period, suggesting a strong stomatal response in the broad-leaved birch to temperature changes. The climate warming-induced radial growth enhancement in white birch is primarily associated with a conservative water-use strategy. In contrast, larch exhibits a profligate water-use strategy. It implies an advantage for white birch over larch in the warming permafrost regions.

Diminishing legacy effects from forest fertilization on stand structure, vegetation community, and soil function

While there is consensus that fertilization with nitrogen (N) is a cost-effective way of increasing both forest biomass yield and timber harvest profitability, the strength and longevity of legacy effects are debated. To quantify legacy effects of past fertilization, we analysed 21 mixed Pinus sylvesteris and Picea abies stands. The stands, on average 23 years old at the time of this study, were either unfertilized (n=7), fertilized with 150 kg N ha−1 once 36 years ago (n=7), or twice, 45 and 36 years ago, respectively (n=7), during the previous stand rotation. We performed measurements on soil N mineralisation and N availability, forest growth, ground vegetation community composition, soil and vegetation C/N ratios and soil C and N stocks, many of which responded to legacy N fertilization earlier in stand development. Our results show that the legacy effects of fertilization during the previous stand rotation have diminished through time, indicating an eventual convergence of stand properties. Specifically, all significant effects present in the previous measurement period (over a decade ago), were weaker or completely absent in the current study (i.e. 36 years after fertilization and 23 years after initiation of the new stands). None-the-less, this indicates a longer legacy effect of N fertilization than what is normally considered and suggests that care should be taken to mitigate unwanted, long-term effects when utilizing N addition to promote tree growth in boreal forests.

Planting density and mechanical site preparation effects on understory composition, functional diversity and planted black spruce growth in boreal forests

Mechanical site preparation (MSP) is used prior to planting to control competing vegetation and enhance soil conditions, particularly in areas prone to paludification. Tree planting density can be adapted to the management context and objectives, as it influences yield and wood quality. However, the combined effects of MSP and planting density on understory vegetation composition, functional traits, and diversity remain uncertain. We thus conducted a study in the Clay Belt region of northwestern Quebec, Canada. After careful logging, the study area was divided into nine sites, each receiving one of three treatments: plowing, disc trenching, or no preparation. Sites were further divided into two, with black spruce (Picea mariana [Mill.] Britton, Sterns & Poggenb.) seedlings planted at either a low planting density of 1100 seedlings ha-1 or a high planting density of 2500 seedlings ha-1. After nine years, we assessed understory composition, diversity, key functional traits, sapling density and growth of planted trees. Careful logging alone led to a higher density of naturally established conifers compared to plowing or disc trenching. The interaction between planting density and MSP significantly influenced understory diversity and composition in plowed plots. Understory composition was affected by the soil C/N ratio, coniferous species, and deciduous species density. The growth of black spruce was notably enhanced with higher planting density in the plow treatment only. Neither planting density nor MSP alone affected tree height and diameter. Our results suggest that combining plowing with high-density planting can enhance stand growth and improve forest productivity. These findings guide future research on paludified forests.

Diversification of forest management can mitigate wind damage risk and maintain biodiversity

Mitigating future forest risks, safeguarding timber revenues and improving biodiversity are key considerations for current boreal forest management. Alternatives to rotation forestry likely have an important role, but how they will perform under a changing climate remains unclear. We used a boreal forest growth simulator to explore how variations on traditional clear-cutting, in rotation length, thinning intensity, and increasing number of remaining trees after final harvest (green tree retention), and on extent of continuous cover forestry will affect stand-level probability of wind damage, timber production, deadwood volume, and habitats for forest species. We used business-as-usual rotation forestry as a baseline and compared alternative management adaptations under the reference and two climate change scenarios. Climate change increased overall timber production and had lower impacts on biodiversity compared to management adaptations. Shortening the rotation length reduced the probability of wind damage compared to business-as-usual, but also decreased both deadwood volume and suitable habitats for our focal species. Continuous cover forestry, and management with refraining from thinnings, and extension of rotation length represent complementary approaches benefiting biodiversity, with respective effects of improving timber revenues, reducing wind damage risk, and benefiting old-growth forest structures. However, extensive application of rotation length shortening to mitigate wind damage risk may be detrimental for forest biodiversity. To safeguard forest biodiversity over the landscape, shortening of the rotation length could be complemented with widespread application of regimes promoting old-growth forest structures.

Increasing atmospheric dryness reduces boreal forest tree growth

Rising atmospheric vapour pressure deficit (VPD) associated with climate change affects boreal forest growth via stomatal closure and soil dryness. However, the relationship between VPD and forest growth depends on the climatic context. Here we assess Canadian boreal forest responses to VPD changes from 1951-2018 using a well-replicated tree-growth increment network with approximately 5,000 species-site combinations. Of the 3,559 successful growth models, we observed a relationship between growth and concurrent summer VPD in one-third of the species-site combinations, and between growth and prior summer VPD in almost half of those combinations. The relationship between previous year VPD and current year growth was almost exclusively negative, while current year VPD also tended to reduce growth. Tree species, age, annual temperature, and soil moisture primarily determined tree VPD responses. Younger trees and species like white spruce and Douglas fir exhibited higher VPD sensitivity, as did areas with high annual temperature and low soil moisture. Since 1951, summer VPD increases in Canada have paralleled tree growth decreases, particularly in spruce species. Accelerating atmospheric dryness in the decades ahead will impair carbon storage and societal-economic services.

Ectomycorrhizal fungi integrate nitrogen mobilisation and mineral weathering in boreal forest soil.

Tree growth in boreal forests is driven by ectomycorrhizal fungal mobilisation of organic nitrogen and mineral nutrients in soils with discrete organic and mineral horizons. However, there are no studies of how ectomycorrhizal mineral weathering and organic nitrogen mobilisation processes are integrated across the soil profile. We studied effects of organic matter (OM) availability on ectomycorrhizal functioning by altering the proportions of natural organic and mineral soil in reconstructed podzol profiles containing Pinus sylvestris plants, using 13 CO2 pulse labelling, patterns of naturally occurring stable isotopes (26 Mg and 15 N) and high-throughput DNA sequencing of fungal amplicons. Reduction in OM resulted in nitrogen limitation of plant growth and decreased allocation of photosynthetically derived carbon and mycelial growth in mineral horizons. Fractionation patterns of 26 Mg indicated that magnesium mobilisation and uptake occurred primarily in the deeper mineral horizon and was driven by carbon allocation to ectomycorrhizal mycelium. In this horizon, relative abundance of ectomycorrhizal fungi, carbon allocation and base cation mobilisation all increased with increased OM availability. Allocation of carbon through ectomycorrhizal fungi integrates organic nitrogen mobilisation and mineral weathering across soil horizons, improving the efficiency of plant nutrient acquisition. Our findings have fundamental implications for sustainable forest management and belowground carbon sequestration.

Predicting Growth of Individual Trees Directly and Indirectly Using 20-Year Bitemporal Airborne Laser Scanning Point Cloud Data

Reviewing forest carbon sinks is of the utmost importance in efforts to control climate change. This study focuses on reporting the 20-year boreal forest growth values acquired with airborne laser scanning (ALS). The growth was examined on the Kalkkinen research site in southern Finland as a continuation of several earlier growth studies performed in the same area. The data for the study were gathered with three totally different airborne laser scanning systems, namely using Toposys-I Falcon in June 2000 and Riegl VUX-1HA and miniVUX-3UAV in June 2021 with approximate point densities of 11, 1360, and 460 points/m2, respectively. The ALS point cloud was preprocessed to identify individual trees, from each of which different features were extracted either for direct or indirect growth measurement. In the direct method, the growth value is predicted based on differences of features, whereas in the indirect method, the growth value is obtained by subtracting the results of two independent predictions of different years. The growth in individual tree attributes, such as growth in height, diameter at breast height (DBH), and stem volume, were calculated for direct estimation. Field reference campaigns were performed in the summer of 2001 and in November 2021 to validate the obtained growth values. The study showed that long-term series growth of height, DBH, and stem volume are possible to record with a high-to-moderate coefficient of determination (R2) of 0.90, 0.48, and 0.45 in the best-case scenarios. The respective root-mean-squared errors (RMSE) values were 0.98 m, 0.02 m, and 0.17 m3, and the biases were −0.06 m, 0.00 m, and 0.17 m3. The direct method produced better metrics in terms of RMSE-% and bias, but the indirect method produced better best-fit lines. Additionally, the mean growth values for height, diameter, and stem volume intervals were compared, and they are presumed to be usable even for forest modelling.

Feasibility of Bi-Temporal Airborne Laser Scanning Data in Detecting Species-Specific Individual Tree Crown Growth of Boreal Forests

The tree crown, with its functionality of assimilation, respiration, and transpiration, is a key forest ecosystem structure, resulting in high demand for characterizing tree crown structure and growth on a spatiotemporal scale. Airborne laser scanning (ALS) was found to be useful in measuring the structural properties associated with individual tree crowns. However, established ALS-assisted monitoring frameworks are still limited. The main objective of this study was to investigate the feasibility of detecting species-specific individual tree crown growth by means of airborne laser scanning (ALS) measurements in 2009 (T1) and 2014 (T2). Our study was conducted in southern Finland over 91 sample plots with a size of 32 × 32 m. The ALS crown metrics of width (WD), projection area (A2D), volume (V), and surface area (A3D) were derived for species-specific individually matched trees in T1 and T2. The Scots pine (Pinus sylvestris), Norway spruce (Picea abies (L.) H. Karst), and birch (Betula sp.) were the three species groups that studied. We found a high capability of bi-temporal ALS measurements in the detection of species-specific crown growth (Δ), especially for the 3D crown metrics of V and A3D, with Cohen’s D values of 1.09–1.46 (p-value < 0.0001). Scots pine was observed to have the highest relative crown growth (rΔ) and showed statistically significant differences with Norway spruce and birch in terms of rΔWD, rΔA2D, rΔV, and rΔA3D at a 95% confidence interval. Meanwhile, birch and Norway spruce had no statistically significant differences in rΔWD, rΔV, and rΔA3D (p-value < 0.0001). However, the amount of rΔ variability that could be explained by the species was only 2–5%. This revealed the complex nature of growth controlled by many biotic and abiotic factors other than species. Our results address the great potential of ALS data in crown growth detection that can be used for growth studies at large scales.

Insect defoliation modulates influence of climate on the growth of tree species in the boreal mixed forests of eastern Canada.

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.

Declining Growth Response of Siberian Spruce to Climate Variability on the Taiga–Tundra Border in the Putorana Mountains (Northwest Siberia)

Global warming is most pronounced at high latitudes where temperatures increase twice as fast as the global average. Boreal forest growth is generally limited by low temperatures, so elevated temperature is supposed to enhance biomass production and carbon sequestration. A large amount of evidence has recently shown inconsistent responses of tree growth derived from annual tree rings to increasing temperature. We studied Siberian spruce growth in the remote and isolated Putorana Mts, Western Siberia in populations at its natural distribution limit. Tree ring cores were sampled along vertical transect in 100, 200 and 350 m a.s.l. as the aim was to identify the tree growth rate at different altitudes. Detailed sampling site descriptions served to identify possible factors controlling the growth rate in extremely heterogeneous environments. Monthly climate data for the period 1900–2020 were extracted from the gridded CRU database. Tree ring chronologies confirmed long-lasting limited growth, and despite high year-to-year ring width variability, synchronous growth at vertical study sites dominantly controlled by climate. The positive tree ring growth response to summer temperature was significant for most of the 20th century but dramatically changed in recent decades, when unusually warm summers were reported. There was no, or even a negative growth rate correlation with precipitation, which indicates a sufficient water supply at the study sites. Elevated temperature in this region with a continental climate might turn the study localities to water-limited areas with many negative consequences on tree growth and related ecosystem services.

Measuring tree growth using terrestrial laser scanning

Forests are dynamic ecosystems that are constantly changing. The most common natural reasons for change in forests are the growth and death of trees, as well as the damage occurring to them. Tree growth appears as an increment of its structural dimensions, such as stem diameter, height, and crown volume, which all affect the structure of a tree. Repeated measurements of tree characteristics enable observations of the respective increments indicating tree growth. According to current knowledge, the tree growth process follows the priority theory, where trees aim to achieve sufficient lightning conditions for the tree crown through primary growth, whereas increment in diameter results from the secondary growth. Tree growth is known to have an effect on the carbon sequestration potential of trees as well as on the quality of timber. To improve the understanding of the underlying cause–effect relations driving tree growth, methods to quantify structural changes in trees and forests are needed. The use of terrestrial laser scanning (TLS) has emerged during the recent decade as an effective tool to determine attributes of individual trees. However, the capacity of TLS point cloud-based methods to measure tree growth remains unexplored. This thesis aimed at developing new methods to measure tree growth in boreal forest conditions by utilizing two-date TLS point clouds. The point clouds were also used to investigate how trees allocate their growth and how the stem form of trees develops, to deepen the understanding of tree growth processes under different conditions and over the life cycle of a tree. The capability of the developed methods was examined during a five- to nine-year monitoring period with two separate datasets consisting of 1315 trees in total. Study I demonstrated the feasibility of TLS point clouds for measuring tree growth in boreal forests. In studies II and III, an automated point cloud-based method was further developed and tested for measuring tree growth. The used method could detect trees from two-date point clouds, with the detected trees representing 84.5% of total basal area. In general, statistically significant changes in the examined attributes, such as diameter at breast height, tree height, stem volume, and logwood volume, were detected during the monitoring periods. Tree growth and stem volume allocation seemed to be more similar for trees growing in similar structural conditions. The findings obtained in this thesis demonstrate the capabilities of repeatedly acquired TLS point clouds to be used for measuring the growth of trees and for characterizing the structural changes in forests. This thesis showed that TLS point cloud-based methods can be used for enhancing the knowledge of how trees allocate their growth, and thus help discover the underlying reasons for processes driving changes in forests, which could generate benefits for ecological or silvicultural applications where information on tree growth and forest structural changes is needed.

Effects of harvesting intensity, vegetation control and fertilization on 5–20 year post-harvest N availability in boreal jack pine and black spruce forest soils in northern Ontario, Canada

The impact of harvesting intensity, vegetation control and fertilization on soil N availability 5–20 years post-harvest was determined at 18 jack pine and black spruce forests in northern Ontario, Canada. The study sites are affiliated with the North American long-term soil productivity (LTSP) study. Boreal forest growth is commonly N limited and young stands approaching crown closure acquire most of their N from the soil. The study sites represented a range of soil types and were harvested with three levels of organic matter removal (stem-only harvest, full-tree harvest and full-tree harvest with organic horizon removal by blading). Three study sites had multiple applications of glyphosate to control competing vegetation and one site included a trial with two applications of N, P, K fertilizer to the full-tree harvest treatment. We used short-term anaerobic laboratory incubations and in-situ closed top tube growing season incubations to determine mineralizable N and net N mineralization, respectively. Profile (organic plus mineral horizons) mineralizable N pools were higher in operational clearcut (stem-only and full-tree harvesting) treatments compared to the uncut reference forest 10–14 years post harvest. Full-tree harvesting with blading significantly reduced profile mineralizable N pools at this time compared to operational harvesting, while the same treatment at one upland site also reduced net N mineralization pools. Profile mineralizable N pools and microbial biomass C and N were lower in full-tree compared to stem-only harvesting treatments, suggesting that increased slash retention with stem-only harvesting was playing a role in enhanced soil N availability at a time when plantations were approaching a period of greater N demand. Fertilization and vegetation control treatments had few effects on N indices, although net N mineralization pools for the stem-only and full-tree harvest that also had the repeated herbicide treatment were consistently, but not significantly, lower than non-herbicided treatments. Soil N indices were useful predictors of dominant height growth for jack pine to a greater degree than for black spruce, and soil N concentrations were as effective at predicting height growth as more resource intensive measurements. Our results point to the importance of continued monitoring of soil and stand conditions at these LTSP sites as treatment effects on soil N cycling and site productivity will likely continue to evolve. At this stage of stand development, full-tree harvesting plus organic horizon removal, followed by operational harvesting has led to lower soil N availability than operational tree-length harvesting, and stands with understory vegetation control are now showing indications of reduced N availability.

How permafrost degradation threatens boreal forest growth on its southern margin?

Boreal forests are adapted to cold climates and are thus especially sensitive and vulnerable to climate change. In this study, we chose 10 plots covering different topographies at the southern edge of a boreal forest. The annual ring widths of the 249 Dahurican larch (Larix gmelinii) tree cores from these plots were measured and used to calculate the basal area increment (BAI) and the interannual sensitivity (Sx). We found that forests in 10 plots showed a significant change in Sx consistently around 1980. The growth of slope forest was significantly correlated with increases in temperature and precipitation, while the wetland forests, including bogs and peatland plateaus, responded negatively to temperature. In terms of precipitation, there was no effect in the peatland plateaus, but a negative effect occurred in bogs. Our results imply that the depth of the frost table could lead to different soil waterlogging from surplus water from thawing permafrost, resulting in different responses of tree growth to climate change.

Biochar amendment increases tree growth in nutrient-poor, young Scots pine stands in Finland

Biochar is charred material formed by the pyrolysis of organic matter. The addition of biochar to soil may offer a chance to mitigate climate change by increasing soil carbon stocks, improving soil fertility and enhancing plant growth. Vast majority of biochar studies are conducted in agricultural soils, and field experiments studying the effect of biochar on tree growth in boreal forests are lacking. We applied spruce biochar amendments of 5 and 10 Mg ha−1 to the soil surface in young boreal Scots pine (Pinus sylvestris L.) forests in Southern Finland and studied the responses in tree growth and needle nitrogen (N) and 15N concentrations during the first three years after treatment. The biochar amendment of 10 Mg ha−1 increased the diameter growth of dominant trees significantly, on average by 1 mm year−1, which corresponded to 25% increase compared to control during the three years study period. The positive growth responses were less pronounced in height than in diameter growth. The biochar amendment of 5 Mg ha−1 increased the height growth of dominant trees by 0.16 m or 12% compared to the control during three years. Biochar amendments did not affect N and 15N concentrations in needles. The results suggest that wood biochar amendment can be a climate-friendly method to increase tree biomass production in nutrient poor, xeric, young Scots pine forests.

Taxonomy, together with ontogeny and growing conditions, drives needleleaf species' sensitivity to climate in boreal North America.

Currently, there is no consensus regarding the way that changes in climate will affect boreal forest growth, where warming is occurring faster than in other biomes. Some studies suggest negative effects due to drought-induced stresses, while others provide evidence of increased growth rates due to a longer growing season. Studies focusing on the effects of environmental conditions on growth-climate relationships are usually limited to small sampling areas that do not encompass the full range of environmental conditions; therefore, they only provide a limited understanding of the processes at play. Here, we studied how environmental conditions and ontogeny modulated growth trends and growth-climate relationships of black spruce (Picea mariana) and jack pine (Pinus banksiana) using an extensive dataset from a forest inventory network. We quantified the long-term growth trends at the stand scale, based on analysis of the absolutely dated ring-width measurements of 2,266 trees. We assessed the relationship between annual growth rates and seasonal climate variables and evaluated the effects of various explanatory variables on long-term growth trends and growth-climate relationships. Both growth trends and growth-climate relationships were species-specific and spatially heterogeneous. While the growth of jack pine barely increased during the study period, we observed a growth decline for black spruce which was more pronounced for older stands. This decline was likely due to a negative balance between direct growth gains induced by improved photosynthesis during hotter-than-average growing conditions in early summers and the loss of growth occurring the following year due to the indirect effects of late-summer heatwaves on accumulation of carbon reserves. For stands at the high end of our elevational gradient, frost damage during milder-than-average springs could act as an additional growth stressor. Competition and soil conditions also modified climate sensitivity, which suggests that effects of climate change will be highly heterogeneous across the boreal biome.

Björn Lindahl.

Björn Lindahl.

Allelopathic effects of dominant ground vegetation species on initial growth of Pinus sylvestris L. seedlings in response to different temperature scenarios

The dominant species of ground vegetation cover in clear-cuts impact the regeneration of Scots pine forests due to the biochemical properties of these dominant species. Environmental conditions in clear-cuts, specifically increased light and temperature, can alter the biochemical impact of dominant species on subsequent Scots pine regeneration processes. To investigate this, plant species diversity, frequency and cover were estimated in order to identify the dominant species of ground vegetation in clear-cut areas over a two-year period. Afterward, the impact of dominant species extracts on pine seed germination and seedling physiology at different temperature scenarios was evaluated. The species Pleurozium schreberi (Brid.) Mitt., Vaccinium vitis-idaea L. and Calluna vulgaris (L.) Hull were recorded as dominant in 1-yr-old clear-cut areas, and Vaccinium vitis-idaea L., Rumex acetosella L., and Calamagrostis epigejos (L.) Roth were dominant in 2-yr-old clear-cut areas. The prepared aqueous extracts of all dominant species exhibited strong inhibitory effects on pine seed germination and seedling morpho-physiological traits (the length of the radicle and hypocotyl; chlorophyll a, b, a/b and carotenoid content), resulting in the reduction of ex situ pine adaptive capacity at higher air temperature (24°C) compared to that at lower temperature (20°C). Significantly (p<0.05) stronger inhibitory effects of root and shoot extracts produced from all dominant species on chlorophyll a, b and a/b content were recorded at higher temperature (22-24°C) compared to lower (20°C) temperature. A significantly lower content of carotenoids was observed in the control (24°C) temperature. Thus, a high increase of ambient temperature may cause unfavourable conditions for Scots pine seedling growth in boreal forests.

Soil solution, foliar concentrations and tree growth response to 8 years of ammonium-nitrate additions in two boreal forests of Quebec, Canada

The boreal forest is recognised as one of the world’s most N-limited ecosystems. Increased N deposition is thought to stimulate boreal forest growth and carbon sequestration. However, evidence is lacking about the impact of increasing N supply over long time scales. Moreover, N deposition in forest ecosystems may cause N saturation, which leads to soil cation depletion and, ultimately, affects tree health. Ammonium nitrate (NH4NO3) was applied during an 8-year period in two stands in the boreal forest in Québec (Canada): one composed of balsam fir (Abies balsamea [Linné] Miller), and the other, of black spruce (Picea mariana [Mill.] B.S.P.). Application rates were 3 and 10 times the atmospheric nitrogen (N) deposition rate measured at each site, which was 6 kg ha−1 year−1 at the balsam fir site and 3 kg ha−1 year−1 at the black spruce site. Soil solution (at depths of 30 and 60 cm), mosses, soil and foliar chemistry as well as tree growth were analyzed. After 8 years of N additions, no signs of N saturation were observed in the soil solution of either site, and no treatment response was observed at the balsam fir site. By contrast, in the treated plots at the black spruce site, N concentration increased in mosses and tree needles. Tree basal areal increment also increased by 21% compared with the control. The different responses between the two forest types could be ascribed to the fact that the black spruce site is located further north and receives less N deposition than the balsam fir site. The tree growth increase at the black spruce site (concomitant with foliar N increase) suggests that C sequestration in tree biomass is more likely to increase in areas having soils with high C:N ratio and receiving low N deposition.

Growth characteristics of natural and planted Dahurian larch in northeast China

Abstract. Dahurian larch (Larix gmelinii Rupr.) is the dominant species in both natural and planted forests in northeast China, which is situated in the southernmost part of the global boreal forest biome and is undergoing great climatically induced changes. Published studies (1965–2015) on tree above-ground growth of Dahurian larch forests in northeast China were collected in this study and critically reviewed, and a comprehensive growth data set was developed from 122 sites, which are distributed between 40.85 and 53.47∘ N in latitude, between 118.20 and 133.70∘ E in longitude and between 130 and 1260 m in altitude. The data set is composed of 743 entries and includes growth data (mean tree height, mean diameter at breast height (DBH), mean tree volume and/or stand volume) and associated information, i.e., geographical location (latitude, longitude, altitude, aspect and slope), climate (mean annual temperature (MAT) and mean annual precipitation (MAP)), stand description (origin, stand age, stand density and canopy density) and sample regime (observation year, plot area and plot number). It provides a quantitative reference for plantation management practices and boreal forest growth prediction under future climate change. The data set is freely available for noncommercial scientific applications, and the DOI for the data is https://doi.org/10.1594/PANGAEA.880984.