Productivity Of Boreal Forests Research Articles

Net Primary Production of Ecoregions Across North America in Response to Drought and Wildfires From 2015 to 2022

AbstractEcosystem models are valuable tools to make climate‐related assessments of change when ground‐based measurements of water and carbon fluxes are not adequate to realistically capture regional variability. The Carnegie‐Ames‐Stanford Approach (CASA) is one such model based on satellite observations of monthly vegetation cover to estimate net primary production (NPP) of terrestrial ecosystems. CASA model predictions from 2015 to 2022 revealed several notable high and low periods in growing season NPP totals in certain biomes. Both Temperate Broadleaf and Boreal Forest production shifted from relatively high average NPP values in 2015 through 2019 to lower levels in 2020, typically representing a loss of 10%–14% of growing season NPP flux. This rapid decline in growing season NPP from 2019 to 2020–2021 was also estimated for the Temperate Grasslands and Savanna, Temperate Conifer Forest, and Tundra biomes. In contrast to the climate patterns in the temperate biomes that developed into severe widespread drought in 2020 and 2021 due to low precipitation totals and extreme hot temperatures, growing season NPP in the Tundra biome was depressed in these same years by colder temperature induced drought conditions at the high latitudes of North America. Drought severity classes were closely associated with different levels of decline in NPP in most biomes. Trends in NPP in areas of the largest wildfires in North America that burned between 2012 and 2021 were examined to assess recovery of vegetation and the resiliency of ecosystems during extreme drought periods.

Predicting net growth rates in boreal forests using Landsat time series and permanent sample plot data

Abstract Increasing temperature and changes in water dynamics are bringing uncertainty regarding the future productivity of boreal forests, even in the absence of stand-replacing disturbances. There is accumulating evidence that water deficits caused by warmer summer temperatures are linked to decreases in the growth rate of boreal tree species in some regions. In this context, it is essential to provide forest professionals with a means of monitoring net forest growth rates in undisturbed areas and at the scale of a management unit in order to determine where and when changes in growth are taking place. This is challenging using conventional forest inventory approaches. In this study, we use Landsat time series and data from permanent sample plots (PSP) to develop spatially explicit estimates of annual net basal area growth at a 30-m spatial resolution for a forest management unit in Canada. An ordinary least square regression model was developed using data from 120 PSPs and validated on an independent set of 60 PSPs, with R2 values of 0.61 and 0.58, respectively. Applying the model over a 586 607-ha study area revealed considerable temporal and spatial variability in the predicted growth rates and their evolution through time. There was an overall decline in predicted growth rates over time, with this trend corroborated by the PSP data and attributed to the ageing demographics of the forests in the study area. This variability was related to forest development stage, species composition, and structural attributes derived from light detection and ranging (LiDAR). The information generated by the suggested approach can help to improve yield predictions, optimize rotation lengths, and allow for the identification of target areas where silvicultural interventions aimed at maintaining or enhancing growth could be conducted.

Initiating the transition from open-canopy lichen woodland to productive forest by transplanting moss, results from a 10-year experiment

Background and aimsLow productivity open lichen (Cladonia spp.) woodlands have been rapidly expanding in the closed-crown feather moss (Pleurozium schreberi (Brid.) Mitt.) boreal forest of eastern Canada. While open-woodland areas are progressing, there is little information on the recoverability of open lichen woodlands back to closed-canopy forests.MethodsAn experimental set-up using moss transplantation was installed on a poor jack pine (Pinus banksiana Lamb.) stand with a lichen ground cover in 2011. Treatments included: 1) lichen cover removed, 2) lichen cover removed and transplantation of a feather moss cover, 3) lichen control, and 4) a natural jack pine site with feather moss cover (moss control). We extracted tree stem increment cores and collected needles and soil samples for nutrient analysis.ResultsThe transplanted-moss treatment can counteract the adverse effects of lichen on jack pine growth. This treatment enhanced foliar nutrition and soil nutrients, especially ammonium (N-NH4+) and nitrate (N-NO3−). With this treatment, the soil conditions (e.g., soil nutrients, soil moisture) and foliar nutrition were closer to that of moss control. Surprisingly, lichen removal treatment did not improve growth and resulted in poorer jack pine growth and harsher soil conditions.ConclusionFeather moss can establish, survive, and remain healthy in an environment previously occupied by lichen. The replacement of lichen by feather moss establishes soil conditions that appear conducive to better tree growth and have the potential of restoring the productivity of boreal forests in open-canopy lichen woodlands.

Climate change may increase Quebec boreal forest productivity in high latitudes by shifting its current composition

Several recent studies point out that climate change is expected to influence boreal forest succession, disturbances, productivity, and mortality. However, the effect of climate change on those processes and their interactions is poorly understood. We used an ecophysiological-based mechanistic landscape model to study those processes and their interactions and predict the future productivity and composition under climate change scenarios (RCP) for 300 years (2010–2310). The effects of climate change and wildfires on forest composition, biomass carbon sequestration and storage, and mortality were assessed in three management units of Quebec boreal forest, distributed along a longitudinal gradient from west to east: North-of-Quebec (MU1), Saguenay–Lac-Saint-Jean (MU2), and Côte-Nord region (MU3). Coniferous mortality variation was explained by competitive exclusion and wildfires, which are related to climate change. In the studied MU, we found a decrease in coniferous pure occupancy at the landscape scale and an increase in mixed deciduous forests in MU1 and MU2, and an increase in mixed coniferous, mainly black spruce and balsam fir in MU3. On the other hand, for extreme scenarios (RCP4.5 and RCP8.5), in the absence of broadleaves dispersal, the open woodland occupancy could increase to more than 8, 22, and 10% in MU1, MU2, and MU3 respectively. Also, climate change might increase overall biomass carbon stock two times for RCP2.6 and RCP4.5 scenarios compared to the baseline this may be explained by the extension of the growing season and the reduction of potential cold-temperature injuries. Generally, western regions were more sensitive to climate changes than the eastern regions (MU3), in fact under RCP8.5 biomass carbon stock will be decreasing in the long-term for MU1 compared to the current climate. This study provides a good starting point to support future research on the multiple factors affecting forest C budget under global change.

Soil organic matter diagenetic state informs boreal forest ecosystem feedbacks to climate change

Abstract. The fate of soil organic carbon (SOC) in boreal forests is dependent on the integrative ecosystem response to climate change. For example, boreal forest productivity is often nitrogen (N) limited, and climate warming can enhance N cycling and primary productivity. However, the net effect of this feedback on the SOC reservoir and its longevity with climate change remain unclear due to difficulty in detecting small differences between large and variable carbon (C) fluxes needed to determine net changes in soil reservoirs. The diagenetic state of SOC – resulting from the physicochemical and biological transformations that alter the original biomolecular composition of detrital inputs to soil over time – is useful for tracing the net response of SOC at the timescales relevant to climate change not usually discernible from fluxes and stocks alone. Here, we test two hypotheses using a mesic boreal forest climate transect: (1) the SOC diagenetic state is maintained across this climosequence, and (2) the maintenance of the SOC diagenetic state is a consequence of coupled soil C and N cycling, signifying the role of enhanced N cycling supporting SOC inputs that maintain SOC stocks within the warmer-climate forests. Shifts in nonvascular to vascular plant inputs with climate observed in these and other boreal forests highlighted the need to carefully separate biogeochemical indicators of SOC source from those signifying diagenetic alteration. We thus evaluated and applied lignin biomarkers to assess the diagenetic alteration of SOC in these boreal forest organic soils and directly compared the lignin diagenetic state with that of soil organic nitrogen (SON) assessed through amino acid composition. The lignin diagenetic state remained constant across the climate transect, indicating a balance between the input and removal of lignin in these mesic boreal forests. When combined with previous knowledge of these forest ecosystems, including the diagenetic state of SON and direct measures of C fluxes and stocks, the results indicate a coupled increase in C and N cycling with climate warming that supports forest productivity and maintains SOC stocks. This balance could markedly shift as other factors begin to limit forest productivity (e.g., trace nutrients, water) with further climate change or affect forest nutrient allocation (e.g., forest age or compositional change). Further application of the approach presented here could be used to detect the limits of this and other ecosystem–climate feedbacks, by providing a tractable and parameterizable index of the lignin state across large spatial scales, necessary for ecosystem-scale parameterizations.

Neighborhood dissimilarity consistently attenuates competition stress on tree growth under altered water availability in a natural boreal forest

Tree growth regulates forest productivity and carbon sequestration. Although there has been evidence showing positive effects of neighborhood diversity on tree growth, few have experimentally demonstrated how tree growth may respond to neighborhood diversity in natural forests under altered water availability, which is anticipated to occur more frequently with ongoing global climate change. We examined three-year individual tree basal area growth under the water alteration treatments of reduced (-25% throughfall during the growing season), ambient and added (+25% throughfall during the growing season) rainfall in the broadleaf stands, conifer stands and their mixtures of a young natural boreal forest. We quantified neighborhood drought tolerance dissimilarity, phylogeny dissimilarity and neighborhood crowding based on individual tree sizes and the distance between them. We found that the tree growth on average did not change significantly with water alteration but decreased with neighborhood crowding, with the negative crowding effect being more pronounced in water reduction treatment than in ambient and water addition treatments. Higher neighborhood dissimilarity of drought tolerance and phylogeny promoted tree growth consistently across all water conditions. Moreover, the positive effects of neighborhood dissimilarity of drought tolerance and phylogeny became more pronounced with increasing neighborhood crowding. Our results suggest that promoting species mixtures in boreal forests may sustain higher boreal forest productivity under changing water availability.

Threshold Responses of Canopy Cover and Tree Growth to Drought and Siberian silk Moth Outbreak in Southern Taiga Picea obovata Forests

The consecutive occurrence of drought and insect outbreaks could lead to cumulative, negative impacts on boreal forest productivity. To disentangle how both stressors affected productivity, we compared changes in tree canopy cover and radial growth after a severe outbreak in Siberian spruce (Picea obovata) southern taiga forests. Specifically, we studied the impacts of the 2012 severe drought followed by a Siberian silk moth (Dendrolimus sibiricus, hereafter SSM) outbreak, which started in 2016, on spruce forests by comparing one non-defoliated site and two, nearby fully defoliated sites, using remote sensing and tree-ring data. The SSM outbreak caused total defoliation and death of trees in the infested stands. We found a sharp drop (–32%) in the normalized difference infrared index and reduced radial growth in the defoliated sites in 2018. The growth reduction due to the 2012 drought was –37%, whereas it dropped to 4% of pre-outbreak growth in 2018. Tree growth was constrained by warm and dry conditions from June to July, but such a negative effect of summer water shortage was more pronounced in the defoliated sites than in the non-defoliated site. This suggests a predisposition of sites where trees show a higher growth responsivity to drought to SSM-outbreak defoliation. Insect defoliation and drought differently impacted taiga forest productivity since tree cover dropped due to the SSM outbreak, whereas tree growth was reduced either by summer drought or by the SSM outbreak. The impacts of abiotic and biotic stressors on boreal forests could be disentangled by combining measures or proxies of canopy cover and radial growth which also allow the investigation of drought sensitivity predisposes to insect damage.

Characterization of a seasonally snow-covered evergreen forest ecosystem

It remains challenging to interpret seasonal profile of vegetation dynamics from empirical indices NDVI and EVI for boreal forests due to confounding impacts of snow, soil and snowmelt in winter and spring. This work aims to characterize the seasonally snow-covered Howland boreal forest ecosystem in Maine, USA with the Moderate Resolution Imaging Spectrometer (MODIS) images. Vegetation cover fraction (VGCF), fractional absorption of photosynthetically active radiation (fAPAR) by all canopy components (fAPARcanopy), fAPAR by canopy chlorophyll (fAPARchl) and fAPAR by canopy non-chlorophyll components (fAPARnon-chl) were extracted from MODIS images in multiple years (2001 - 2014). Snow exposed during December to April. Top of canopy viewable snow cover fraction in April of multiple years varied between 0.02 and 0.16 (0.06 ± 0.04). Seasonal VGCF and fAPARcanopy showed a summer plateau (VGCF: 0.97 ± 0.01; fAPARcanopy: 0.90 ± 0.01). Both seasonal fAPARchl and fAPARnon-chl changed with time, and seasonal fAPARnon-chl had a bimodal shape. Spring VGCF varied between 0.54 and 0.69 (0.61 ± 0.04). Spring fAPARchl and fAPARnon-chl were 0.22 ± 0.03 and 0.21 ± 0.02, respectively. Peak summer fAPARchl was 0.58 ± 0.02. The lowest summer fAPARnon-chl was 0.32 ± 0.02. Replacing fAPARcanopy with fAPARchl to simulate boreal forest ecosystem gross primary production (GPP) could reduce uncertainties in GPP simulations.

Distinct response of gross primary productivity in five terrestrial biomes to precipitation variability

Climate change will impact precipitation variability, potentially accelerating climate-terrestrial carbon feedbacks. However, the response of ecosystems to precipitation variability is difficult to constrain due to myriad physiological and abiotic variables that limit terrestrial productivity. Based on a combination of satellite imagery and a global network of daily precipitation data, we present here a statistical framework to isolate the impact of precipitation variability on the gross primary productivity of five biomes that collectively account for 50% of global land area. The productivity of mesic grasslands and forests decreases by ~28% and ~7% (respectively) in response to more irregular rain within the year, while the sensitivity is halved in response to higher year-to-year variability. Xeric grasslands are similarly impacted by intra-annual rainfall variance, but they show an increase in productivity with higher interannual rainfall variability. Conversely, the productivity of boreal forests increases under higher variability on both timescales. We conclude that projected changes in precipitation variability will have a measurable global impact on the terrestrial carbon sink.

Транспорт, распределение и потребление 14С -ассимилятов у сосны и ели в северотаежных фитоценозах при различном световом и азотном питании

A rational system of cuttings, mineral nutrition of plants, and preservation of softwood undergrowth during felling is one of the ways to increase the productivity of boreal forests. The physiological methods of research used in forestry and, first of all, the study of donoracceptor interactions, allow obtaining the necessary information on the efficiency level of forestry measures in the stands of pine and spruce undergrowth quickly enough (within 1–2 years) without waiting for the final result in the form of an increment in shoots. The dynam ics of accumulation, transportation and distribution of 14C in organs and tissues of a tree under the influence of selective cuttings and a nitrogen dose has been studied in spruce and pine undergrowth in stands of different species composition and age. The study objects have been: blueberry spruce and birch forests of the forest site types with varying intensity of cutting; lichen pine forests formed from pine undergrowth preserved during the cutting. It were found that, in the summer period under the canopy of spruce and birch forests, the illumination does not exceed 8–10 % of the open area and does not correspond to the biological norm for spruce. The lack of solar radiation and soil nitrogen in the undergrowth of these tree species leads to a decrease in the intensity of photosynthesis and the violation of donoracceptor bonds, delays evacuation of assimilates from a leaf and their transportation to the active zones, suppresses the growth activity and the associated with it “demand” for macronutrients sent to the leaves declines the vitality of plants. The efficiency of the root system deteriorates, the formation of a new assimilation apparatus is delayed, and the growth processes of vegetative organs are violated due to the lack production of assimilates and the disruption of their distribution system. Nitrogen introduced into the stands and selective cuttings in pine and spruce undergrowth activate the synthetic and absorption activity of the root system, improve the functioning of the assimilation apparatus, strengthen the donoracceptor interactions between the aboveground and underground organs of a tree, significantly accelerate the outflow of photosynthetic products from a leaf, and positively influence on the growth processes. It has been found that, according to the level of activity of the donor-acceptor system for the pine and spruce undergrowth, it is possible within 1–2 years to set up a system of felling and a dose of nitrogen introduced into the forest stands that most closely meet the needs of these species. For citation: Konovalov V.N., Zarubina L.V. Transport, Distribution and Consumption of Pine and Spruce 14C-Assimilates in Northern Phytocenoses under Different Illumination and Nitrogen Nutrition. Lesnoy Zhurnal [Russian Forestry Journal], 2020, no. 4, pp. 77–94. DOI: 10.37482/0536-1036-2020-4-77-94

Do recent NDVI trends demonstrate boreal forest decline in Alaska?

Remote sensing analyses of boreal forest regions have found widespread decreasing or increasing trends in normalized difference vegetation index (NDVI). Initially, these trends were attributed to climate change induced shifts in primary productivity. It is emerging, however, that fire disturbance and subsequent succession also strongly impact the optical properties of boreal forests. Here we use NDVI time series data from Landsat (1999–2018) paired with surveys of 102 forest stands with known recent fire history to investigate the relationship between NDVI and forest structure during succession. We found that NDVI varies systematically with stand age as a result of successional changes in forest structure and composition and that the proportion of deciduous (broad-leaved) trees in the upper canopy is a better predictor of NDVI than leaf area index. Recent fire disturbance led to strong NDVI decreases and early post-fire recovery of herbaceous and deciduous vegetation to strong NDVI increases. The mid-succession transition from deciduous to evergreen (needle-leaved) stands led to weak NDVI decreases, while mid-to-late succession thinning of evergreen canopies led to weak NDVI increases. Thus, both increasing and decreasing NDVI stands occur naturally across the landscape, and do not necessarily reflect a large-scale shift in boreal forest productivity.

Effects of Temperature and Water Availability on Northern European Boreal Forests

Boreal forests are warming twice as fast as the rest of the planet. Do the benefits of higher temperatures and longer growing seasons for forest productivity exceed the negative effects of more frequent dry spells and heat waves, shifting precipitation patterns and higher evaporative demands? And are the effects uniformly distributed geographically? To answer to these questions, we use NDVI as a proxy of forest productivity and explore, via Partial Least Square (PLS) regression analyses, the relation between climatic variables and forest productivity at the regional scale. We focus on Northern Europe, a region for which contrasting findings on the effects of warming have been reported and that has so far been overlooked by systematic large-scale explorations of the linkages between boreal forest productivity and climatic conditions. Our results show that the effects of warmer temperatures on boreal forest productivity are not uniformly positive and that water stress is already negatively affecting the productivity of these forests. Indeed, increased temperatures appear beneficial in northern and wetter regions, while warmer temperatures mostly reduce forest productivity in southern and drier areas. These results are suggestive of already existing limitations due to water availability and warm temperatures, even in mesic regions like Northern Europe. These conditions are expected to become more frequent and intense in the future, potentially reducing the ability of boreal forests to provide their essential ecosystem services unless forest management practices are adapted to the new conditions.

Spatiotemporal variation in the relationship between boreal forest productivity proxies and climate data

The impacts of climate change on high-latitude forest ecosystems are still uncertain. Divergent forest productivity trends have recently been reported both at the local and regional level challenging the projections of boreal tree growth dynamics. The present study investigated (i) the responses of different forest productivity proxies to monthly climate (temperature and precipitation) through space and time; and (ii) the local coherency between these proxies through time at four high-latitude boreal Scots pine sites (coastal and inland) in Norway. Forest productivity proxies consisted of two proxies representing stem growth dynamics (radial and height growth) and one proxy representing canopy dynamics (cumulative May-to-September Normalized Difference Vegetation Index (NDVI)). Between-proxy and climate-proxy correlations were computed over the 1982–2011 period and over two 15-yr sub-periods. Over the entire period, radial growth significantly correlated with current year July temperature, and height growth and cumulative NDVI significantly correlated with previous and current growing season temperatures. Significant climate responses were quite similar across sites, despite some higher sensitivity to non-growing season climate at inland sites. Significant climate-proxy correlations identified over the entire period were temporarily unstable. Local coherency between proxies was generally insignificant. The spatiotemporal instability in climate-proxy correlations observed for all proxies underlines evolving responses to climate and challenges the modelling of forest productivity. The general lack of local coherency between proxies at our four study sites suggests that forest productivity estimations based on a single proxy should be considered with great caution. The combined use of different forest growth metrics may help circumvent uncertainties in capturing responses of forest productivity to climate variability and improve estimations of carbon sequestration by forest ecosystems.

Quantifying and easing conflicting goals between interest groups in natural resource planning

Management of natural resources at the regional level is a compromise between a variety of objectives and interests. At the local level, management of the forests depends upon the ownership structure, with forest owners using their forests as they see fit. A potential conflict occurs if the forest owners’ management decisions are counter to the interests of society in general or the industry that relies on the forest resource as their raw material. We explore the intensity of this conflict at the regional level in several large boreal forest production landscapes. To explore the conflict, we investigate three main interest groups: (i) economically oriented forest owners; (ii) industry groups (focusing on maintaining an even timber supply); and (iii) a group representing general public interests (focusing on enhancing ecosystem services and biodiversity protection). The severity of conflicts differs between interest groups; we found a minor conflict between the economically oriented forest owners and industry and a severe conflict among general public interests and the other groups. By quantifying the conflicts, visualizing the impacts shared among interest groups, we anticipate that through shared discovery and understanding, forests can be managed to lessen the conflicts between interest groups.

Tree density influences ecohydrological drivers of plant–water relations in a larch boreal forest in Siberia

AbstractTranspiration and stomatal conductance in deciduous needleleaf boreal forests of northern Siberia can be highly sensitive to water stress, permafrost thaw, and atmospheric dryness. Additionally, north‐eastern Siberian boreal forests are fire‐driven, and larch (Larix spp.) are the sole tree species. We examined differences in tree water use, stand characteristics, and stomatal responses to environmental drivers between high and low tree density stands that burned 76 years ago in north‐eastern Siberia. Our results provide process‐level insight to climate feedbacks related to boreal forest productivity, water cycles, and permafrost across Arctic regions. The high density stand had shallower permafrost thaw depths and deeper moss layers than the low density stand. Rooting depths and shallow root biomass were similar between stands. Daily transpiration was higher on average in the high‐density stand 0.12 L m−2 day−1 (SE: 0.004) compared with the low density stand 0.10 L m−2 day−1 (SE: 0.001) throughout the abnormally wet summer of 2016. Transpiration rates tended to be similar at both stands during the dry period in 2017 in both stands of 0.10 L m−2 day−1 (SE: 0.002). The timing of precipitation impacted stomatal responses to environmental drivers, and the high density stand was more dependent on antecedent precipitation that occurred over longer periods in the past compared with the low density stand. Post‐fire tree density differences in plant–water relations may lead to different trajectories in plant mortality, water stress, and ecosystem water cycles across Siberian landscapes.

Recent loss of sensitivity to summer temperature constrains tree growth synchrony among boreal Eurasian forests

High-latitude terrestrial ecosystems are crucial to the global climate system and its regulation by vegetation. Since productivity of boreal forests is much limited by low summer temperatures, it is expected that trees subjected to warming are progressively decreasing their regional growth coherence in the last decades. In this study, we used a comprehensive network of indexed ring-width records to assess 20th-century spatiotemporal patterns of climatic sensitivity of forest growth around the Urals mountain range above 60 °N (ca. 750,000 km2). This area offers an excellent opportunity to test for warming effects as most north Eurasian conifers (including Larix, Picea and Pinus species) are found along a north-to-south temperature gradient across contrasting soil hydrothermal regimes (permafrost and permafrost-free). We observed positive associations between indexed ring-width and summer temperature over the past century, decreasing southwards. However, weaker (permafrost) or non-significant (permafrost-free) relationships were consistently found at the local and regional scales after 1960. A cointegration analysis indicated that tree-growth release from cold limitation significantly reduced the degree and spatial extent of synchronous growth at short- (annual) and long-term (decadal) scales, most likely by exposing forests to endogenous (local) factors (e.g., competition, soil properties, nutrient availability) and species-specific reactions. Whereas the loss of temperature sensitivity progressively reduced non-permafrost synchrony by 50% over the whole 20th century, permafrost forests decreased their synchrony only after the 1960s, by 20%. Radial growth was enhanced in permafrost sites, as suggested by increasing basal area increment. Our results unequivocally link a substantial decrease in temporal coherence of forest productivity in boreal ecosystems to a growth release from cold limitation that is concurrent with regional warming trends. This emerging pattern points to increasing dependence on local drivers of the carbon balance and the role as carbon sinks of forests in the northern Ural region.

Disentangling the effect of drought on stand mortality and productivity in northern temperate and boreal forests

Abstract It has recently been reported that changing precipitation patterns increase tree mortality and reduce biomass accumulation in northern temperate and boreal forests. Functional diversity can mitigate the impacts of climate extremes in different types of ecosystems, but few studies have focused on tree functional diversity in forest ecosystems. It is critical to identify functional traits that could help anticipate the impact of drought on tree mortality, and how these affect above‐ground tree biomass productivity at the ecosystem level. We tested how three functional traits (ratio of dry leaf mass per unit area [LMA], xylem pressure at which 50% of stem xylem conductivity is lost through cavitation [Ψ50] and leaf area to sapwood ratio) influence severe drought impacts on stand mortality and productivity. We used structural equation modelling to compare the effect of a latent variable composed of these three traits between plots that had suffered drought and plots that had not, on the mortality and productivity of northern temperate and boreal forests of Québec, Canada. A latent variable composed of LMA and Ψ50 significantly explained drought‐induced tree mortality, but did not explain stand productivity response to severe drought. Therefore, even if these traits relate to the ability of species to survive drought (drought tolerance) at the tree level, they did not affect the maintenance of plant productivity during drought events (drought resistance) at the stand scale. We hypothesize that the effect of tree mortality on productivity was likely compensated by the formation of canopy openings that stimulate the growth of surviving trees. Synthesis and applications. Our results show that mitigation of water loss and xylem resistance to cavitation contribute similarly to severe drought tolerance in trees within northern temperate and boreal forests. Therefore, including drought‐resistant trees in fine‐scale mixtures might mitigate the impacts of drought on forest productivity due to the survivors’ response to released resources.

Decomposing sources of uncertainty in climate change projections of boreal forest primary production

We are bound to large uncertainties when considering impacts of climate change on forest productivity. Studies formally acknowledging and determining the relative importance of different sources of this uncertainty are still scarce, although the choice of the climate scenario, and e.g. the assumption of the CO2 effects on tree water use can easily result in contradicting conclusions of future forest productivity. In a large scale, forest productivity is primarily driven by two large fluxes, gross primary production (GPP), which is the source for all carbon in forest ecosystems, and heterotrophic respiration. Here we show how uncertainty of GPP projections of Finnish boreal forests divides between input, mechanistic and parametric uncertainty. We used the simple semi-empirical stand GPP and water balance model PRELES with an ensemble of downscaled global circulation model (GCM) projections for the 21st century under different emissions and forcing scenarios (both RCP and SRES). We also evaluated the sensitivity of assumptions of the relationships between atmospheric CO2 concentration (Ca), photosynthesis and water use of trees. Even mean changes in climate projections of different meteorological variables for Finland were so high that it is likely that the primary productivity of forests will increase by the end of the century. The scale of productivity change largely depends on the long-term Ca fertilization effect on GPP and transpiration. However, GCM variability was the major source of uncertainty until 2060, after which emission scenario/pathway became the dominant factor. Large uncertainties with a wide range of projections can make it more difficult to draw ecologically meaningful conclusions especially on the local to regional scales, yet a thorough assessment of uncertainties is important for drawing robust conclusions.

Nutrient optimization of tree growth alters structure and function of boreal soil food webs

Nutrient optimization has been proposed as a way to increase boreal forest production, and involves chronic additions of liquid fertilizer with amounts of micro- and macro-nutrients adjusted annually to match tree nutritional requirements. We used a short-term (maintained since 2007) and a long-term (maintained since 1987) fertilization experiment in northern Sweden, in order to understand nutrient optimization effects on soil microbiota and mesofauna, and to explore the relationships between plant litter and microbial elemental stoichiometry. Soil microbes, soil fauna, and aboveground litter were collected from the control plots, and short- and long-term nutrient optimization plots. Correlation analyses revealed no relationships between microbial biomass and litter nutrient ratios. Litter C:N, C:P and N:P ratios declined in response to both optimization treatments; while only microbial C:P ratios declined in response to long-term nutrient optimization. Further, we found that both short- and long-term optimization treatments decreased total microbial, fungal, and bacterial PLFA biomass and shifted the microbial community structure towards a lower fungi:bacterial ratio. In contrast, abundances of most fungal- and bacterial-feeding soil biota were little affected by the nutrient optimization treatments. However, abundance of hemi-edaphic Collembola declined in response to the long-term nutrient optimization treatment. The relative abundances (%) of fungal-feeding and plant-feeding nematodes, respectively, declined and increased in response to both short-term and long-term treatments; bacterial-feeding nematodes increased relative to fungal feeders. Overall, our results demonstrate that long-term nutrient optimization aiming to increase forest production decreases litter C:N, C:P and N:P ratios, microbial C:P ratios and fungal biomass, whereas higher trophic levels are less affected.

Contrasting shrub species respond to early summer temperatures leading to correspondence of shrub growth patterns

The Arctic-alpine biome is warming rapidly, resulting in a gradual replacement of low statured species by taller woody species in many tundra ecosystems. In northwest North America, the remotely sensed normalized difference vegetation index (NDVI), suggests an increase in productivity of the Arctic and alpine tundra and a decrease in productivity of boreal forests. However, the responses of contrasting shrub species growing at the same sites to climate drivers remain largely unexplored.Here, we test growth, climate, and NDVI relationships of two contrasting species: the expanding tall deciduous shrub Salix pulchra and the circumarctic evergreen dwarf shrub Cassiope tetragona from an alpine tundra site in the Pika valley in the Kluane Region, southwest Yukon Territories, Canada.We found that annual growth variability of both species at this site is strongly driven by early summer temperatures, despite their contrasting traits and habitats. Shrub growth chronologies for both species were correlated with the regional climate signal and showed spatial correspondence with interannual variation in NDVI in surrounding alpine and Arctic regions. Our results suggest that early summer warming represents a common driver of vegetation change for contrasting shrub species growing in different habitats in the same alpine environments.