Boreal Tree Research Articles

The Identification of Small RNAs Differentially Expressed in Apple Buds Reveals a Potential Role of the Mir159-MYB Regulatory Module during Dormancy.

Winter dormancy is an adaptative mechanism that temperate and boreal trees have developed to protect their meristems against low temperatures. In apple trees (Malus domestica), cold temperatures induce bud dormancy at the end of summer/beginning of the fall. Apple buds stay dormant during winter until they are exposed to a period of cold, after which they can resume growth (budbreak) and initiate flowering in response to warmer temperatures in spring. It is well-known that small RNAs modulate temperature responses in many plant species, but however, how small RNAs are involved in genetic networks of temperature-mediated dormancy control in fruit tree species remains unclear. Here, we have made use of a recently developed ARGONAUTE (AGO)-purification technique to isolate small RNAs from apple buds. A small RNA-seq experiment resulted in the identification of 17 micro RNAs (miRNAs) that change their pattern of expression in apple buds during dormancy. Furthermore, the functional analysis of their predicted target genes suggests a main role of the 17 miRNAs in phenylpropanoid biosynthesis, gene regulation, plant development and growth, and response to stimulus. Finally, we studied the conservation of the Arabidopsis thaliana regulatory miR159-MYB module in apple in the context of the plant hormone abscisic acid homeostasis.

Foliar nutrient resorption dynamics of trembling aspen and white birch during secondary succession in the boreal forest of central Canada

Nutrient resorption is a key strategy in plant conservation, which minimizes nutrient loss and enhances productivity. However, the effects of stand development and the neighbourhood species diversity on the nutrient resorption of boreal tree species remain unclear. Using a detailed, replicated chronosequence spanning 7, 15, 33, 98, 146, and 209 years following fire, we examined the dynamics of leaf nitrogen (NRE), phosphorus (PRE), and potassium (KRE) resorption efficiencies of two common broadleaved boreal tree species (Populus tremuloides and Betula papyrifera) associated with stand development and overstory composition type (monoculture and mixedwood) in a central boreal forest of Canada. We found that the leaf NRE did not change; however, the PRE and KRE decreased with overstory succession. Further, we observed a higher leaf PRE in younger mixed stands, but a lower leaf PRE in older mixed stands, compared to monocultures for Populus tremuloides. Our results also revealed that the leaf NRE/PRE and NRE/KRE ratios increased with overstory succession, particularly in mixed stands, which demonstrated strong N limitations in old mixed boreal forests. Moreover, the overstory succession- and type-dependent dynamics of nutrient resorption efficiencies were driven by soil nutrient changes and relative tree growth rates associated with stand development. Nutrient resorption efficiency patterns may imply a transition from more P and K limitations in early succession, to more N limitations in late succession. Our findings highlight the need for modelling long-term primary productivity to account for overstory succession- and type-specific changes in nutrient resorption efficiencies.

Chironomid‐inferred summer temperature development during the late Rissian glacial, Eemian interglacial and earliest Würmian glacial at Füramoos, southern Germany

Eemian pollen records from central Europe describe a transition from thermophilous tree taxa in the early Eemian to boreal tree taxa in the late Eemian with forest opening in the subsequent stadial. Available summer‐temperature reconstructions for the mid‐ to late Eemian transition show decreasing values during that time. We present a new chironomid record from southern Germany that covers the mid‐Eemian to the end of the first Würmian stadial (c. 125–105 ka) and also parts of the late Rissian glaciation and early Brörup interstadial of the early Würmian glaciation. Based on this record we describe lake development in the former Füramoos palaeolake and quantitatively reconstruct July air temperature during the examined interval. Late Rissian sediments are dominated by two chironomid taxa, Sergentia coracina‐type and Micropsectra radialis‐type, indicating very cold conditions. Following an uncertain interval, probably including a hiatus at the late Rissian/Eemian transition, mid‐Eemian sediments contain Tanytarsus glabrescens‐type and Tanytarsus mendax‐type suggesting relatively high July air temperatures. During the late Eemian, typically thermophilic taxa such as Tanytarsus glabrescens‐type disappear, suggesting decreasing temperatures. Stadial A is associated with increases in Microtendipes pedellus‐type suggesting more oligotrophic conditions. Early Brörup sediments contain Tanytarsus glabrescens‐type, suggesting a slight increase in July air temperature. Reconstructed July air temperatures show temperatures of 7–8 °C during the late Rissian and a decline from ~15.5–12 °C during the mid‐ to late Eemian associated with decreasing Northern Hemisphere July insolation. July air temperature values vary between 12 and 14 °C in the late Eemian, while reconstructed temperatures remain within 12–13.5 °C during Stadial A. Our new chironomid‐based temperature reconstruction provides valuable corroboration and new quantification of temperature development from the mid‐Eemian to the early Brörup interstadial as well as for sections of the late Rissian from the alpine foreland of southern Germany.

Boreal tree species change as a climate mitigation strategy: impact on ecosystem C and N stocks and soil nutrient levels

AbstractTo increase the annual uptake of CO2 as well as the long‐term storage of carbon (C) in forests, the Norwegian government consider large‐scale replacements of native, deciduous forests with faster‐growing species like Norway spruce. To assess the effects of tree species change on ecosystem C and nitrogen (N) stocks and soil chemistry, we used a paired plot approach including stands of native downy birch and planted 45‐ to 60‐yr‐old Norway spruce. The birch stands were used as reference for the assessment of differences following the tree species change. We found significantly higher C and N stocks in living tree biomass in the spruce stands, whereas no significant differences were found for dead wood. The cover of understory species groups, and the C and N stocks of the aboveground understory vegetation were significantly higher in the birch stands. The tree species change did not affect the soil organic carbon (SOC) stock down to 1 m soil depth; however, the significantly higher stock in the forest floor of the spruce stands suggested a re‐distribution of SOC within the profile. There was a significant positive correlation between the SOC stock down to 30 cm soil depth and the total ecosystem C stock for the birch stands, and a negative correlation for the spruce stands. Significant effects of tree species change were found for C and N concentrations, C/N, exchangeable acidity, base saturation, and exchangeable Ca, K, Mg, Na, S, and Fe in the organic horizon or the upper mineral soil layer. The total ecosystem C stock ranged between 197 and 277 Mg/ha for the birch stands, and 297 and 387 Mg/ha for the spruce stands. The ecosystem C accumulation varied between 32 and 142 Mg/ha over the past 45–60 yr, whereas the net ecosystem C capture was considerably lower and potentially negative. Our results suggest that the potential to meet the governments' targets to increase C sequestration depend on the C debt incurred from the removed birch stands, the rotation length, and potentially also the susceptibility of the different stand types to future risk factors related to climate change.

Impacts of pre-fire conifer density and wildfire severity on ecosystem structure and function at the forest-tundra ecotone.

Wildfire frequency and extent is increasing throughout the boreal forest-tundra ecotone as climate warms. Understanding the impacts of wildfire throughout this ecotone is required to make predictions of the rate and magnitude of changes in boreal-tundra landcover, its future flammability, and associated feedbacks to the global carbon (C) cycle and climate. We studied 48 sites spanning a gradient from tundra to low-density spruce stands that were burned in an extensive 2013 wildfire on the north slope of the Alaska Range in Denali National Park and Preserve, central Alaska. We assessed wildfire severity and C emissions, and determined the impacts of severity on understory vegetation composition, conifer tree recruitment, and active layer thickness (ALT). We also assessed conifer seed rain and used a seeding experiment to determine factors controlling post-fire tree regeneration. We found that an average of 2.18 ± 1.13 Kg C m-2 was emitted from this fire, almost 95% of which came from burning of the organic soil. On average, burn depth of the organic soil was 10.6 ± 4.5 cm and both burn depth and total C combusted increased with pre-fire conifer density. Sites with higher pre-fire conifer density were also located at warmer and drier landscape positions and associated with increased ALT post-fire, greater changes in pre- and post-fire understory vegetation communities, and higher post-fire boreal tree recruitment. Our seed rain observations and seeding experiment indicate that the recruitment potential of conifer trees is limited by seed availability in this forest-tundra ecotone. We conclude that the expected climate-induced forest infilling (i.e. increased density) at the forest-tundra ecotone could increase fire severity, but this infilling is unlikely to occur without increases in the availability of viable seed.

Field experiment demonstrates the potential utility of satellite-derived reflectance indices for monitoring regeneration of boreal forest communities

Large-scale reforestation efforts require scalable and affordable monitoring technologies to track progress. Boreal forests disturbed by oil and gas development provide an opportunity to utilize ongoing reclamation efforts to assess new monitoring technologies with potential utility to other regions. In this study, we tested the application of reflectance indices derived from high spatial resolution satellite imagery to monitor early-stage reforestation. We installed a randomized nursery experiment in Alberta, Canada wherein the total stocking density and relative abundance of four common boreal tree species were manipulated. Satellite imagery from Maxar's WorldView-2 and WorldView-3 satellites were collected over the experimental plots annually from 2017 through 2020. From these reflectance data, we calculated one texture index, three greenness indices, and one index that integrated both texture and greenness. Finally, two sets of regression models were developed, one of which included only manipulated stem density while the other set included both the density and the mean height of stems in a given year. In the first year of the experiment, we detected no significant differences in any reflectance index across the range of manipulations. Thereafter, all indices showed significant differences, with the integrated texture-greenness index demonstrating the best relative performance among those tested. Our results illustrate the potential for satellite data to yield information regarding tree density and height during the critical early years of restoration. We suggest specific steps that future researchers could use extend this work to other regions and enable potential adoption of this monitoring approach at scale.

Two pollen-based methods of Eemian climate reconstruction employed in the study of the Żabieniec-Jagodne palaeolakes in central Poland

The aim of the present study was to estimate the range of climate variability in Central Poland during the Eemian Interglacial (MIS 5e). High resolution pollen records are available for all seven RPAZs typical of this interglacial in Central Europe. These pollen records were used to estimate total annual precipitation and mean annual temperature, as well as the mean temperatures of the warmest and the coldest months based on the plant indicator method and the modern analogue technique. The reconstructions indicate that the combination of these two methods provides a much clearer insight into the climate changes of the Eemian optimum. The results confirmed no drastic drop in temperature in the Middle Eemian, with high total precipitation in the hazel phase, and that this was followed by a decrease in temperature in the Late Eemian (fir-spruce and pine phases). A drop in precipitation occurred during the youngest part of the Carpinus phase, while coincided with a marked lowering of the water level in lakes and their transformation into peatbogs. The last part of the Eemian is characterized by decreasing temperature, particularly during the coldest month, rising water levels due to a combination of lower evaporation and higher air humidity, and a transition to glacial conditions. Principal Component Analysis found all investigated sites to follow the same pattern of changes. Mean winter temperature, annual temperature and precipitation were found to have a strong positive correlation with the occurrence of thermophilus temperate broadleaved trees (hazel, linden, and ash) typical for the mid-Eemian optimum. These three factors also have a negative correlation with the presence of cold tolerant boreal trees (birch and pine) and open land taxa characteristic of the beginning and end of the interglacial. Mean July temperature was found to be positively correlated with the occurrence of temperate broadleaved trees (oak, ash and elm) typical for early stages of the Eemian and negatively with that of coniferous trees (fir and spruce) characteristic for later stages of the interglacial.

Organic nitrogen enhances nitrogen nutrition and early growth of Pinus sylvestris seedlings.

Boreal trees are capable of taking up organic nitrogen (N) as effectively as inorganic N. Depending on the abundance of soil N forms, plants may adjust physiological and morphological traits to optimize N uptake. However, the link between these traits and N uptake in response to soil N sources is poorly understood. We examined Pinus sylvestris L. seedlings’ biomass growth and allocation, transpiration and N uptake in response to additions of organic N (the amino acid arginine) or inorganic N (ammonium nitrate). We also monitored in situ soil N fluxes in the pots following an addition of N, using a microdialysis system. Supplying organic N resulted in a stable soil N flux, whereas the inorganic N resulted in a sharp increase of nitrate flux followed by a rapid decline, demonstrating a fluctuating N supply and a risk for loss of nitrate from the growth medium. Seedlings supplied with organic N achieved a greater biomass with a higher N content, thus reaching a higher N recovery compared with those supplied inorganic N. In spite of a higher N concentration in organic N seedlings, root-to-shoot ratio and transpiration per unit leaf area were similar to those of inorganic N seedlings. We conclude that enhanced seedlings’ nutrition and growth under the organic N source may be attributed to a stable supply of N, owing to a strong retention rate in the soil medium.

Cold Acclimation Increases Freeze Tolerance in Acacia koa, a Tropical Tree Species Occurring over a Wide Elevational Gradient

Frost damage is among the major limitations to reforestation and forest restoration projects worldwide. Investigations of environmental and genetic effects on frost resistance have focused on boreal and temperate tree species rather than tropical trees. Koa (Acacia koa A. Gray) is a valuable tropical hardwood tree species endemic to the Hawaiian Islands, USA. Koa occurs across a wide elevational gradient, and newly planted trees are subject to winter frost at high elevations. We sought to determine whether different koa populations show variation in freeze hardiness as a cold-tolerance mechanism, and whether exposure to hardening conditions prior to frost exposure can modify koa cold-tolerance adaptation. Seeds from 13 populations of koa (Acacia koa A. Gray) were collected across an elevational range (603–2050 m) on the Island of Hawai’i. Four-month-old seedlings grown from the 13 population seed sources were divided into control (non-acclimated) and cold-acclimated treatments, maintained at 26 °C/22 °C (day/night) or exposed to gradually decreasing temperatures to 8 °C/4 °C (day/night), respectively. After six weeks, control and cold-acclimated seedlings from each population were tested for freeze tolerance by electrolyte leakage at five test temperatures ranging from 5 °C (control) to −20 °C. Treatment effects were mainly observed at the lowest test temperatures (−15 and −20 °C). A higher index of cold damage occurred in the non-acclimated seedlings for most of the populations. Several of our higher elevation populations showed greater cold tolerance than populations from lower elevations, particularly when cold-acclimated. Our results suggest that cold acclimation may increase frost hardiness in a tropical forest tree species, and that there is likely some adaptive variation in frost tolerance among populations from different elevations. Cold acclimation could be a useful tool to prepare koa seedlings to be planted in high-elevation sites prone to freezing winter temperatures.

Earlier and more variable spring phenology projected for eastern Canadian boreal and temperate forests with climate warming

Climate change affects timing and variability of spring phenology, as well as occurrence of spring frosts, and therefore influences forest structure, function, and management practices. In this study, we evaluated changes in budburst timing, sequential order of budburst, budburst temperatures, and frosts, from 1981–2010 baseline to different future climates (+2, +4, and +6 °C for moderate, high, and extremely high warming) for selected boreal and temperate species in eastern Canada. We used species-specific budburst models that are developed from large-scale forcing experiments.Budburst averaged 10–15 days earlier per 2 °C increase of temperatures, except for temperate yellow birch and eastern white pine that slowed down to 5–7 days in budburst advancement from +4 to +6 °C. Earlier budburst was associated with greater annual and interspecific variations in budburst timing, lower budburst temperatures, more frosts, and less annual variations in sequential order of budburst. Compared to temperate trees, boreal trees had greater budburst advances and annual variations, but less interspecific variations in timing and sequential order of budburst. Early flushing species had greater phenological changes and annual variations, as well as more frosts. Our results suggest that budburst advance that has occurred will continue with greater among years/species variability but is unlikely to converge between boreal and temperate regions in eastern Canada under anticipated levels of climate warming.

Large seasonal fluctuations in whole-tree carbohydrate reserves: is storage more dynamic in boreal ecosystems?

Carbon reserves are a critical source of energy and substrates that allow trees to cope with periods of minimal carbon gain and/or high carbon demands, conditions which are prevalent in high-latitude forests. However, we have a poor understanding of carbon reserve dynamics at the whole-tree level in mature boreal trees. We therefore sought to quantify the seasonal changes in whole-tree and organ-level carbon reserve pools in mature boreal Betula papyrifera. Non-structural carbohydrate (NSC; soluble sugars and starch) tissue concentrations were measured at key phenological stages throughout a calendar year in the roots, stem (inner bark and xylem), branches and leaves, and scaled up to estimate changes in organ and whole-tree NSC pool sizes. Fine root and stem growth were also measured to compare the timing of growth processes with changes in NSC pools. The whole-tree NSC pool increased from its spring minimum to its maximum at bud set, producing an average seasonal fluctuation of 0.96 kg per tree. This fluctuation represents a 72 % change in the whole-tree NSC pool, which greatly exceeds the relative change reported for more temperate conspecifics. At the organ level, branches accounted for roughly 48-60 % of the whole-tree NSC pool throughout the year, and their seasonal fluctuation was four to eight times greater than that observed in the stemwood, coarse roots and inner bark. Branches in boreal B. papyrifera were the largest and most dynamic storage pool, suggesting that storage changes at the branch level largely drive whole-tree storage dynamics in these trees. The greater whole-tree seasonal NSC fluctuation in boreal vs. temperate B. papyrifera may result from (1) higher soluble sugar concentration requirements in branches for frost protection, and/or (2) a larger reliance on reserves to fuel new leaf and shoot growth in the spring.

Capital Return Rate and Carbon Storage on Forest Estates of Three Boreal Tree Species

In this study, the capital return rate and carbon storage on forest estates with three boreal tree species are discussed. A growth model is applied, along with verified yield models of sawlogs and veneer logs. Using the normal forest principle, thinning schedules and rotation ages maximizing the estate-level capital return rate are clarified. Regeneration expenses are amortized at the end of any rotation. Capitalizations are greater and rotations longer than in recent studies. The capital return rate is a weak function of initial stem count and rotation age but differs by tree species. The initial stem count strongly contributes to biomass stored in trees. Omission of thinnings increases carbon storage very effectively but requires financial compensation. The most promising way of increasing the capital return rate is the reduction of regeneration expenses. Thinnings are triggered by stand volumes of at least 200 m3/ha. The average commercial trunk volume of trees removed in thinnings always exceeds 200 L. Risk aversion theory proposes short rotations and low stem count in seedling planting unless carbon storage compensation exists. Even a small carbon storage compensation justifies increased seedling counts and extended rotations.

Premature leaf discoloration of European deciduous trees is caused by drought and heat in late spring and cold spells in early fall

While leaf discoloration of many temperate and boreal tree species has been delayed in response to global warming, climatic stress such as severe drought can cause premature leaf discoloration. We investigated whether leaf discoloration is synchronized within and across tree species and under which climatic conditions premature leaf discoloration occurs (i.e. exceptionally early dates of leaf discoloration within the lower 5 % percentile). We analyzed 16’865 observations of leaf discoloration from nine deciduous tree species (beech, silver birch, horse chestnut, larch, large-leaved linden, rowan, small-leaved lime, sweet chestnut, sycamore) from a phenological network in Switzerland. We considered observations of leaf discoloration from 222 stations that were located between 200 and 1933 m and covered the period 1971–2018. Leaf discoloration of most species has been significantly delayed by 1.5 to 3.3 days per decade from 1996 to 2018. We detected a synchronous temporal pattern of leaf discoloration both within and across species. For most species, premature leaf discoloration occurred in years with very warm and dry spring-summer conditions or with very cool fall temperatures. Premature leaf discoloration was observed in 10-40 % of all stations during these years, whereas the long-term mean was 4-6 %. Growing degree days in May and June were significantly higher and the climatic water balance in June was significantly lower in years with premature leaf discoloration compared to years with regular leaf discoloration, especially for drought sensitive species such as beech. Similarly, minimum temperatures in September were consistently lower in years with premature leaf discoloration. Our study provides the first systematic and long-term assessment under which climatic conditions premature leaf discoloration occurs. We expect that the future increase in the frequency and severity of droughts in Europe will likely increase the risk of premature leaf discoloration.

Growth variations of Dahurian larch plantations across northeast China: Understanding the effects of temperature and precipitation

Climate change is affecting the growth and distribution of trees in the Chinese boreal forest. Such changes in China, the southern terminus of the extensive Eurasian boreal forests, reflect on the changes that could occur further north under a warming climate. Most studies have found that tree growth increases with increasing temperature and precipitation in boreal forests, but there is little observational evidence of the climate thresholds that might slow these growth rates at the more extreme temperatures which are predicted to occur under future global warming. Here, we examine growth responses of this dominant boreal tree species (Larix gmelinii) to climate based on the data from plantation sample plots across a broad region (40° 51′–52° 58′N, 118° 12′E−133° 42′E) in northeast China. From statistically significant fits to quadratic equations, temperature and precipitation are the important climatic factors determining tree growth in L. gmelinii plantations at two age classes (<10 year and 10–30 year-old stands). The maximum rates of tree height and diameter at breast height (DBH) were about 0.53 m/year and 0.46 cm/year at <10 year stands, and about 0.63 m/year and 0.60 cm/year at 10–30 year stands, respectively. For stands with the highest values of mean annual increment (MAI), the corresponding optimal mean annual temperature (MATopt) focused between 0.66 °C and 1.57 °C. The optimal mean annual precipitation (MAPopt) between 663 mm and 708 mm produced the maximal growth increments. With mean annual temperature of −2.4 °C and precipitation of 470 mm averaged over 1954–2005 in Chinese boreal forest region as baseline, we conservatively estimated that trees in Chinese boreal forest appear to have higher growth potentials with the maximum temperature increase of 3.6 °C and precipitation increase of 40%.

New insight to the role of microbes in the methane exchange in trees: evidence from metagenomic sequencing.

Methane (CH4 ) exchange in tree stems and canopies and the processes involved are among the least understood components of the global CH4 cycle. Recent studies have focused on quantifying tree stems as sources of CH4 and understanding abiotic CH4 emissions in plant canopies, with the role of microbial in situ CH4 formation receiving less attention. Moreover, despite initial reports revealing CH4 consumption, studies have not adequately evaluated the potential of microbial CH4 oxidation within trees. In this paper, we discuss the current level of understanding on these processes. Further, we demonstrate the potential of novel metagenomic tools in revealing the involvement of microbes in the CH4 exchange of plants, and particularly in boreal trees. We detected CH4 -producing methanogens and novel monooxygenases, potentially involved in CH4 consumption, in coniferous plants. In addition, our field flux measurements from Norway spruce (Picea abies) canopies demonstrate both net CH4 emissions and uptake, giving further evidence that both production and consumption are relevant to the net CH4 exchange. Our findings, together with the emerging diversity of novel CH4 -producing microbial groups, strongly suggest microbial analyses should be integrated in the studies aiming to reveal the processes and drivers behind plant CH4 exchange.

Tower‐Based Remote Sensing Reveals Mechanisms Behind a Two‐phased Spring Transition in a Mixed‐Species Boreal Forest

AbstractThe boreal forest is a major contributor to the global climate system, therefore, reducing uncertainties in how the forest will respond to a changing climate is critical. One source of uncertainty is the timing and drivers of the spring transition. Remote sensing can provide important information on this transition, but persistent foliage greenness, seasonal snow cover, and a high prevalence of mixed forest stands (both deciduous and evergreen species) complicate interpretation of these signals. We collected tower‐based remotely sensed data (reflectance‐based vegetation indices and Solar‐Induced Chlorophyll Fluorescence [SIF]), stem radius measurements, gross primary productivity, and environmental conditions in a boreal mixed forest stand. Evaluation of this data set shows a two‐phased spring transition. The first phase is the reactivation of photosynthesis and transpiration in evergreens, marked by an increase in relative SIF, and is triggered by thawed stems, warm air temperatures, and increased available soil moisture. The second phase is a reduction in bulk photoprotective pigments in evergreens, marked by an increase in the Chlorophyll‐Carotenoid Index. Deciduous leaf‐out occurs during this phase, marked by an increase in all remotely sensed metrics. The second phase is controlled by soil thaw. Our results demonstrate that remote sensing metrics can be used to detect specific physiological changes in boreal tree species during the spring transition. The two‐phased transition explains inconsistencies in remote sensing estimates of the timing and drivers of spring recovery. Our results imply that satellite‐based observations will improve by using a combination of vegetation indices and SIF, along with species distribution information.

Model analysis of temperature impact on the Norway spruce provenance specific bud burst and associated risk of frost damage

The annual growth cycle of boreal trees is synchronized with seasonal changes in photoperiod and temperature. A warmer climate can lead to an earlier bud burst and increased risk of frost damage caused by temperature backlashes. In this study we analysed site- and provenance specific responses to interannual variation in temperature, using data from 18 Swedish and East-European provenances of Norway spruce (Picea abies), grown in three different sites in southern Sweden. The temperature sum requirements for bud burst, estimated from the provenance trials, were correlated with the provenance specific place of origin, in terms of latitudinal and longitudinal gradients. Frost damage had a significant effect on tree height development. Earlier timing of bud burst was linked to a higher risk of frost damage, with one of the sites being more prone to spring frost than the other two. The estimated provenance specific temperature sum requirements for bud burst were used to parametrize a temperature sum model of bud burst timing, which was then used together with the ensemble of gridded climate model data (RCP8.5) to assess the climate change impact on bud burst and associated risk of frost damage. In this respect, the simulated timing of bud burst and occurrence of frost events for the periods 2021–2050 and 2071–2100 were compared with 1989–2018. In response to a warmer climate, the total number of frost events in southern Sweden will decrease, while the number of frost events after bud burst will increase due to earlier bud burst timing. The provenance specific assessments of frost risk under climate change can be used for a selection of seed sources in Swedish forestry. In terms of selecting suitable provenances, knowledge on local climate conditions is of importance, as the gridded climate data may differ from local temperature conditions. A comparison with temperature logger data from ten different sites indicated that the gridded temperature data were a good proxy for the daily mean temperatures, but the gridded daily minimum temperatures tended to underestimate the local risk of frost events, in particular at the measurements 0.5 m above ground representing the height of newly established seedlings.

Warming induces divergent stomatal dynamics in co-occurring boreal trees.

Climate warming will alter photosynthesis and respiration not only via direct temperature effects on leaf biochemistry but also by increasing atmospheric dryness, thereby reducing stomatal conductance and suppressing photosynthesis. Our knowledge on how climate warming affects these processes is mainly derived from seedlings grown under highly controlled conditions. However, little is known regarding temperature responses of trees growing under field settings. We exposed mature tamarack and black spruce trees growing in a peatland ecosystem to whole-ecosystem warming of up to +9°C above ambient air temperatures in an ongoing long-term experiment (SPRUCE: Spruce and Peatland Responses Under Changing Environments). Here, we report the responses of leaf gas exchange after the first two years of warming. We show that the two species exhibit divergent stomatal responses to warming and vapor pressure deficit. Warming of up to 9°C increased leaf N in both spruce and tamarack. However, higher leaf N in the warmer plots translate into higher photosynthesis in tamarack but not in spruce, with photosynthesis being more constrained by stomatal limitations in spruce than in tamarack under warm conditions. Surprisingly, dark respiration did not acclimate to warming in spruce, and thermal acclimation of respiration was only seen in tamarack once changes in leaf N were considered. Our results highlight how warming can lead to differing stomatal responses to warming in co-occurring species, with consequent effects on both vegetation carbon and water dynamics.

Assessing the relevant time frame for temperature acclimation of leaf dark respiration: A test with 10 boreal and temperate species.

Plants often adjust their leaf mitochondrial ("dark") respiration (Rd ) measured at a standardized temperature such as 20°C (R20 ) downward after experiencing warmer temperatures and upward after experiencing cooler temperatures. These responses may help leaves maintain advantageous photosynthetic capacity and/or be a response to recent photosynthate accumulation, and can occur within days after a change in thermal regime. It is not clear, however, how the sensitivity and magnitude of this response change over time, or which time period prior to a given measurement best predicts R20 . Nor is it known whether nighttime, daytime, or 24-hour temperatures should be most influential. To address these issues, we used data from 1620 Rd temperature response curves of 10 temperate and boreal tree species in a long-term field experiment in Minnesota, USA to assess how the observed nearly complete acclimation of R20 was related to past temperatures during periods of differing lengths. We hypothesized that R20 would be best related to prior midday temperatures associated with both photosynthetic biochemistry and peak carbon uptake rates that drive carbohydrate accumulation. Inconsistent with this hypothesis, prior night temperatures were the best predictors of R20 for all species. We had also hypothesized that recent (prior 3-10days) temperatures should best predict R20 because they likely have stronger residual impacts on leaf-level physiology than periods extending further back in time, whereas a prior 1- to 2-day period might be a span shorter than one to which photosynthetic capacity and Rd adjust. There was little to no support for this idea, as for angiosperms, long time windows (prior 30-60 nights) were the best predictors, while for gymnosperms both near-term (prior 3-8 nights for pines, prior 10-14 nights for spruce/fir) and longer-term periods (prior 45 nights) were the best predictors. The importance of nighttime temperatures, the relatively long "time-averaging" that best explained acclimation, and dual peaks of temporal acclimation responsiveness in some species were all results that were unanticipated.

Nutrient loads and ratios both explain the coexistence of dominant tree species in a boreal forest in Xinjiang, Northwest China

The resource ratio hypothesis (RRH) and the nutrient-load hypothesis (NLH) state that species coexistence is mediated by the ratios and loads (availability), respectively, of multiple limiting resources. However, few attempts to quantify these effects in mature forest ecosystems have been attempted to date. Data were collected from 300 quadrats (20 m × 20 m) spread across a 12-ha (400 m × 300 m) boreal forest dynamics plot in the Kanas Reserve of Xinjiang, Northwest China. Torus translation tests were used to analyze associations between dominant tree species and specific microhabitats. Linear mixed models were used to assess how soil nutrient availability and nutrient ratios affected the abundance and basal area of specific tree species, as well as how these effects varied across life stages. Two shade-tolerant species, Picea obovata and Pinus sibirica, exhibited opposite relationships with soil nitrogen (N) and phosphorus (P) content. Picea obovata was associated with microhabitats with high N:P ratios, while P. sibirica preferred microhabitats with low N:P ratios. The light-demanding Betula pendula was most abundant at low N and P sites. Another light-demanding species, Larix sibirica, did not show any significant habitat preferences. Furthermore, N:P ratios mainly affected species abundance, while N and P contents largely impacted basal area. Soil nutrient effects on species distributions weakened with tree age (from saplings to adults). Different microhabitats in the boreal forest had distinct soil resources. Both nutrient ratios and loads were correlated with the distribution of dominant boreal tree species, though these effects weakened as trees grew, especially for shade-tolerant species. Overall, niche partitioning due to resource variation may alleviate interspecific competition and contribute to species coexistence, and this study provides convincing proof of the importance of both nutrient ratios and loads on maintaining boreal forest diversity.