With climate change, climatic optima are shifting poleward more rapidly than tree migration processes, resulting in a mismatch between species distributions and bioclimatic envelopes. Temperate hardwood tree species may take advantage of the release of climate constraints and forest management to migrate into the boreal forest. Here, we use the SORTIE-ND forest simulation model to determine the potential for the persistence of three temperate species (sugar maple, red maple and yellow birch) when introduced at seedling stage in typical balsam fir–paper birch (BF–PB) bioclimatic domain stands of eastern Canada, quantifying the consequences on the native species composition. SORTIE-ND is a spatially explicit, individual-based forest stand model that simulates tree growth, regeneration and mortality. We performed a novel parameterization of the SORTIE-ND tree growth equation allowing for the inclusion of climate modifiers on tree growth. After validating our model with data from permanent forest inventory plots, we modeled the dynamics of unharvested stands at different successional stages, as well as post-harvest stands, after the addition of sugar maple, red maple and yellow birch seedlings at different densities. Our results show that current BF–PB domain climate conditions do not limit growth and survival of temperate species in boreal stands. Of the temperate species introduced, sugar maple had the lowest ability to grow and survive by the end of the simulation. Species assemblages of host stands were impacted by the presence of temperate species when the addition of seedlings was above 5000 temperate seedlings per hectare at the beginning of the simulation. For stands that were recently clear cut, temperate seedlings were unable to grow due to intense competition from aspen regeneration. Our results suggest that both current climate and competitive interactions between temperate species and boreal species should not impede the ability of temperate species to grow and survive in the BF–PB domain.

We investigated how the surrounding environment influences the growth of dominant trees and their responses to temperature and insect epidemics in boreal forests of eastern Canada. We focused on 82 black spruce and jack pine focal trees in stands spanning a double gradient of species diversity and soil texture within a 36 km2 area of western Québec. For these trees, we compared their diameter at breast height, growth rates, temperature-growth relations, and growth during insect defoliator epidemics. We used linear models to study how surrounding tree attributes and soil properties affected the growth of focal trees. Models showed that tree growth responses and responses to temperature and insect epidemics were generally negative with higher intraspecific competition and positive with greater tree species diversity. Growth of both species benefitted from lower soil sand content. Our research offers novel insights on the potential role of the surrounding environment, notably tree competition and species diversity, in mitigating the vulnerability of eastern Canada’s boreal trees to anthropogenic climate change and insect epidemics.

Although global and Northern Hemisphere temperature reconstructions are coherent with climate model simulations over the last millennium, reconstructed temperatures tend to diverge from simulations at smaller spatial scales. Yet, it remains unclear to what extent these regional peculiarities reflect region-specific internal climate variability or inadequate proxy coverage and quality. Here, we present a high-quality, millennial-long summer temperature reconstruction for northeastern North America, based on maximum latewood density, the most temperature-sensitive tree-ring proxy. Our reconstruction shows that a large majority (31 out of 44) of the coldest extremes can be attributed to explosive volcanic eruptions, with more persistent cooling following large tropical than extratropical events. These forced climate variations synchronize regional summer temperatures with hemispheric reconstructions and simulations at the multidecadal time scale. Our study highlights that tropical volcanism is the major driver of multidecadal temperature variations across spatial scales.

Sapwood characteristics, such as sapwood area as well as thermal and hydraulic conductivity, are linked to species-specific hydraulic function and resource allocation to water transport tissues (xylem). These characteristics are often unknown and thus a major source of uncertainty in sap flow data processing and transpiration estimates because bulk rather than species-specific values are usually applied. Here, we analyzed the sapwood characteristics of fifteen common tree species in eastern North America from different taxonomic (i.e., angiosperms and gymnosperms) and xylem porosity groups (i.e., tracheid-bearing, diffuse- or ring-porous species) and we assessed how uncertainties in sapwood characteristics involved in sap flow calculations are propagated in tree water use estimates. We quantified their sapwood area changes with stem diameter (allometric scaling) and thermal conductivity. We combined these measurements with species-specific values of wood density and hydraulic conductivity found in the literature and assessed the role of wood anatomy in orchestrating their covariation. Using an example sap flow dataset from tree species with different xylem porosity, we assessed the sensitivity of tree water use estimates to sapwood characteristics and their interactions. Angiosperms (ring- and diffuse-porous species), with specialized vessels for water transport, showed a steeper relationship (scaling) between tree stem diameter and sapwood area in comparison to gymnosperms (tracheid-bearing species). Gymnosperms (angiosperms) were characterized by lower (higher) wood density and higher (lower) sapwood moisture content, resulting in non-significant differences in sapwood thermal conductivity between taxonomic and xylem porosity groups. Clustering of species sapwood characteristics based on taxonomic or xylem porosity groups and constraining these parameters could facilitate more accurate sap flow calculations and tree water use estimates. When combined with an increasing number of sap flow observations, these findings should improve tree- and landscape-level transpiration estimates, leading to more robust partitioning of terrestrial water fluxes.

Monitoring of forest response to gradual environmental changes or abrupt disturbances provides insights into how forested ecosystems operate and allows for quantification of forest health. In this chapter, we provide an overview of Smartforests Canada, a national-scale research network consisting of regional investigators who support a wealth of existing and new monitoring sites. The objectives of Smartforests are threefold: (1) establish and coordinate a network of high-precision monitoring plots across a 4400 km gradient of environmental and forest conditions, (2) synthesize the collected multivariate observations to examine the effects of global changes on complex above- and belowground forest dynamics and resilience, and (3) analyze the collected data to guide the development of the next-generation forest growth models and inform policy-makers on best forest management and adaptation strategies. We present the methodological framework implemented in Smartforests to fulfill the aforementioned objectives. We then use an example from a temperate hardwood Smartforests site in Quebec to illustrate our approach for climate-smart forestry. We conclude by discussing how information from the Smartforests network can be integrated with existing data streams, from within Canada and abroad, guiding forest management and the development of climate change adaptation strategies.

In boreal ecosystems, phenological events display seasonal patterns. These patterns allow for the development of tissues during the short time window available for growth in cold climates. Primary and secondary growth, two expensive processes for plants, are supposedly modulated in time to optimize allocation of carbon to bud and woody tissues. We aimed to assess the phenology of primary and secondary meristems, testing their relationship over the closed black spruce stands of the commercially exploited forest region in Quebec, Canada.

Location

Quebec, Canada.

Time period

2002–2016.

Major taxa studied

Gymnospermae.

Methods

We combined weekly scaled field observations with Moderate Resolution Imaging Spectroradiometer (MODIS) time series of the normalized difference vegetation index (NDVI) to extract timings of photosynthesis and meristem growth in five black spruce [Picea mariana (Mill.) B.S.P.] stands located along a latitudinal gradient and to assess their relationship. We then tested empirical models based on geographical position and seasonal temperatures to predict wood phenology (i.e., the onset and ending of earlywood and latewood formation), and compared its spatial patterns with existing predictions of bud phenology for the same study area.

Results

Photosynthesis started at the beginning of May, 3 weeks before bud reactivation and the onset of wood growth. Latewood formation started in mid-July, after shoot elongation was completed. For wood phenology models, the residual standard error ranged from 1 week to 12 days. Growth dynamics spatialized across the boreal forest of Quebec varied with the transition between the subarctic and humid continental climate.

Main conclusions

Shoot elongation and latewood formation were temporally separated, providing evidence of a trade-off in structural carbon allocation between primary and secondary growth in trees. Spatial patterns of wood phenology predicted for the black spruce polygons are consistent with spatial patterns of bud phenology, demonstrating synchronized temporal dynamics of meristems at the regional scale.

Tree-ring chronologies underpin the majority of annually-resolved reconstructions of Common Era climate. However, they are derived using different datasets and techniques, the ramifications of which have hitherto been little explored. Here, we report the results of a double-blind experiment that yielded 15 Northern Hemisphere summer temperature reconstructions from a common network of regional tree-ring width datasets. Taken together as an ensemble, the Common Era reconstruction mean correlates with instrumental temperatures from 1794–2016 CE at 0.79 (p < 0.001), reveals summer cooling in the years following large volcanic eruptions, and exhibits strong warming since the 1980s. Differing in their mean, variance, amplitude, sensitivity, and persistence, the ensemble members demonstrate the influence of subjectivity in the reconstruction process. We therefore recommend the routine use of ensemble reconstruction approaches to provide a more consensual picture of past climate variability.

Tree-rings are one of the most commonly used proxies for reconstructing past climates at annual resolution. The climate information is generally deduced from tree-rings using statistical relationships, but the assumed linearity and stationarity may be inadequate. Process-based models allow for non-stationarity and non-linearity; however, many challenges are associated with their application for global scale reconstructions. In this study, we aim to test the feasibility of using the mechanistic model MAIDEN at the global scale for paleoclimate reconstructions based on data assimilation by applying it to the PAGES2k tree-ring width database. We also compare its performance with the simpler model VS-Lite, often used in global applications. Both models are skillful in terms of calibration and verification correlations for a similar number of sites (63 and 64 for VS-Lite and MAIDEN, respectively). VS-Lite tends to perform better for sites where the climate signal in tree-rings is strong and clear. By contrast, MAIDEN’s performance is likely mostly limited by the lack of data (for example, daily Gross Primary Production data or phenological timings) needed to accurately calibrate the model. However, when the calibration is robust, both models reproduce well the observed link between climate and tree-growth. In general, VS-Lite tends to overestimate the climate signal in tree-rings compared to MAIDEN, which better reproduces the magnitude of the climate signal on average. Our results show that both models are complementary and can be applied at the global scale to reconstruct past climates using an adequate protocol designed to exploit existing tree-ring data.

We investigated whether stand species mixture can attenuate the vulnerability of eastern Canada’s boreal forests to climate change and insect epidemics. For this, we focused on two dominant boreal species, black spruce [Picea mariana (Mill.) BSP] and trembling aspen (Populus tremuloides Michx.), in stands dominated by black spruce or trembling aspen (“pure stands”), and mixed stands (M) composed of both species within a 36 km2 study area in the Nord-du-Québec region. For each species in each stand composition type, we tested climate-growth relations and assessed the impacts on growth by recorded insect epidemics of a black spruce defoliator, the spruce budworm (SBW) [Choristoneura fumiferana (Clem.)], and a trembling aspen defoliator, the forest tent caterpillar (FTC; Malacosoma disstria Hübn.). We implemented linear models in a Bayesian framework to explain baseline and long-term trends in tree growth for each species according to stand composition type and to differentiate the influences of climate and insect epidemics on tree growth. Overall, we found climate vulnerability was lower for black spruce in mixed stands than in pure stands, while trembling aspen was less sensitive to climate than spruce, and aspen did not present differences in responses based on stand mixture. We did not find any reduction of vulnerability for mixed stands to insect epidemics in the host species, but the non-host species in mixed stands could respond positively to epidemics affecting the host species, thus contributing to stabilize ecosystem-scale growth over time. Our findings partially support boreal forest management strategies including stand species mixture to foster forests that are resilient to climate change and insect epidemics.

Tree-ring archives are one of the main sources of information to reconstruct climate variations over the last millennium with annual resolution. The links between tree-ring proxies and climate have usually been estimated using statistical approaches, assuming linear and stationary relationships. Both assumptions may be inadequate, but this issue can be overcome by ecophysiological modelling based on mechanistic understanding. In this respect, the model MAIDEN (Modeling and Analysis In DENdroecology) simulating tree-ring growth from daily temperature and precipitation, considering carbon assimilation and allocation in forest stands, may constitute a valuable tool. However, the lack of local meteorological data and the limited characterization of tree species traits can complicate the calibration and validation of such a complex model, which may hamper palaeoclimate applications. The goal of this study is to test the applicability of the MAIDEN model in a palaeoclimate context using as a test case tree-ring observations covering the 20th century from 21 Eastern Canadian taiga sites and 3 European sites. More specifically, we investigate the model sensitivity to parameter calibration and to the quality of climatic inputs, and we evaluate the model performance using a validation procedure. We also examine the added value of using MAIDEN in palaeoclimate applications compared to a simpler tree-growth model, i.e. VS-Lite. A Bayesian calibration of the most sensitive model parameters provides good results at most of the selected sites with high correlations between simulated and observed tree growth. Although MAIDEN is found to be sensitive to the quality of the climatic inputs, simple bias correction and downscaling techniques of these data improve significantly the performance of the model. The split-sample validation of MAIDEN gives encouraging results but requires long tree ring and meteorological series to give robust results. We also highlight a risk of overfitting in the calibration of model parameters that increases with short series. Finally, MAIDEN has shown higher calibration and validation correlations in most cases compared to VS-Lite. Nevertheless, this latter model turns out to be more stable over calibration and validation periods. Our results provide a protocol for the application of MAIDEN to potentially any site with tree-ring width data in the extratropical region.

Climate reconstructions for the Common Era are compromised by the paucity of annually-resolved and absolutely-dated proxy records prior to medieval times. Where reconstructions are based on combinations of different climate archive types (of varying spatiotemporal resolution, dating uncertainty, record length and predictive skill), it is challenging to estimate past amplitude ranges, disentangle the relative roles of natural and anthropogenic forcing, or probe deeper interrelationships between climate variability and human history. Here, we compile and analyse updated versions of all the existing summer temperature sensitive tree-ring width chronologies from the Northern Hemisphere that span the entire Common Era. We apply a novel ensemble approach to reconstruct extra-tropical summer temperatures from 1 to 2010 CE, and calculate uncertainties at continental to hemispheric scales. Peak warming in the 280s, 990s and 1020s, when volcanic forcing was low, was comparable to modern conditions until 2010 CE. The lowest June–August temperature anomaly in 536 not only marks the beginning of the coldest decade, but also defines the onset of the Late Antique Little Ice Age (LALIA). While prolonged warmth during Roman and medieval times roughly coincides with the tendency towards societal prosperity across much of the North Atlantic/European sector and East Asia, major episodes of volcanically-forced summer cooling often presaged widespread famines, plague outbreaks and political upheavals. Our study reveals a larger amplitude of spatially synchronized summer temperature variation during the first millennium of the Common Era than previously recognised.

The most accurate understanding of forest functioning during drought is crucial to improve the forecast of future forest productivity. Here we investigate the ecophysiological responses (i.e. primary production, evapotranspiration and water use efficiency) of European beech to drought events with the ecosystem model MuSICA, using as benchmark the observed fluxes at the experimental forest Hesse (France). We show that MuSICA is able to realistically simulate observed drought-induced limitations. Subsequently we use simulation experiments to provide: (1) a quantification of the reduction of ecosystem fluxes during the 2003 drought, (2) a partitioning of heat stress and water limitations during droughts, (3) an analysis of the impact of specific drought trajectories, and (4) an evaluation of the potential impact of projected climate change on the studied forest and (5) over the beech distributional range. Our results show that the 2003 drought resulted in a 17% reduction of annual gross primary production and in a 21% reduction of evapotranspiration at Hesse. The studied forest ecosystem is mostly sensitive to negative precipitation anomalies (82% of the reduced forest productivity in 2003) and almost insensitive to heat stress due to high temperatures (16%). Moreover, we show that the ecosystem fluxes are limited more by fast drought onsets in the early growing season (June–July) than by onsets later in the season. Deciphering the impact of future climate change on beech productivity is complicated by large uncertainties in projected future precipitation and in the severity of extreme dry years. Drastic reduction of ecosystem fluxes is only predicted with climate projections that show marked reductions in precipitation. However, increased CO2 fertilization in the future will counterbalance negative drought impacts. This modelling-based study improves our understanding of the functioning of an emblematic European tree species during extreme events and informs on potential future forest responses to projected climate change.