Phenotypic plasticity enables considerable acclimation to heat and drought in a cold-adapted boreal forest tree species

Increasing frequencies of severe heat waves and drought are expected to influence the composition and functioning of ecosystems globally. Our ability to predict and mitigate these impacts depends on our understanding of species- and age-specific responses to these stressors. To assess the adaptive capacity of balsam fir to climate change, a cold-adapted boreal tree species, we conducted a climate-controlled greenhouse experiment with four provenances originating from across the species biogeographic range, 12 temperature treatments ensuring a minimum of +11°C warming, and five drought treatment intensities. We found considerable acclimation to temperature and drought treatments across all provenances, with steady gains in biomass under temperatures well-beyond the “worst-case” (RCP 8.5) climate forcing scenario within the species natural range. Acclimation was supported by high phenotypic plasticity in root:shoot ratio (RSR) and photosynthesis, which were greatly increased with warming, but were not affected by drought. Our results suggest that regardless of the observed provenance variation, drought and heat are not limiting factors of the current-year balsam fir seedling growth, instead, these factors may be more impactful on later stages of regeneration or previously stressed individuals, thus highlighting the necessity of incorporating the factors of ontogeny and provenance origin in future research regarding plant and climate interactions

The changing culture of silviculture

Changing climates are altering the structural and functional components of forest ecosystems at an unprecedented rate. Simultaneously, we are seeing a diversification of public expectations on the broader sustainable use of forest resources beyond timber production. As a result, the science and art of silviculture needs to adapt to these changing realities. In this piece, we argue that silviculturists are gradually shifting from the application of empirically derived silvicultural scenarios to new sets of approaches, methods and practices, a process that calls for broadening our conception of silviculture as a scientific discipline. We propose a holistic view of silviculture revolving around three key themes: observe, anticipate and adapt. In observe, we present how recent advances in remote sensing now enable silviculturists to observe forest structural, compositional and functional attributes in near-real-time, which in turn facilitates the deployment of efficient, targeted silvicultural measures in practice that are adapted to rapidly changing constraints. In anticipate, we highlight the importance of developing state-of-the-art models designed to take into account the effects of changing environmental conditions on forest growth and dynamics. In adapt, we discuss the need to provide spatially explicit guidance for the implementation of adaptive silvicultural actions that are efficient, cost-effective and socially acceptable. We conclude by presenting key steps towards the development of new tools and practical knowledge that will ensure meeting societal demands in rapidly changing environmental conditions. We classify these actions into three main categories: reexamining existing silvicultural trials to identify key stand attributes associated with the resistance and resilience of forests to multiple stressors, developing technological workflows and infrastructures to allow for continuous forest inventory updating frameworks, and implementing bold, innovative silvicultural trials in consultation with the relevant communities where a range of adaptive silvicultural strategies are tested. In this holistic perspective, silviculture can be defined as the science of observing forest condition and anticipating its development to apply tending and regeneration treatments adapted to a multiplicity of desired outcomes in rapidly changing realities

A three-year increase in soil temperature and atmospheric N deposition has minor effects on the xylogenesis of mature balsam fir

Tree growth in most boreal forests is strongly regulated by temperature and nitrogen (N) availability. The expected increases in soil temperature and N deposition over the next decades have the potential to affect the phenology of tree growth and xylogenesis. To test for these changes on xylogenesis of balsam fir (Abies balsamea (L.) Mill), 12 mature trees were subjected to a combination of experimentally increased soil temperature (+4 °C) with an earlier snowmelt (2-3 weeks) and N deposition (3 × ambient rain N concentrations using NH4-NO3 in artificial precipitation) over a 3-year period. Increased soil temperature and atmospheric N deposition had no significant effect on the number of tracheids produced (38-51), tracheid diameter (27.2-29.0 μm) and cell wall thickness (2.5-3.1 μm). For the 3 years of treatment, xylogenesis was initiated at minimum and average daily air temperatures of 0.6 ± 0.5 and 6.5 ± 0.6 °C, respectively, with inter-annual differences of 17 days in the onset of xylogenesis. The earlier snowmelt induced by soil warming did not hasten resumption of xylogenesis, and the time dynamics of xylogenesis was not affected by higher N deposition. Our results suggest that soil temperature and the timing of snowmelt have no direct influence on the breaking of cambium dormancy in balsam fir. The short-term effects of increased soil temperature and N deposition on xylogenesis of mature balsam fir appear to be small compared with the effects of air temperature and are likely to be associated with a persistent N limitation

Beneficial effects of climate warming on boreal tree growth may be transitory

The productivity of boreal forests in Eastern North America is predicted to increase with warming under sufficient moisture supply. Here D’Orangeville et al. study seven tree species and predict that growth enhancements may be seen up to 2 °C warming, but would decline if temperatures exceed this

Data_Sheet_1_Phenotypic plasticity enables considerable acclimation to heat and drought in a cold-adapted boreal forest tree species.docx

Increasing frequencies of severe heat waves and drought are expected to influence the composition and functioning of ecosystems globally. Our ability to predict and mitigate these impacts depends on our understanding of species- and age-specific responses to these stressors. To assess the adaptive capacity of balsam fir to climate change, a cold-adapted boreal tree species, we conducted a climate-controlled greenhouse experiment with four provenances originating from across the species biogeographic range, 12 temperature treatments ensuring a minimum of +11°C warming, and five drought treatment intensities. We found considerable acclimation to temperature and drought treatments across all provenances, with steady gains in biomass under temperatures well-beyond the “worst-case” (RCP 8.5) climate forcing scenario within the species natural range. Acclimation was supported by high phenotypic plasticity in root:shoot ratio (RSR) and photosynthesis, which were greatly increased with warming, but were not affected by drought. Our results suggest that regardless of the observed provenance variation, drought and heat are not limiting factors of the current-year balsam fir seedling growth, instead, these factors may be more impactful on later stages of regeneration or previously stressed individuals, thus highlighting the necessity of incorporating the factors of ontogeny and provenance origin in future research regarding plant and climate interactions.</p

Summary of the selection process leading to the formation of valid black spruce chronologies (EPS ≥ 0.85, 1900–2012) from the broadest to the finest spatial scales with and without considerations of the soil physical environment type (+soil).

<p>Summary of the selection process leading to the formation of valid black spruce chronologies (EPS ≥ 0.85, 1900–2012) from the broadest to the finest spatial scales with and without considerations of the soil physical environment type (+soil).</p

Comparison of the ring-width index (rwi) from ten crossdated reference chronologies (grey) against neighbouring new chronologies (red) built at the district scale (median area: 203 km²) and landscape scale (median area: 2,259 km²) grouped by soil physical environment type.

<p>Only chronologies located within a 40-km radius (district scale) or 80-km radius (landscape scale) of the reference chronologies were used. Radius was determined as the distance between the centroid of the spatial unit and the location of the stand sampled for the reference chronology. Average Spearman correlations (ρ) and associated standard deviations are shown.</p

Map showing the location of tree-ring sample sites.

<p>Red triangles show the locations of the ten reference chronologies while green points show the locations where cores of black spruce trees were sampled in national forest inventories in Quebec, Canada.</p

Schematic classification of soil physical environments described in Quebec.

<p>Schematic classification of soil physical environments described in Quebec.</p

Characteristics of the reference crossdated black spruce chronologies used to validate the newly formed chronologies.

<p>Characteristics of the reference crossdated black spruce chronologies used to validate the newly formed chronologies.</p