<p>Vulnerability curves to cavitation (VC) and their derived parameters, such as <em>P₅₀</em>, are increasingly used and reported to assess forest vulnerability to drought and predict forest responses to climate change. Forest practitioners and policy-makers are encouraged to rely on these parameters to support species selection based on their sensitivity to drought. However, in the majority of studies, we consider that the variability of VC parameters is not clearly reported nor considered, which can lead to counterproductive decisions. In this opinion paper, we demonstrate the importance of precisely reporting the variability around VC parameters and the sources of this variability (plant materials, methods, etc.). We also identify the information that should be provided when reporting mean values of VC parameters. To support our argument, we built VCs for three <em>Picea</em> species and <em>Pinus strobus</em>, using different methods, and compared the value of <em>P₅₀</em> determined in our experiments with values from a literature review.</p>

The future climate of northern temperate forests is projected to be drier and warmer by the end of this century. As a result, more drought-induced forest dieback events are anticipated in northeastern North America, and assessing the vulnerability of dominant tree species to drought is critical for understanding the future composition of these forests. In a greenhouse experiment, we exposed two-year-old seedlings of Picea glauca (Moench) Voss, Picea mariana (Mill.) B.S.P. and Pinus strobus L. to three future climate treatments for southern Quebec, Canada, and evaluated their mortality, growth, and foliage water status responses to soil water availability and atmospheric drought. Using a unique approach, climate treatments emulated droughts of different frequencies, durations, and intensities. Treatments closely simulated one growing season, with changes in air temperature and relative humidity every six hours and daily adjustment in the amount of water delivered to the seedlings. The three species experienced high mortality (75%) in all water-limited treatments compared to a control treatment that provided non-limiting soil moisture (0% mortality). The biomass of the seedlings that survived was 40% lower than that of control seedlings. Our results confirmed that the hydraulic safety margins, defined as the difference between seasonal minimum water potential and xylem water potential leading to 12, 50 and 88% of hydraulic conductivity loss, were good predictors of probability of tree mortality. Therefore, hydraulic safety margins are useful functional traits that can be used to compare the vulnerability of various species to drought and then provide crucial information to practitioners and policymakers to adjust forest management to climate change. We showed that three dominant conifer species of northern temperate forests were highly vulnerable to drought in future climates. Because drought is projected to be a significant threat to forests, understanding potentially adaptive physiological responses to drought, such as hydraulic safety margins of tree seedlings, is important for predicting the response of forest regeneration and composition in warmer and drier climates.

1. Considering intraspecific trait variability (ITV) in ecological studies has improved our understanding of species persistence and coexistence. These advances are based on the growing number of leaf ITV studies over local gradients, but logistical constraints have prevented a solid examination of ITV in root traits or at scales reflecting species’ geographic ranges.
2. We compared the magnitude of ITV in above? and below?ground plant organs across three spatial scales (biophysical region, locality and plot). We focused on six understorey species (four herbs and two shrubs) that occur both in disturbed and undisturbed habitats across boreal and temperate Canadian forests. We aimed to document ITV structure over broad ecological and geographical scales by asking: (a) What is the breadth of ITV across species range?scale? (b) What proportion of ITV is captured at different spatial scales, particularly when local scale disturbances are considered? and (c) Is the variance structure consistent between analogous leaf and root traits, and between morphological and chemical traits?
3. Following standardized methods, we sampled 818 populations across 79 forest plots simultaneously, including disturbed and undisturbed stands, spanning four biophysical regions (~5,200 km). Traits measured included specific leaf area (SLA), specific root length (SRL) and leaf and root nutrient concentrations (N, P, K, Mg, Ca). We used variance decomposition techniques to characterize ITV structure across scales.
4. Our results show that an important proportion of ITV occurred at the local scale when sampling included contrasting environmental conditions resulting from local disturbance. A certain proportion of the variability in both leaf and root traits remained unaccounted for by the three sampling scales included in the design (36% on average), with the largest amount for SRL (54%). Substantial differences in magnitude of ITV were found among the six species, and between analogous traits, suggesting that trait distribution was influenced by species strategy and reflects the extent of understorey environment heterogeneity.
5. Even for species with broad geographical distributions, a large proportion of within?species trait variability can be captured by sampling locally across ecological gradients. This has practical implications for sampling design and trait selection for both local studies and continental?scale modelling.

Sustainable management of boreal ecosystems involves the establishment of vigorous tree regeneration after harvest. However, two groups of understory plants influence regeneration success in eastern boreal Canada. Ericaceous shrubs are recognized to rapidly dominate susceptible boreal sites after harvest. Such dominance reduces recruitment and causes stagnant conifer growth, lasting decades on some sites. Additionally, peat accumulation due to Sphagnum growth after harvest forces the roots of regenerating conifers out of the relatively nutrient rich and warm mineral soil into the relatively nutrient poor and cool organic layer, with drastic effects on growth. Shifts from once productive black spruce forests to ericaceous heaths or paludified forests affect forest productivity and biodiversity. Under natural disturbance dynamics, fires severe enough to substantially reduce the organic layer thickness and affect ground cover species are required to establish a productive regeneration layer on such sites. We succinctly review how understory vegetation influences black spruce ecosystem dynamics in eastern boreal Canada, and present a multi-scale research model to understand, limit the loss and restore productive and diverse ecosystems in this region. Our model integrates knowledge of plant-level mechanisms in the development of silvicultural tools to sustain productivity. Fundamental knowledge is integrated at stand, landscape, regional and provincial levels to understand the distribution and dynamics of ericaceous shrubs and paludification processes and to support tactical and strategic forest management. The model can be adapted and applied to other natural resource management problems, in other biomes.

Abstract: Plant species distribution and plant community composition vary along environmental gradients. At the continental scale, climate plays a major role in determining plant distribution, while at the local and regional scales vegetation patterns are more strongly related to edaphic and topographic factors. The projected global warming and alteration of the precipitation regime will influence tree physiology and phenology, and is likely to promote northward migration of tree species. However the influence of soil characteristics on tree species migration is not as well understood. Considering the broad tolerance of most tree species to variations in soil factors, soils should not represent a major constraint for the northward shift of tree species. However, locally or regionally, soil properties may constrain species migration. Thus, while climate change has the potential to induce a northward migration of tree species, local or regional soil properties may hinder their migratory response. These antagonistic forces are likely to slow down potential tree migration in response to climate change. Because tree species respond individualistically to climate variables and soil properties, new tree communities are likely to emerge from climate change.

Résumé : La distribution des espèces et la composition des communautés végétales varient le long de gradients environnementaux. À l’échelle continentale, le climat joue un rôle primordial dans la distribution des espèces végétales, alors qu’aux échelles locale et régionale la distribution des espèces est surtout influencée par les variables édaphiques et la topographie. Le réchauffement climatique projeté et l’altération du régime des précipitations sont susceptibles d’influencer la physiologie et la phénologie des arbres, et d’entraîner leur migration vers le nord. Toutefois, l’influence des caractéristiques des sols sur le potentiel migratoire des arbres est peu connue. Puisque la plupart des espèces d’arbres possèdent une grande tolérance aux variations édaphiques, les sols ne devraient pas constituer un obstacle majeur à leur migration vers le nord. Toutefois, localement ou régionalement, les sols pourraient posséder des propriétés qui pourraient restreindre la migration des espèces. Alors que les changements climatiques ont le potentiel de provoquer la migration des espèces vers le nord, localement ou régionalement les propriétés des sols pourraient limiter leur capacité migratoire. Ces forces en opposition pourraient restreindre le potentiel migratoire des arbres en réponse aux changements climatiques. Parce que les espèces d’arbres répondent de manière individuelle aux variables climatiques et aux propriétés des sols, de nouvelles communautés d’arbres pourraient résulter des changements climatiques.

The development of a coherent system of criteria and indicators (C&I) requires collaboration and communication among scientists, government, the public, certifying organizations, and the forest industry. It also demands the integration of knowledge from many fields of study, which is foreign to the disciplinary nature of most forestry research. There needs to be greater effort to link groups of indicators and to favour those that are assimilative in nature. Modelling tools adapted to a multi-disciplinary approach and collaborative development will help to integrate knowledge from various fields and institutions. Specific challenges for implementation of C&I have been identified, including: leadership and vision in the evolution towards sustainable forest management (SFM); linking of grass-roots and higher level CI initiatives; streamlining and co-ordinating different certification initiatives and agencies; technology transfer; and collaboration among disciplines.