Intra-annual tracheid production in balsam fir stems and the effect of meteorological variables

Tracheid production of balsam fir in the Québec boreal forest (Canada) was studied by repeated cell analysis to investigate the influence of meteorological variables during the growing seasons 1998 to 2000. Wood micro-cores were extracted on a weekly basis throughout the growing season and sections were prepared in order to count the total number of cells produced. From the weekly cell number obtained, the rate of tracheid production was calculated and correlated with meteorological variables. The average total number of cells produced per year was reasonably uniform, increasing only from 36.6 in 1998, to 41.1 in 2000. However, different cell production rates were noted during the growing season. Regression analysis revealed that the cell production rate was largely dependent on minimum air and soil temperature during most of the cell production period. Mean and maximum temperature had less influence on cell production. Moreover, the influence of temperature was higher during earlywood production mainly from the end of May to mid-July. Lagging the weather data by 1-5 days decreased the relationship between temperature and cell production, showing the high correspondence with the same interval where cell production was measured. These results suggest a fast response of the cambium to temperature variation during tree-ring formation

Chronologie 14C et développement des combes à neige du lac à l’Eau Claire, Québec nordique

Deux types de combes à neige ont été répertoriés au lac à l'Eau Claire: 1) des combes à neige sur substrat minéral formées après le passage de feux dans les grandes îles ; 2) des combes à neige sur substrat tourbeux, localisées dans les petites îles envahies par des tourbières ombrotrophes dont l'existence est redevable au mésoclimat créé par la grande masse d'eau du lac. La chronologie du développement des combes à neige construite à l'aide des datations 14C de charbons de bois, ainsi que de morceaux de bois conservés dans la tourbe, diffère fortement entre les substrats minéraux et tourbeux. Les combes à neige sur substrat minéral sont apparues après des feux survenus au cours des deux derniers millénaires, et tout particulièrement après 800 BP. Elles se sont formées plus récemment que celles des autres régions étudiées dans le Nord québécois. Au cours du XXe siècle, elles ont été colonisées par l'épinette noire, surtout au cours des décennies 1950 et 1960. Les combes à neige sur substrat tourbeux se sont développées de façon continue depuis 3000 BP, mais surtout au cours du dernier millénaire. À l'instar des combes à neige précédentes, leur formation est étroitement liée au refroidissement néoglaciaire, mais aussi au développement graduel d'un dénivelé topographique, créé par une croissance accélérée de la partie est des tourbières, qui a favorisé une grande accumulation de neige et un déneigement tardif empêchant l'établissement des épinettes noires.Two different types of snow patches were found in the Lac à l'Eau Claire area: 1) post-fire snow patches developed on mineral soil on large islands: 2) snow patches developed on peat in small islands where ombrotrophic peatlands expanded because of the humid lake mesoclimate. Timing in the development of snow patches, using 14C dating of charcoal in mineral soil and wood remains in peat sections, differed between these two habitats. Snow patches on mineral soil developed during the last two millenia, particularly after 800 BP, after site deforestation because of a lack in conifer post-fire regeneration. These snow patches were formed more recently than those already reported in northern Québec. During the XXth century, they were colonized by black spruce, mostly during the 1950's and 1960s. Snow patches on peat developed continuously since 3000 BP, and particularly during the last thousand ,4C-years. Although their formation was closely associated with Neoglacial cooling, the gradual development of a topographic depression, induced by accelerated growth of the eastern part of peatlands, was instrumental in favoring deep-snow accumulation and late-snow melting inimical to spruce establishment.Zwei Typen von Schneeschluchten wurden am Lac à l'Eau Claire wahrgenommen: 1) Schneeschluchten auf mineralischem Substrat, welche sich nach den Branden auf den gro(ien lnseln gebildet haben: 2) Schneeschluchten auf Torf-Substrat, welche sich auf den kleinen lnseln befinden, auf denen sich ombrotrophe Torfmoore ausgedehnt haben, die dem durch die gro|ie Wassermasse des Sees geschaffenen Mesoklima ihre Existenz verdanken. Die Chronologie der Entwicklung der Schneeschluchten, die mit Hilfe der 14C-Datierungen von Holzkohlen sowie mit im Torf konservierten Holzstucken erstellt wurde, ist sehr unterschiedlich zwischen den mineralischen Substraten und den Torfsubstraten. Die Schneeschluchten auf mineralischem Substrat haben sich nach Branden, die im Laufe der zwei letzten Jahrtausende stattgefunden haben, gebildet, insbesondere nach 800 v.u.Z. Sie haben sich in jungerer Zeit gebildet als die Schneeschluchten der anderen, im Norden Québecs untersuchten Gebiete. Im Laufe des 20. Jahrhunderts sind sie von Picea mariana besiedelt worden, vor allem im Lauf der fiinfziger und sechziger Jahre. Die Schneeschluchten auf Torfsubstrat haben sich konstant seit 3000 BP entwickelt, aber vor allem im Laufe des Letzten Jahrhunderts. So wie die vorherigen Schneeschluchten hàngt ihre Bildung eng mit der glazialen Abkûhlung zusammen, auperdem aber auch mit der allmâhlichen Entwicklung einer topographischen Senkung, welche durch ein beschleunigtes Anwachsen des Osteils der Torfmoore geschaffen wurde und eine grope Schneeanhaufung und spate Schneeschmelze begunstigte und so die Ansiedlung von Picea mariana verhinderte

Cellular phenology of annual ring formation of Abies balsamea in the Quebec boreal forest (Canada)

Cell formation in growth rings of balsam fir (Abies balsamea (L.) Mill) in the boreal forest was studied to describe the timing of ring formation and the development patterns of earlywood and latewood. Wood micro-cores were extracted during the growing season from 1998 to 2000. The micro-cores were stained with cresyl fast violet to facilitate counting the number of cells in the radial enlargement, wall thickening phases, and mature cell phases. The periods required to complete these various phases were then estimated. Variations in the beginning of the growing season (May 7 - June 7), the earlywood-latewood transition (July 2 - July 19), and the end of the growing season (August 20 - September 20) were observed. Short cell enlargement durations of less than a week for earlywood and 5-10 days for latewood were observed. Time required for cell wall thickening was about 20 days for earlywood and longer than 10-15 days for latewood. A certain flexibility was observed in the ring formation patterns and in the cell development rate, providing an advantage in the boreal forest where optimal growth conditions change from year to year. These findings on the spatial and temporal patterns of ring development may be useful for understanding tree relationships with climate or other environmental parameters. La formation des cellules dans les cernes annuels du sapin baumier (Abies balsamea (L.) Mill.) en forêt boréale a été étudiée pour décrire la chronologie de la formation des cernes et le développement du bois initial et final. Des micro-carottes ont été extraites pendant les saisons 1998 à 2000. Des sections ont été colorées à l'aide du « cresyl fast violet » pour faciliter le dénombrement des cellules dans les phases d'accroissement radial, de formation des parois secondaires et les cellules matures. Le temps requis pour que les cellules pour complètent ces phases a été estimé. Des variations dans le début de la croissance (7 mai au 7 juin), dans la transition du bois initial au bois final (2 juillet au 19 juillet) et dans la fin de la croissance (20 août au 20 septembre) ont été observées. De courtes périodes d'accrois sement, de moins d'une semaine pour le bois initial et de 5 à 10 jours pour le bois final ont été observées. Le temps nécessaire à l'épais sissement des parois secondaires était d'environ 20 jours pour le bois initial et plus de 30 à 35 jours pour le bois final respectivement. Les résultats mettent en évidence une certaine flexibilité dans le développement des cernes, ce qui procure un avantage en forêt boréale où les conditions optimales de croissance changent d'années en années. Ces résultats pourront être utiles pour mieux comprendre la relation entre les arbres et le climat ou d'autres paramètres environnementaux

Multi-scale influence of snowmelt on xylogenesis of black spruce

Snowmelt is considered to affect growth of the boreal forest. So, we tested the hypothesis that late snowmelts delay the onset of xylogenesis and reduce xylem production in trees. Timings of xylem formation were compared to the dates of complete snowmelt combining a 7-year monitoring of cambial activity with meteorological records in four plots of Picea mariana in Quebec, Canada. The spatial and temporal variability in snowfall was analyzed separately, so taking into account both the long- and short-term effects. Snowfall occurred from October to May, with a snow cover lasting 173-199 days. Overall, xylogenesis lasted 99-117 days, with onsets ranging from late May to mid-June. The highest cell productions were observed in the warmest site, where the longest periods of growth were observed. Although at long-term the effects of snowmelt were significant for both onset and duration of xylogenesis and cell production, at short-term only the relationship between the onset of xylogenesis and the date of complete snowmelt was significant. The initial hypothesis could be confirmed only partially. The different responses to the long- and short-term analyses demonstrate the multi-scale influence of snowfall on tree growth and the determinant role of nutrient cycling in the productivity of boreal ecosystems

Causes and correlations in cambium phenology: towards an integrated framework of xylogenesis

Although habitually considered as a whole, xylogenesis is a complex process of division and maturation of a pool of cells where the relationship between the phenological phases generating such a growth pattern remains essentially unknown. This study investigated the causal relationships in cambium phenology of black spruce [Picea mariana (Mill.) BSP] monitored for 8 years on four sites of the boreal forest of Quebec, Canada. The dependency links connecting the timing of xylem cell differentiation and cell production were defined and the resulting causal model was analysed with d-sep tests and generalized mixed models with repeated measurements, and tested with Fisher’s C statistics to determine whether and how causality propagates through the measured variables. The higher correlations were observed between the dates of emergence of the first developing cells and between the ending of the differentiation phases, while the number of cells was significantly correlated with all phenological phases. The model with eight dependency links was statistically valid for explaining the causes and correlations between the dynamics of cambium phenology. Causal modelling suggested that the phenological phases involved in xylogenesis are closely interconnected by complex relationships of cause and effect, with the onset of cell differentiation being the main factor directly or indirectly triggering all successive phases of xylem maturation

Boreal old-growth forest structural diversity challenges aerial photographic survey accuracy

The erosion of old-growth forests in boreal managed landscapes is a major issue currently faced by forest managers; however, resolving this problem requires accurate surveys. The intention of our study was to determine if historic operational aerial forest surveys accurately identified boreal old-growth forests in Quebec, Canada. We first compared stand successional stages (even-aged vs. old-growth) in two aerial surveys performed in 1968 (preindustrial aerial survey) and 2007 (modern aerial survey) on the same 2200 km2 territory. Second, we evaluated the accuracy of the modern aerial survey by comparing its results with those of 74 field plots sampled in the study territory between 2014 and 2016. The two aerial surveys differed significantly; 80.8% of the undisturbed stands that were identified as “old-growth” in the preindustrial survey were classified as “even-aged” in the modern survey, and 60% of the stands identified as “old-growth” by field sampling were also erroneously identified as “even-aged” by the modern aerial survey. The scarcity of obvious old-growth attributes in boreal old-growth forests, as well as poorly adapted modern aerial survey criteria (i.e., criteria requiring high vertical stratification and significant changes in tree species composition along forest succession), were the main factors explaining these errors. It is therefore likely that most of Quebec’s boreal old-growth forests are currently not recognized as such in forest inventories, challenging the efficacy of sustainable forest management policies. L’érosion des superficies des vieilles forêts boréales est actuellement un enjeux majeurs pour les gestionnaires forestier. Répondre efficacemment à cette problématique demande néanmoins l’accès à des données d’inventaires fiables. Ainsi, l’objectif de cette étude était de déterminer si les inventaires forestiers aériens identifient correctement les vieilles forêts dans les paysages boréaux du Québec, Canada. Nous avons comparé les stades de succession (forêt équienne ou vieille forêt) de deux inventaires aériens réalisés en 1968 (inventaire aérien préindustriel) et en 2007 (inventaire aérien moderne) sur un territoire de 2200 km2. Nous avons aussi comparé les résultats de l’inventaire aérien moderne avec ceux obtenus à partir de 74 placettes de terrain échantillonnés entre 2014 et 2016. Les deux inventaires aériens étaient très incohérents : 80.8% des peuplements non-perturbés identifiés comme « vieilles forêts » par l’inventaire préindustriel étaient classés comme « équiens » par l’inventaire moderne. 60% des placettes de terrain identifiées comme vieilles forêts étaient aussi classées « équiens » par l’inventaire aérien moderne. Le manque d’attributs de vieilles forêts évidents ainsi que l’utilisation de critères inadaptés (c’est-à-dire nécessitant une forte complexité verticale et d’importants changements de composition en espèces arborescentes durant la succession forestière) étaient les principaux éléments expliquant ces erreurs. Il est ainsi possible que la majorité des vieilles forêts boréales du Québec ne soient pas identifiés comme telles, limitant l’efficacité des stratégies de gestion durable

High-resolution analysis of stem radius variations in black spruce [Picea mariana (Mill.) BSP] subjected to rain exclusion for three summers

Future climate warming is expected to produce more severe and frequent periods of drought with consequent water stresses for boreal species. In this paper, we present a high-resolution analysis of stem radius variations in black spruce under rain exclusion. All summer long rain exclusions were applied for three consecutive summers to mature trees on four sites along a latitudinal gradient. The stem radius variations of control and treated trees were monitored year-round at an hourly resolution with automatic point dendrometers. The seasonal patterns of shrinking and swelling were analyzed using a sequential analysis technique and the daily patterns of contraction and expansion were extracted. Overall, the treated trees followed their diurnal cycles of contraction and expansion during the rain exclusions and no significant cumulative difference in stem expansion between control and treated trees was observed over the 3 years. In the third year trees subjected to rain exclusion showed larger stem contractions in summer on three out of four sites and larger winter contractions were observed on the northern sites. This study shows that repeated summer rain exclusion does not necessarily lead to a direct evident stress reaction, showing the resilience of the boreal forest. Key message: A rain exclusion repeated for 3 years resulted in larger summer stem contractions in three of the sites in the third year of the experiment and in larger winter contractions in the northern sites. However, there was no pronounced stress reaction in the stem radius variations of mature black spruce since total stem expansion was not reduced

Fine roots and ectomycorrhizal colonization in black spruce subjected to reductions in soil moisture

Growth and survival of boreal tree species depends on fungal symbionts, namely mycorrhizas. Due to their rapid turnover, fine roots and their symbionts can be prompt and sensitive indicators of the effects of environmental changes on forest ecosystems. We investigated how a reduction in soil moisture influences fine roots and ectomycorrhizas (ECM) of black spruce [Picea mariana (Mill.) BSP] after three years of rain exclusion in four mature stands of the boreal forest of Quebec, Canada. Rain was excluded with transparent plastic under-canopy roofs installed during the growing seasons 2010-2012. Soil moisture was strongly affected by the treatment, with water content being reduced from 40 to 21% on average. As expected, the experimental trees showed a decrease in the proportion of vital root tips and ECM with respect to controls. The proportion of vital ECM was also globally reduced from 73.8 to 68.6%, but the effects of the treatment were significant only in two sites. The expected drier conditions due to current warming will likely influence the presence of mycorrhizas, with potential consequences on the root vitality of boreal tree species. La croissance et la survie d’espèces d’arbres de la forêt boréale dépendent de la présence de symbiotes fongiques, à savoir les mycorhizes. À cause de leur taux de renouvellement rapide, les radicelles et leurs symbiotes peuvent constituer des indicateurs rapides et sensibles des effets de changements environnementaux sur les écosystèmes forestiers. Les auteurs ont examiné comment une réduction de l’humidité du sol influence les radicelles et les ectomycorhizes (ECM) de l’épinette noire [Picea maniana (Mill.) BSP] après trois ans d’exclusion aux précipitations de quatre peuplements forestiers matures de la forêt boréale au Québec, Canada. L’exclusion des précipitations a été réalisée par l’installation de toiles plastiques transparentes sous le couvert forestier durant les saisons de croissance de 2010 à 2012. L’humidité du sol était fortement affectée par le traitement, le contenu en eau étant réduit de 40 à 21 % en moyenne. Comme prévu, les arbres soumis aux conditions expérimentales présentaient une diminution de la proportion d’extrémités radiculaires vigoureuses et d’ECM comparativement aux contrôles. La proportion d’ECM vigoureux était aussi globalement réduite de 73,8 à 68,6 %, mais les effets du traitement n’étaient significatifs que sur deux sites seulement. Les conditions de sécheresse prédites par le réchauffement actuel influenceront probablement la présence de mycorhizes, ce qui aura des conséquences potentielles sur la vitalité des racines des espèces d’arbres de la forêt boréale

Inter- and intra-annual patterns of seed rain in the black spruce stands of Quebec, Canada

Divergent reproductive strategies of tree species generate differences in the dynamics of seed production and dispersion. The spatial and temporal variability in seed rain abundance and viability was monitored during the period 2000-2007 in four boreal stands in Quebec, Canada. The aim was to compare the inter-and intra-annual patterns of seed dispersal between species with diverging adaptive characteristics and reproductive strategies by testing the hypothesis that sympatric species can exhibit different patterns of seed dispersal according to specific ecological adaptations. The coefficient of variation (CV), representing the inter-annual variability in seed rain, was close to or higher than 1 in balsam fir (Abies balsamea [L.] P. Mill.) and white birch (Betula papyrifera Marsh.) and confirmed the mast seeding habit of the two species. In contrast, CV in black spruce (Picea mariana [Mill.] BSP) ranged between 0.24 and 0.54, indicating a more homogeneous inter-annual amount of seed dispersal because of its semiserotinous cones that preserve seeds for an indefinite period of time. The species showed divergent intra-annual patterns of seed dispersal. Most seed dispersal of the companion species was observed in September-November, while black spruce concentrated seed rain in spring, when the proportion of germinated seeds was higher. Boreal stands experience annual seed rains constituted by a gradual dispersal of seeds of different ages and originating from cones belonging to multiple cohorts. However, asynchronous seed rains in terms of quantity and quality can occur if companion species are associated to the dominant black spruce

Unveiling the diversity of tree growth patterns in boreal old-growth forests reveals the richness of their dynamics

Research Highlights: Radial growth patterns of trees growing in old-growth boreal forests in eastern Canada can be grouped into a small number of simple patterns that are specific to different old-growth forest types or successional stages. Background and Objectives: Identifying the main radial growth trends in old-growth forests could help to develop silvicultural treatments that mimic the complex dynamics of old-growth forests. Therefore, this study aimed to identify the main radial growth patterns and determine how their frequencies change during forest succession in old-growth forests, focusing on boreal landscapes in eastern Canada. Materials and Methods: We used dendrochronological data sampled from 21 old-growth stands in the province of Quebec, Canada. Tree-ring chronologies were simplified into chronologies of equal length to retain only primary growth trends. We used k-means clustering to identify individual growth patterns and the difference in growth-pattern frequency within the studied stands. We then used non-parametric analyses of variance to compare tree or stand characteristics among the clusters. Results: We identified six different growth patterns corresponding to four old-growth forest types, from stands at the canopy breakup stage to true old-growth stands (i.e., when all the pioneer cohort had disappeared). Secondary disturbances of low or moderate severity drove these growth patterns. Overall, the growth patterns were relatively simple and could be generally separated into two main phases (e.g., a phase of limited radial increment size due to juvenile suppression and a phase of increased radial increment size following a growth release). Conclusions: The complexity of old-growth forest dynamics was observed mainly at the stand level, not at the tree level. The growth patterns observed in true old-growth forests were similar to those observed following partial or stem-selection cuts in boreal stands; thus, these silvicultural treatments may be effective in mimicking old-growth dynamics