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

Dendrometer and intra-annual tree growth : what kind of information can be inferred?

Dendrometer measurements provide time series composed of the rhythm of water storage fluctuations over the year and seasonal tree growth. For slow-growing trees, however, difficulties have been found in the identification of crucial events such as growth onset, stem growth period and cessation, rendering it necessary to define what can be measured and at which time scale. Time scale means the time interval (from one day to one month) at which stem radius variation is extracted. In this study, two conifer species were monitored by an automatic band dendrometer to assess several time scales and analysis approaches. Data were collected from 8 trees of Picea abies (L.) Karst and Larix decidua L., growing at 1020 and 2080 m a.s.l. in the eastern Italian Alps, from 2000 to 2003. Time series of stem radius variation were extracted with different approaches, such as the stem cycle, daily mean and daily maximum. Several approaches can be used, as very similar time series of stem radius variations were produced with high coefficients of correlation among the series. At lower altitude, the approximate onset was identified at the beginning of May with a 10-day time scale, when the distribution of stem radius variation differed from zero. The main growth period, from May to June-July, corresponded mainly with earlywood cell formation. At higher altitude, a time scale of at least 15 days facilitated identification of the main period of stem growth only, corresponding with earlywood cell formation. Even if latewood cells were produced in August at both altitudes, the variability in stem radius changes was higher than the amount of growth in terms of cell-wood production. For a slow-growing species in a cold environment, an understanding of the growth period, assessed with several time scales, is necessary when using time series of stem radius variation to assess growth and climate relationships. The period used for growth and climate analysis should correspond only with the main period of stem growth

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

Diel growth dynamics in tree stems : linking anatomy and ecophysiology

Impacts of climate on stem growth in trees are studied in anatomical, ecophysiological, and ecological disciplines, but an integrative framework to assess those impacts remains lacking. In this opinion article, we argue that three research efforts are required to provide that integration. First, we need to identify the missing links in diel patterns in stem diameter and stem growth and relate those patterns to the underlying mechanisms that control water and carbon balance. Second, we should focus on the understudied mechanisms responsible for seasonal impacts on such diel patterns. Third, information on stem anatomy and ecophysiology should be integrated in the same experiments and mechanistic plant growth models to capture both diel and seasonal scales

Carbon allocation during defoliation : testing a defense-growth trade-off in balsam fir

During repetitive defoliation events, carbon can become limiting for trees. To maintain growth and survival, the resources have to be shared more efficiently, which could result in a trade-off between the different physiological processes of a plant. The objective of this study was to assess the effect of defoliation in carbon allocation of balsam fir [Abies balsamea (L.) Mill.] to test the presence of a trade-off between allocation to growth, carbon storage, and defense. Three defoliation intensities [control (C-trees, 0% defoliation), moderately (M-trees, 41-60%), and heavily (H-trees, 61-80%) defoliated] were selected in order to monitor several variables related to stem growth (wood formation in xylem), carbon storage in stem and needle (non-structural soluble sugars and starch), and defense components in needles (terpenoids compound) from May to October 2011. The concentration of starch was drastically reduced in both wood and leaves of H-trees with a quasi-absence of carbon partitioning to storage in early summer. Fewer kinds of monoterpenes and sesquiterpenes were formed with an increasing level of defoliation indicating a lower carbon allocation for the production of defense. The carbon allocation to wood formation gradually reduced at increasing defoliation intensities, with a lower growth rate and fewer tracheids resulting in a reduced carbon sequestration in cell walls. The hypothesis of a trade-off between the allocations to defense components and to non-structural (NCS) and structural (growth) carbon was rejected as most of the measured variables decreased with increasing defoliation. The starch amount was highly indicative of the tree carbon status at different defoliation intensity and future research should focus on the mechanism of starch utilization for survival and growth following an outbreak

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

Dynamique de la croissance radiale et influence météorologique quotidienne chez le sapin baumier (Abies balsamea (L.) Mill.) en forêt boréale

La formation du cerne de croissance du sapin de baumier (Abies balsamea (L.) Mill.) a été étudiée en forêt boréale dans le but de décrire son développement et de déterminer les facteurs météorologiques qui l'influencent. La croissance radiale a été mesurée à l'aide de deux techniques soit les analyses cellulaires et la dendrométrie. Des micro-carottes ont été extraites à toutes les semaines pendant les saisons de croissance de 1998 à 2000 afin d'effectuer des analyses cellulaires. Des sections ont été colorées à l'aide du cresyl fast violet pour dénombrer les cellules dans la phase d'élargissement radial, la phase de formation des parois secondaires et les cellules matures, permettant une reconstruction du développement du cerne. L'accroissement radial journalier a été étudié au cours des saisons de 1998 à 2001 à l'aide de dendromètres électroniques. Les analyses des cellules ont permis d'observer des variations dans le début de la croissance (7 mai-7 juin), dans la transition bois initial-bois final (2 juillet-19 juillet) et dans la fin de la lignification des parois cellulaires (20 août-20 septembre). L'élargissement radial a été de courte durée, de moins d'une semaine pour le bois initial et de 5 à 10 jours pour le bois final. Le temps requis pour l'épaississement des parois secondaires a été d'environ 20 jours et 30-35 jours pour le bois initial et le bois final respectivement. L'accroissement radial mesuré par les dendromètres a été divisé en 4 périodes suivant le cycle diurne du tronc soit (1) contraction, (2) expansion, (3) accroissement et (4) cycle total. Les conditions météorologiques moyennes de chacune des périodes ont été comparées à l'accroissement à l'aide de corrélations et de fonctions de réponse. Les conditions météorologiques prévalant de 16-17 h jusqu'à 8-9 h, correspondant aux périodes d'expansion et d'accroissement, ont eu le plus d'impact sur l'accroissement. Les fonctions de réponse étaient fortement linéaires avec une variance expliquée de 80-90 % et ont confirmé la majeure partie des résultats obtenus à l'aide des corrélations. L'accroissement a été positivement corrélé aux précipitations durant les phases 2 à 4 du cycle. Seule la température maximale de la période d'accroissement a eu un effet positif, suggérant que les températures nocturnes sont les plus importantes. Les résultats des analyses climat-croissance à l'aide des mesures cellulaires ont montré que dans le bois initial, la largeur des cellules est fortement reliée avec la température maximale, particulièrement celle du mois de juin où la majorité du cerne est formée. L'épaisseur des parois cellulaires dans le bois initial est aussi fortement reliée à la température à partir de la mi-juin jusqu'à la mi-juillet. Les résultats des analyses cellulaires montrent une flexibilité dans le développement du cerne procurant un avantage en forêt boréale où les conditions optimales de croissance changent d'années en années. La variation de la dimension des trachéides formées durant la saison de croissance est principalement affectée par la température journalière au tout début de sa différentiation. Par la suite, les processus d'élargissement cellulaire sont principalement influencés par les conditions nocturnes de précipitations et de températures

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

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

Cambial phenology, wood formation and temperature thresholds in two contrasting years at high altitude in Southern Italy

Xylogenesis was monitored during 2003 and 2004 in a timberline environment in southern Italy to assess links between temperature, cambial phenology and wood formation on a short-time scale. Wood microcores were collected weekly from May to October from 10 trees of Pinus leucodermis Ant., histological sections were cut with a rotary microtome and anatomical features of the developing and mature tracheids were observed and measured along the growing tree ring. Spring 2003 was hotter than spring 2004, with temperatures up to 2.6°C above historical means. The hotter conditions resulted in an earlier onset of cambial activity and all differentiation phases of about 20 days, resulting in an increased duration of xylogenesis of about 23 days. Air and stem temperatures at which xylogenesis had a 0.5 probability of being active were calculated with logistic regressions fitted on binary responses. In both years, similar thresholds were estimated with daily mean values of 8.2 and 9.5°C for air and stem temperatures, respectively. The observed convergent responses of cambium phenology to temperature during the two contrasting springs confirm the key role of this environmental factor in determining the onset and duration of wood formation in timberline areas. The intra-annual dynamics of ring-width increase differed between years, with significantly narrower rings formed in 2004 than in 2003. These differences were mainly related to cell size because larger early wood tracheids were produced in 2003. This study demonstrates the plasticity of tree-ring formation in response to high temperatures as a result of modifications in the onset and duration of differentiation