Annie Deslauriers

Daily weather response of balsam fir ( Abies balsamea (L.) Mill.) stem radius increment from dendrometer analysis in the boreal forests of Québec (Canada)

Daily stem radial growth of balsam fir [Abies balsamea (L.) Mill.] was studied between 1998 and 2001 using automated point dendrometers to investigate meteorological influence. By dividing the dendrometer day-night variation, the diurnal growth pattern was resolved into the three phases of (1) contraction, (2) expansion and (3) stem radius increment (SRI). The entire circadian cycle (4) defined by the three previous phases was considered as a fourth phase. The mean weather conditions of each phase were compared with the SRI using simple correlation and response function analysis. It was found that the weather conditions prevailing from 1600/1700 hours to 0800/0900 hours corresponding with the expansion-SRI phases had greater impact on SRI. Response function results confirmed most of the correlation analyses and explained up to 95% of the variance of the SRI series. Total rainfall in phases 2, 3 and 4 was correlated positively with SRI, and hence verifies the importance of daily water balance. The importance of water was also demonstrated by the negative effect of high vapour pressure deficit of phase 2, decreasing the possibility of cell radial expansion. The maximum temperature of phase 3 was the only temperature variable having a positive impact on SRI suggesting that night temperature was more important than day temperature in controlling radial growth. These results may influence the process of cell enlargement and reflect only the mechanical aspect of growth.

ASSESSMENT OF CAMBIAL ACTIVITY AND XYLOGENESIS BY MICROSAMPLING TREE SPECIES: AN EXAMPLE AT THE ALPINE TIMBERLINE

SUMMARY Mechanisms of cell production and maturation and dynamics of xylem formation have been widely studied in trees in order to better characterize stem radial growth. Histological analyses have been used in this study to describe cambial activity and xylem cell differentiation in Larix decidua, Pinus cembra and Picea abies at the Alpine timberline. Wood microcores were collected weekly from April to October 2003 and cross sections prepared to distinguish xylem cells of the growing tree ring and to determine the number of cells in the cambial zone, radial cell enlargement, secondary wall thickening and lignification and the number of mature tracheids. The anatomical changes characterizing the phases of xylem cell production and differentiation during the year are described and discussed. All species showed the same trend of xylem formation. Three delayed bell-shaped curves and an S-shaped curve were observed for cambium, enlarging and wall thickening cells and mature cells, respectively. Cells divided in the cambial zones from April-May to August, depending on the species. From 100 to 130 days were required to complete cell differentiation. Tree-ring formation ended during September. The average periods spent on radial enlargement, and secondary cell wall thickening and lignification were estimated at 7–10 and 20–25 days, respectively.

Application of the Gompertz equation for the study of xylem cell development

Summary To assess daily relationships between xylem development and the environment, precise knowledge of tree-ring development is required. This includes accurate estimations of the rate and duration of the cell enlargement and wall thickening phases. This paper presents an application where the Gompertz equation is used to calculate cell number increase and to estimate both the rates and periods of the differentiation phases on a daily scale during the growing season. Tests were performed on two coniferous species, Abies balsamea (L.) Mill. and Pinus cembra L., growing in the Canadian boreal forest and on the Italian Alpine treeline respectively. Wood micro-cores, including the most recent tree-rings, were collected during the growing season of 2001. Cross-sections were cut with a microtome and stained with cresyl fast violet to differentiate xylem cells of the growing tree-ring. The number of cells within the zones of radial cell enlargement, secondary wall thickening and mature xylem were counted. The normalised cell number increases were fitted into a Gompertz function [y=f(t)]. The results showed that the function provided suitable descriptions of the cell increase for both Abies balsamea and Pinus cembra with R2 ranging between 0.60 and 0.90. Subsequently, to assess the development phases in time, the Gompertz equation was expressed in function of the independent variable [t=f(y)]. The use of only one equation for the estimation of both cell division and differentiation throughout the vegetative season has demonstrated to be an important improvement.

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.

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.

Xylem phenology and wood production: resolving the chicken-or-egg dilemma

Delays in the start of the growing season reduce the period available for growth and the amount of xylem production. However, a higher number of developing tracheids could prolong cell differentiation and, consequently, lengthen the growing season. The relationship between the amount and duration of cell production in the xylem remains an unresolved issue. The aim of this study was to resolve the chicken-or-egg causality dilemma about duration of growth and cell production through simple- and double-cause models. This was achieved by (1) analysing the intra-annual growth dynamics of the xylem in Picea mariana (Mill.) BSP during 2006-2009 in two contrasting sites of the boreal forest of Quebec, Canada, and (2) extracting the dates of onset and ending of xylem formation and the number of radial cells along the tree ring. A higher number of cells was linked to an earlier onset (r=0.74) and later ending (r=0.61) of cell differentiation. The absence of a relationship between the residuals of the onset and ending of xylogenesis (r(p)=-0.06) indicated that cell production influenced the correlation between the two phenophases of the xylem. These results demonstrated that a higher number of cells produced delay the ending of xylem maturation, so extending the duration of wood formation.

Woody biomass production lags stem-girth increase by over one month in coniferous forests

Wood is the main terrestrial biotic reservoir for long-term carbon sequestration(1), and its formation in trees consumes around 15% of anthropogenic carbon dioxide emissions each year(2). However, the seasonal dynamics of woody biomass production cannot be quantified from eddy covariance or satellite observations. As such, our understanding of this key carbon cycle component, and its sensitivity to climate, remains limited. Here, we present high-resolution cellular based measurements of wood formation dynamics in three coniferous forest sites in northeastern France, performed over a period of 3 years. We show that stem woody biomass production lags behind stem-girth increase by over 1 month. We also analyse more general phenological observations of xylem tissue formation in Northern Hemisphere forests and find similar time lags in boreal, temperate, subalpine and Mediterranean forests. These time lags question the extension of the equivalence between stem size increase and woody biomass production to intra-annual time scales(3, 4, 5, 6). They also suggest that these two growth processes exhibit differential sensitivities to local environmental conditions. Indeed, in the well-watered French sites the seasonal dynamics of stem-girth increase matched the photoperiod cycle, whereas those of woody biomass production closely followed the seasonal course of temperature. We suggest that forecasted changes in the annual cycle of climatic factors(7) may shift the phase timing of stem size increase and woody biomass production in the future.

Widening of xylem conduits in a conifer tree depends on the longer time of cell expansion downwards along the stem

The diameter of vascular conduits increases towards the stem base. It has been suggested that this profile is an efficient anatomical feature for reducing the hydraulic resistance when trees grow taller. However, the mechanism that controls the cell diameter along the plant is not fully understood. The timing of cell differentiation along the stem was investigated. Cambial activity and cell differentiation were investigated in a Picea abies tree (11.5 m in height) collecting microsamples at nine different heights (from 1 to 9 m) along the stem with a 4 d time interval. Wood sections (8–12 μm thick) were stained and observed under a light microscope with polarized light to differentiate the developing xylem cells. Cell wall lignification was detected using cresyl violet acetate. The first enlarging cells appeared almost simultaneously along the tree axis indicating that cambium activation is not height-dependent. A significant increase in the duration of the cell expansion phase was observed towards the tree base: at 9 m from the ground, xylem cells expanded for 7 d, at 6 m for 14 d, and at 3 m for 19 d. The duration of the expansion phase is positively correlated with the lumen area of the tracheids (r2=0.68, P < 0.01) at the same height. By contrast, thickness of the cell wall of the earlywood did not show any trend with height. The lumen area of the conduits down the stem appeared linearly dependent on time during which differentiating cells remained in the expansion phase. However, the inductive signal of such long-distance patterned differentiation remains to be identified.

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.

Comparing needle and shoot phenology with xylem development on three conifer species in Italy.

Abstract• Since the 1960s biologists have been trying to assess factors affecting cambial activity and latewood formation. A comparison of the phenology of the apical and lateral meristems could add new clues in order to understand mechanisms of the growth dynamic in conifers.• This study compared needle and shoot growth with xylogenesis in Larix decidua, Pinus cembra and Picea abies during 2001 to verify if cambial resumption occurred after shoot and needle growth had begun, and if latewood initiation occurred when shoot lengthening was complete.• In pine and spruce, needle and shoot lengthening was synchronous between mid-June and August while larches showed an early development of the needles at the end of May with a later shoot lengthening. Cell production lasted from mid-May to the beginning of August, with the first differentiating cells observed at the end of May, which indicated a earlier reactivation of the cambium with respect to needle and shoot growth.The first latewood cells began wall thickening between July and August, during the termination of needle and shoot lengthening.• The hypothesis that cambial resumption occurs after shoot and needle elongation was rejected. The separation in time between apical growth and secondary wall thickening of latewood suggested the presence of an internal competition in resource allocation.Résumé• Depuis les années soixante, les biologistes étudient les facteurs contrôlant l’activité cambiale et la formation du bois final. Une comparaison entre la phénologie des méristèmes apicaux et latéraux pourrait ajouter de nouveaux indices afin de mieux comprendre les mécanismes et la dynamique de croissance des conifères.• Cette étude compare la croissance des aiguilles et des nouvelle tiges avec la xylogénèse de trois espèces de conifères, Larix decidua, Pinus cembra et Pices abies dans le but de vérifier si la réactivation cambiale survient après la croissance des aiguilles et des nouvelles pousses et si la formation du bois final survient lorsque la croissance des méristèmes apicaux et latéraux est terminée.• Chez le pin et l’épicéa, la croissance des aiguilles et des nouvelles pousses était synchrone, se produisant de la mi-juin jusqu’au début août. Chez le mélèze par contre, le développement des aiguilles a été plus précoce, débutant fin mai avec une croissance différée des tiges latérales. La production cellulaire du cambium a durée de mi-mai jusqu’à début août, avec apparition des premières cellules en différentiation fin mai, indiquant une réactivation plus précoce du cambium que de la croissance des aiguilles et des nouvelles pousses. L’épaississement des parois secondaires chez les premières cellules de bois final a débuté entre juillet et août, à la fin de l’élongation des aiguilles et des nouvelles pousses.• L’hypothèse que la réactivation cambiale survient après la croissance des aiguilles et des nouvelles pousses a été rejetée. La séparation dans le temps entre les croissances apicale et latérale et l’épaississement des parois secondaires du bois final a suggéré la présence d’une compétition interne dans l’allocation des ressources.