Aim To identify temperatures at which cell division and differentiation are active in order to verify the existence of a common critical temperature determining growth in conifers of cold climates. Location Ten European and Canadian sites at different latitudes and altitudes. Methods The periods of cambial activity and cell differentiation were assessed on a weekly time-scale on histological sections of cambium and wood tissue collected over 2 to 5 years per site from 1998 to 2005 from the stems of seven conifer species. All data were compared with daily air temperatures recorded from weather stations located close to the sites. Logistic regressions were used to calculate the probability of xylogenesis and of cambium being active at a given temperature. Results Xylogenesis lasted from May to October, with a growing period varying from 3 to 5 months depending on location and elevation. Despite the wide geographical range of the monitored sites, temperatures for onset and ending of xylogenesis converged towards narrow ranges with average values around 4-5, 8-9 and 13-14 degrees C for daily minimum, mean and maximum temperature, respectively. On the contrary, cell division in the cambium stopped in July-August, when temperatures were still high. Main conclusions Wood formation in conifers occurred when specific critical temperatures were reached. Although the timing and duration of xylogenesis varied among species, sites and years, the estimated temperatures were stable for all trees studied. These results provide biologically based evidence that temperature is a critical factor limiting production and differentiation of xylem cells in cold climates. Although daily temperatures below 4-5 degrees C are still favourable for photosynthesis, thermal conditions below these values could inhibit the allocation of assimilated carbon to structural investment, i.e. xylem growth

Summary• Intra-annual radial growth rates and durations in trees are reported to differ greatly in relation to species, site and environmental conditions. However, very similar dynamics of cambial activity and wood formation are observed in temperate and boreal zones.• Here, we compared weekly xylem cell production and variation in stem circumference in the main northern hemisphere conifer species (genera Picea , Pinus , Abies and Larix ) from 1996 to 2003. Dynamics of radial growth were modeled with a Gompertz function, defining the upper asymptote ( A ), x -axis placement ( β ) and rate of change ( κ ).• A strong linear relationship was found between the constants β and κ for both types of analysis. The slope of the linear regression, which corresponds to the time at which maximum growth rate occurred, appeared to converge towards the summer solstice.• The maximum growth rate occurred around the time of maximum day length, and not during the warmest period of the year as previously suggested. The achievements of photoperiod could act as a growth constraint or a limit after which the rate of tree-ring formation tends to decrease, thus allowing plants to safely complete secondary cell wall lignification before winter.

In this review, we focus on the biotic parameters that are crucial to an understanding of the recruitment dynamics of North American boreal tree species following natural (fire, budworm infestation, windthrow) or human-induced (clearcut, partial cut) disturbances. The parameters we emphasize are (i) the production of seeds and asexual stems (both of which, we argue, are a function of basal area density), (ii) the dispersal of seeds by wind (or the dispersion of asexual stems) as a function of distance from source, (iii) dormant seed bank capacity, (iv) organic layer depth as a determinant of germinant mortality and asexual bud response, and (v) shade tolerance as a partial arbiter of the density of advanced regeneration. Having identified the gaps in our knowledge, we conclude by suggesting a short-term research agenda whose completion would lead to the parameterized functions that would constitute the recruitment subroutine in a landscape-scale forest dynamics simulator.

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.

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