Detailed analyses of cambium activity and wood formation during growth need repeated sampling of newly formed xylem. In order to be minimally invasive, wood samples are extracted as microcores. Despite the research done on xylem cell development and the increasing interest in intra-annual studies of xylogenesis, few tools are available for microcore sampling. Methods originally designed for other purposes have often been used, but no details are available on their efficiency and usefulness. Information is also lacking on laboratory preparation techniques for cell analysis of tree-ring formation, leading to difficulties in carrying out these experiments. The advantages and limits of the tools used up to now are described. A new tool, named Trephor (patent pending n° PD2004A000324), specifically designed for long-lasting use is presented. Trephor is chisel-shaped for a fast recovery of 2 mm diameter microcores. The cutting tube is inserted into the wood using a hammer and no other accessory is required. Simple technical characteristics allow high quality samples to be collected from both softwood and hardwood species with minimum damage to the sampled trees. Trephor was tested during the 2004 growing season, demonstrating good resistance to wear and tear and mechanical stress. Embedding the microcores in paraffin for fast section preparation is described.

• 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.

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 earlywood 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.

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.