Wood Science and Technology

, Volume 41, Issue 6, pp 463–475

Regular cambial activity and xylem and phloem formation in locally heated and cooled stem portions of Norway spruce

Authors

  • Jožica Gričar
    • Department of Wood Science and Technology, Biotechnical FacultyUniversity of Ljubljana
  • Martin Zupančič
    • Department of Wood Science and Technology, Biotechnical FacultyUniversity of Ljubljana
  • Katarina Čufar
    • Department of Wood Science and Technology, Biotechnical FacultyUniversity of Ljubljana
    • Department of Wood Science and Technology, Biotechnical FacultyUniversity of Ljubljana
ORIGINAL

DOI: 10.1007/s00226-006-0109-2

Cite this article as:
Gričar, J., Zupančič, M., Čufar, K. et al. Wood Sci Technol (2007) 41: 463. doi:10.1007/s00226-006-0109-2

Abstract

The effect of heating (23–25°C) and cooling (9–11°C) on regular cambial activity and xylem and phloem formation in the stem portion of Norway spruce was investigated. Adult trees were sampled at 21-day intervals during the 2005 vegetation period. Continuously elevated temperatures increased the rate of cell division in the first part of the growing season, but did not significantly prolong cambial activity at the end of the vegetation period in the heated tree. Low-temperature treatments shortened regular cambial activity and slowed down cell production. The xylem growth ring was wider in the heated sample and narrower in the cooled sample compared to the control. The temperature in the cambial region was only negligibly transferred along the stem from the site of its application. In general, the temperature in the cambium was affected by a long-term rise or drop in air temperatures. Both experiments affected the structure and width of phloem growth increments. The tangential band of the axial parenchyma was not continuous in the cooled sample. The number of late phloem cells was reduced in the cool-treated sample and increased in the heat-treated sample. Our experiments confirmed the effect of constantly increased or decreased temperatures on regular cambial activity in Norway spruce.

Introduction

Wood formation (xylogenesis) begins with periclinal cell divisions in the vascular cambium, followed by differentiation. In conifers, the majority of the newly formed cells differentiates into axial tracheids, responsible for water conduction and mechanical support (Panshin and de Zeeuw 1980; Larson 1994; Chaffey 2002). Wood formation is a plastic process, a function of epigenetic status and the physico-chemical environment within the cell (Savidge 2000). Environmental factors provide physical conditions for the growth and development of trees. In temperate climatic regions, cambial activity is a periodic process, which usually occurs from late spring to late summer (Denne and Dodd 1981; Larson 1994; Savidge 1996, 2000; Wodzicki 2001). Environmental factors interact in a complex way under natural conditions, which complicates analysis of the effects of selected factors on xylogenesis. However, the effect of individual factors on xylogenesis has been successfully demonstrated in experiments with shoots, stem cuttings or intact stem portions growing under controlled conditions (Little and Bonga 1974; Little 1981; Savidge and Wareing 1981; Riding and Little 1984, 1986; Mellerowicz et al. 1992; Savidge and Barnett 1993; Barnett and Miller 1994; Oribe and Kubo 1997; Oribe et al. 2001, 2003, 2004; Wodzicki 2001; Rensing and Samuels 2004; Begum et al. 2005; Gričar et al. 2006).

It has already been demonstrated that the application of temperature can cause alterations in regular cambial activity (Barnett and Miller 1994; Oribe and Kubo 1997; Oribe et al. 2001, 2004; Begum et al. 2005; Gričar et al. 2006). However, the application of heat revealed differences in the response of dormant cambium to treatments among different species of evergreen and deciduous habit (Barnett and Miller 1994; Oribe et al. 2001, 2003, 2004). Cambial cells of evergreen conifers (Abies sachalinensis, Cryptomeria japonica, Picea abies and Picea sitchensis) at the quiescent stage of cambial dormancy, which is imposed by external factors, can re-initiate cell division independently of the growth of new shoots and the development of buds in spring (Oribe and Kubo 1997; Oribe et al. 2001, 2003; Gričar et al. 2006). The effect of localized heating of stems of A. sachalinensis on the extent of cell division in heat-reactivated cambium was not as distinct as that in naturally reactivated cambium. In addition, heat-reactivated cambium ceased cell division soon after a few cells had been produced and no differentiating xylem cells were observed (Oribe and Kubo 1997). In dormant evergreen P. sitchensis saplings, cambial reactivation occurred only in the heated region of the stem if needles and buds were left intact (Barnett and Miller 1994). In the evergreen C. japonica, cambial reactivation often occurred in the heated portion of the stem (Oribe and Kubo 1997). This response gradually increased as the dormant season passed from winter to spring. It has been suggested that heating directly triggers the breaking of cambial dormancy in evergreen conifers. Hence, applying heat stimulated divisions in the dormant cambium and led to xylem and phloem formation (Oribe and Kubo 1997; Oribe et al. 2001, 2004). Cambial reactivation in the stem of A. sachalinensis and P. abies occurred first on the phloem side (Oribe et al. 2001, 2003; Gričar et al. 2006). In the stem of adult Norway spruce, cambial reactivation was restricted to the heated region, which suggests that temperature is not transmitted along the stem from the site of its application (Gričar et al. 2006). The absence of cambial response to heat treatment in the deciduous Larix leptolepis indicated that cambial reactivation is limited by several factors associated with bud break in this species (Oribe and Kubo 1997).

The influence of the cooling of tree stems on cambial activity and cell differentiation during the growing season has been less investigated. Gričar et al. (2006) cooled the stem portion of Norway spruce at the height of cambial activity for 1 month. The treatment caused earlier formation of late wood. The drop in temperatures shortened cambial activity, resulting in a lower portion of late wood in the current xylem increment. It appeared that short-term heat and cooling treatments did not affect the width and structure of phloem growth increments in Norway spruce (Gričar et al. 2006). Moreover, heating and cooling a stem portion of Norway spruce did not provoke any alterations in the pattern of secondary cell wall formation and lignification, or in lignin structure at the ultrastructural and topochemical levels (Gričar et al. 2006).

To summarize, experiments under controlled conditions have been mainly performed over short periods of time. Moreover, the effect of elevated temperature on cambial activity has been mostly studied during dormancy. The purpose of this research was to evaluate the response of active cambium of Norway spruce (P. abies) to experimentally increased (23–25°C) and decreased (9–11°C) temperatures during the entire 2005 growing season. The response of the cambium and the formation of xylem and phloem to both treatments were investigated at 21-day intervals by means of light microscopy. In order to reveal the effect of treatments on the general anatomy of xylem and phloem growth rings, samples were taken at the end of the 2005 vegetation period. In addition, the temperature of the cambial region in the isolated stem portion, below and above the system, was recorded at each sampling date.

Materials and methods

Three 70-year-old Norway spruce test trees (DBH = 33 cm) in the urban Norway spruce/beech forest Rožnik in Ljubljana (46°03′N, 14°28′E, 323 m a.s.l.) were selected for the experiment. One tree was used for the heating experiment and one for the cooling treatment. The third tree was not treated and served as a control. The number of test trees was defined in accordance with experimental limitations when working with large forest trees. The distance between the test trees was about 6 m. Average maximum and minimum daily air temperatures were recorded by a weather station located 50 m away from the test trees during the two experiments. Localized heating of the stem portion of Norway spruce was performed in the period between 24 April 2005 and 11 October 2005, and cooling between 24 April and 20 September 2005. The stem portion of the first Norway spruce was heated with a 15 m long electric heat cable (FSM-17, 17 W/ + 5°C, 11 W/25°C, 230 V) wrapped around 1 m of the length of the stem. The lower part of the heating system was 70 cm above the ground. Insulation material was wrapped around the electric heat cable to prevent energy loss. The temperature between the bark and the insulation was adjusted to 23–25°C, and monitored daily with a thermometer sensor. For cooling the stem portion of the second experimental spruce, we used a circulating pump that pushed cooled water through copper tubes, which were wrapped around 1 m of the length of the stem. The system was carefully insulated. The temperature between the stem and the insulation was set to 9–11°C, monitored and regulated daily with a capillary thermostat.

Blocks of tissue (10 × 10 × 30 mm3) containing inner phloem, cambium and outer xylem were taken at the beginning of each of the treatments and thereafter at 21-day intervals. Samples were taken at breast height (1.3 m above ground) from treated trees and the control tree. The distance between neighbouring samples was at least 10 cm in horizontal direction to avoid influence of wounds on tissues of the next sampling location. At the same time, the temperature of the cambial tissue was measured in an isolated stem portion, 10 cm below and above using thermo-cables. The insulation was carefully placed in the original position after each sampling. Additionally, samples were taken on 1 November 2005, 21 days after termination of both experiments. After sampling, the blocks of tissue were immediately fixed in formaldehyde-ethanol-acetic acid (FAA) solution, after being dehydrated for 1 week in a graded series of ethanol and embedded in paraffin (Rossi et al. 2006). For light microscopy, permanent cross sections of 13 μm thickness were prepared on a rotary microtome Leica RM 2245, using disposable Feather N35H blades, stained with safranin and astra blue and mounted in Euparal. A Nikon Eclipse 800E light microscope was used for anatomical observations. Radially flattened cells of dormant cambium were easily distinguished from terminal latewood tracheids and differentiated secondary phloem. An increase in the number of cells in the cambium indicated its divisional activity. Expanding cells with thin primary walls between the cambium and a zone of xylem and phloem cells with secondary walls were considered to be xylem and sieve cells in the stage of post-cambial growth. The initiation of the secondary wall thickening was determined by birefringence in the cell walls under polarized light. Blue-stained cell walls and the protoplasmic content in the cell lumina indicated incompletely developed tracheids. Fully matured tracheids had red-stained cell walls and empty lumina.

Results

Climatic conditions during experiment

The daily average maximum and minimum air temperatures and amount of precipitation recorded in the area from 1 April to 31 October 2005 are shown in Fig. 1. When starting with both experiments at the end of April, air temperatures were relatively high, with a maximum above 25°C. In the first part of May and June, short-term colder periods were recorded, when maximum temperatures fell below 20°C and minimum below 5°C. The summer of 2005 was considered wet and not too hot compared to long-term annual climatic conditions in the area. Maximum temperatures, thus, rarely exceeded 30°C. Since the average temperature decreased below 15°C at the end of September, we ended the cooling experiment at that time. Three weeks later, we also terminated the heat treatment. The driest month during the experiments was October (56 mm/month). In August and September, above average monthly amounts of precipitation were observed: 230 and 275 mm, respectively.
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Fig. 1

Average, maximum and minimum daily air temperatures and amount of precipitation in Ljubljana (46°03′N, 14°28′E, 323 m a.s.l.) during the two experiments. The duration of the heating and cooling experiments is indicated by arrows

Xylem

Examination of the material removed at the beginning of the research on 26 April showed that the experiments were set after the initiation of cell divisions in the cambium in all trees. The active cambium consisted of 6–8 layers of radially flattened cells (Figs. 2, 3, 4). Xylem cells in the phase of post-cambial growth were not detected, while on the phloem side up to two layers of sieve cells were already in the phase of differentiation. Three weeks later on 17 May 2005, 5–6 layers of xylem cells were in the phase of post-cambial growth in all trees. However, 9–11 layers of cells were undergoing secondary cell wall formation and lignification in the heated sample (Fig. 2). In addition, one layer of fully matured tracheid was observed. In contrast, no cells in the stage of secondary cell wall formation or full developed were found in the cooled and control trees (Figs. 3, 4). At the beginning of June, 3–5 layers of xylem cells were in post-cambial growth in all samples. Deposition of secondary cell wall and lignification had occurred in 8–9 layers of cells of heated sample, in 9–10 layers of cells in the control and in 2–3 layers of cells in the cooled sample. At that time, 11–13 layers of cells were fully developed in the heated sample, 2–4 layers of cells in the control and none in the cooled sample. After 2 months of heat and cool treatments (on 28 June 2005), 2–4 layers of cells were in post-cambial growth in the heated and control samples, and only 1–2 layers in the cooled sample. Wall thickening had occurred in 7–8 layers of cells in the heated sample, up to 11 layers of cells in the control and 5–6 layers of cells in the cooled sample. More than ten layers of xylem cells were fully matured in the heated and control samples, but still none in the cooled sample. In the middle of July, the number of cambial cells in the cooled sample had decreased to five or six layers of cells. One layer of cells was in the stage of post-cambial growth, indicating the cessation of cell division in the cambium of the cooled sample (Fig. 3). In contrast, 2–3 layers of radially expanding cells were detected in the heated and control samples (Figs. 2, 4). More than ten layers of cells undergoing secondary wall deposition and lignification were noted in the heated and control samples, and around five layers of cells. By that time, ∼18 layers of mature tracheids had formed in the heated and control samples, and only up to four layers in the cooled sample. On 9 August 2005, 1–2 layers of cells in post-cambial growth were found only in the heated sample, whereas cambial activity seemed to have been completed in the cooled and control samples. Dormant cambium of those trees consisted of five, rarely six, layers of radially flattened cells. More than ten layers of cells were undergoing cell wall thickening and lignification in the heated and control samples, and fewer than five in cooled ones. At the end of August, cambium appeared to be dormant in all samples. However, more than ten layers of cells were in the phase of secondary cell wall deposition and lignification in the heated and control samples, and fewer than five layers of xylem cells in the cooled sample. On 20 September, the development of the 2005 xylem growth ring of the cooled sample had been completed (Fig. 5b). It was composed of ten layers of cells. Meanwhile, synthesis of the secondary cell wall and lignification was still occurring in more than five layers of cells of the heated and control samples. Xylem growth ring formation in 2005 was completed in the heated and control samples at the beginning of November. In the heated sample, the xylem growth ring numbered around 35 layers of cells and in the control around 27 layers of cells (Fig. 5a, c). All samples were free of wound-wood, traumatic resin canals and compression wood.
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Fig. 2

Seasonal dynamic of the xylem growth ring formation in heated sample. CC cambial cells, PG cells in post-cambial growth, SL cells undergoing secondary cell wall formation and lignification, MT mature tracheids, T total number of xylem cells

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Fig. 3

Seasonal dynamic of xylem growth ring formation in the cooled sample. CC cambial cells, PG cells in post-cambial growth, SL cells undergoing secondary cell wall formation and lignification, MT mature tracheids, T total number of xylem cells

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Fig. 4

Seasonal dynamic of xylem growth ring formation in the control sample. CC cambial cells, PG cells in post-cambial growth, SL cells undergoing secondary cell wall formation and lignification, MT mature tracheids, T total number of xylem cells

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Fig. 5

Light micrographs of the xylem and phloem increments in the a heated, b cooled and c control samples at the end of the vegetation period in the year 2005. P phloem, CC cambial cells, SL cells undergoing secondary cell wall formation and lignification, MT mature tracheids. Scale bars 100 μm

Phloem

On the phloem side, we did not detect any differences in the structure of early phloem or the duration of its development among the samples (Table 1). In all cases, it was composed of 3–4 layers of sieve cells with large radial dimensions and thin walls. Sieve cells of Pinaceae are characterized by unlignified secondary cell walls. The first solitary parenchyma cells separating early phloem from late phloem sieve cells were laid down by mid-May in all samples. Late phloem cells subsequently started to form. The tangential band of parenchyma cells was continuous in the heated and control trees, but not in cooled ones. By the end of cambial activity, 2–3 layers of late phloem sieve cells had developed in the cooled sample. In the heated sample, 5–7 layers of cells had formed and 4–5 in the control.
Table 1

Seasonal dynamics of the phloem formation in the heated, cooled and control samples in 2005

 

Heating

Cooling

Control

26 April

2 EP

2 EP

2 EP

17 May

3–4 EP APa

3–4 EP APa

3–4 EP APa

7 June

3–4 EP AP 1–2 LP

3–4 EP APa 1 LP

3–4 EP AP 2 LP

28 June

3–4 EP AP 2–3 LP

3–4 EP APa 1 LP

3–4 EP AP 2–3 LP

19 July

3–5 EP AP 3 LP

3–4 EP APa 2 LP

3–4 EP AP 3–4 LP

9 August

3–4 EP AP 4 LP

3–4 EP APa 2–(3) LP

3–4 EP AP 4–(5) LP

30 August

3–4 EP AP 4–6 LP

3–4 EP APa 2–(3) LP

3–4 EP AP 4–5 LP

20 September

3–4 EP AP 5–7 LP

3–4 EP APa 2–(3) LP

3–4 EP AP 4–5 LP

11 October

3–4 EP AP 5–7 LP

3–4 EP APa 2–(3) LP

3–4 EP AP 4–5 LP

1 November

3–4 EP AP 5–7 LP

3–4 EP APa 2–(3) LP

3–4 EP AP 4–5 LP

EP early phloem, LP late phloem, AP tangential band of axial parenchyma

aNot continuous tangential band of axial parenchyma

Temperatures in the cambium

At each sampling date, we measured the temperatures in the cambial tissue of isolated stem portions during heating and cooling, 10 cm below and above the heated or cooled portion. Measured temperatures are shown in Table 2. Temperatures in the cambial region of isolated stem portions oscillated in line with air temperatures, although the adjusted temperatures of the experimental system remained constant. There was a conspicuous trend of slow increase in temperatures in the cambial region from May to June (Fig. 1). With dropping air temperatures during an extended period in August, the temperature in the cambium also decreased by about 5°C. A rise in air temperatures at the end of August resulted in elevated temperatures in the cambial region of the heated sample. Temperatures of the cambial region below and above the isolated stem portions showed a similar pattern. They also oscillated according to the air temperatures. Above the isolated stem portion, temperatures were on average 2°C higher than below it in the case of the heating experiment, and vice versa in the cooling experiment (Table 2).
Table 2

Temperatures in the cambium in the isolated stem portion, and 10 cm above and below it. They were recorded during heating and cooling in the growing season of 2005

Temperature in cambium (°C)

Date

17 May

7 June

28 June

19 July

9 August

30 August

Heating

23.3

24.5

26.1

25.8

20.7

24.0

Above heating

17.0

18.1

21.8

23.2

20.5

19.0

Below heating

14.9

15.8

21.3

22.0

18.0

17.8

Cooling

11.7

11.6

12.0

14.4

12.1

12.5

Above cooling

14.4

14.2

19.3

21.0

15.2

17.7

Below cooling

15.0

15.3

20.2

21.9

15.8

17.8

Discussion

We demonstrated that the application of high- or low-temperatures caused alterations in regular cambial activity and cell differentiation in Norway spruce, which is in agreement with previous observations (Oribe and Kubo 1997; Oribe et al. 2001, 2003, 2004; Gričar et al. 2006). Experimental heating of the stem portion of Norway spruce during the entire 2005 growing season did not cause drastic changes in regular cambial activity or in the process of differentiation. Regular cambial activity was not significantly prolonged, despite continuously elevated temperatures in the second part of the growing season. Divisions in the cambium stopped in the first part of August, a few days later than in the control tree. However, the rate of cell divisions was increased at the beginning of the growing season compared to the control tree. It seems that higher temperatures were favourable for cambial cell production at the very beginning of the growing season. Cells undergoing secondary cell wall thickening and mature tracheids were observed much earlier in the heated sample compared to the control. On the other hand, no effect of the elevated temperatures on cambial activity in the second part of the growing season indicates that the influence of temperature on cambial activity varies over the vegetation period. As a result, the number of the cells in the completed xylem growth ring was slightly higher in the heated tree (35 layers of cells) than in the control tree (27 layers of cells). Application of cooling during the entire growing season in 2005 shortened regular cambial activity in comparison with the control tree. In addition, the rate of division in the cambium slowed down. The first fully developed xylem cells were detected approximately 1 month later than in the control tree. The xylem increment was reduced by about one-third (ten layers of cells). It seems that temperature is crucial for cambial activity and cell development at the beginning of the growing season, while other factors prevail in the second part.

Rossi et al. (2006) reported a maximum growth rate around the time of maximum day length, and not during the warmest period of the year, as previously suggested. The authors propose that maximum photoperiod could act as a growth constraint or limit, after which the rate of cell production decreases, which enables plants to complete differentiation of the cell walls before winter. If, in a cold environment, tracheid production was only influenced by temperature, warm periods in August, September or even October could lead plants to maintain high rates of cell division in late summer and they would not succeed in completing the development of the latest formed cells before winter (Rossi et al. 2006). Temperature has also been proposed as the main factor affecting growth onset (Vaganov et al. 1999; Kirdyanov et al. 2003). Environmental factors influencing xylogenesis can have different effects at different periods of the growing season. As growth rate tends to decrease after the summer solstice, the temperature in the first increasing part of the growth curve is expected to have a major impact on tree ring formation and wood production (Rossi et al. 2006).

Locally heating or cooling the stem portion of Norway spruce did not influence cambial activity and cell differentiation above the treated area (Gričar et al. 2006). Earlier resumption of cambial cell division, or earlier late wood formation, was restricted to the treated region, which confirmed the previous observations of Barnett and Miller (1994) about the non-transference of temperature along the stem from the site of its application (Gričar et al. 2006). Measurements of the temperature in the cambial region of the isolated stem portion and 10 cm below and above it in 2005 showed only negligible transfer of temperature along the stem from the site of its application. The temperature above and below the treated stem portion deviated on average ±2°C. The temperature in the cambial region, including the isolated stem portions, oscillated proportionally to the air temperatures, with a certain delay in regard to oscillations of air temperatures. In general, the temperature in the cambium was affected by a long-term rise or drop in air temperatures. We assume that the temperatures in the cambial region are also affected by the temperature of the soil and ground water, which was not investigated in this study.

Short-term temperature treatment in 2004 did not affect the widths and anatomical structure of the phloem increments (Gričar et al. 2006). In contrast, long-term experiments in 2005 affected the structure and the width of the phloem growth increments. The structure of early phloem was comparable in cooled, heated and control samples. Early phloem was formed in all trees by mid-May. However, the tangential band of axial parenchyma cells separating early and late phloem was not continuous, in the cooled sample, which is typical of trees growing under stress conditions (Holdheide 1951). In addition, the temperature affected the number of late phloem cells. The number of cells was reduced in the cool-treated sample and increased in the heat-treated sample when compared to the control sample. According to our observations, the temperature affected phloem production mainly in the second part of the growing season. It appears that the rate of phloem production stayed the same in treated and control trees, but distinctions in the number of late phloem cells arose from the different times of cessation of cambial activity among the samples.

Our results and the results obtained by other scientists confirm the importance of external factors on cambial activity and corresponding cell differentiation. However, internal factors, such as phytohormones, sugars, etc., should also be taken into consideration (Little and Bonga 1974; Riding and Little 1984; Mellerowicz et al. 1992; Savidge and Barnett 1993; Savidge 1996, 2000; Kozlowsky and Pallardy 1997; Lachaud et al. 1999; Aloni et al. 2000; Krabel 2000; Uggla et al. 2001; Wodzicki 2001; Oribe et al. 2003). The endogenous auxin present in the dormant cambium of mature stems is sometimes sufficient for cambial reactivation, but not in all cases. After cambial reactivation, a continuous supply of IAA through the polar transport system is needed for the maintenance of cambial cell divisions and cell development (Oribe and Kubo 1997; Sundberg et al. 2000; Oribe et al. 2003). Auxin stimulates cambial growth by acting as a positional signal, increasing the population of dividing cambial cells through its radial distribution pattern and by stimulating the rate of cambial cell cycling through an increased concentration in the meristem (Sundberg et al. 2000). Initiation of late wood formation and cessation of cambial cell divisions are not a consequence of decreased IAA concentrations dividing and differentiating cells (Uggla et al. 2001). Nevertheless, the variation of cambial sensitivity to auxin is also very important and is probably directly linked to the ability of cambial cells to polar transport (Lachaud et al. 1999). Exogenous transport of auxin depending on the temperature is thus qualitatively modified at the end of summer, when cambial activity ceases. We can therefore assume that lower temperatures in our experiment affected the speed of a basipetal auxin transport. Physical factors of the environment act on the seasonal activity of the cambium by way of hormonal messengers (Lachaud et al. 1999). For xylem and phloem development, an availability of sucrose is required. Steep concentration gradients of sugars across developing vascular tissues suggest a role for sugar signalling in vascular development (Uggla et al. 2001).

Conclusions

Constantly increased or decreased temperatures affect regular cambial activity and cell differentiation in Norway spruce. However, temperatures have greater impact on cambial cell production at the very beginning of the growing season, while other factors prevail in the period of late wood formation. Phloem production is less dependent on temperatures. Besides the effect of external factors on cambial activity and corresponding cell differentiation, internal factors play a very important role. A precise knowledge of the effect of temperature on all internal processes needs further investigation.

Acknowledgments

We are grateful to our colleagues Peter Cunder, Lena Marion and Maks Merela for their helpful field assistance. We are indebted to the Slovenian Forestry Institute for enabling experimental work in the field. We thank Dr. Tom Levanič of the Slovenian Forestry Institute for providing the meteorological data. The work was funded by the Slovenian Research Agency, the Ministry of Higher Education, Science and Technology of the Republic of Slovenia and by the World Federation of Scientists and the Slovenian Science Foundation. The research was done in the framework of the Research Programme, Wood Science and Technology.

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