Les/Wood, Vol. 66, No. 2, November 2017 15
Vol. 66, No. 2, 15-27
DOI: hBps://doi.org/10.26614/les-wood.2017.v66n02a02
WOOD FORMATION IN NORWAY SPRUCE ON A LOWLAND SITE IN SLOVENIA
IN 2015 AND COMPARISON WITH OTHER CONIFERS ALL OVER EUROPE
NASTAJANJE LESA SMREKE NA NIŽINSKEM RASTIŠČU V SLOVENIJI
V LETU 2015 IN PRIMERJAVA Z IGLAVCI PO EVROPI
Fernando Useros1, Angela Balzano2, Peter Prislan3, MarAn de Luis4, Jožica Gričar3, Maks Merela2,
Katarina Čufar2*
Abstract / Izvleček
Abstract: We present xylem forma7on in Norway spruce (Picea abies (L.) Karst.) in 2015 at Panška reka, near Ljubljana,
Slovenia (ca. 46°00’N, 14°40’E, 400 m a.s.l.) with a temperate climate. The research was a part of a long-term project,
which involves different sites and tree species. We measured the widths of cambium and the forma7on of xylem
growth rings with differen7a7ng cells in postcambial (PC) and secondary wall forma7on (SW) phases, and mature
cells (MT). The results for 2015 were compared with those for previous years (2009–2011) in the same species and
site, and with the published data from wood forma7on studies in conifers from all over Europe. The la8er were used
in two-factor regression models which confirmed the effects of la7tude and al7tude on the cri7cal dates of onset,
end, and dura7on of xylem produc7on. The models thus helped us to predict/reconstruct phenology of wood forma7on
in conifers in Europe.
Keywords: Norway spruce (Picea abies (L.) Karst.), phenology, cambium, xylogenesis, model, conifers, Europe
Izvleček: Študija prikazuje nastajanje lesa smreke (Picea abies (L.) Karst.) v rastni sezoni 2015 na ras7šču Panška reka
v bližini Ljubljane (pribl. 46°00’N, 14°40’E, 400 m n.v.). Raziskava je del večletnega projekta spremljanja nastajanja
lesa več drevesnih vrst na različnih ras7ščih. V okviru raziskave smo spremljali širino kambijeve cone ter nastajajoče
branike lesa, kjer smo ločili celice v fazah postkambialne ras7 (PC) in nastajanja sekundarne celične stene (SW) ter
zrele (MT) celice. Rezultate smo primerjali z ugotovitvami študije, opravljene na smreki z istega ras7šča v rastnih se-
zonah 2009–2011 ter s podatki, objavljenimi za več vrst iglavcev z različnih ras7šč po Evropi. Primerjave smo uporabili
za izdelavo regresijskih modelov z dvema neodvisnima spremenljivkama. Modeli so potrdili vpliv zemljepisne širine in
nadmorske višine na kri7čne datume začetka in konca ter na trajanje nastajanja lesa. Modeli so omogočili rekonstruk-
cijo fenologije nastajanja lesa iglavcev po Evropi.
Ključne besede: smreka (Picea abies (L.) Karst.), fenologija, kambij, ksilogeneza, modeliranje, iglavci, Evropa
long-term carbon sequestraAon, and has a great im-
pact on terrestrial ecosystems (Cuny et al., 2015).
Variability and potenAal changes in the wood for-
maAon process thus have ecological and economic
implicaAons. Wood, or secondary xylem, is an im-
portant tree Assue, with mechanical and physiolo-
gical funcAons (Wilson & White, 1986; Cuny et al.,
2014). It is produced by cell division in cambium,
which is a secondary meristem that produces secon-
dary xylem and secondary phloem. Axial tracheids
are the main cells in secondary xylem of conifers.
A.er their formaAon, tracheids undergo the process
of differenAaAon with the phases of (I) cell enlarge-
ment, (II) wall deposiAon and lignificaAon and (III)
programmed cell death (Plomion et al., 2001).
1 INTRODUCTION
1 UVOD
Wood producAon (xylogenesis) is closely rela-
ted to the funcAon and survival of trees and forest
ecosystems (Cuny et al., 2012). It is also linked to
1 University of Alicante, Faculty of Sciences, Road of San
 Vicente del Raspeig s/n, 03690 San Vicente del Raspeig,
 Alicante, Spain
2 University of Ljubljana, Biotechnical Faculty, Department
of Wood Science and Technology, Jamnikarjeva 101, 1000
Ljubljana, Slovenia
* e-mail: katarina.cufar@bf.uni-lj.si
3 Slovenian Forestry InsAtute, Večna pot 2, SI-1000 Ljubljana,
Slovenia
4 University of Zaragoza, Department of Geography, Pedro
Cerbuna 12, 50009 Zaragoza, Spain
UDK 630*811.13:174.7 (Picea abies L. Karst.) Original scienAfic arAcle / Izvirni znanstveni članek
Les/Wood, Vol. 66, No. 2, November 2017
Useros, F., Balzano, A., Prislan, P., de Luis, M., Gričar, J., Merela, M., & Čufar, K.: Nastajanje lesa smreke na
nižinskem rasAšču v Sloveniji v letu 2015 in primerjava z iglavci po Evropi
16
Secondary meristems are highly plasAc. They
can respond to environmental sAmuli and allow mo-
dificaAons of the plant morphology during its life-
cycle in order to adapt to the local condiAons (Gričar
et al., 2014). Plants try to maximize the use of re-
sources while minimizing the effects of various
harmful events. Growth occurs when the condiAons
are favourable (Cuny et al., 2012). In temperate and
boreal biomes, cold temperatures are the main li-
miAng factor of growth (Rossi et al., 2008), and cam-
bium is usually acAve from spring unAl autumn, with
a shorter growth period in boreal ones. Wood incre-
ment is deposited in the form of annual growth
rings, with clear growth ring boundaries, which are
studied in disciplines like dendrochronology (e.g.,
Novak, 2016; Wilson & White, 1986). Besides tem-
perature, precipitaAon, photoperiod and other fac-
tors also affect wood formaAon. A lack of water is,
for instance, the main factor impacAng growth in
xeric ecosystems, while fires, aBacks by parasites or
herbivores, air polluAon or human acAvity are also
important (Novak, 2016; de Luis et al., 2011). All
these factors impact the physiological and structural
properAes of wood, which are important for its use
(Humar et al., 2008; Wilson & White, 1986). There-
fore, understanding the wood formaAon process
and the factors influencing it can help us predict the
performance and survival of trees as well as the pro-
perAes of the related wood.
Many of the menAoned factors, especially the
climaAc ones, are influenced by the laAtude and al-
Atude of the sites where the trees grow. MarAnez
del CasAllo et al. (2016) showed a clear negaAve re-
laAon between laAtude and xylogenesis duraAon in
Fagus sylva7ca (shorter at lower laAtudes) and Pinus
sylvestris (higher at lower laAtudes). AlAtude also af-
fects the growth, especially through duraAon of xy-
logenesis (Rossi et al., 2008; Cocozza et al., 2016).
Understanding relaAonships between xylem forma-
Aon phenology and intra-annual climaAc/weather
condiAons could help us to beBer understand the
potenAal of species to adapt to future climate con-
diAons.
Norway spruce (Picea abies (L.) Karst.) is a spe-
cies whose wood formaAon dynamics has been in-
vesAgated on a great variety of sites in Europe. The
natural area of this species is mainly in the NE boreal
zone and mountainous areas of Europe (EUFORGEN,
2009). Since the 18th century it has been widely cul-
Avated for Amber producAon all over Europe, also
outside its natural range, such as on the lowland
sites in Slovenia (Zavod za gozdove Slovenije, 2017).
In Slovenia spruce has a great importance from
both economic and ecological perspecAves (Brus,
2012; Caudullo et al., 2016). Its phenology is affec-
ted by climaAc change which is in Slovenia mainly
characterized by increasingly mild winters which fa-
vour the early onset of spring phenology (Čufar et
al., 2008, 2012; de Luis et al., 2014), frosts which
o.en interrupt spring phenology (Sinjur, 2017), as
well as increasing temperatures with hot spells and
frequent drought periods during the vegetaAon pe-
riod (Cegnar, 2015 a, b).
The objecAve of this study was to invesAgate
the wood formaAon and dynamics of cambial acA-
vity in Picea abies on a site near Ljubljana, Slovenia
in 2015. The second objecAve was to compare the
obtained results with the published data from wood
formaAon studies in Picea abies and other conifers
from all over Europe, and to apply models to eva-
luate the effects of laAtude and alAtude and to pre-
dict/reconstruct phenology of wood formaAon in
conifers in Europe.
2 MATERIALS AND METHODS
2 MATERIAL IN METODE
2.1 STUDY SITE, TREES, TISSUES AND ANALYSES
2.1 RASTIŠČA, DREVESA, TKIVA IN ANALIZE
The dynamics of xylem producAon were stu-
died in six mature Norway spruce (Picea abies (L.)
Karst.) trees growing at Panška reka, near Ljubljana
(ca. 46°00’N, 14°40’E, 400 m a.s.l.). The site is a
mixed forest of the HacqueAo-Fagetum typicum
type, containing mainly Fagus sylva7ca, Acer pseu-
doplatanus and Picea abies, with dolomites as the
predominant rock type in the area (Gričar et al.,
2014). The climate at the site is temperate humid.
The site is included in the long-term monitoring of
xylem and phloem formaAon and cambial acAvity in
different species and sites which started in 2006
(Čufar et al., 2015).
The microcores were collected from the stems
at 1.1–1.7 m above ground from March unAl October
2015. The methodology of sample collecAon and As-
sue preparaAon followed the procedure described by
Prislan et al. (2014). According to this methodology,
Useros, F., Balzano, A., Prislan, P., de Luis, M., Gričar, J., Merela, M., & Čufar, K.: Wood formaAon in Norway spruce
on a lowland site in Slovenia in 2015 and comparison with other conifers all over Europe
Les/Wood, Vol. 66, No. 2, November 2017 17
the micro cores were collected with a Trephor tool at
weekly intervals, fixed in a soluAon of ethanol, for-
malin and aceAc acid, and then stored in ethanol. Af-
terwards, the microcores were trimmed, dehydrated
in a graded series of ethanol, infiltrated with D-limo-
nene and paraffin, and embedded in paraffin blocks.
The 9 µm thick transverse secAons were cut, trans-
ferred to object glasses, stained with safranin and
astra blue and embedded in Euparal. The Assues
were observed with a Nikon Eclipse E800 light mi-
croscope, and analysed with the NIS Elements image
analysis program.
We measured the width (in µm) and counted the
number of cells in different Assue porAons: cambium
(CC), current xylem increment with postcambial cells
(PC), cells in the phase of secondary cell wall deposi-
Aon and lignificaAon (SW) and mature (MT) xylem
cells (Fig. 2). We also measured/counted the width
and cell number in the three previous xylem rings.
2.2 DATA PROCESSING
2.2 OBDELAVA PODATKOV
For each tree and date, measurements were
taken along three radial rows and then averaged. To
study the behaviour of the trees as a whole popula-
Aon we performed quality control of the data, which
showed that two of six trees contained numerous
anomalies due to wounding, therefore we averaged
the data of four trees and calculated the basic staA-
sAcs.
The data were then processed by using the R
package “CaviaR” (Rathgeber et al., 2011). The
Gompertz funcAon was fiBed to the data to des-
cribe the producAon of new cells in the wood (Rossi
et al., 2003):
2.3 CLIMATIC DATA
2.3 KLIMATSKI PODATKI
Monthly sums of precipitaAon as well as mini-
mum and maximum mean temperatures from 1974
unAl 2015 were obtained using KNMI Climate explo-
rer (Trouet & Van Oldenborgh, 2013). The staAon
found by the gridded system was situated at 45°30’
-46°N and 14°30’-15°E. Comparing the data of 2015
with the average data of the previous 30 years, we
noAced an increase in the average minimum and
maximum temperatures during summer and winter
months, and a noAceable decrease in the precipita-
Aon of March and April (Figure 4).
In addiAon, we also collected daily climaAc data
for 2015 from the Grosuplje meteorological staAon
(45°58’N, 14°39’E, 350 m a.s.l.) located near the fo-
rest site using the website of the Slovenian Environ-
ment Agency (ARSO) (meteo.si), and checked the
reports on climaAc characterisAcs for the year 2015
in Slovenia (Cegnar, 2015 a, b).
2.4 EFFECTS OF LATITUDE AND ALTITUDE ON
PHENOLOGY OF XYLOGENESIS
2.4 VPLIV ZEMLJEPISNE ŠIRINE IN NADMORSKE
VIŠINE NA FENOLOGIJO NASTAJANJA LESA
To place the wood formaAon data of Picea
abies at Panška reka in 2015 in a wider context, we
compared them with the published data on conifers
growing on a wide range of sites all over Europe. We
collected the data on onset, cessaAon and duraAon
of xylem producAon from 47 studies involving Picea
abies and seven other coniferous species growing
at laAtudes ranging from 38.1 to 68.5°N and alAtu-
des from 15 to 2156 m a.s.l. (Table 3).
Using the R package “lmtest” we used two-fac-
tor linear regression models to check the correlaAon
of laAtude, alAtude and their interacAon with the
dates of onset, end, and duraAon of the cell produc-
Aon. Variables were selected using a stepwise pro-
cess. Absence of significant autocorrelaAon in the
residuals was checked with the Durbin-Watson test.
We calculated the models with the dependent va-
riable unchanged, transformed logarithmically, and
square rooted, and then chose the one with beBer
fiCng (using AIC criteria). Finally, we applied the ob-
tained models to the whole European conAnent and
mapped the predicted values of onset, end and du-
raAon of the cell producAon (Rossi et al., 2011).
where N(t) is the number of tracheids (all pha-
ses) at a given Ame t; A the upper asymptote, indi-
caAng the final number of tracheids; β the x-axis
placement parameter, reflecAng Ame 0; and k the
rate of change parameter, which determines the
spread of the curve.
We also used CaviaR for determining the criAcal
dates of xylogenesis: onset, cessaAon and duraAon
of PC, SW and MT phases of xylem formaAon (Rath-
geber et al., 2011).
Les/Wood, Vol. 66, No. 2, November 2017
Useros, F., Balzano, A., Prislan, P., de Luis, M., Gričar, J., Merela, M., & Čufar, K.: Nastajanje lesa smreke na
nižinskem rasAšču v Sloveniji v letu 2015 in primerjava z iglavci po Evropi
18
3 RESULTS AND DISCUSSION
3 REZULTATI IN RAZPRAVA
3.1 WOOD FORMATION IN SPRUCE AT PANŠKA
REKA IN 2015
3.1 NASTAJANJE LESA SMREKE NA PANŠKI REKI
LETA 2015
Assessment of cambial acAvity was based on
number of cambial cells (CC) (Figure 1, Figure 2a).
We recorded 5.11±0.69 CC on 10 April (DOY 100).
A.erwards, the number of CC increased and sho-
wed a peak in the second week of May (8.22±3.09
cells) and another in the second week of July
(7.55±0.60 cells). Therea.er the number of CC de-
creased (Figure 2a).
The first postcambial cells (PC) which indicate
the onset of wood producAon were observed on 24
April (DOY 114) (Figures 2b, 3). The maximum num-
ber of PC cells was observed from the end of May
to the first weeks of June. In most trees PC produc-
Aon finished between 24 August and 5 September
(DOY 236-248). However, in some samples indivi-
dual PC cells could be observed unAl the end of the
sampling period in October 2015 (Figure 2b).
The first SW cells, undergoing secondary cell
wall deposiAon and lignificaAon, were observed bet-
ween 12 and 23 May (DOY 132-143) (Figures 1, 2c).
Between 28 May and 9 June (DOY 148-160) we ob-
served the first fully mature MT cells (Figures 1, 2d).
Figure 1. Cross-sec7ons of Norway spruce (Picea abies) 7ssues on different dates of the year in 2015 at Panška reka
site near Ljubljana with varying numbers of cambial cells (CC), cells of secondary xylem in various phases of
differen7a7on (PC and SW), and mature cells (MT). The newly formed secondary phloem increment consists of varying
por7ons of early (EP) and late (LP) phloem, divided by axial parenchyma (AP).
Slika 1. Prečni prerezi tkiv smreke (Picea abies) z ras7šča Panška reka v okolici Ljubljane na različne datume v letu 2015.
Opazujemo lahko različno število celic v kambiju (CC) in nastajajoči braniki lesa, kjer je mogoče loči7 celice v različnih
fazah diferenciacije (PC in SW) in zrele celice (MT). Novo nastali floem vsebuje različne deleže ranega (EP) in kasnega
(LP) floema, ki ju razmejuje aksialni parenhim (AP).
Useros, F., Balzano, A., Prislan, P., de Luis, M., Gričar, J., Merela, M., & Čufar, K.: Wood formaAon in Norway spruce
on a lowland site in Slovenia in 2015 and comparison with other conifers all over Europe
Les/Wood, Vol. 66, No. 2, November 2017 19
Figure 2. Average number of cells or widths of differen7a7ng and mature xylem in Norway spruce (Picea abies) from
Panška reka near Ljubljana on different days of the year (DOY) in 2015: (a) cambial cells (CC), (b) xylem cells in post-
cambial growth (PC), (c) xylem cells in phase of secondary cell wall deposi7on and lignifica7on (SW), and (d) mature
xylem (MT) expressed in number of cells (MT-gray line) and micrometers (MTm-black line) and 7me expressed in DOY
and calendar dates. Lines represent the average of four trees, and bars show standard devia7ons.
Slika 2. Povprečno število celic oz. širine diferencirajočega in zrelega lesa nastajajoče branike smrek (Picea abies) na
Panški reki v bližini Ljubljane v letu 2015: (a) kambijeve celice (CC), (b) ksilemske celice v fazi post-kambialne ras7 (PC),
(c) ksilemske celice v fazi odlaganja sekundarne celične stene in lignifikacije (SW) in (d) zrele celice lesa (MT), izražene
kot število celic (MT-siva črta) in širina v mikrometrih (MTm-črna črta). Grafi predstavljajo povprečne vrednos7 podatkov
za 4 drevesa, ročaji prikazujejo standardni odklon. Podatki so prikazani za različne dneve v letu (DOY), na (d) pa so pod
DOY pripisani tudi koledarski datumi.
Figure 3. Cri7cal dates for (a) onset and cessa7on of wood differen7a7on phases and (b) their dura7on. PC: xylem cells
in post-cambial growth, SW: secondary cell wall deposi7on and lignifica7on, MT mature cells, X: dura7on of xylogenesis.
In (a) the ends of the line indicate maximum and minimum values, the corners of the diamond the first and third
quar7les, and the middle of the diamond the median. In (b) different points on the line represent, minimum, Q1, median,
Q3 and maximum dura7on.
Slika 3. Kri7čni datumi za (a) začetek in zaključek faz PC, SW in MT ter (b) njihovo trajanje. PC: celice lesa v postkambialni
ras7, SW: razvoj sekundarne celične stene in lignifikacija, MT: zrele celice, X: trajanje nastajanja lesa. Na grafu (a) konca
črt kažeta maksimalne in minimalne vrednos7, vogali romba predstavljajo prvi in tretji kvar7l (Q1 in Q3), sredina romba
pa mediano. V grafu (b) točke na čr7 prikazujejo minimalno, Q1, mediano, Q3 in maksimalno trajanje posamezne faze.
Les/Wood, Vol. 66, No. 2, November 2017
Useros, F., Balzano, A., Prislan, P., de Luis, M., Gričar, J., Merela, M., & Čufar, K.: Nastajanje lesa smreke na
nižinskem rasAšču v Sloveniji v letu 2015 in primerjava z iglavci po Evropi
20
DifferenAaAon of the cells was completed by mid-
September to mid-October (Figures 2c, d, 3). The
final xylem growth ring of 2015 was on average
1468 µm wide and composed of 37 cells (Figure 2d).
AddiAonal parameters of wood formaAon dy-
namics in 2015, as calculated by the Gompertz func-
Aon, showed that the trees on average needed 125
days to produce 90% of the growth ring, produced
0.32 cells per day, and that the maximum produc-
Aon was 0.53 cells per day (Table 1).
3.2 WOOD FORMATION IN 2015 COMPARED TO
THAT OF PREVIOUS YEARS
3.2 NASTAJANJE LESA V LETU 2015 V PRIMERJAVI
S PREDHODNIMI LETI
As the wood formaAon has been followed over
a longer period in Picea abies at Panška reka (Gričar
et al., 2014, 2015; Čufar et al., 2015), the results for
2015 could be compared to those available for
2009, 2010 and 2011, while the data for 2012, 2013,
and 2014 are sAll being processed. The comparison
showed (Table 2) that on average xylem cell produc-
Aon in 2015 started five-11 days later than in pre-
vious years (2009–2011). On the other hand, maxi-
mum cell producAon and cessaAon of cell produc-
Aon occurred at approximately the same Ame as in
previous years. Consequently, the duraAon of cell
producAon was shorter and the tree ring was nar-
rower in 2015 than in 2009, 2010 and 2011.
Later onset of cell producAon in 2015 could
possibly be due to extremely low precipitaAon in
March and especially April 2015, which were below
the average of the last 30 years and lower than in
the years 2009, 2010, and 2011 (Figure 4). Although
previous invesAgaAons mainly confirmed the impor-
tance of spring temperatures for the onset of cam-
bial producAon in spruce (Gričar et al., 2006, 2014,
2015), the extremely dry condiAons in March and
April 2015 could be a reason for the later onset of
wood producAon. According to published reports
on the climaAc characterisAcs of 2015, it should also
be noAced that summer was parAcularly warm, with
five heat waves with daily temperatures of above
30 °C (Cegnar, 2015 a, b). Due to this, more detailed
studies are needed to clarify the effects of high tem-
peratures on the phenology of wood formaAon.
A β k tip Dt90 r90 rmax R2
Average 43.59 5.32 0.035 154.75 124.75 0.32 0.53 0.913
SD 10.90 1.28 0.010 8.26 35.98 0.06 0.088 0.075
Table 1. Parameters calculated with the Gompertz func7on showing averages and standard devia7on (SD)
for four trees. Dt90 indicates the number of days needed to produce from 5% to 95% of the xylem ring. r90
is the average growth rate (cells/day) in that period and rmax the maximum growth rate.
Preglednica 1. Parametri, izračunani s pomočjo Gompertzove funkcije, ki kažejo povprečja in standardne
odklone (SD) š7rih dreves. Dt90 prikazuje število dni, potrebnih za nastanek od 5 % do 95 % celotne branike.
r90 je povprečni prirastek (celic/dan) v tem obdobju in rmax je maksimalni prirastek.
2009 2010 2011 2015
Onset of cell producAon (DOY) 106.17±4.58 109.00±0 103.17±2.86 114.00±0.00
Maximal cell producAon (DOY) 154.15±5.24 157.91±10.77 164.11±22.41 154.75±8.26
CessaAon of cell producAon (DOY) 243.00±14.68 243.17±8.18 237.50±17 241.50±15.83
DuraAon of cell producAon (days) 136.83±15.38 134.17±8.18 134.33±19.63 124.75±35.98
Number of cells in final xylem ring 68.66±31.72 55.47±12.38 63.49±27.37 37.50±13.77
Table 2. Parameters of xylogenesis for Norway spruce (Picea abies) at the forest site Panška reka, near
Ljubljana, Slovenia in 2015 compared with the published data for previous years (Gričar et al., 2014).
Preglednica 2. Kazalniki nastajanja lesa smreke (Picea abies) na Panški reki v bližini Ljubljane (Slovenija) v
letu 2015 in primerjava z razpoložljivimi podatki, objavljenimi za predhodna leta (Gričar et al., 2014).
Useros, F., Balzano, A., Prislan, P., de Luis, M., Gričar, J., Merela, M., & Čufar, K.: Wood formaAon in Norway spruce
on a lowland site in Slovenia in 2015 and comparison with other conifers all over Europe
Les/Wood, Vol. 66, No. 2, November 2017 21
3.3 COMPARISON OF XYLEM PHENOLOGY ON
DIFFERENT SITES - EFFECT OF LATITUDE AND
ALTITUDE
3.3 PRIMERJAVE FENOLOGIJE NASTAJANJA LESA
MED RASTIŠČI – VPLIV ZEMLJEPISNE ŠIRINE
IN NADMORSKE VIŠINE
Although the results obtained for one year and
locaAon provide valuable insights into the wood for-
maAon process, we wanted to relate these findings
to wood formaAon data for a larger sample from the
published literature (Table 4). As Picea abies mainly
grows on higher elevaAons and laAtudes (EUFOR-
GEN, 2009), and its wood formaAon data cover laA-
tudes from 45.93 to 67.50°N, we collected also the
data of other conifers (mainly Pinus sp.) to widen the
interval to 38.10 – 68.5°N and alAtudes from 15 to
2156 m a.s.l. The collected data showed high varia-
bility of onset (DOY 47 – 168, 16 February-17 June),
cessaAon (DOY 198-334, 17 July-30 November) and
duraAon (49–287 days) of wood producAon (Table 4).
Figure 4. Monthly sum of precipita7on (P) and average minimum and maximum temperatures (Tmin and Tmax)
in 2015 and 2009–2011, compared with the averaged 1974–2015 data (P30, Tmin30, Tmax30), with the
standard devia7on represented by error bars. Source: KNMI Climate explorer (Trouet & Van Oldenborgh, 2013).
Slika 4. Mesečne padavine (P) in povprečne minimalne in maksimalne temperature (Tmin in Tmax) v le7h 2015
in 2009–2011 v primerjavi s povprečnimi vrednostmi za obdobje 1974–2015 (P30, Tmin30, Tmax 30). Ročaji
prikazujejo standardni odklon. Vir: KNMI Climate explorer (Trouet & Van Oldenborgh, 2013).
Table 3. Models showing the effects of la7tude (l) and al7tude (a) on phenology (onset, end and dura7on)
of wood produc7on.
Preglednica 3. Modeli, ki prikazujejo vpliv zemljepisne širine (l) in nadmorske višine (a) na fenologijo (začetek
(onset), konec (end) in trajanje (dura7on)) kambijeve produkcije celic lesa.
Model
Onset DOY =
3,6447 l +
0,0289 a -
77,3589
End DOY =
365,515 -
2,2280 l -
0,0218a
Log(Dura7on) =
1.008 ∙ 10-5 l * a -
2,574 ∙ 10-2 l -
6,525 ∙ 10-4 a + 3,428
R2 0.8275 0.3810 0.803
p-value 2.2 ∙ 10-16 9.84 ∙ 10-6 7.1 ∙ 10-16
F 111 on 2 and 44 DF 15.15 on 2 and 44 DF 63.8 on 3 and 43 DF
Les/Wood, Vol. 66, No. 2, November 2017
Useros, F., Balzano, A., Prislan, P., de Luis, M., Gričar, J., Merela, M., & Čufar, K.: Nastajanje lesa smreke na
nižinskem rasAšču v Sloveniji v letu 2015 in primerjava z iglavci po Evropi
22
Table 4. Onset, cessa7on and dura7on of cambial cell produc7on in Picea abies and other conifers from
various sites in Europe used for the regression models.
Preglednica 4. Podatki o začetku, koncu in trajanju kambijeve celične produkcije smreke in drugih iglavcev
na različnih ras7ščih po Evropi, ki so bili uporabljeni v regresijskem modelu.
Species Country
Latitude
(°N)
Longitude
(°E)
Altitude
(m a.s.l.)
Onset
(DOY)
End
(DOY)
Duration
(days)
Reference
Picea abies Slovenia 46.00 14.65 400 114 241 125 This study
Picea abies Italy 45.93 11.05 680 106 258 152 Cocozza et al., 2016
Picea abies Slovenia 46.18 14.38 392 126 225 99 Rossi et al., 2008
Picea abies Slovenia 46.27 16.80 1200 117 228 111 Gričar et al., 2014
Picea abies Slovenia 46.35 13.97 1280 144 221 77 Rossi et al., 2008
Picea abies Italy 46.35 11.48 1780 168 256 88 Cocozza et al., 2016
Picea abies Italy 46.45 12.13 2156 157 225 68 Rossi et al., 2008
Picea abies Italy 46.45 12.13 2085 155 233 78 Rossi et al., 2008
Picea abies Austria 47.23 10.84 750 119 238 119 Swidrak et al., 2014
Picea abies France 48.48 7.15 643 122 221 99 Cuny et al., 2012
Picea abies Czech Rep. 49.29 16.67 650 112 232 120 Gričar et al., 2014
Picea abies Finland 60.20 25.00 60 152 229 77 Jyske et al., 2014
Picea abies Finland 60.20 25.00 60 142 232 90 Jyske et al., 2014
Picea abies Finland 61.20 25.60 120 163 229 66 Jyske et al., 2014
Picea abies Finland 61.20 25.60 120 163 237 74 Jyske et al., 2014
Picea abies Finland 61.90 24.30 181 152 215 63 Jyske et al., 2014
Picea abies Finland 61.90 24.30 181 147 217 70 Jyske et al., 2014
Picea abies Finland 62.40 27.30 110 158 220 62 Jyske et al., 2014
Picea abies Finland 62.40 29.40 84 159 217 58 Jyske et al., 2014
Picea abies Finland 62.40 27.30 110 153 217 64 Jyske et al., 2014
Picea abies Finland 63.00 29.50 183 166 216 50 Jyske et al., 2014
Picea abies Finland 66.20 26.40 140 165 221 56 Jyske et al., 2014
Picea abies Finland 66.20 29.50 140 168 226 58 Jyske et al., 2014
Picea abies Finland 67.50 29.40 390 163 215 52 Jyske et al., 2014
Pinus halepensis Spain 38.10 -0.65 15 47 334 287 de Luis et al., 2007
Pinus halepensis Spain 38.52 -0.64 845 75 310 235 de Luis et al., 2011
Pinus halepensis Spain 39.16 -1.15 850 93 198 105 de Luis et al., 2007
Pinus leucodermis Italy 39.90 16.20 2053 155 220 65 Rossi et al., 2008
Pinus sylvestris Spain 41.79 -1.82 1600 100 245 145 MarAnez del CasAllo et al., 2016
Pinus sylvestris Spain 41.80 -1.81 1200 87 246 160 MarAnez del CasAllo et al., 2016
Pinus sylvestris Austria 47.23 10.84 750 111 248 137 Gruber et al.,2010 
Pinus sylvestris Austria 47.23 10.84 750 119 279 160 Gruber et al.,2010 
Pinus sylvestris Austria 47.23 10.84 750 107 214 107 Oberhuber et al., 2011
Pinus sylvestris Austria 47.23 10.84 750 108 236 128 Swidrak et al., 2014
Pinus sylvestris France 48.40 6.32 270 111 259 148 Rathgeber et al., 2011
Pinus sylvestris France 48.48 7.15 643 116 250 134 Cuny et al., 2014
Pinus sylvestris Finland 66.37 26.72 150 160 219 59 Rossi et al., 2008
Pinus sylvestris Finland 68.50 27.50 220 166 215 49 Rossi et al., 2008
Pinus uncinata Italy 45.05 6.67 2030 143 204 61 Rossi et al., 2008
Abies alba France 48.48 7.15 643 117 243 126 Cuny et al., 2012
Larix decidua Italy 45.05 6.67 2030 155 208 53 Rossi et al., 2008
Larix decidua Italy 46.45 12.13 2156 154 218 64 Rossi et al., 2008
Larix decidua Italy 46.45 12.13 2085 150 226 76 Rossi et al., 2008
Larix decidua Austria 47.23 10.84 750 120 217 98 Swidrak et al., 2014
Pinus cembra Italy 45.05 6.67 2030 139 201 62 Rossi et al., 2008
Pinus cembra Italy 46.45 12.13 2156 145 223 78 Rossi et al., 2008
Pinus cembra Italy 46.45 12.13 2085 141 223 82 Rossi et al., 2008
Useros, F., Balzano, A., Prislan, P., de Luis, M., Gričar, J., Merela, M., & Čufar, K.: Wood formaAon in Norway spruce
on a lowland site in Slovenia in 2015 and comparison with other conifers all over Europe
Les/Wood, Vol. 66, No. 2, November 2017 23
The models applied to data obtained from 47
sites confirmed the staAsAcally significant effects of
laAtude and alAtude on the onset, cessaAon, and
duraAon of xylem producAon (Table 3). LaAtude and
alAtude both have a posiAve influence on the date
of onset (higher laAtude and alAtude, later onset),
and a negaAve one on the cessaAon date (higher la-
Atude and alAtude, earlier cessaAon). The correla-
Aons are higher for the onset than for the cessaAon
of cell producAon.
LaAtude proved to be the most important fac-
tor for explaining the variability in phenology of cell
producAon in conifers on different sites. The effect
of alAtude proved to be important as well, but it
only became evident a.er filtering the effect of la-
Atude. AlAtude therefore explained the residuals of
the laAtude model. As such, the model including
both laAtude and alAtude gave the best predicAons.
These results support the observaAon that tem-
perature is likely the main determinant of the onset,
cessaAon and duraAon of wood producAon in tem-
Figure 5. Maps of Europe showing predicted values for (a) onset - start, (b) cessa7on – end, and (c) dura7on of
cell produc7on in wood according to the two-factor, al7tude and la7tude, regression models (see Tables 3 and 4).
Slika 5. Zemljevidi Evrope in napoved vrednos7 za (a) začetek, (b) konec in (c) trajanje kambijeve produkcije celic
lesa, izračunanimi s pomočjo dvofaktorskega regresijskega modela (faktorja: zemljepisna širina in nadmorska
višina) (glejte tudi preglednici 3 in 4).
Les/Wood, Vol. 66, No. 2, November 2017
Useros, F., Balzano, A., Prislan, P., de Luis, M., Gričar, J., Merela, M., & Čufar, K.: Nastajanje lesa smreke na
nižinskem rasAšču v Sloveniji v letu 2015 in primerjava z iglavci po Evropi
24
perate ecosystems, although extremely dry condi-
Aons during the spring may also affect the onset of
this. Trees at different laAtudes are adapted to local
condiAons, and mainly to the duraAon of the warm
period (Rossi et al., 2006; Cocozza et al., 2016).
These relaAons with temperature are highest for the
onset of cell producAon, whereas cessaAon of cell
producAon seems to be more dependent on indivi-
dual factors (e.g. tree vitality, social status, soil pro-
perAes, topography) and therefore shows a weaker
correlaAon in the model. Increasing laAtude and al-
Atude also decrease the duraAon of cell producAon
by the cambium.
These results confirm the high plasAcity of Picea
abies and other conifers, which allows them to adapt
to the environmental condiAons of a wide range of
sites (Gričar et al. 2014, 2015; MarAnez del CasAllo
et al., 2016). An increase in laAtude is generally ac-
companied by a decrease in temperature and shorter
duraAon of the warm period that allows growth. Va-
riaAon in laAtude also entails differences in day
length (photoperiod), whereas variaAon in alAtude
in a parAcular area does not include this effect.
As we worked with data from various publica-
Aons, it should be noted that the predicAons may
be less reliable in some areas, for instance in the
Mediterranean, where the data are scarce, and lack
of water availability is generally more important for
limitaAon of growth than temperature (Novak et al.,
2013).
4 CONCLUSIONS
4 ZAKLJUČKI
Xylogenesis in Picea abies at Panška reka near
Ljubljana during 2015 followed the paBern which
could be fiBed to the Gompertz funcAon. The cha-
racterisAcs of wood formaAon in 2015 were a de-
layed onset of cell producAon, shorter duraAon of
cell producAon and narrower tree ring width com-
pared to the years 2009, 2010, and 2011. This can
possibly be ascribed to the very dry March and April
in 2015, whereas this study could not explain the ef-
fects of that year’s hot summer on the xylogenesis
phases, with five heat waves having occurred.
Maximal tracheid producAon occurred around
4 June 2015 (± ca. 1 week), which is comparable to
the situaAon in 2009–2011 at the same site, and
earlier than reported for conifers in colder climates,
which generally show maximal tracheid producAon
around the summer solsAce, i.e. 21 June (Rossi,
2006). The factors involved in cessaAon of cell pro-
ducAon seem to be less evident than those involved
in its onset.
Comparison of wood formaAon in Picea abies
on the site near Ljubljana with the phenology of
xylem producAon in the same species and other co-
nifers (mainly Pinus sp.) all over Europe showed a
significant correlaAon with laAtude and alAtude. An
increase in laAtude and alAtude as a rule resulted in
a delayed onset, earlier cessaAon and shorter dura-
Aon of cambial producAon. LaAtude proved to be
more important for explaining the variability in phe-
nology of cell producAon than the alAtude.
This study is part of long-term monitoring pro-
ject of wood formaAon in Slovenia, and shows that
long-term observaAons of the same species and site
are necessary to predict the intra-annual and inter-
annual growth of trees, which is crucial for their sur-
vival and producAvity in changing climate as well as
for wood quality.
5 SUMMARY
5 POVZETEK
Vaskularni kambij je sekundarni meristem, ki
proizvaja sekundarni ksilem (les) in sekundarni
floem. Nastajanje lesa in floema sta osnovna pro-
cesa, potrebna za rast in preživetje dreves, kar po-
sledično omogoča obstoj gozdnih ekosistemov.
Nastajanje lesa (ksilogeneza) vpliva tudi na kroženje
ogljika in na kakovost lesa. Kambijeva akAvnost in
produkcija celic lesa sta odvisna od okoljskih dejav-
nikov. V zmernem podnebnem pasu temperatura in
padavine pomembno vplivajo na kambijevo produ-
kAvnost. Kambij praviloma deluje periodično in je
akAven v toplejšem ter mirujoč v hladnejšem ob-
dobju leta, zato v enem koledarskem letu praviloma
nastane ena prirastna plast oz. branika lesa. Perio-
dično letno priraščanje je osnova za dendrokrono-
loške študije.
Namen te študije je bil proučiA dinamiko nasta-
janja lesa smreke (Picea abies (L.) Karst.) na rasAšču
Panška reka v bližini Ljubljane v letu 2015. Cilj je bil
tudi primerjaA pridobljene rezultate s podatki o na-
stajanju lesa v isA vrsA na istem rasAšču v preteklih
leAh (2009–2011), ter s podatki o nastajanju lesa v
Useros, F., Balzano, A., Prislan, P., de Luis, M., Gričar, J., Merela, M., & Čufar, K.: Wood formaAon in Norway spruce
on a lowland site in Slovenia in 2015 and comparison with other conifers all over Europe
Les/Wood, Vol. 66, No. 2, November 2017 25
smreki in drugih iglavcih iz študij za rasAšča po Ev-
ropi. Namen primerjave je bil preveriA vpliv zemlje-
pisne širine in nadmorske višine na nastajanje lesa
ter pripraviA model za napovedovanje ali rekon-
strukcijo fenologije nastanka lesa iglavcev v Evropi.
Proces nastajanja lesa smo spremljali na 6
smrekah v rastni sezoni 2015 na Panški reki v bližini
Ljubljane (pribl. 46°00’N, 14°40’E, 400 m n.m.v.). V
ta namen smo od aprila do oktobra 2015 tedensko
odvzemali mikro izvrtke iz debel izbranih dreves na
višini 1,1 m–1,7 m nad tlemi. Izvrtke smo fiksirali v
raztopini formalina, ocetne kisline in etanola (FAA),
dehidrirali in prepojili s parafinom. Z rotacijskim mi-
krotomom smo odrezali 9 µm debele preparate
preč nega prereza tkiv, jih obarvali s kombinacijo bar-
vil safranin ter astra modro in jih vklopili v euparal
(Prislan et al., 2014). Na izdelanih anatomskih pre-
paraAh smo vzdolž treh radialnih nizov merili in šteli
celice kambija (CC), lesa nastajajoče branike ter
predhodnih branik. V nastajajoči braniki smo razli-
kovali celice v fazi postkambialne rasA (PC), odlaga-
nja in lignifikacije sekundarne celične stene (SW) in
zrele celice (MT) (slika 1). Po opravljenih meritvah
smo podatke preverili in zaradi rastnih anomalij iz-
ločili 2 vzorčni drevesi. Podatke smo staAsAčno ob-
delali, za izravnavo podatkov pa smo uporabili tudi
Gompertzovo funkcijo.
Vremenske razmere (mesečne padavine, mini-
malne ter maksimalne temperature) v letu 2015
smo primerjali z razmerami v obdobju 1974–2014.
Za leto 2015 so bile značilne podpovprečne pada-
vine v marcu in še posebej v aprilu ter nadpovpre-
čne temperature pozimi in poleA (slika 4). Po nižinah
je bilo leto 2015 drugo ali tretje najtoplejše v Slove-
niji, poleA pa se je zvrsAlo 5 vročinskih valov, ko so
temperature dosegle vsaj 30 °C (Cegnar, 2015 a, b).
Ocena akAvnosA kambija je temeljila na številu
celic v kambijevi coni (CC) (slika 1, slika 2a). 10. aprila
(na 100. dan v letu, DOY) je število CC znašalo 5,11 ±
0,69. Potem se je število CC povečalo z vrhom v dru-
gem tednu maja (8,22 ± 3,09). Po drugem tednu ju-
lija, ko je število CC znašalo 7,55 ± 0,60, se je število
začelo zmanjševaA (slika 2a).
24. aprila (DOY 114) smo opazili prve celice v fazi
postkambialne rasA (PC), kar je pomenilo, da se je za-
čela kambijeva produkcija celic lesa (slika 2b, 3). Naj-
večje število PC celic smo opazili od sredine maja do
prvih tednov junija. V večini dreves je bilo nastajanje
PC končano med 24. avgustom in 5. septembrom
(DOY 236–248). Prve SW celice, kjer sta potekala
odlaganje sekundarne celične stene in lignifikacija,
smo opazili med 12. in 23. majem (DOY 132–143)
(slika 1, 2c). Med 28. majem in 9. junijem (DOY 148–
160) smo opazili prve popolnoma zrele MT celice
(slika 1, 2d). Diferenciacija celic je bila zaključena od
sredine septembra do sredine oktobra (slika 2c, d, 3).
Povprečna ksilemska branika smreke v letu 2015 je
bila široka 1468 μm in je vsebovala 37 celic – traheid
v radialnem nizu (slika 2d). Parametri, izračunani s
pomočjo Gompertzove funkcije, so pokazali, da so
drevesa v povprečju potrebovala 125 dni, da so proiz-
vedla 5 %–95 % branike; v povprečju so proizvedla
0,32 celice na dan, največja proizvodnja je znašla 0,53
celice na dan in je bila zabeležena 4. junija (DOY
154.75±8.26) (preglednica 1 in 2).
Podatke nastajanja lesa smreke na Panški reki
smo lahko primerjali z razpoložljivimi podatki iz pred-
hodnih let za isto drevesno vrsto in rasAšče (Gričar et
al., 2014, 2015, Čufar et al., 2015). Primerjava je po-
kazala (preglednica 2), da se je proizvodnja ksilemskih
celic v letu 2015 v povprečju začela 5–11 dni kasneje
kot v predhodnih leAh (2009–2011). Maksimum in
zaključek produkcije celic smo v letu 2015 zabeležili
ob približno istem času kot v predhodnih leAh. Posle-
dično je bilo trajanje produkcije celic krajše, branika
2015 pa je bila v splošnem ožja kot branike, nastale v
leAh 2009, 2010 in 2011.
Kasnejši začetek produkcije celic v letu 2015 bi
bil lahko posledica suše v marcu in zlasA aprilu 2015,
vpliva poletne vročine pa v tej študiji nismo mogli
potrdiA.
Modeli za obdelavo podatkov smreke in drugih
iglavcev s 47 lokacij po Evropi so zajeli območje med
38.1° in 68.5° severne zemljepisne širine in nadmor-
ske višine od 15 do 2156 m n.m.v. Potrdili so staAsA-
čno značilen učinek zemljepisne širine in nadmorske
višine na začetek, zaključek in trajanje kambijeve
produkcije lesa (preglednica 3). Večja zemljepisna ši-
rina in višja nadmorska višina vplivata na poznejši za-
četek in bolj zgoden zaključek ter posledično na
krajši čas kambijeve produkcije, kar nakazuje velik
vpliv temperatur na omenjene fenološke faze. Rezul-
taA kažejo, da so zveze bolj značilne za začetek kot
za zaključek celične produkcije. Omenjeni rezultaA
dvofaktorskih linearnih regresijskih modelov za do-
ločanje datumov začetka in konca ter trajanja kam-
bijeve produkcije lesa so prikazani na zemljevidih
Evrope (slika 5 a, b, c).
Predstavljena študija je del dolgoročnega pro-
jekta spremljanja nastajanja lesa (in floema) v smreki
in bukvi (Fagus sylva7ca) na dveh rasAščih različnih
nadmorskih višin v Sloveniji (Čufar et al., 2015). Re-
zultaA kažejo, da je večletno spremljanje nastajanja
lesa na isA drevesni vrsA in rasAšču pomembno za ra-
zumevanje rasA dreves znotraj enega leta ter razlik
med leA. Razlike so v veliki meri odvisne od letne in
medletne variabilnosA klimatskih dejavnikov in izjem-
nih klimatskih razmer. Procesi nastajanja lesa in flo -
ema, na katere vplivajo tudi klimatske spremembe,
so ključni za preživetje in produkAvnost dreves ter
nenazadnje za kakovost lesa.
ACKNOWLEDGEMENTS
ZAHVALA
The study was supported by the Slovenian Re-
search Agency (programs P4-0015 and P4-0107 and
project Z4-7318). InternaAonal cooperaAon was
supported by the LLP Erasmus bilateral agreement
between the University of Ljubljana and the Univer-
sity of Alicante (supporAng the project work of Fer-
nando Useros and cooperaAon of Katarina Čufar,
MarAn De Luis and Josep Raventós) and by the COST
AcAon FP1106, STReESS. We thank Luka Krže, for his
valuable work in the field and laboratory, and Prof.
Dr. Josep Raventos for his immense support for the
cooperaAon among the co-authors. We are grateful
to two anonymous referees for their valuable com-
ments which helped us to improve the manuscript,
and to Paul Steed for English language ediAng.
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