ANATOMICAL CHANGES INDUCED BY FIRE-DAMAGED CAMBIUM IN TWO NATIVE TREE SPECIES OF THE CHACO REGION, ARGENTINA
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IAWA Journal, Vol. 31 (3), 2010: 283–292
Anatomical changes induced by fire-damaged
cambium in two native tree species of the
Chaco Region, Argentina
Sandra Bravo
Cátedra de Botánica, Facultad de Ciencias Forestales, Universidad Nacional de Santiago del Estero,
Avenida Belgrano 1912 (s), 4200 Santiago del Estero, Argentina
[E-mail: sjbravo@unse.edu.ar]
Summary
This study examined anatomical responses to fire damage of the cam-
bium in Schinopsis lorentzii and Aspidosperma quebracho-blanco.
Bole cross sections were extracted from specimens with external signs
of fire damage. Samples were taken from zones designated normal,
discoloured and wound altered. The vessel, fibre, axial and ray paren-
chyma percentages, tangential vessel diameter, vessels per mm 2, rays
per mm, and ray width and height of these zones were compared. Fire
scars and fire marks were identified on cross sections of S. lorentzii and
A. quebracho-blanco. The fire marks reflect minor wounds that did not
affect wood formation. The fire scars, on the other hand, are the result
of wounds that interrupted cambial activity thus affecting the shape of
the bole and causing discolouration of pre-existing wood adjacent to
wounds. The wood formed after fire damage included callus, barrier
zones at fire scar edges and the formation of ribs of wound wood. The
wound altered zone was characterised by a decrease in the percentage
of vessels and fibres, an increase in the percentage of axial parenchyma,
the formation of grouped rays, a decrease in vessel tangential diameter,
and occurrence of fibres with atypical structure. Disorientation in the
axial xylem system was observed in the barrier zone. The anatomical
responses to cambium damage and formation of discoloured wood and
woundwood ribs suggest that wood quality and utilisable volume of bole
in the studied species is affected by fire.
Key words: Schinopsis, Aspidosperma, cambium, fire damage, barrier
zone, discoloured wood.
Introduction
Scars and/or marks resulting from fire that occur in woody plants are useful for dating
past fires (Agee 1993; Kitzberger et al. 2000; Smith & Sutherland 2001; Bravo et al.
2001a). After fire damage plants begin physiological processes important for defense
against pathogen attack and recovery from wounding. These processes can vary with
fire characteristics, plant species and individuals. The most common responses to
Associate Editor: Susan Anagnost
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cambial damage are the formation of barrier
zones and the compartmentalization of the
injured area (Larson 1994; Gill 1995; Smith Argentine
Chaco Region
& Sutherland 2001). Injuries caused by fire
usually lead to wood decay, reduced com- Santiago
del Estero
mercial value and also may affect tree health
(e.g., Rademacher et al. 1984; Gill 1995;
Sutherland & Smith 2000; Bravo et al.
2001a; 2006).
The Chaco region extends through Ar-
gentina, Paraguay, Bolivia and a small part
of Brazil. In Argentina, the Chaco region is
approximately 600,000 km2 and includes
the provinces of Salta, Tucumán, Jujuy, Ca-
tamarca, Santiago del Estero, Córdoba,
Chaco, Santa Fe and Formosa (Hueck 1978;
de la Balze et al. 2004; Fig. 1). The vegeta-
tion of the Chaco is a mosaic of forests,
woodlands, savannas and shrublands (Mo-
rello & Adámoli 1968; Bucher 1982). Fire
and floods are the major disturbances in the
region (Bucher 1982; Bravo et al. 2001b;
Tálamo & Caziani 2003). Fires usually start
in savannas and may spread to forests and Figure 1. Location of the study area in the
woodlands when environmental conditions Argentine Chaco region.
are extreme (Kunst & Bravo 2003).
The forests in the western Chaco region, Argentina, are dominated by the decidu-
ous species Schinopsis lorentzii (Griseb.) Engler (quebracho colorado santiagueño)
of the Anacardiaceae and the evergreen Aspidosperma quebracho-blanco Schltdl.
(quebracho blanco) of the Apocynaceae. Both species are 16–20 m tall, produce high
quality diffuse porous wood and form annual growth rings (Giménez & Ríos 1999;
Moglia 2000). Schinopsis lorentzii has been overexploited during the last century for
tannin extraction, firewood, posts, railway sleepers and carpentry materials, and thus
may be at risk (Hueck 1978). Aspidosperma quebracho-blanco has been exploited
for similar uses although with lesser intensity. Wood defects such as fire scars and
fire marks are common in the wood of native species of the Chaco region (Giménez
et al. 1998; Moglia 2000; Bravo et al. 2001a; 2006) and make the woods unsuitable
for many uses because the anatomical structure and physical properties are altered.
The volume and degree of wood discolouration and pathogen attack may be affected
by the size and severity of the wounds and the vigour and age of individuals (Lee
et al. 1988). Bravo et al. (2008) found that fire wounds observed in some native woody
species of the Chaco region generally affect up to 20% of the bole perimeter.
The objective of this study was to determine how fire damage affected the anatomy
and wood quality of S. lorentzii and A. quebracho-blanco.
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Materials and methods
The research area was located in La Maria Experimental Ranch belonging to the
Instituto Nacional de Tecnologia Agropecuaria, Santiago del Estero, Argentina (28°
3' S and 64° 15 ' N). The vegetation is composed of patches of forests, savannas and
grasslands and could be considered typical of the Chaco region, Santiago del Estero
(Fig. 1). The climate is semiarid and strongly seasonal. Rainfall occurs mainly in sum-
mer, and the dry season extends from April to October. Average rainfall is 550 mm.
The average temperature is 26.9 °C for the hottest month and 12.4 °C for the coldest.
Extreme temperatures range between 42 and 45 °C in summer and -7 and -8 °C in winter
(Boletta et al. 2006).
Sampling was carried out in an ecotonal band located between the forests and the
savannas. This area was selected because fires that start in the savannas usually end
in the ecotone (Kunst & Bravo 2003). Cross sections of the bole of six specimens of
Schinopsis lorentzii and six specimens of Aspidosperma quebracho-blanco that had
external signs of fire damage (fire scars or scorched barks) were taken at 0.3 m height.
It was not feasible to core trees because of the species’ high density. Moreover, cross
sections allowed better views of fire injuries. Samples defined as normal wood, dis-
coloured wood, and altered wood were extracted from the cross sections (Fig. 2). The
wood defined as normal was formed before the fire, it has an anatomy that is typical
for the species. Discoloured wood is wood affected by heat transference and with less
coloration than normal wood. The altered wood zone included the barrier zone and
wound wood ribs generated by fire scars.
Discoloured wood
Wound altered wood zone
Normal wood
Figure 2. Sample extraction zones for microscopic sections in woody species boles.
Transverse, radial longitudinal and tangential longitudinal sections from each zone
were made using a Leitz sliding microtome following the methodology proposed by
Johansen (1940). The sections had a thickness of 10–15 µm and were stained with
safranin and fast green. Percentages of vessels, fibres, axial and radial parenchyma,
tangential diameter of vessels (µm), number of vessel per mm2, number of rays per
mm, maximum ray width (cell number) and height (µm) were measured as per the
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recommendations of Rademacher et al. (1984) and Gourlay and Grime (1994). The
percentages of vessels, fibres, parenchyma and rays were obtained following Quirk
and Smith’s methodology (1975). Photomicrographs were taken using Leitz-OM 100
equipment.
A parametric t test for matched samples was used to assess the significance of the
anatomical differences between the normal wood and the wound-altered wood. Analyses
were done using the statistical program Statistica 6.0.
A B
C D
E F
Figure 3. Fire-altered wood. – A: Fire scar in Schinopsis lorentzii. – B: Fire scar in Aspidosperma
quebracho-blanco. – C: Fire mark in S. lorentzii. – D: Fire mark in A. quebracho-blanco. –
E: Tannin secretion in compartmentalizated wood of S. lorentzii. – F: Cicatricial callus in
A. quebracho-blanco.— Scale bars in E & F = 50 µm.
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A B
C D
E F
Figure 4. Wood altered by fire scars. – A: Cross section of callus and barrier zone in Schinopsis
lorentzii. – B: Cross section of barrier zone in S. lorentzii. – C: Longitudinal radial section with
aggregated rays in Aspidosperma quebracho-blanco. – D: Cross section of barrier zone with crys-
tals in S. lorentzii. – E: Cross section of bar zone with atypical fibres in S. lorentzii. –
F: Longitudinal radial section with disorientation in axial xylem system in A. quebracho-blanco.
— Scale bars for A & C = 150 µm, for B, D–F = 50 µm.
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RESULTS
Fire scars and fire marks were identified on transverse sections of both species (Fig.
3A, B). Due to death of the cambium cells, wood formation was interrupted and fire
scars formed in both species. Fire scars altered the bole shape and remained open long
after the fire. Very close to the fire injury, the wood tissue became discoloured (Fig. 2,
3). Evidence of pathogen attacks (e.g. by insects and fungi) were also observed on the
discoloured wood (Fig. 3A, B).
Fire marks were minor wounds affecting isolated sectors of growth rings. Here the
fire did not kill the cambium or interrupt bole growth and, only occasionally, the wood
was discoloured (Fig. 3 C, D). The wood present at the time of fire damage showed
considerable accumulation of dark substances. Wood formed after fire damage differed
from the wood formed before the damage and consequently when samples dried the
two zones separated.
In fire scars, the discoloured wood was compartmentalized by the secretion of dark
substance, likely tannins or gums, between the unaltered and the altered wood tissue.
The secretion products were deposited in the axial and radial parenchyma, vessels, and
fibres of S. lorentzii, whereas in A. quebracho-blanco only parenchyma accumulated
secretion products (Fig. 3E). The wood formed after cambium damage showed a cal-
lus on the edge of the fire scar, barrier zones and ribs of wound wood (Fig. 3, 4). This
callus was formed by a proliferation of parenchyma cells. The altered wood zone cells
spread forming a convex mass of tissue, from the fire scar edges to the damaged surface
(Fig. 4A, B). Subsequently, the cambium produced ribs of wound wood with atypical
wide growth rings (Fig. 3A, B). The wound altered wood zone showed a decrease in
the percentage of vessels and fibres, an increase in the percentage of axial parenchyma
cells and grouped rays (Fig. 4A, B; Table 1). This zone accumulated secretion substances
similar to the ones observed at the boundary between normal and discoloured wood.
In S. lorentzii the ray parenchyma percentage and the ray width were reduced sig-
nificantly (p < 0. 05) in response to cambium damage. The number of rays/mm and ray
height in this species increased as did the crystal abundance. Aspidosperma quebracho-
blanco did not show significant differences in the ray parenchyma percentage. However,
it showed a significant increase in ray height and width (Table 1; Fig. 4 C).
The tangential diameter of vessels was reduced in both species. The fibres had thin-
ner cell walls and atypically wide lumens (Fig. 4 E). In addition the axial system was
disoriented (Fig. 4 F).
In the fire mark zone, the wood formed after the fire appears similar to that of
wound altered zone. The thickness of this zone varied between 200 and 250 µm in A.
quebracho-blanco and between 200 and 400 µm in S. lorentzii. A large accumulation
of dark substances occurred in response to cambium damage (Fig. 5 B).
DISCUSSION
The fire scars observed in Aspidosperma quebracho-blanco and Schinopsis lorentzii
are similar to those described in other species (e.g. Barret & Arno 1988; Agee 1993;
Larson 1994; Gill 1995; Smith & Sutherland 2001; Kitzberger et al. 2000; Grau et al.
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Table 1. Anatomical features of the normal and wound altered wood in Aspidosperma
quebracho-blanco and Schinopsis lorentzii.
A = average, SD = standard deviation. * = marginally significant, ** = significant, *** = highly sig-
nificant.
A. quebracho-blanco S. lorentzii
–––––––––––––––––––––––––––– –––––––––––––––––––––––––––
Variables Normal wood Wound altered Normal wood Wound altered
wood wood
––––––––––– ––––––––––– ––––––––––– –––––––––––
A SD A SD A SD A SD
% of vessels 14.6 5.3 7.8 5.3** 11.2 2.7 7.4 2.6 **
% of fibres 57.6 4.5 25.8 7.3 *** 65.5 4.1 17.4 3.9 ***
% of axial parenchyma 8 3.1 46.8 12.2*** 4.9 2.6 68 7.1***
% of radial parenchyma 19.8 2.7 19.6 5.9 18.4 2.9 6.4 3.7 ***
Tangential diameter of
vessels (µm) 68.8 13.5 53.3 14 *** 81 17.5 52.5 15.2 ***
No. of vessels per
ocular area (×40) 3.4 1.0 2.4 1.5 5.8 2.5 5 2.1*
No. of rays/mm 12.3 1.7 10.9 1.5 9.8 1.6 13 2 ***
Ray width (No. of cells) 2.9 0.4 5 1.2 *** 3.6 1.0 3.2 0.9 *
Ray height (µm) 246.8 77.5 256.5 109.3 * 240.3 48.8 263.3 60.9 *
Ray width (µm) 40.2 6.1 62 22.1*** 24.3 4.8 19.3 4.8 ***
2003). Comparable fire injuries have been observed also in other native woody species
of the Chaco region (Giménez et al. 1997, 1998; Bravo et al. 2001a, b; Bravo et al.
2006). Fire marks were described as minor fire wounds in Prosopis alba and P. nigra
(Bravo et al. 2001a) and they seem to be frequent in species with a thick bark, such
as A. quebracho-blanco and S. lorentzii (Bravo et al. 2008). Bark can be effective in
A B
Figure 5. Wood altered by fire marks. – A: Cross section with wood formed before and after fire
in Aspidosperma quebracho-blanco. – B: Cross section of barrier zone associated to fire mark
in Schinopsis lorentzii. — Scale bars = 150 µm.
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preventing fire damage to the cambium because it reduces thermal shock (Hengst &
Dawson 1994; Gill 1995; Sutherland & Smith 2000). In the fire marks described in
this work, the cambium remained functionally active allowing continuity of wood pro-
duction. However, when the wood is dried, fissures would develop between the wood
formed before and after fire damage and this would be a serious defect.
The discoloration observed in the wood present at the time of injury in A. quebracho-
blanco and S. lorentzii could be related to changes in moisture content caused by
wounds or heat transference during the fire. Rademacher et al. (1984) and Smith and
Sutherland (2001) stated that these changes favour reactions such as the breakdown of
phenolic compounds and the growth of pathogenic microorganisms, which can cause
discoloration. Discoloration associated with fire wounds has been observed in other
native species of the Chaco region and cited as a common wood defect (Giménez et al.
1997; Bravo et al. 2001a, 2006; Giménez 2003).
The accumulation of dark substances in the boundary layer between discoloured
and normal wood is a part of the compartmentalization process (Schmitt et al. 1995;
Rademacher et al. 1984). Compartmentalization reduces water loss and minimises the
spreading of wood decay (Sutherland & Smith 2000). Widespread tannin accumulation
in wood of S. lorentzii could be associated with the radial canals present in this spe-
cies and which likely react to fire damage (Giménez & Moglia 1995). Aspidosperma
quebracho-blanco normally does not produce coloured heartwood (Moglia & López
2001), but wounds caused by fire are dark-coloured indicating the synthesis of new
compounds or changes in common wood compounds (Fig. 3 B). The nature of these
chemical changes is yet to be analyzed.
The callus observed in fire-scar edges could originate from living wood parenchyma
and phloem cells (Smith & Sutherland 2001). In these two Chaco species the barrier zone
consisted of parenchyma cells, fibres and larger rays. The increase in axial parenchyma
in the barrier zone and wound altered wood zone probably resulted from enhanced
cell divisions of vascular cambium and phellogen (Larson 1994). The decrease in ray
parenchyma percentage in S. lorentzii (Table 1) differs from the response observed in
other species in which the percentage of ray parenchyma increases after cambial dam-
age (Rademacher et al. 1984; Gourlay & Grime 1994).
The ray parenchyma percentage and number of rays per mm in the barrier zone
and wound altered wood of A. quebracho-blanco was not significantly different from
‘normal wood’, but ray height and width were greater than in ‘normal wood’. The in-
crease of parenchyma in response to cambial damage has been interpreted as forming
an anatomical and chemical boundary to resist the moisture loss and to avoid the spread
of wood-inhabiting fungi (e.g., Rademacher et al. 1984; Smith & Sutherland 2001).
The parenchyma secretes substances such as tannins, gums and others into vessels and
fibres as a response to damage (Schmitt et al. 1995).
The significant decrease in the percentage of vessels and their tangential diameter in
A. quebracho-blanco and S. lorentzii is a response that would reduce water transport
and axial spread of pathogens (e.g., Rademacher et al. 1984). The atypical appearance
of the fibres could represent cells that are a transitional state between parenchyma and
fibres (Larson 1994). It has been suggested that the tree prioritises new cell production
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at the expense of cell differentiation (wall thickness) so as to quickly close wounds. The
locally wide growth rings of woundwood ribs are interpreted as important for closing
the wounds and reducing notch stresses (Smith & Sutherland 2001).
The results of this research indicate that fire scars reduce the utilisable volume of
the tree bole due to the formation of discoloured wood, interruption in wood formation,
and production of barrier zones. The anatomical changes in the barrier zone and wound
altered wood surely affect wood quality and the wood’s technological properties such
as durability, strength, and dimensional changes during drying. Although fire marks
represent minor wounds and do not represent interrupted cambial activity, the wood
in this region differs and can be considered a defect.
These results suggest that, if fire is used as a management tool in native forests or
commercial plantations, the risk of affecting wood quality needs to be considered. The
relation between fire wounds and the health of the tree (e.g., susceptibility to bacterial,
fungal and insects attacks) needs further research in the Chaco region.
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