Faculty of Sciences and Mathematics (Niš). Depattment of Biology and Ecology
Abstract
Picea omorika (Pančić) Purkynĕ is Balkan endemic coniferous species and
Tertiary relict of the European flora. Its natural habitat is fragmented and reduced to
the middle and upper courses of the Drina River, in Western Serbia and Eastern
Bosnia and Herzegovina. This region represents species long-term, cryptic and last
refugium. The current limited natural range of Serbian spruce is mainly the result of
the species poor competing ability. It retreats to areas less inhabitable by its
competitors, predominantly Picea abies and Fagus orientalis. It inhabits open
habitats, comprising cliffs and forest clearings, characterized by strong northerlywind,
snow and rockfalls. Since the middle of the 19th century, its natural range declines
continuously. Planting Serbian spruce outside its natural range has a long tradition,
either as an ornamental tree species, or for afforestation, throughout Central and
Northern Europe. Despite its endemism, P. omorika is considered as one of the most
adaptable spruces.
Wood properties are determined by cell arrangement, size and shape, and cell
wall structure and thickness. Conifers, as a response to mechanical stress, such as
wind and stem lean, form reaction wood called compression wood (CW). Its
formation occurs on the lower side of the leaning stem, resulting in eccentric growth.
Wood opposite to the CW in the same growth ring is termed opposite wood (OW),
while wood from growth rings that do not contain any CW is termed normal wood
(NW). CW is characterized by reduced tracheid length, rounder cell cross-sectional
profile, presence of intercellular spaces, absence of the S3 cell wall layer and presence
of helical cavities in S2 layer. Higher lignification, as one of the main characteristics
of CW, is associated with changed lignin composition, increased amounts of phydroxyphenyl monomers and increased condensation of monomer units in the
polymer. Consequently, CW contains less cellulose, with greatly increased amounts
of galactan, and slightly lower amounts of mannan and xylan, and with higher angle
of cellulose microfibrils in the S2 layer of the cell wall, compared to NW.
CW occurs in a range of gradations from near NW to severe CW (SCW), mild
CW (MCW) forming a continuum between NW and SCW. The degree of
development of particular features of CW does not necessarily change in parallel to
each other, so the severity of a given tracheid is represented as a function of the
degrees of development of individual features, mainly lignification, helical cavities
and cell wall thickness. Visual detection of compression wood severity, more
precisely the determination of MCW, is difficult. As the severity of CW affects
mechanical and chemical properties of wood in forest products industry it is desirable
to be able to measure CW severity.
We developed different morphometric and non-morphometric methods for
distinguishing wood samples on a compression severity scale. They are based on
tracheid double wall thickness, cellulose microfibrils order (distribution and
alignment of cellulose microfibrils), or variation in lignin structure. We used confocal
fluorescence microscopy and spectroscopy, and fluorescence-detected linear
dichroism (FDLD) microscopy, combined with development of new algorithms and
statistical analysis. We tested our methods on stem samples of P. omorika juvenile
trees exposed to long term static bending. P. omorika belongs to slow-growing conifer
species in which CW typically occurs in a severe form, while juvenile conifer wood
is characterized by randomly distributed MCW, NW often being absent. These are the
features that suggest P. omorika juvenile wood a good choice of samples for
evaluation of the precision of methods suggested for estimation of compression wood
severity.
Our methods for distinguishing wood samples on a compression severity scale
provide a fine gradation of juvenile P. omorika wood samples from NW to the severest
form of CW, compression severity scales being partially different. The presented
results qualify our methods for use in estimation of compression wood severity in
forest products industries, individually or in combination, and confirm juvenile P.
omorika stem samples as a good choice of samples for evaluation of the precision of
methods suggested for compression wood severity estimation
