Abstract
The Fynbos biome is one of five biomes with a mediterranean climate type. The climate regime
of these biomes is characterized by winter rains and summer droughts, with 90% or more of
the annual precipitation falling in the six months around winter. The fynbos biome occupies
approximately 70000 km2 near the southernmost extremity of the African continent, in the
South African provinces of Western Cape and Eastern Cape. The fynbos is a core of the Cape
Floristic Region (CFR) of about 90 000 km2 having an exceptional plant diversity (over 9000
species, almost 70% of which are endemics).
The present study aimed to explore the diversity of wood structure in southern African
Proteoideae (Proteaceae), Diosmeae (Rutaceae) and Erica (Ericaceae), i.e. in three large
endemic lineages of the Cape Floristic Region making essential contribution into the species
richness of fynbos vegetation, to clarify the evolutionary and ecological trends of wood trait
variation within these lineages. We also intended to assess the incidences of the wood
anatomical traits in the fynbos vegetation, and to compare them with those based on available
published data for the Californian chaparral and Mediterranean maquis to clarify the impact of
different environmental factors on the diversification of wood structure in these mediterraneantype
biomes.
The wood structure of 124 samples representing 97 species from 21 genera of Proteoideae (50
species, 11 genera), Diosmeae (21 species, 9 genera) and Erica (26 species) has been examined
and analyzed. Apart from that, the anatomical study of 30 samples from 16 species of 7 genera
of Thymelaeaceae has been carried out; these data were used only for the overall assessment
of the wood anatomical diversity in fynbos.
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Three studied plant groups (Proteoideae, Diosmeae and Erica) are distinctive from each other
in their wood anatomy, but each of them is uniform in its wood structure. The diversity patterns
within each of three lineages are more related to plant stature, climatic factors, and fire-survival
strategies than to their taxonomy or phylogeny.
Within Proteoideae, the longer and wider imperforate tracheary elements with larger bordered
pits are associated with mild maritime climates with year-round rainfall, rather than with more
continental climates with severe dry seasons. These trends may imply the evolutionary
transition from libriform fibres to fibre tracheids in mediterranean-type climatic region. This
presumable gain of hydraulic function of imperforate tracheary elements in the Proteoideae
could be explained by adaptation to freeze–thaw stress during the late Quaternary. The
occurrence of sclerified cells or/and irregularly arranged (occasionally circular) tracheary
elements in broad rays of some species are thought to be responses to excessive sucrose
transport in secondary phloem.
The shrubby species of Diosmeae are distinctive from closely related canopy tree Calodendrum
capense and large riparian shrub Empleurum unicapsulare in their wood traits. Among these
shrubby species, the vessel grouping weakly increases with increasing water limitation along
the gradient from a semi-arid winter-dry climate to a Mediterranean winter-wet climate. The
presence of spherocrystals and acicular crystals in the axial parenchyma and rays in Adenandra,
Agathosma, Coleonema, Diosma and Macrostylis provides indirect evidence of the presence
of flavonoids which may have important pharmacological properties.
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In the African Erica, the loss of scalariform perforation plates could be driven by increased
aridity and seasonality in the mid-Miocene, and its re-gain can represent an adaptation to
freezing in the high elevation species E. nubigena. This scenario is consistent with Carlquist’s
‘ratchet hypotheses. Increase of ray frequency in habitats with prominent dry and hot season
likely facilitates refilling of vessels after embolism caused by water stress. Wider rays are
ancestral for the lineage comprising African Erica and the Mediterranean E. australis.
Within Proteoideae and Erica, the species with resprouter fire response have wider rays than
the seeders. We hypothesize that the larger rays in overground stems of sprouters serve as the
sites of epicormic buds that provide for resprouting after fire or other damage. The negative
correlation between ray width and expression of summer drought is consistent with Ojeda’s
(1998) model explaining the diversification of seeders and resprouters among southern African
Erica.
The comparison between the incidences of wood anatomical traits in fynbos, chaparral, and
maquis shoed that the lack of ring-porosity and very low incidence of semi-ring-porous woods
is the most prominent difference of the fynbos from other two vegetation types. This feature
was attributed to the paucity of deciduous plants in fynbos which is probably associated with
the poor-nutrient soils in this biome. Apart from that, fynbos differs from chaparral and maquis
also in the greatest percentage of the species having no tracheids in their wood as well as with
the lowest incidence of helical thickenings on the vessel walls. These differences could be
explained by the lower tolerance of fynbos shrubs to seasonal drought due to relatively mild
dry season in this biome. We also found some differences between fynbos, chaparral, and
maquis in the incidences of fibre tracheids and vasicentric/vascular tracheids associated with
large vessel groups can be at least partly explained by different systematic floristic
compositions. Finally, the higher percentage of distinct growth rings, greater vessel diameter,
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lower vessel frequency, and higher mesomorphy index in maquis compared with fynbos and
chaparral can be attributed to a common occurrence of relatively tall trees in this vegetation.