Axial sampling height outperforms site as predictor of wood trait variation
2019
Covariation amongst wood traits along the stem axis is important to maintain
hydraulic integrity ensuring sufficient sap flow to the canopy. Here, we test how
wood traits (co)vary along the trunk and whether two seasonally dry Brazilian
habitats (cerrado and caatinga) influence this variation in two co-occurring species, Tocoyena formosa (Rubiaceae) and Tabebuia aurea (Bignoniaceae). The
samples were collected at five heights along the main trunk of three individuals per species in both sites. We used light, scanning and transmission electron
microscopy to observe the wood traits. Out of 13 wood traits, nine show relationships with sampling height: eight traits predict height in T. formosa and five
in T. aurea. Contrastingly, only three traits show differences between sites and
only for T. formosa. The intratrunk wood variation is reflected by the hydraulically weighted vessel diameter showing a curvilinear relationship, disagreeing
with the prediction of a continuous vessel widening from tip to base. In both
species, the largest vessels are linked to the thinnest intervessel pit membranes.
Wood density increases basipetally for both species, being site-dependent and
correlated with vessel traits in T. formosa, and site-independent and determined
by fiber wall thickness in T. aurea. Furthermore, the functional role of rays was
found to be different for each species, and may be related to the marked difference in ray composition. In conclusion, both species show a unique adaptation to
deal with height-related constraints using species-specific co-variation amongst
wood traits, while site does not contribute much to the wood variation.
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