Abstract
Increasing or decreasing wood density (WD) from pith to bark is commonly observed in tropical tree species. The different types of WD radial variations, long been considered to depict the diversity of growth and mechanical strategies among forest guilds (heliophilic vs. shade-tolerant), were never analyzed in the light of heartwood (HW) formation. Yet, the additional mass of chemical extractives associated to HW formation increases WD and might affect both WD radial gradient (i.e., the slope of the relation between WD and radial distance) and pattern (i.e., linear or nonlinear variation). We studied 16 legumes species from French Guiana representing a wide diversity of growth strategies and positions on the shade-tolerance continuum. Using WD measurements and available HW extractives content values, we computed WD corrected by the extractive content and analyzed the effect of HW on WD radial gradients and patterns. We also related WD variations to demographic variables, such as sapling growth and mortality rates. Regardless of the position along the shade-tolerance continuum, correcting WD gradients reveals only increasing gradients. We determined three types of corrected WD patterns: (1) the upward curvilinear pattern is a specific feature of heliophilic species, whereas (2) the linear and (3) the downward curvilinear patterns are observed in both mid- and late-successional species. In addition, we found that saplings growth and mortality rates are better correlated with the corrected WD at stem center than with the uncorrected value: taking into account the effect of HW extractives on WD radial variations provides unbiased interpretation of biomass accumulation and tree mechanical strategies. Rather than a specific feature of heliophilic species, the increasing WD gradient is a shared strategy regardless of the shade tolerance habit. Finally, our study stresses to consider the occurrence of HW when using WD.
Highlights
Basic wood density (WD), which is defined as the ratio of oven-dried wood mass to fresh volume [1]is an effective biophysical property that is commonly used by wood scientists and technologists.The consideration of WD has recently been extended to other disciplines and is widely used as a functional trait in the field of functional ecology [2,3,4,5]
Our sampling covered a wide range of WD values at both sample level
T. guianensis (Benth.) Zarucchi & Herend. was the only HW-free species that exhibited no differences between inn WD and out WD (Figure 2)
Summary
Basic wood density (WD), which is defined as the ratio of oven-dried wood mass to fresh volume [1]is an effective biophysical property that is commonly used by wood scientists and technologists.The consideration of WD has recently been extended to other disciplines and is widely used as a functional trait in the field of functional ecology [2,3,4,5]. The success of WD can be largely attributed to its ability to relate the carbon investment per unit volume of stem and to its integrative power of diverse properties, characteristics, traits and strategies of the materials, and organisms under study. WD is related to both mechanical and hydraulic properties of wood: it positively correlates with bending and compression strength or stiffness [3,6,7], negatively with capacitance and water storage [8,9], and positively with embolism resistance [8,10,11,12]. Since the performance of mechanical and hydraulic functions is a good predictor of the growth and mortality of forest species [3,14,15], WD is considerated as a surrogate to establish the position of a species in the growth-mortality trade-off [4,15], altough some reports relate significant but weak correlations (e.g., [16])
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