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A perennial woody stem is the basic feature that sets woody plants aside from herbaceous plants. Recently, stem traits are emerging as core plant functional traits because of their importance for the stability, defence, architecture, hydraulics, carbon gain and growth potential of plants (Santiago et al., 2004; Jacobsen et al., 2008; Chave et al., 2009). Wood density is often taken as a starting point to compare species, because it is easy to measure, readily available for a large number of species, and because it has strong ecological implications (Falster, 2006). Wood density partly underlies, for example, the growth–survival trade-off that is observed among woody plants (Kitajima, 1994; Kobe et al., 1995; Poorter & Bongers, 2006). Low wood density is associated with fast growth because of cheap volumetric construction costs of the wood, whereas high wood density is associated with high survival because of biomechanical and hydraulic safety (Putz et al., 1983; Hacke et al., 2001; Sterck et al., 2006) and resistance against physical damage by herbivores, pathogens and falling woody debris (Augspurger & Kelly, 1984; Van Gelder et al., 2006; McCarthy-Neumann & Kobe, 2008).
Analogously to specific leaf area for leaves, wood density is associated with a variety of morphological and physiological stem traits that are closely related to the functioning of trees. In angiosperm tree species, wood or xylem is built up of three different tissue types that fulfil different functions: vessels provide longitudinal water transport; parenchyma as living, physiologically active cells provide carbohydrate storage and local radial transport; and fibres provide mainly strength. Investments in these different tissue types therefore imply trade-offs between the different functions they deliver. Such trade-offs, however, might be compensated by the size, number and structure of the elements that make up these tissue types. For example, hydraulic conductance depends not only on the stem cross-sectional area occupied by vessels but also on the size and number of these vessels. According to the Hagen–Poiseuille law, the hydraulic conductance scales with the fourth power of the vessel radius. Wider vessels therefore contribute to a larger hydraulic conductance (Sperry et al., 2006) which, in turn, facilitates higher stomatal conductance and more photosynthetic carbon gain (Santiago et al., 2004). By contrast, smaller vessels imbedded in a matrix of dense tissue lead to a higher hydraulic safety because of less risk of vessel implosion (Hacke et al., 2001) and cavitation (as small vessels have lower risk of air-seeding because they have a smaller pit membrane area (Hacke et al., 2006)).
The picture that emerges is that trees can solve strength and hydraulic limitations in several ways, but little is known how anatomical traits (co)vary across sympatrically occurring tree species. Some authors suggest that there is a spectrum in wood properties paralleling the economics spectrum found for leaves (Chave et al., 2009), but the exact nature of this wood spectrum is less clear as many wood traits can vary independently from each other and from wood density (Curtis & Ackerly, 2008). Moreover, the nature of these trait associations may vary from community to community (Jacobsen et al., 2008). Although the adaptive value is often inferred, it is not clear how these wood traits relate to whole-plant performance in the field or to the life history strategies of tree species. Interspecific comparisons are often made by comparing different species measured in different sites (Maherali et al., 2004), thus potentially confounding interspecific and environmental effects.
For closed forests, the shade tolerance of juvenile trees and the stature of reproductive adult trees are considered to be two of the most important life history strategies (Turner, 2001; Poorter et al., 2006). Variation in shade tolerance allows species to partition the horizontal light gradient at the forest floor (Kitajima & Poorter, 2008), whereas differences in adult stature allow species to partition the vertical height gradient in the forest canopy (Poorter et al., 2008a; Kohyama & Takada, 2009). Initial results suggest that fast-growing and shade-intolerant species have large vessels and high hydraulic conductance (Castro-Diez et al., 1998; Tyree et al., 1998; Sack et al., 2005), while tall species have large vessels and a low vessel density (Preston et al., 2006), although the latter has only been observed for woody plants from a Mediterranean-type climate.
In this study we compare quantitative wood traits of 42 coexisting rainforest species. We take advantage of large-scale permanent sample plot data to calculate species-specific growth and survival rates. Wood anatomical traits are related to each other, then to growth and survival rates, and finally to quantitative indices of shade tolerance and adult stature. The rationale behind this approach is that wood traits should affect plant performance and that wood traits together with plant performance shape the life history variation across species. The following three corresponding hypotheses are addressed:
Wood density increases with fibre cross-sectional area because fibres make up most of the solid wood mass, and wood density decreases with vessel cross-sectional area as more open conduit spaces should lead to less dense material. Kp increases with vessel cross-sectional area, and especially with vessel diameter.
Diameter growth rate increases with the water transport capacity (vessel cross-sectional area, vessel diameter and Kp), and decreases with the volumetric stem construction cost (wood density) of the species. Survival rate increases with the stem material strength (fibre cross-sectional area and wood density) and carbon storage potential (parenchyma cross-sectional area) of the species.
Both light-demanding species and tall species have exposed crowns and therefore need high Kp and associated traits to meet their high transpirational demands. At the same time they have stem properties associated with fast growth (high Kp and low wood density). By contrast, both shade-tolerant species and small species have stem properties that are associated with high survival (large fibre cross-sectional area and high wood density).