Functional ecology

Functional ecology is a field of ecology that has grown considerably in the past two decades. Functional ecology can offer a more mechanistic and predictive understanding of how traits are related with demographic tree patterns such as survival, growth and mortality rates. This approach is based on the study of functional traits. A functional trait is defined as any morphological, phenological, or physiological features of an individual that affect the growth, survival, reproduction, and fitness of that individual (Violle et al. 2007).

Functional traits in trees and forest
There is growing consensus that wood density (WD), the maximum height of adult trees, the specific leaf area (a measure of the surface produced to intercept the solar radiation per unit of dry weight invested in the construction of the leaves) and seed volume are the functional traits that best explain demographic attributes such as tree growth and mortality rates (Chao et al. 2008, King et al. 2006, Poorter et al. 2008). WD is the most functional trait analyzed because it is easy to measure and is strongly correlated with growth rates (lower density and, therefore, lower construction cost per unit xylem volume was associated with high growth rates) and mortality rates (Poorter et al. 2008).

Wood density as a functional trait
WD also has a positive relationship with some mechanical properties of wood (van Gelder et al. 2006), such as modulus of elasticity (MOE), modulus of rupture (MOR), toughness and hardness. High MOE values indicate that a tree can maintain its structural integrity with a low risk of mechanical failures. Also, MOE affects self-supporting capacity as it influences the critical height of trees (Fournier et al. 2013). Woods with high MOR and MOE values have a high breaking strength, which reduces the risk of buckling. This is critical for the mechanical integrity of the stems under variable loads, such as falling branches and trees, liana colonization of tree crowns and wind speed (King et al. 2006). Toughness is important because dynamic loads can change substantially and unpredictably over short periods of time and induce tree fall (Niklas 2016). On the other hand, susceptibility to coleopteran insects’ attack relates to wood and bark tree resistance to penetration, which can be estimated by wood and bark hardness (Paine et al. 2010). Species with low WD have low MOR and are more susceptible to breaking than species with high WD and high MOR (Putz et al. 1983).

WD can also be related to the hydraulic architecture of trees (Bucci et al. 2004, Campanello et al. 2008, Chave et al. 2009). Water transport efficiency (hydraulic conductivity per xylem area) correlates positively with vessel size and negatively with WD (Bucci et al., 2004); although in some studies no relationship was found between hydraulic conductivity and WD (Poorter et al. 2010). The area of the vessels, in general, decreases with the increase in WD (Preston et al. 2006). Resistance to cavitation, a measure of water stress resistance, could be associated in some cases with a higher WD (Hacke & Sperry 2001). However, other studies do not agree with this hypothesis (Cochard et al. 1999, Pratt et al. 2007).