Physiological trade-offs in the complexity of pine tree defensive chemistry

Abstract

Like all organisms on Earth, trees must finely tune the relative allocation of resources to their living functions (namely growth, maintenance, defence and reproduction), seeking to optimize the costs and benefits (Bazzaz et al. 1987). As resources are limited and those allocated to one trait cannot be allocated to another, conflicts in resource allocation may result in trade-offs among different functions or traits (Agrawal et al. 2010). Such trade-offs might emerge as negative phenotypic correlations between pairs of traits with a shared source (reviewed by Saeki et al. 2014). Thus, patterns of phenotypic covariation (i.e., individual-based correlations) between traits help reveal possible conflicts in resource allocation and shared regulatory processes. One example in tree ecology and physiology is the relative allocation of resources to chemical defences (e.g., Keinanen et al. 1999, Koricheva et al. 2004, Donaldson et al. 2006, Agrawal 2011). Because of their life-history characteristics (such as being long-lived, large and forming extensive and stable populations) trees usually support a particularly diverse, extensive and temporally variable community of herbivores and pathogens. The selective pressure imposed by their antagonists has led to the evolution of effective resistance mechanisms, which include both constitutive and inducible defences (Zangerl and Bazzaz 1992). Constitutive defences, which are permanently expressed irrespective of the incidence of herbivores and pathogens, represent the first line of resistance. In contrast, induced resistance traits are activated, synthesized or mobilized in response to biotic challenges or cues of biotic damage. Plant resistance based on inducible defences, although energy saving, is a risky strategy as its benefits are based on the reliable identification of biotic cues (Karban 2011). Moreover, a plant could remain vulnerable for a period of time while induced defences are not activated or synthesized (e.g., Gómez et al. 2010). Therefore, plants need to combine a defensive …