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As massive, sessile and long-lived organisms, Mediterranean pines are exposed to a wide array of insect herbivores that feed on their tissues causing damage and ultimately, tree death. To protect themselves against biotic risks, pines have a complex battery of chemical and physical defenses, that act directly to kill or repel the antagonist organisms. Defense production, however, is not constant throughout time, but can be fine-tuned to the particular biotic risk to which the individuals and populations are exposed, producing the so called induced defenses. Moreover, the defensive state of a tree might also be determined by mutualistic interactions between the tree and other organisms that act as indirect defenses by means of top-down effects on the herbivore populations. Knowledge about these intriguing interactions in Mediterranean pines is still very limited but most of the theoretical progress made in model plant species could be hopefully translated to these long-lived plants. The defensive arsenal that a tree produces requires large amounts of resources that are no longer available for other life functions (i.e. growth, reproduction). Pine resistance to biotic threats must thus be understood as a general function integrated with other life-history traits, where defensive allocation, ultimately leading to biotic resistance, is shaped by the optimal resource allocation across life functions according to the particular biotic and abiotic environment. Thus, environmental variation across the distribution range of Mediterranean pines generates patterns of phenotypic diversity in defensive investment, by means of both adaptive (genetic) and plastic responses. Genetic variation and plasticity in the expression of defenses in Meditteranean pines have inmidiate implications in the resistance capacity of these long-lived species. Understanding the sources of phenotypic variation driving allocation to defenses in Mediterranean pines is therefore specially relevant to forecast species responses to ongoing environmental changes, particularly those casued by increased biotic stresses associated with global change. |
