Some trees produce defensive resin and chemicals all the time and are always prepared to ward off enemies, but others have figured out how to conserve their energy and only mount defenses when under attack. Some don’t bother protecting themselves at all. Why the variation? It seems like the most advantageous strategy is to defend when attacked, so why hasn’t this ability developed in all species?
Researchers at the Doñana Biological Station in Spain conducted an experiment to answer this question by observing how defensive abilities have evolved in pine trees. They grew seventeen pine species from North America and Eurasia together in a greenhouse and then tried to elicit a response by exposing them to a chemical that signals the trees that they are under attack. By measuring the base amount of resin and chemicals produced as well as the amount produced after provocation, the scientists could determine which strategy each species uses to defend itself and whether closely related species share similar strategies.
It turns out that the baseline amount of toxic chemicals produced in needles and resin in the stem is a characteristic that is similar among species that are more closely related. So, once acquired, these defenses are retained by species that evolve subsequently. However, the ability to produce more resin or chemicals when under attack appears to crop up randomly on the pine family tree. Despite the potential advantage of being able to conserve energy until it is needed, these “inducible defenses” are not retained when species split. Perhaps there is a hidden cost to maintaining this defensive flexibility? Maybe it is not always beneficial to wait a threat is present before committing resources to defense? The explanation will have to wait for another clever experiment or perhaps a clever model. In the meantime I will give thanks for the docility of my own holiday evergreen with a prayer for the continued absence of human-induced defenses.
You can find this article at:
Carrillo-Gavil an A., Moreira X., Zas R., Gonzalez-Voyer A., Vila M. & Sampedro L. (2014). Phylogenetic and biogeographical patterns in defensive strategies and quantitative allocation to checmical defenses in Palaearctic and Nearactic pine trees, Journal of Biogeography, DOI: 10.1111/jbi.12444