The green thorns of Ulex europaeus play both defensive and photosynthetic roles: consequences for predictions of the Enemy Release Hypothesis

Abstract Ulex europaeus , a thorny shrub native to NW Europe, is one of the worst invasive species worldwide. The mechanisms underlying its invasive success are not completely understood. According to the Enemy Release Hypothesis (ERH), lower pressure by vertebrate herbivores in the invaded areas should lead to lower investment in physical defenses, allowing the plant to invest more in growth and/or reproduction. However, antiherbivore physical defenses in U. europaeus (thorns) are also the main photosynthetic tissue of the plant. Therefore, reduced investment in thorns could compromise growth of Ulex europaeus in the invaded range. We hypothesized that changes in physical defenses of U. europaeus in invaded ranges should reflect reduced effectiveness (e.g., softer, and more palatable tissues) but not reduced biomass allocation. We compared U. europaeus plants from the invaded (Chile) and native (Spain) ranges of distribution regarding: i) spinescence traits (thorn length, width, biomass, slenderness and bending strength) in adult plants, ii) thorn fiber content and digestibility (as proxies for palatability) in adult plants, and iii) spine density in seedlings grown in a common garden. As expected, plants in the invaded range invested more mass in larger thorns, which contained less cellulose, were slenderer and easier to bend than those from plants in the native range. Likewise, seedlings from the invaded range showed lower spine density and more diameter growth, thus supporting the ERH. Our study shows functional changes in spinescence traits between distribution ranges that account for the fact that thorns are both defensive and photosynthetic organs in U. europaeus, and these changes may contribute to explain its invasiveness.