The green thorns of Ulex europaeus play both defensive and photosynthetic roles: consequences for predictions of the enemy release hypothesis

14 páginas, 5 figuras, 3 tablas

The widespread invasive success of Ulex europaeus, a thorny shrub native to NW Europe, remains to be understood from a functional perspective. According to the Enemy Release Hypothesis (ERH), lower pressure by vertebrate herbivores in the invaded areas should lead to lower investment in (costly) physical defenses, allowing plants to invest more in growth and/or reproduction. While U. europaeus seedlings have spines, adult plants have thorns, which are the main photosynthetic tissue (leaves are reduced to small phyllodes). Therefore, reduced biomass investment in thorns could compromise photosynthesis and growth in the invaded range. We hypothesized that U. europaeus plants in invaded ranges should show a reduction in the defensive components of thorns (e.g., softer and less fibrous tissues), but not reduced biomass allocation. We compared U. europaeus plants from the invaded (Chile) and native (Spain) distribution ranges regarding: (i) spinescence traits (thorn length, width, biomass, slenderness and bending strength) in adult plants, (ii) thorn fiber content and digestibility (proxies for palatability) in adult plants, and (iii) spine density in seedlings grown in a common garden. As expected, plants in the invaded range showed 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. We found functional changes in spinescence traits between distribution ranges consistent with the dual role of thorns in U. europaeus, and these changes may partly explain its invasiveness.

This work and SMV were funded by FONDECYT grant 3180289 (Chile). BRVA acknowledges funding from project ‘‘CLU-2019-05 – IRNASA/CSIC Unit of Excellence’’, funded by the Junta de Castilla y León and cofinanced by the European Union (ERDF ‘‘Europe drives our growth’’). AH was supported by the University of Alcala´’s Own Research Programme’s 2019/20 Postdoctoral Grant, DARE research project (reference RTI 2018-096884-B-C32) financed by the Spanish Ministry of Science Innovation and Universities and Basque Country Government funding support to FisioClima CO2 (IT1022-16) research group. EPC acknowledge REMEDINAL3-CM S2013/MAE-2719 network (Comunidad de Madrid) and the funding of UCM Research Group Program (2018, 2020, Research group 91034. Plant Evolutionary Ecology and Restoration Ecology). EG acknowledges funding from FONDECYT grant 1180334. We thank Marina Briones Rizo and Antonio Requena Serrano for their help during sampling processing and Carlos Silva for his help with maps. We are greatful to the support of Red Fabáceas Invasoras en Iberoamérica (FFII): perspectiva alelopática e histoquímica (REDI170025, CONICYT, Chile).

Peer reviewed