Potential Germination Success of Exotic and Native Trees Coexisting in Central Spain Riparian Forests

We compared potential germination success (i.e., percentage of produced seeds that germinate under optimal conditions), the percentage of empty and insect-damaged seeds, germinability (Gmax), and time to germination (Tgerm) between the exotics Ailanthus altissima, Robinia pseudoacacia, and Ulmus pumila and two coexisting native trees (Fraxinus angustifolia and Ulmus minor) in the riparian forests of Central Spain. Additionally, we tested the effect of seed age, seed bank type (canopy or soil) and population on Gmax and Tgerm of A. altissima and R. pseudoacacia, which are seed-banking species. Species ranked by their potential germination success were A. altissima > U. pumila > R. pseudoacacia > U. minor > F. angustifolia. The combination of a high Gmax and negligible seed insect-damage provided A. altissima with a potential germination advantage over the natives, which were the least successful due to an extremely high percentage of empty seeds or a very low Gmax. R. pseudoacacia showed high vulnerability to insect seed predation which might be compensated with the maintenance of persistent seed banks with high Gmax. Gmax and Tgerm were strongly affected by seed age in the seed-banking invaders, but between-seed bank variation of Gmax and Tgerm did not show a consistent pattern across species and populations

Comparing the sexual reproductive success of two exotic trees invading spanish riparian forests vs. a native reference

A widely accepted hypothesis in invasion ecology is that invasive species have higher survival through the early stages of establishment than do non-invasive species. In this study we explore the hypothesis that the sexual reproductive success of the invasive trees Ailanthus altissima (Mill.) Swingle and Robinia pseudoacacia L. is higher than that of the native Fraxinus angustifolia Vahl., all three species coexisting within the riparian forests of Central Spain. We compared different stages of the early life cycle, namely seed rain, seed infestation by insects, seed removal by local fauna, seed germination under optimal conditions and seedling abundance between the two invasive trees and the native, in order to assess their sexual reproductive success. The exotic species did not differ from the native reference (all three species displaying high seed rain and undergoing seed losses up to 50% due to seed removal by the local fauna). Even if the exotic R. pseudoacacia showed a high percentage of empty and insect-parasited seeds along with a low seedling emergence and the exotic A. altissima was the species with more viable seeds and of higher germinability, no differences were found regarding these variables when comparing them with the native F. angustifolia. Unsuitable conditions might have hampered either seedling emergence and survival, as seedling abundance in the field was lower than expected in all species -especially in R. pseudoacacia-. Our results rather suggest that the sexual reproductive success was not higher in the exotic trees than in the native reference, but studies focusing on longterm recruitment would help to shed light on this issue.Ministerio de Economía y CompetitividadJunta de Comunidades de Castilla-La ManchaComunidad de Madri

Análisis de la invasión del hábitat ribereño por tres árboles exóticos en España

Este estudio diagnostica y valora la invasión de las especies exóticas Ailanthus altissima (Mill.) Swingle, Robinia pseudoacacia L. y Ulmus pumila L. en hábitats ribereños españoles. Se recorrieron las riberas de los ríos Henares, Oria y el tramo medio del río Ebro en busca de rodales de estas tres especies, con la intención de determinar el grado de invasión de cada zona y especie, así como las características de los rodales y de los hábitats que ocupan cada una de ellas. Se anotó el origen (plantado/espontáneo), tamaño, densidad y características del hábitat (bióticas, abióticas e influencia humana) de todos ellos. El grado de invasión fue mayor en la ribera del Henares, donde existe una mayor influencia antrópica y densidad de fuentes de propágulos. Cada especie presentó un mayor grado de invasión en una zona diferente. Los rodales espontáneos de las riberas del Henares y del Ebro son pequeños comparados con los del Oria. Las tres especies coexisten en hábitats con alta disponibilidad de luz, baja competencia y fuerte influencia antrópica, pero también pueden ocupar zonas de mayor cobertura arbórea. Las diferentes condiciones del hábitat en que se plantaron las especies y el hábitat donde se naturalizan podrían explicarse en gran medida por los mecanismos de dispersión utilizados por cada una

Stages of the sexual reproductive cycle of a plant.

<p>Seed losses at early stages of the plant cycle are represented in dark grey.</p

Analysis of the riparian habitat invasion by three tree exotic species in Spain

Este estudio diagnostica y valora la invasión de las especies exóticas Ailanthus altissima (Mill.) Swingle, Robinia pseudoacacia L. y Ulmus pumila L. en hábitats ribereños españoles. Se recorrieron las riberas de los ríos Henares, Oria y el tramo medio del río Ebro en busca de rodales de estas tres especies, con la intención de determinar el grado de invasión de cada zona y especie, así como las características de los rodales y de los hábitats que ocupan cada una de ellas. Se anotó el origen (plantado/espontáneo), tamaño, densidad y características del hábitat (bióticas, abióticas e influencia humana) de todos ellos. El grado de invasión fue mayor en la ribera del Henares, donde existe una mayor influencia antrópica y densidad de fuentes de propágulos. Cada especie presentó un mayor grado de invasión en una zona diferente. Los rodales espontáneos de las riberas del Henares y del Ebro son pequeños comparados con los del Oria. Las tres especies coexisten en hábitats con alta disponibilidad de luz, baja competencia y fuerte influencia antrópica, pero también pueden ocupar zonas de mayor cobertura arbórea. Las diferentes condiciones del hábitat en que se plantaron las especies y el hábitat donde se naturalizan podrían explicarse en gran medida por los mecanismos de dispersión utilizados por cada una.This study provides a diagnosis and assessment on the invasion of Spanish riparian habitats by Ailanthus altissima (Mill.) Swingle, Robinia pseudoacacia L. and Ulmus pumila L. The riparian corridors of the Henares and Oria rivers and a stretch of the middle Ebro River were surveyed to locate all the stands of these three species. Origin (planted/spontaneous), size, density and habitat characteristics (biotic, abiotic and human influence) of every stand were considered to determine the degree of invasion of each zone and species. Data on stand and habitat characteristics were also collected. The degree of invasion was greater in the Henares River riparian corridors, where human influence and propagule density were higher. Each species showed a greater degree of invasion in a different study area. Spontaneous stands of the Henares and Ebro riparian vegetation were smaller than those of the Oria River. All three species coexist in habitats with high light availability, low competition and strong human influence; however, they may also occur in areas with a very dense canopy. Differences in habitat characteristics between planted and spontaneous stands could largely be explained by specific dispersal mechanisms operating at each species.Ministerio de Ciencia e InnovaciónJunta de Comunidades de Castilla-La ManchaComunidad de Madri