What determines the number and size of the seeds produced by a plant? How often should it reproduce them? How often should a plant produce them? Why and how are seeds dispersed, and what are the implications for the diversity and composition of vegetation? These are just some of the questions tackled in this wide-ranging review of the role of seeds in the ecology of plants. The authors bring together information on the ecological aspects of seed biology, starting with a consideration of reproductive strategies in seed plants and progressing through the life cycle, covering seed maturation, dispersal, storage in the soil, dormancy, germination, seedling establishment, and regeneration in the field. The text encompasses a wide range of concepts of general relevance to plant ecology, reflecting the central role that the study of seed ecology has played in elucidating many fundamental aspects of plant community function.

The ecology of seeds

Herbivory by domestic and wild ungulates is a major driver of global vegetation dynamics. However, grazing is not considered in dynamic global vegetation models, or more generally in studies of the effects of environmental change on ecosystems at regional to global scale. An obstacle to this is a lack of empirical tests of several hypotheses linking plant traits with grazing. We, therefore, set out to test whether some widely recognized trait responses to grazing are consistent at the global level. We conducted a meta-analysis of plant trait responses to grazing, based on 197 studies from all major regions of the world, and using six major conceptual models of trait response to grazing as a framework. Data were available for seven plant traits: life history, canopy height, habit, architecture, growth form (forb, graminoid, herbaceous legume, woody), palatability, and geographic origin. Covariates were precipitation and evolutionary history of herbivory. Overall, grazing favoured annual over perennial plants, short plants over tall plants, prostrate over erect plants, and stoloniferous and rosette architecture over tussock architecture. There was no consistent effect of grazing on growth form. Some response patterns were modified by particular combinations of precipitation and history of herbivory. Climatic and historical contexts are therefore essential for understanding plant trait responses to grazing. Our study identifies some key traits to be incorporated into plant functional classifications for the explicit consideration of grazing into global vegetation models used in global change research. Importantly, our results suggest that plant functional type classifications and response rules need to be specific to regions with different climate and herbivory history.

Plant trait responses to grazing – a global synthesis

Mountain Mediterranean landscape evolution caused by the abandonment of traditional primary activities: a study of the Spanish Central Pyrenees

The relationship between above and belowground species composition has been researched in forests, grasslands, and wetlands to understand what mechanisms control community composition. I thoroughly reviewed 108 articles published between 1945 and 2006 that summarized and provided specific values on similarities between above and belowground communities to identify common trends among ecosystems. Using Sorenson's index of similarity, I found that standing vegetation and its associated seed bank was the least similar in forest ecosystems, most similar in grasslands, and of intermediate similarity in wetlands. I also discovered that species richness was not related to seed bank - vegetation similarity in any of the three ecosystems. Disturbances were a common mechanism driving community composition in all ecosystems, where similarity decreased with time since disturbance in forest and wetland ecosystems and increased with time since disturbance in grasslands. Knowing the relationships between seed bank and standing vegetation may help conservationists to manage against exotic species, plan for community responses to disturbances, restore diversity, and better understand the resilience of an ecosystem.

A review of similarity between seed bank and standing vegetation across ecosystems

Abandonment in grazing systems: Consequences for vegetation and soil

. The similarity in species composition between seed bank and vegetation was analysed in Mediterranean grasslands in relation to altitude, topography and grazing. Soil samples were collected in permanent plots in autumn at the end of the summer drought period and in spring, before the new seed fall and after the natural winter seed stratification. The seed bank composition was determined by greenhouse germination over a nine-month period. Presence/absence of species in the standing vegetation throughout the complete annual cycle, and the percentage area of bare ground in October, were recorded in the same plots. The species composition of the standing vegetation is clearly determined by altitude, topography and grazing, while the floristic composition of the seed banks is only related to altitude and topography in the case of autumn seed bank and with any of the three factors in the spring seed bank. Relative abundances of grasses, legumes and forbs also show different patterns in vegetation and seed bank data. Sorensen similarity between the autumn seed bank and the vegetation declines as altitude rises, but there are no significant differences for topography and grazing. This similarity decreases in the case of the spring seed bank and does not show any significant relationship with any of the factors. The perennial/ annual ratio and the proportion of bare soil in October are proposed as explanatory variables in a predictive model of similarity between the seed composition of the seed bank and vegetation.

Similarity between seed bank and vegetation in Mediterranean grassland: a predictive model

The size and dynamics of seed banks were studied in grazed and ungrazed Mediterranean pastures at different altitudes and topography positions. The soil samples were collected in autumn and spring and the seed banks composition was determined by greenhouse germination over a 9-month period. The percentage of bare ground and the presence of new seedlings were recorded monthly from October to July in the field. A fall in seed density and species richness in the banks and a tendency for seeds to remain in the banks were linked to a rise in altitude. Germination in lower pastures mainly occurred in October in the numerous gaps left by the summer drought. At higher altitudes, the scarcity of gaps and the harsh climate led to an autumn–spring segregation of germination. On a local scale, the low slope positions and the ungrazed plots had a larger number of persistent seed bank species and a lower percentage of bare ground where seeds could germinate than their respective plots in the upper positions and grazed plots. A higher seed density in ungrazed than grazed plots was only detected in the three highest plots. No seed bank species richness trend was detected. In populations of the same species in different types of environments, the seasonal variation of seed numbers was environment-dependent for the majority of the species. In general, perennial grassland and its related low gaps availability appear to favour persistent seed banks.



Se estudian las diferencias en el tamano y dinamica de los bancos de semillas de pastizales Mediterraneos sometidos o no a consumo por herbivoros en diferentes altitudes y posiciones topograficas. Se determino el tamano y composicion de los bancos de semillas mediante la toma de muestras de suelo en otono y primavera y conteo de las germinaciones en invernadero por un periodo de 9 meses. Ademas se registro mensualmente en el campo el porcentaje de suelo desnudo y de germinaciones en el periodo de Octubre a Julio. El aumento de altitud esta asociado a una disminucion de la densidad de semillas y de la riqueza de especies de los bancos y a la tendencia de las semillas a persistir en los bancos. En los pastizales mas bajos la germinacion ocurre fundamentalmente en Octubre, en los numerosos huecos dejados por la sequia estival. A mayores altitudes se observa una segregacion de la germinacion entre los periodos de otono y primavera que esta relacionada probablemente con la baja disponibilidad de huecos y con la dureza climatica invernal. A escala local, las zonas bajas de las laderas y las privadas del consumo por herbivoros presentan un mayor nuAÂmero de especies con banco persistente y un menor porcentaje de suelo desnudo que sus respectivas zonas altas de ladera y zonas pastoreadas. A esta escala no se observa ninguna tendencia respecto a la riqueza de especies en los bancos de semillas y solo se detecta una mayor densidad de semillas en las zonas clausuradas frente a las pastoreadas en las tres posiciones mas altas del gradiente altitudinal. La variacion estacional en el nuAÂmero de semillas de poblaciones de diferentes ambientes resulto ser dependiente de estos para la mayoria de las especies. En general se observa que el aumento de herbaceas perennes, relacionado con la baja disponibilidad de huecos, parece favorecer la existencia de bancos persistentes.

M. Ortega

Papers

Similarity between seed bank and vegetation in Mediterranean grassland: a predictive model

Seasonal dynamics of Mediterranean pasture seed banks along environmental gradients