The effect of grazing and management measures on the vegetation of a dehesa – an agro-ecosystem formed during centuries by agro-sylvopastoral exploitation

摘要

The Mediterranean dehesas are an example of an agro-ecosystem with very high biodiversity. The management regimes of the Spanish and Portuguese dehesas are complex due to their traditional, multi-factorial use by different livestock, extensive agricultural exploitation as well as production of firewood and cork over many centuries. The formerly huge areas covered with a dehesa type ecosystem in Mediterranean Spain are nowadays reduced to about 3 – 3.5 Mio. ha. However, the decline of open, semi-natural grasslands like the Spanish dehesas as well as their typical and species-rich flora and fauna is caused both by abandonment and agricultural intensification and represents a global problem. udA special role in the pasturing system of a dehesa plays the Iberian pig, which roam traditionally free on large pastures in herds mixed with cattle. Besides their grazing activities, pigs have a special impact on vegetation due to their digging in soil. With the disruption of the normally dense vegetation cover, digging increases patchiness and vegetation dynamics. Therewith, in the present work, the effect of changes of the traditional grazing system of a Mediterranean dehesa with mixed herds of cattle and pigs in single pig grazing and abandonment on vegetation was studied. The different management regimes resulted not only in different species numbers, but also in specific plant compositions. After four years, species number was highest on pig pasture, followed by cattle and pig pasture, being lowest at fallow. Whereas species composition changed after change in grazing system, the influence of slope remained clearly visible throughout the years. udThe effects of other traditional managements (burning, ploughing and a combination of both), often being used against shrub encroachment in grazed Mediterranean dehesas, were also studied. Both, species composition as well as species number differed between the managed areas as well as between the managed areas and the solely grazed area. As species composition differed until the end of the project compared to the solely grazed areas, some kind of memory of the vegetation regarding disturbances was indicated through several persistent species. Cistus spec. as typical phryganic plants from the Mediterranean Basin showed an increase on the burned and burned and ploughed areas compared to the ploughed and solely grazed pastures, indicating the management problems of encroachment of these unwanted shrubs after fire hazards.udTo reveal the mechanisms behind the effect of management regimes on vegetation patterns and dynamics, dispersal in space and time were also studied. Therefore, endozoochorous dispersal for both cattle and pigs was analysed. A broad range of species was dispersed through the dung of both livestock with partly high numbers of seedlings per species. Both seedling number as well as species number emerged from pig dung was lower than from cattle dung. However, due to the broad range of species germinated from both pig and cattle dung, both pigs and cattle must be considered as important dispersal vectors for Mediterranean dehesa vegetation. Since resilience of plant species gains in importance with increasing disturbance intensity and frequency, the composition of soil seed bank from differently managed areas were investigated, too. Here, the investigation focused on the influence of management on soil seed bank composition and similarity of soil seed bank and actual vegetation. Species number and seedling number varied both between soil depth as well as management of the respective area. Similarity of soil seed bank composition and actual vegetation was generally low, and similarity decreased with increasing disturbance intensities. Besides grazing, fire is one of the major factors shaping Mediterranean type ecosystems. Taking this fact into account the effect of heat and smoke on the germination of 27 selected species has been tested in a laboratory experiment. Altogether, species tested in this experiment showed a wide variation in their response to heat and smoke treatments. Using the results of this experiment as a basis for interpretation, the influence of the management tool fire as well as an increasing number of wildfires should have serious influence on species composition of dehesa grasslands. udInterpreting the results of the present study, processes like climate change and fragmentation of the landscape have to be considered as well, in order to find effective conservation measures. In addition, if management influences the adaptiveness of Mediterranean species to changing climate, an adapted management may play a decisive factor for the survival of these ecosystems on a longer perspective. As efficient dispersal mechanisms may be decisive both in changing environments as well as with increasing fragmentation, conservation measures should be implemented which support and restore these dispersal mechanisms.