R. Fernández-Alés

Soil nitrogen heterogeneity in a Dehesa ecosystem

The Dehesa are savannah-like ecosystems that occupy extensive areas in the mediterranean regions of Southern Spain and Portugal. The changes in the soil nitrogen (N) potential mineralisation rate, microbial biomass-N and available mineral N in soils were studied over 1 year in a Dehesa ecosystem, in the SW of Spain. The temporal variability of soil samples was compared by using the coefficient of variation of samples taken on a monthly basis. Spatial variability of the soil samples was measured by comparing the variograms generated with geostatistical techniques. The potential N mineralisation reached a maximum in the spring and summer samples, and a minimum in the autumn. However, the soil NH4+, NO3- and microbial biomass-N showed maximum levels in winter, compared with minimum levels in the spring and summer. The potential mineralisation rate showed a higher temporal variability in the samples taken under tree canopies, due to highest maximum and similar minimum levels reached during the sampling period. The potential mineralisation rate, microbial biomass-N and soil mineral N were higher under the tree canopies than in the open grassland. The spatial distribution of the soil organic matter and available NH4+ was strongly influenced by tree canopy size. The semivariograms were adjusted to a spherical model, indicating a maximum NH4+ variability at a certain distance from the tree crown diameter. The same was not the case for the spatial distribution of the soil NO3- from the tree crown. Our results indicate that the tree component of a Dehesa ecosystem is a major influence in the spatial and temporal heterogeneity of soils.

Variability in seedling water status during drought within a Quercus ilex subsp. ballota population, and its relation to seedling morphology

Abstract Differences in morphology, growth and plant water status during drought were studied in greenhouse-grown Quercus ilex subsp. ballota seedlings produced by different mother trees occurring in a savannah-like forest (locally named dehesa) in southern Spain. There were significant differences among mother trees in the biomass of their seeds (mother tree level accounted for 62% of total variance in this trait), in the percentage of seedlings that emerged and in the time of shoot emergence. The progeny that had high emergence rates (80–90%) also had short emergence times while the opposite was true for the progeny that had low (43%) and intermediate (70%) emergence rates. Acorn biomass covaried significantly with seedling total biomass and with the biomass of different parts of the seedlings, but not with the partitioning of biomass among roots and shoots (root/shoot ratio) nor to morphological seedling traits such as shoot height, total leaf area, average leaf size, specific leaf area (SLA) and leaf number. All these traits varied significantly among the seedlings produced by different mother trees. The conductance to water vapour (glw) of the foliage of the seedlings also varied significantly among mother trees and was dependent on the root/shoot ratio as on the total leaf area of the seedlings, but not on the total seedling biomass. One of the six trees that were studied produced seedlings which had a significantly higher root/shoot ratio, smaller leaf area, smaller leaves, higher specific leaf area and shorter stems than the seedlings produced by any other tree. In addition, the former set of seedlings had significantly higher glw 15 and 30 days after drought, and higher leaf water content at the end of the experiment than the other seedlings. We discuss the value of using single seedling traits related to drought tolerance such as root/shoot ratio and leaf area in the framework of the restoration of savannah-like oak forest in the Mediterranean areas. We also discuss the functional implications of the diversity of genotypes within the oak populations in the framework of the global climate change.

Post-dispersive losses of acorns from Mediterranean savannah-like forests and shrublands

Abstract We studied post-dispersive losses of holm-oak acorns (Quercus ilex subsp. ballota) due to wild and domestic ungulates and to small mammals in two types of Mediterranean ecosystems in south-western Spain, savannah-like forests, locally named dehesa, and shrublands. Savannah-like forests which are used for sheep and cattle grazing and for acorn fattening of Iberian pig show a lack of oak regeneration. However, these ecosystems are considered as a model of sustainable resource use in all the fragile area of the Mediterranean region. During the 2 years, 1999 and 2000, we carried out a field experiment in three savannah-like forest and three shrubland sites. At each site, we added marked acorns and studied their loss from plots that were either accessible to any animal (no exclosure), protected against ungulates but accessible to small mammals (partial exclosure), or protected against big and small mammals and other vertebrates (complete exclosure). Results indicated that in the no exclosure treatment (i.e., the normal environment) acorn recovery was very low at any site (0–2.4%) without significant differences between savannah-like forests and shrublands. In the partial exclosure treatment, with small mammals having access to the plots only, acorn recovery varied from 0 to 42%. In this treatment, acorn recovery tended to be higher in the savannah-like forests (0.5–42%) when compared to the shrubland sites (0–2.5%). However, the three savannah-like forest sites, in different years, experienced >92% of acorn loss due to small mammals. In addition, many complete exclosures were disrupted, presumably by rodents, and lost 60% of the acorns. These results indicate that loss of acorns due to small mammals is very important in the two type of ecosystems, suggesting that proper management of oak self-regeneration in Mediterranean savannah-like forests should take into account the effect of small mammals, as well as livestock, especially during the seeding stage.

Modelling leaf gas exchange in holm-oak trees in southern Spain

Abstract This paper presents a mechanistically based C 3 leaf CO 2 assimilation model linked with an empirical stomatal model to simulate Quercus ilex leaf net photosynthesis and transpiration in oak-savannah ecosystems of southern Spain. The model estimates (time integration) daily and seasonal changes in carbon fixation, transpiration, and water use efficiency of a single leaf in free air. Simulations were carried out on two trees for a dry year. Results shown in our study demonstrate that the modelling approach, compared to field measurements, provides a realistic description of diurnal and seasonal patterns of leaf gas-exchange response to different environmental conditions, and as affected by water availability.

A vegetation management experiment: Goats grazing shrublands in Doñana Natural Park

We have studied the effect of goat grazing on the shrub understory of a pine forest situated in a protected area (Doñana Natural Park). Along three years we have studied the changes in phytovolume, flammability, species richness and diversity in a grazed shrubland and in a control area not grazed by the goats. We studied also food preferences of the goats and the ability of the goats to disperse the seeds of the plants they eat. Goat grazing significantly reduced shrub biomass and flammability, decreasing the risk of forest fire. Goats selected the species they eat, changing the pattern of selection throughout the year. Grazing reduced shrub diversity, but did not affected species richness. The goats can also potentially disperse the seeds of some of the species they eat, since the seeds were able to pass through the goat’s gut and germinated afterwards. Goat grazing on scrub vegetation can be used as an effective tool for the control of shrubs in protected forest areas, without losing biodiversity.