Agustín Rubio

Factors affecting establishment of a gypsophyte: the case of Lepidium subulatum (Brassicaceae)

The restriction of vascular plants to gypsum-rich soils under arid or semiarid climates has been reported by many authors in different parts of the world. However, factors controlling the presence of gypsophytes on these soils are far from understood. We investigated the establishment of Lepidium subulatum, a gypsophyte, in a nondisturbed semiarid gypsum-soil landscape in central Spain, both from spatial and temporal perspectives. Over 1400 seedlings were tagged, and their growth and survival were monitored for a 2-yr period. Several biotic and abiotic variables were measured to determine the factors controlling the emergence and early survival. These variables included the cover of annual plants, bryophytes, lichens, litter, gypsum crystals, bare fraction and cover of each perennial plant, and several soil properties (gravel, fine gravel, and fine-earth fraction, conductivity, pH, gypsum content, organic matter and penetrometer soil resistance). Our results support the linkage of gypsophily with some physical properties of the surface crust. Seedlings tended to establish on the gypsum surface crust, and their survival was size dependent, probably as a consequence of the necessity of rooting below the surface crust before summer drought arrives. However, once seedlings emerged, a higher survival rate occurred on the alluvial soils of the piedmont-slope boundary where soil crusts are absent or thinner. We conclude that Lepidium subulatum may be considered a refuge model endemic with a distribution range that occupies a reduced fraction of a wider habitat from which it is probably excluded by competition.

Small-scale patterns of abundance of mosses and lichens forming biological soil crusts in two semi-arid gypsum environments

Despite important advances in the understanding of biological soil crusts and their key role in ecosystem processes in arid and semi-arid environments, little is known about those factors driving the small-scale patterns of abundance and distribution of crust-forming lichens and mosses. We used constrained ordination techniques (RDAs) to test the hypothesis that the spatial patterning of lichens and mosses is related to surface and subsurface soil variables in two semi-arid gypsum environments of Spain. Our results show that the abundance of mosses and lichens forming biological soil crusts was related to a limited set of variables (cover of bare soil and litter, soil respiration, potassium content and aggregate stability). Moreover, they provide some insights into the importance of these variables as drivers of biological soil-crust composition and abundance in semi-arid gypsum environments.

Small-scale spatial soil-plant relationship in semi-arid gypsum environments

Studies on soil patterning on a small scale in arid and semi-arid regions have rarely been conducted. Many papers implicitly assume that plant distribution is controlled by some soil variables acting at small scales. We have directly tackled the relationships between soil and some biotic variables including plant distribution at small scales in an Iberian semi-arid gypsum environment. This has been carried out by means of Canonical Correspondence Analysis as a hypothesis-testing tool. CCA models show that the spatial data matrix is able to explain a relevant fraction of the soil data set (P < 0.001). The most important variable, as firstly selected in the CCA stepwise selection procedure, suggests the existence of a vegetation-elevation gradient in relation to soil physical properties; the rest of selected variables indicates the existence of other spatial trends which may be related to certain microgeomorphological features. On the other hand, only the cover of annuals and the cover of litter are selected in the case of the biotic data set as constraining matrix, but not the cover of any perennial plant. Partial CCA models indicated that the remaining information explained by the spatial data set after adjusting the biotic set as covariables is also significant (p < 0.001). This variability is not related to the existence of vegetation bands as shown by the two selected variables in the case of the partial CCA models. The primary source of spatial soil variation is related to the existence of three community bands and these differences are able to explain even the change of plant life forms in vegetated band. The soil parameters controlling the changes are mainly related to texture and surface features. However, we detected other sources of spatial soil variation out of this primary model. This hierarchical spatial pattern seems to be related to some geomorphological traits of the landscape, such as soil crust strength, presence of gypsum crystals or bare zones, and not to the presence of mature gypsophytes (at least the five most frequent) which might ameliorate the soil environment. Furthermore, the biotic data set is not able to explain any new fraction of soil variability out of that already explained by the spatial data set.

Patch Dynamics and Islands of Fertility in a High Mountain Mediterranean Community

Abstract Vegetation in high Mediterranean mountains usually consists of patchy communities. Patch structures have been interpreted as a result of the prevalence of facilitation phenomena in highly stressful environments. Several mechanisms have been proposed in order to explain the factors that control the existence of these clumped structures. However, they have not been evaluated in these mountains. Our hypothesis is that patchy structure in high mountain Mediterranean vegetation is a consequence of facilitative and competitive interactions in a very harsh environment which ultimately involve strong localized effects on soil properties. Our results show that levels of soil nutrients were higher under vegetation patches than in bare ground areas, confirming the hypothesis of an amelioration of soil resources under canopies. Pairwise associations and repulsions suggest the existence of two contrasting composition stages. Contrasting models relating patch species composition (cover and biomass) and soil resources indicated a weak relationship between species features and soil nutrient levels. Finally, structural modeling showed that patch size has a relevant but indirect effect on soil resource levels through grass and total biomass. We conclude that patch structure and dynamics in high Mediterranean mountain communities may be partly controlled by an endogenous process involving facilitation and competition for soil key resources. These interactions may operate through some community traits related to patch size but not to composition.

Are soil characteristics and understorey composition controlled by forest management

Data from 30 plots of Castanea sativa Miller stands were analysed to study the relationships among soil, understorey composition and management. Floristic cover percentage and edaphic features were separately studied by means of PCA. After that, correlations between floristic and edaphic axes were carried out by means of Pearson product‐moment correlation; and finally, correlations between floristic or edaphic axes and silvicultural characteristics were also calculated. PCA of soil variables showed differences between stands located on granitic rocks and floristic PCA showed a clear difference between managements. There were correlations between floristic axes and soil or silvicultural measurements, while correlations between soil axes and silvicultural measurements were not significant. The results indicated there were no general trends between soil properties and forest management probably due to deepness and development of soils. The management is the first source of variation on understorey composition: high forests were poorer than coppice stands. There were also differences within coppice stands related to the type of post-harvest practice. Finally temporal changes in nutrient rates were related to the understorey composition, as well. # 1999 Elsevier Science B.V. All rights reserved.

Clear-cut effects on chestnut forest soils under stressful conditions: lengthening of time-rotation

Forestry practices produce intense alterations on soil processes, which in turn could lead to more intense transformations of the system in relation to the availability of resources. From this point of view we conducted research in the critical Mediterranean environment of central Spain to check if length of time-rotation on chestnut (Castanea sativa Mill.) coppice stand allows recovery towards pre-disturbance conditions. According to the management scheme for chestnut forests in the zone, the quadrants which were clear-cut in 1957, 1967, 1982 and 1993 were used to build a chronosequence. Ten plots were allocated and physiographic, soil and forestry variables were measured to build a linear model in which the time since the last clear-felling was considered as a fixed variable. Almost all soil variables showed no significant differences between the studied stand-ages, but organic matter, nitrogen and potassium content showed an intense depletion after clear-cutting. The sustainability of this management scheme, as is shown in our results, is primarily based on the ability of chestnut trees to rapidly recover their canopy structure from rootstock as well as, from a forestry perspective, on a twofold increase in timerotation (in more mesic chestnut stands of Mediterranean Spain, clear-cuts are conducted after 20 years). Under stressful conditions of our stands, maximum productivity (20 years) does not coincide with the time to soil recovery (40‐50 years) and consequently this management scheme has lengthened the time-rotation interval to guarantee viability of the stand. # 2003 Elsevier Science B.V. All rights reserved.

Soil evaluation for Castanea sativa afforestation in Northeastern Spain

This paper is focused on the soil characterisation of forest sites where Castanea sativa wood stands are located in Catalonia (NE of Spain) as a former and essential requirement to future afforestation programs with this species. From 28 sample plots field survey, several soil indexes have been calculated and, their central and marginal values have been established, defining a range of soil conditions where this species can successfully live in Catalonia. So, we can see that soils are developed on acid bedrock as parent material, basically igneous rocks such as granodiorite and granite, but they occur on schist as well. Their predominant texture is loamy-sand. Soils have low water holding capacity, mainly due to their high stoniness and steep slopes. Nevertheless, regular rainfall distribution all year around, avoids meteorological drought as well as reduces physiological drought. Annual rainfall distribution and steep slopes are responsible for the dwarf development of argic horizons. Soils are relatively poor in organic matter, and despite being generally very acid they have forest mull humus forms.

Análisis ecológico comparativo de los castañares de Extremadura y de la región cántabro-astur (España)

A partir de los datos ecologicos de los territorios ocupados por Castanea sativa Miller en las Comunidades Autonomas espnaolas de Extremadura, Cantabria y en el Principado de Asturias se efectua un estudio conjunto de sus caracteres generales. Ademas, se buscan los parametros que son significativamente distintos en las regiones extremena y cantabro-astur. Mediante analisis de componentes principales y discriminantes se pone de manifiesto en que radica, esencialmente, la diversidad de comportamiento de ambas poblaciones.

Evaluation of the potential index model to predict habitat suitability of forest species: the potential distribution of mountain pine ( Pinus uncinata ) in the Iberian peninsula

Characterization of the suitability or potentiality of a territory for forest tree species is an important source of information for forest planning and managing. In this study, we compared a relatively simple methodology to generate potential habitat distribution areas that has been traditionally used in Spain (the potential index model) with a statistical modelling approach (generalized linear model). We modelled the potential distribution of mountain pine (Pinus uncinata) in the Iberian peninsula as a working example. The potential index model generated a map of habitat suitability according to the values of an index of potentiality, whose distribution has usually divided into four categories based on quartiles (from optimum to low suitability). Considering all values of the index of potentiality as presences of mountain pine resulted in a low to moderate degree of agreement between the potential index model and the generalized linear model according to the kappa coefficient. Using the cut-off value of the index of potentiality that maximized the degree of agreement between both modelling approaches resulted in a substantial similarity between the maps of the predicted distribution of mountain pine. This cut-off value did lie in the upper-third quartile of the potential index distribution (high suitability category), and roughly coincided with the upper 30th percentile. The use of statistical techniques, which have proved to be powerful and versatile for species distribution modelling, is recommended. However, the potential index model, together with the adjustments proposed here, could be a reasonably simple methodology to predict the potential distribution of forest tree species that forest managers should take into account when evaluating forestation and afforestation projects.

Soil CO2 Efflux in a Mixed Pine-Oak Forest in Valsaín (Central Spain)

Soil-surface CO2 efflux and its spatial and temporal variation were investigated in a southern Mediterranean, mixed pine-oak forest ecosystem on the northern slopes of the Sierra de Guadarrama in Spain from February 2006 to July 2006. Measurements of soil CO2 efflux, soil temperatures, and moisture were conducted in nine 1963-m2 sampling plots distributed in a gradient around the ecotone between Pinus sylvestris L. and Quercus pyrenaica Lam. forest stands. Total soil organic matter, Walkey-Black C, particulate organic matter, organic matter fraction below 53 μm, total soil nitrogen content, total soil organic carbon content, and pH were also measured under three representative mature oak, pine, and mixed pine-oak forest stands. Soil respiration showed a typical seasonal pattern with minimums in winter and summer, and maximums in spring, more pronounced in oak and oak-pine stands. Soil respiration values were highest in pine stands during winter and in oak stands during spring and summer. Soil respiration was highly correlated with soil temperatures in oak and pine-oak stands when soil moisture was above a drought threshold of 15%. Below this threshold value, soil moisture was a good predictor of soil respiration in pine stands. Greater soil organic matter, particulate organic matter, Walkey-Black C, total organic C, and total N content in pine compared to oak sites potentially contributed to the greater total soil CO2 efflux in these stands during the winter. Furthermore, opposing trends in the organic matter fraction below 53 μm and soil respiration between plots suggest that in oak stands, the C forms are less affected by possible changes in use. The effects of soil properties on soil respiration were masked by differences in soil temperature and moisture during the rest of the year. Understanding the spatial and temporal variation even within small geographic areas is essential to assess C budgets at ecosystem level accurately. Thus, this study bears important implications for the study of large-scale ecosystem dynamics, particularly in response to climatic change.