Figueroa I., López B.C., López A., Potrony D. (2009) What happens to ptarmigan when marmots arrive?. Ethology Ecology and Evolution. 21: 251-260.EnllaçDoi: 10.1080/08927014.2009.9522480
Rock ptarmigan is a vulnerable species in the southern Pyrenees, with less than 300 pairs and two unconnected populations. Alpine marmot was introduced in the northern Pyrenees between 1955 and 1988, but they rapidly colonised the southern slopes, with an actual estimated population of around 10,000 individuals. Both species are mainly herbivores, develop their activities on the ground and have their offspring at the same time, so they may compete at various levels. We studied the possible interactions between the two species based on fleld observations, bibliographic data and Geographical Information System (GIS) analysis. We found that alpine marmot has colonised all habitats occupied by rock ptarmigan in the southern Pyrenees, but their plant diet is absolutely differentiated. We also analysed the composition of golden eagle nests and conflrmed that this predator predates the two species. From fleld observations in areas where the two species have coexisted for more than 10 years, we observed no behavioural interaction between the two species. So, although both species share space and time in the alpine communities, both diet differentiation and probably behavioural avoidance permits their coexistence.
López B.C., Figueroa I., Pino J., López A., Potrony D. (2009) Potential distribution of the alpine marmot in Southern Pyrenees. Ethology Ecology and Evolution. 21: 225-235.EnllaçDoi: 10.1080/08927014.2009.9522477
Alpine marmots were introduced in the French Pyrenees between 1948 and 1988. The exact number of re-introduced individuals is unknown, but it oscillated around 400. The likely preference of marmots for the southern sunny slopes rapidly facilitated their expansion to the southern Pyrenees, where the lack of both natural predators and of important interspeciflc competitors also likely facilitated an important expansion of this species. There is only one attempt to broadly calculate the population of marmots in the southern Pyrennes, estimating a population of around 10,000 individuals. However, there exist no reliable data to calculate the potential distribution of this new colonizing species in the southern Pyrenees, and a map of the potential distribution of the species is necessary to see whether alpine marmots can potentially establish in sites where it might be necessary to manage its populations for various reasons. We developed a map of potential distribution based on census carried out in summer of 2007 in an area of more than 600 km. We censused more around 300 colonies together with around 300 random points to characterized habitat selection variables. A map with a pixel size of 15 × 15 m has been obtained based on preferred habitats and also on distances to other habitats for the whole southeastern Pyrenees.
López B.C., Gracia C.A., Sabaté S., Keenan T. (2009) Assessing the resilience of Mediterranean holm oaks to disturbances using selective thinning. Acta Oecologica. 35: 849-854.EnllaçDoi: 10.1016/j.actao.2009.09.001
Climate change will increase the frequency and the intensity of droughts in the Mediterranean region, likely reducing growth and increasing mortality of holm oaks (Quercus ilex), one of the most abundant species of Mediterranean forests. In water-limited systems such as those of the Mediterranean, carbon allocation patterns strongly favour belowground accumulation, especially in large subterranean structures called lignotubers. The resilience of these forests depends largely on the replenishment rate of these carbon reserves after disturbances. An experimental thinning, with two intensities (removal of 40% and 80% of basal area), was performed in 1992 in a holm oak forest at the Prades Experimental Complex of Catchments (NE Spain). In 2002, a second thinning was carried out in subplots within the former experimental 0.5 ha plots. Samples from the lignotubers of holm oak trees were analyzed for starch, and both mobile and immobile chemical components, in order to assess the resilience of holm oaks to repeated disturbances. Our results show that after 10 years, starch stocks in the lignotubers have only recovered to half their former values. Removing 40% of the basal area instead of 80% is suggested to be the better managing option for this kind of forests. © 2009 Elsevier Masson SAS. All rights reserved.
López B.C., Holmgren M., Sabaté S., Gracia C.A. (2008) Estimating annual rainfall threshold for establishment of tree species in water-limited ecosystems using tree-ring data. Journal of Arid Environments. 72: 602-611.EnllaçDoi: 10.1016/j.jaridenv.2007.10.012
In arid and semi-arid ecosystems, water availability is discontinuous, highly variable, and characterized by discrete pulse events separated by long periods of limited resource availability. Plant recruitment in these ecosystems is also episodic and dependent on the water available during and after these discrete rainfall events. Precipitation thresholds for plant establishment have been estimated mainly for herbaceous plants and tree seedlings, but extrapolation of short-term results based on seedlings to natural tree populations is difficult. Nevertheless, estimations of water availability thresholds for tree recruitment are essential for successful policies on forest conservation and restoration. We propose a methodology to estimate precipitation thresholds for adult tree populations using tree-ring series and precipitation data. We used this methodology with two Prosopis species from South America: Prosopis pallida and Prosopis chilensis. Results indicate a precipitation threshold of around 85 mm for the establishment of P. pallida trees, whereas the threshold for P. chilensis is likely to be much higher. © 2007 Elsevier Ltd. All rights reserved.
Martínez-Vilalta J., López B.C., Adell N., Badiella L., Ninyerola M. (2008) Twentieth century increase of Scots pine radial growth in NE Spain shows strong climate interactions. Global Change Biology. 14: 2868-2881.EnllaçDoi: 10.1111/j.1365-2486.2008.01685.x
Stem radial growth responds to environmental conditions, and has been widely used as a proxy to study long-term patterns of tree growth and to assess the impact of environmental changes on growth patterns. In this study, we use a tree ring dataset from the Catalan Ecological and Forest Inventory to study the temporal variability of Scots pine (Pinus sylvestris L.) stem growth during the 20th century across a relatively large region (Catalonia, NE Spain) close to the southern limit of the distribution of the species. Basal area increment (BAI) was modelled as a function of tree size and environmental variables by means of mixed effects models. Our results showed an overall increase of 84% in Scots pine BAI during the 20th century, consistent with most previous studies for temperate forests. This trend was associated with increased atmospheric CO2 concentrations and, possibly, with a general increase in nutrient availability, and we interpreted it as a fertilization effect. Over the same time period, there was also a marked increase in temperature across the study region (0.19°C per decade on average). This warming had a negative impact on radial growth, particularly at the drier sites, but its magnitude was not enough to counteract the fertilization effect. In fact, the substantial warming observed during the 20th century in the study area did not result in a clear pattern of increased summer drought stress because of the large variability in precipitation, which did not show any clear time trend. But the situation may change in the future if temperatures continue to rise and/or precipitation becomes scarcer. Such a change could potentially reverse the temporal trend in growth, particularly at the driest sites, and is suggested in our data by the relative constancy of radial growth after ca. 1975, coinciding with the warmer period. If this situation is representative of other relatively dry, temperate forests, the implications for the regional carbon balance would be substantial. © 2008 The Authors Journal compilation © 2008 Blackwell Publishing.
Holmgren M., López B.C., Gutiérrez J.R., Squeo F.A. (2006) Herbivory and plant growth rate determine the success of El Niño Southern Oscillation-driven tree establishment in semiarid South America. Global Change Biology. 12: 2263-2271.EnllaçDoi: 10.1111/j.1365-2486.2006.01261.x
While climatic extremes are predicted to increase with global warming, we know little about the effect of climatic variability on biome distribution. Here, we show that rainy El Niño Southern Oscillation (ENSO) events can enhance tree recruitment in the arid and semiarid ecosystems of north-central Chile and northwest Peru. Tree-ring studies in natural populations revealed that rainy El Niño episodes have triggered forest regeneration in Peru. Field experiments indicate that tree seedling recruitment in Chile is much less successful than in Peru due mostly to larger mortality caused by herbivores. The dramatic impact of herbivores in Chile was derived from the combined result of slower plant growth and the presence of exotic herbivores (European rabbits and hares). The interplay of herbivory and climatic effects we demonstrated implies that rainy ENSO events may represent 'windows of opportunity' for forest recovery if herbivore pressure is minimized at the right moment. © 2006 Blackwell Publishing Ltd.
López B.C., Rodríguez R., Gracia C.A., Sabaté S. (2006) Climatic signals in growth and its relation to ENSO events of two Prosopis species following a latitudinal gradient in South America. Global Change Biology. 12: 897-906.EnllaçDoi: 10.1111/j.1365-2486.2006.01138.x
Semiarid environments throughout the world have lost a major part of their woody vegetation and biodiversity due to the effects of wood cutting, cattle grazing and subsistence agriculture. The resulting state is typically used for cattle production, but the productivity of these systems is often very low, and erosion of the unprotected soil is a common problem. Such dry-land degradation is of great international concern, not only because the resulting state is hardly productive but also because it paves the way to desertification. The natural distribution of the genus Prosopis includes arid and semiarid zones of the Americas, Africa and Asia, but the majority of the Prosopis species are, however, native to the Americas. In order to assess a likely gradient in the response of tree species to precipitation, temperature and their connection to El Niño southern oscillation (ENSO) events, two Prosopis species were chosen along a latitudinal gradient in Latin America, from northern Peru to central Chile: Prosopis pallida from a semi-arid land in northern and southern Peru and P. chilensis from a semiarid land in central Chile. Growth rings of each species were crossdated at each sampling site using classical dendrochronological techniques. Chronologies were related with instrumental climatic records in each site, as well as with SOI and N34 series. Cross-correlation, spectral and wavelet analysis techniques were used to assess the relation of growth with precipitation and temperature. Despite the long distance among sites, the two Prosopis species presented similar responses. Thus, the two species' growth is positively correlated to precipitation, while with temperature it is not. In northern Peru, precipitation and growth of P. pallida present a similar cyclic pattern, with a period of around 3 years. On the other hand, P. pallida in southern Peru, and P. chilensis also present this cyclic pattern, but also another one with lower frequency, coinciding with the pattern of precipitation. Both cycles are within the range of the ENSO band. © 2006 Blackwell Publishing Ltd.
López B.C., Sabaté S., Gracia C.A., Rodríguez R. (2005) Wood anatomy, description of annual rings, and responses to ENSO events of Prosopis pallida H.B.K., a wide-spread woody plant of arid and semi-arid lands of Latin America. Journal of Arid Environments. 61: 541-554.EnllaçDoi: 10.1016/j.jaridenv.2004.10.008
Prosopis pallida H.B.K. is one of the most economically and ecologically important tree species in the arid and semi-arid lands of the American continent. Sections of P. pallida were used to describe its wood anatomy and to determine whether annual rings were visible or not. Results showed that P. pallida has well-differentiated annual growth rings and is therefore suitable for dendrochronological studies. Tree ring chronologies correlate well with precipitation events related to El Niño Southern Oscillation phases. A master chronology for the northern area of Peru was built with these data, and some physiological derivations from the anatomy of P. pallida wood are discussed. © 2004 Published by Elsevier Ltd.
López B., Sabaté S., Gracia C.A. (2001) Fine-root longevity of Quercus ilex. New Phytologist. 151: 437-441.EnllaçDoi: 10.1046/j.0028-646X.2001.00189.x
Fine-root longevity and phenology were studied in a Quercus ilex (holm oak) forest in Prades (NE Spain). Differences were investigated among roots that had appeared in different seasons and at different depth intervals, differentiating between white and brown roots. Using minirhizotrons installed in March 1994, 1211 roots were monitored every 3 wk from June 1994 to March 1997. Mean and median fine-root longevity were 125 ± 4 d and 67 d, respectively. Longevity of summer and winter roots was greater than that of spring and autumn roots. Although roots appeared and disappeared throughout the year, the rate of appearance was greatest in spring. Maximum longevity occurred at a soil depth of 20-30 cm. Differences among 10-cm depth intervals were due to the number of days that roots were brown, since roots remained white for a similar number of days, independent of soil depth. Temperate soil temperatures and lignotubers permitted the appearance of roots throughout the year. Holm oak fine roots might be more efficient during the first 50 d of their life before the white roots become less efficient brown roots. © New Phytologist (2001).
López B., Sabaté S., Gracia C.A. (2001) Annual and seasonal changes in fine root biomass of a Quercus ilex L. forest. Plant and Soil. 230: 125-134.EnllaçDoi: 10.1023/A:1004824719377
The biomass, production and mortality of fine roots (roots with diameter <2.5 mm) were studied in a typical Mediterranean holm oak (Quercus ilex L.) forest in NE Spain using the minirhizotron methodology, A total of 1212 roots were monitored between June of 1994 and March of 1997. Mean annual fine root biomass in the holm oak forest of Prades was 71 ±8 g m-2 yr-1. Mean annual production for the period analysed was 260+11 g m-2 yr-1. Mortality was similar to production, with a mean value of 253±3 g m-2 yr-1. Seasonal fine root biomass presented a cyclic behaviour, with higher values in autumn and winter and lower in spring and summer. Production was highest in winter, and mortality in spring. In summer, production and mortality values were the lowest for the year. Production values in autumn and spring were very similar. The vertical distribution of fine root biomass decreased with increasing depth except for the top 10-20 cm, where values were lower than immediately below. Production and mortality values were similar between 10 and 50 cm depth. In the 0-10 cm and the 50-60 cm depth intervals, both production and mortality were lower.
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