Barba J., Curiel Yuste J., Martinez-Vilalta J., Lloret F. (2013) Drought-induced tree species replacement is reflected in the spatial variability of soil respiration in a mixed Mediterranean forest. Forest Ecology and Management. 306: 79-87.EnllaçDoi: 10.1016/j.foreco.2013.06.025
As episodes of drought-induced forest mortality are being increasingly reported worldwide and may become more frequent in the future as a result of climate change, it is essential to characterize their functional implications in terms of ecosystem carbon and water fluxes. We investigated the spatial variability of soil respiration in a mixed Mediterranean forest located on rugged terrain, where Scots pine (Pinus sylvestris) is affected by drought-induced dieback and appears to have been replaced by Holm oak (Quercus ilex) as the dominant tree species. Soil respiration was measured in spring 2010 on two plots (16.2×16.2m) using a static closed chamber method (soda lime technique) and a systematic sampling (1.8-m grid) including 100 points per plot. Biotic and abiotic variables, such as soil moisture, soil temperature, soil organic matter content, stoniness, pH, fine root C:N ratio and biomass, tree basal area and tree species and health condition of nearest neighbouring tree were also recorded. Our results showed that the spatial variability of soil respiration under optimal environmental conditions (spring) was high and showed no spatial autocorrelation on the scale studied (1-18m). A mixed-effects model applied to explain the spatial variability of soil respiration indicated that only the variables related to forest structure (i.e., health condition and basal area) explained any of the observed variability of soil respiration (R2=0.45). Our model revealed that soil respiration was highest in soils close to dead pines and under Holm oak trees, suggesting that tree mortality and species replacement of pine trees by Holm oak may lead to higher soil respiration fluxes. The direct effect of tree mortality on soil respiration may be a transitory response caused by fine root mortality. Furthermore, the fact that tree species replacement as a result of drought-induced die-off is accompanied by concomitant changes in soil respiration has important implications for soil and ecosystem carbon balance. © 2013 Elsevier B.V.
Barbeta A., Ogaya R., Penuelas J. (2013) Dampening effects of long-term experimental drought on growth and mortality rates of a Holm oak forest. Global Change Biology. 19: 3133-3144.EnllaçDoi: 10.1111/gcb.12269
Forests respond to increasing intensities and frequencies of drought by reducing growth and with higher tree mortality rates. Little is known, however, about the long-term consequences of generally drier conditions and more frequent extreme droughts. A Holm oak forest was exposed to experimental rainfall manipulation for 13 years to study the effect of increasing drought on growth and mortality of the dominant species Quercus ilex, Phillyrea latifolia, and Arbutus unedo. The drought treatment reduced stem growth of A. unedo (-66.5%) and Q. ilex (-17.5%), whereas P. latifolia remained unaffected. Higher stem mortality rates were noticeable in Q. ilex (+42.3%), but not in the other two species. Stem growth was a function of the drought index of early spring in the three species. Stem mortality rates depended on the drought index of winter and spring for Q. ilex and in spring and summer for P. latifolia, but showed no relation to climate in A. unedo. Following a long and intense drought (2005-2006), stem growth of Q. ilex and P. latifolia increased, whereas it decreased in A. unedo. Q. ilex also enhanced its survival after this period. Furthermore, the effect of drought treatment on stem growth in Q. ilex and A. unedo was attenuated as the study progressed. These results highlight the different vulnerabilities of Mediterranean species to more frequent and intense droughts, which may lead to partial species substitution and changes in forest structure and thus in carbon uptake. The response to drought, however, changed over time. Decreased intra- and interspecific competition after extreme events with high mortality, together with probable morphological and physiological acclimation to drought during the study period, may, at least in the short term, buffer forests against drier conditions. The long-term effects of drought consequently deserve more attention, because the ecosystemic responses are unlikely to be stable over time.Nontechnical summaryIn this study, we evaluate the effect of long-term (13 years) experimental drought on growth and mortality rates of three forest Mediterranean species, and their response to the different intensities and durations of natural drought. We provide evidence for species-specific responses to drought, what may eventually lead to a partial community shift favoring the more drought-resistant species. However, we also report a dampening of the treatment effect on the two drought-sensitive species, which may indicate a potential adaptation to drier conditions at the ecosystem or population level. These results are thus relevant to account for the stabilizing processes that would alter the initial response of ecosystem to drought through changes in plant physiology, morphology, and demography compensation. © 2013 John Wiley & Sons Ltd.
Basnou C., Alvarez E., Bagaria G., Guardiola M., Isern R., Vicente P., Pino J. (2013) Spatial patterns of land use changes across a mediterranean metropolitan landscape: Implications for biodiversity management. Environmental Management. 52: 971-980.EnllaçDoi: 10.1007/s00267-013-0150-5
Land use and land cover change (LUCC) is an acknowledged cause of the current biodiversity crisis, but the link between LUCC and biodiversity conservation remains largely unknown at the regional scale, especially due to the traditional lack of consistent biodiversity data. We provide a methodological approach for assessing this link through defining a set of major pressures on biodiversity from LUCC and evaluating their extent, distribution, and association with a set of physical factors. The study was performed in the Metropolitan Region of Barcelona (MRB, NE of Spain) between 1956 and 2000. We generated a LUCC map for the time period, which was reclassified into a set of pressures on biodiversity (forestation, deforestation, crop abandonment, and urbanization). We then explored the association of these pressures with a set of physical factors using redundancy analysis (RDA). Pressures encompassed 38.8 % of the MRB area. Urbanization and forestation were the dominating pressures, followed by crop abandonment and deforestation. RDA showed a significant distribution gradient of these pressures in relation to the studied physical factors: while forestation and deforestation are concentrated in remote mountain areas, urbanization mainly occurs in lowlands and especially on the coast, and close to previous urban centers and roads. Unchanged areas are concentrated in rainy and relatively remote mountain areas. Results also showed a dramatic loss of open habitats and of the traditional land use gradient, both featuring Mediterranean landscapes and extremely important for their biodiversity conservation. Implications of these results for biodiversity management are finally discussed. © 2013 Springer Science+Business Media New York.
Batriu E., Ninot J.M., Rovira P., Pino J. (2013) Plant communities partially reflect environmental gradients in humanized landscapes: A case study in the Llobregat delta marshes. Phytocoenologia. 43: 183-191.EnllaçDoi: 10.1127/0340-269X/2013/0043-0550
This study took place in a vegetation mosaic of the Llobregat delta (near Barcelona, Spain), which includes both natural and spontaneously restored coastal marshes. We used a dataset comprising vegetation relevés, soil parameters and water table conditions, and explored their correlation. More precisely, we classifi ed the relevés into three phytosociological alliances (namely Phragmition communis, Salicornion fruticosae and Juncion maritimi) combining the previous information available and the use of PCA ordinations. We studied whether these alliances refl ect the main operating environmental gradients (conductivity, sodium absorption rate and water table dynamics) using a GLMM (generalized linear mixed model). The PCA refl ected a fair distinctiveness of Salicornion and Phragmition, whereas Juncion was a more heterogeneous unit, related to the varying dominance of different plant species. The three alliances exhibited signifi cant differences in environmental variables and therefore the phytosociological ordination refl ected at least partially the main operating environmental gradients. However the assemblage of alliances over the gradients varied between locations, suggesting that the initial species pool or plant dispersal capacity to some extent shapes the dynamics of spontaneous restoration in these marshlands. © 2013 Gebrüder Borntraeger.
Beauchamp K., Mencuccini M., Perks M., Gardiner B. (2013) The regulation of sapwood area, water transport and heartwood formation in Sitka spruce. Plant Ecology and Diversity. 6: 45-56.EnllaçDoi: 10.1080/17550874.2012.702359
Background: The mechanism by which water transport, tree growth and heartwood formation are balanced is poorly understood.Aims: To test the hypothesis that xylem formation drives heartwood production through changes in water transport to regulate sapwood area.Methods: We measured changes in sap flux at multiple depths across the sapwood to heartwood boundary in Sitka spruce (Picea sitchensis) from May 2009-October 2010 using the Heat Field Deformation method.Results: The radial sap flux profile was peaked with maximum flux occurring 1-2 cm below bark before reducing to the heartwood boundary (tail). Changes occurred in two stages. The depth of peak sap flux (D) extended outwards as new xylem formed during the growing season, giving an annual increment (AI). Water transport reduced in the tail sometime during dormancy, from November to March. The correlation between AI and D was good however, these variables and the increase in sapwood area correlated poorly with the extent of heartwood formed.Conclusions: Heartwood formed during the dormant period in Sitka spruce in Great Britain. Xylem formation did not directly drive heartwood production; however, changes in specific conductivity need consideration. Reduced transport in the inner sapwood could provide a temporal signal for heartwood formation in pre-conditioned cells. © 2013 Copyright Crown copyright 2013.
Bernal S., Belillas C., Ibanez J.J., Avila A. (2013) Exploring the long-term response of undisturbed Mediterranean catchments to changes in atmospheric inputs through time series analysis. Science of the Total Environment. 458-460: 535-545.EnllaçDoi: 10.1016/j.scitotenv.2013.04.072
The aim of this study was to gain insights on the potential hydrological and biogeochemical mechanisms controlling the response of two nested Mediterranean catchments to long-term changes in atmospheric inorganic nitrogen and sulphate deposition. One catchment was steep and fully forested (TM9, 5.9ha) and the other one had gentler slopes and heathlands in the upper part while side slopes were steep and forested (TM0, 205ha). Both catchments were highly responsive to the 45% decline in sulphate concentration measured in atmospheric deposition during the 1980s and 1990s, with stream concentrations decreasing by 1.4 to 3.4μeqL-1y-1. Long-term changes in inorganic nitrogen in both, atmospheric deposition and stream water were small compared to sulphate. The quick response to changes in atmospheric inputs could be explained by the small residence time of water (4-5months) in these catchments (inferred from chloride time series variance analysis), which was attributed to steep slopes and the role of macropore flow bypassing the soil matrix during wet periods. The estimated residence time for sulphate (1.5-3months) was substantially lower than for chloride suggesting unaccounted sources of sulphate (i.e., dry deposition, or depletion of soil adsorbed sulphate). In both catchments, inorganic nitrogen concentration in stream water was strongly damped compared to precipitation and its residence time was of the order of decades, indicating that this essential nutrient was strongly retained in these catchments. Inorganic nitrogen concentration tended to be higher at TM0 than at TM9 which was attributed to the presence of nitrogen fixing species in the heathlands. Our results indicate that these Mediterranean catchments react rapidly to environmental changes, which make them especially vulnerable to changes in atmospheric deposition. © 2013 Elsevier B.V.
Bernal V., Espadaler X. (2013) Invasive and socially parasitic ants are good bioindicators of habitat quality in Mediterranean forest remnants in northeast Spain. Ecological Research. 28: 1011-1017.EnllaçDoi: 10.1007/s11284-013-1083-4
We surveyed ants in 16 forest remnants in the Vallès lowlands north of Barcelona, Spain: eight with invasive exotic ants (IE = Linepithema humile and/or Lasius neglectus) present, seven with native parasitic ants (P = Lasius meridionalis, Lasius carniolicus, Plagiolepis xene, Chalepoxenus muellerianus, and/or Polyergus rufescens) present, and one remnant with both invasive exotic and parasitic ants present. Forest remnants with IE ants were smaller, more isolated, had greater perimeter/area, lower ant species density, and lower ant species richness than remnants with P ants. The community composition was also significantly different, with greater dissimilarity within remnants with alien, invasive species. The presence of some species is bio-indicator of low disturbance, whereas others are indicative of high disturbance. Our findings underscore the value of different types of ants as bio-indicators of fragmentation and habitat quality. © 2013 The Ecological Society of Japan.
Brotons L., Aquilue N., de Caceres M., Fortin M.-J., Fall A. (2013) How Fire History, Fire Suppression Practices and Climate Change Affect Wildfire Regimes in Mediterranean Landscapes. PLoS ONE. 8: 0-0.EnllaçDoi: 10.1371/journal.pone.0062392
Available data show that future changes in global change drivers may lead to an increasing impact of fires on terrestrial ecosystems worldwide. Yet, fire regime changes in highly humanised fire-prone regions are difficult to predict because fire effects may be heavily mediated by human activities We investigated the role of fire suppression strategies in synergy with climate change on the resulting fire regimes in Catalonia (north-eastern Spain). We used a spatially-explicit fire-succession model at the landscape level to test whether the use of different firefighting opportunities related to observed reductions in fire spread rates and effective fire sizes, and hence changes in the fire regime. We calibrated this model with data from a period with weak firefighting and later assess the potential for suppression strategies to modify fire regimes expected under different levels of climate change. When comparing simulations with observed fire statistics from an eleven-year period with firefighting strategies in place, our results showed that, at least in two of the three sub-regions analysed, the observed fire regime could not be reproduced unless taking into account the effects of fire suppression. Fire regime descriptors were highly dependent on climate change scenarios, with a general trend, under baseline scenarios without fire suppression, to large-scale increases in area burnt. Fire suppression strategies had a strong capacity to compensate for climate change effects. However, strong active fire suppression was necessary to accomplish such compensation, while more opportunistic fire suppression strategies derived from recent fire history only had a variable, but generally weak, potential for compensation of enhanced fire impacts under climate change. The concept of fire regime in the Mediterranean is probably better interpreted as a highly dynamic process in which the main determinants of fire are rapidly modified by changes in landscape, climate and socioeconomic factors such as fire suppression strategies. © 2013 Brotons et al.
Brunner I., Bakker M.R., Björk R.G., Hirano Y., Lukac M., Aranda X., Børja I., Eldhuset T.D., Helmisaari H.S., Jourdan C., Konôpka B., López B.C., Miguel Pérez C., Persson H., Ostonen I. (2013) Fine-root turnover rates of European forests revisited: An analysis of data from sequential coring and ingrowth cores. Plant and Soil. 362: 357-372.EnllaçDoi: 10.1007/s11104-012-1313-5
Background and Aims: Forest trees directly contribute to carbon cycling in forest soils through the turnover of their fine roots. In this study we aimed to calculate root turnover rates of common European forest tree species and to compare them with most frequently published values. Methods: We compiled available European data and applied various turnover rate calculation methods to the resulting database. We used Decision Matrix and Maximum-Minimum formula as suggested in the literature. Results: Mean turnover rates obtained by the combination of sequential coring and Decision Matrix were 0. 86 yr-1 for Fagus sylvatica and 0. 88 yr-1 for Picea abies when maximum biomass data were used for the calculation, and 1. 11 yr-1 for both species when mean biomass data were used. Using mean biomass rather than maximum resulted in about 30 % higher values of root turnover. Using the Decision Matrix to calculate turnover rate doubled the rates when compared to the Maximum-Minimum formula. The Decision Matrix, however, makes use of more input information than the Maximum-Minimum formula. Conclusions: We propose that calculations using the Decision Matrix with mean biomass give the most reliable estimates of root turnover rates in European forests and should preferentially be used in models and C reporting. © 2012 The Author(s).
Canellas-Bolta N., Rull V., Saez A., Margalef O., Bao R., Pla-Rabes S., Blaauw M., Valero-Garces B., Giralt S. (2013) Vegetation changes and human settlement of Easter Island during the last millennia: A multiproxy study of the Lake Raraku sediments. Quaternary Science Reviews. 72: 36-48.EnllaçDoi: 10.1016/j.quascirev.2013.04.004
Earlier palynological studies of lake sediments from Easter Island suggest that the island underwent a recent and abrupt replacement of palm-dominated forests by grasslands, interpreted as a deforestation by indigenous people. However, the available evidence is inconclusive due to the existence of extended hiatuses and ambiguous chronological frameworks in most of the sedimentary sequences studied. This has given rise to an ongoing debate about the timing and causes of the assumed ecological degradation and cultural breakdown. Our multiproxy study of a core recovered from Lake Raraku highlights the vegetation dynamics and environmental shifts in the catchment and its surroundings during the late Holocene. The sequence contains shorter hiatuses than in previously recovered cores and provides a more continuous history of environmental changes. The results show a long, gradual and stepped landscape shift from palm-dominated forests to grasslands. This change started c. 450 BC and lasted about two thousand years. The presence of Verbena litoralis, a common weed, which is associated with human activities in the pollen record, the significant correlation between shifts in charcoal influx, and the dominant pollen types suggest human disturbance of the vegetation. Therefore, human settlement on the island occurred c. 450 BC, some 1500 years earlier than is assumed. Climate variability also exerted a major influence on environmental changes. Two sedimentary gaps in the record are interpreted as periods of droughts that could have prevented peat growth and favoured its erosion during the Medieval Climate Anomaly and the Little Ice Age, respectively. At c. AD 1200, the water table rose and the former Raraku mire turned into a shallow lake, suggesting higher precipitation/evaporation rates coeval with a cooler and wetter Pan-Pacific AD 1300 event. Pollen and diatom records show large vegetation changes due to human activities c. AD 1200. Other recent vegetation changes also due to human activities entail the introduction of taxa (e.g. Psidium guajava, Eucalyptus sp.) and the disappearance of indigenous plants such as Sophora toromiro during the two last centuries. Although the evidence is not conclusive, the American origin of V. litoralis re-opens the debate about the possible role of Amerindians in the human colonisation of Easter Island. © 2013 Elsevier Ltd.
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