Arnan X., Comas L., Gracia M., Retana J. (2014) Composition and habitat use of small mammals in old-growth mountain forests. Journal of Natural History. 48: 481-494.EnllaçDoi: 10.1080/00222933.2013.800611
Old-growth mountain forests in the Pyrenees have natural gap dynamics, a well-developed shrub layer and a large amount of dead wood. Small mammal communities in two types of old-growth forests, silver fir and mountain pine, were studied in July and September in 2006 and 2007. Four species were trapped: bank vole (Myodes glareolus), wood mouse (Apodemus sylvaticus), garden dormouse (Eliomys quercinus) and common shrew (Sorex araneus). Bank voles and wood mice were most commonly trapped (78% of all captures). There were no differences in community composition in the two forest types, although the bank vole was more often captured in the silver fir than in the mountain pine forest. Mammals were more frequently captured at trap stations with high shrub cover, high tree regeneration cover and low herbaceous cover. Our results show that forest structure and, to a lesser extent, forest type determine small mammal community structure, and specifically fine-scale occurrence patterns, in these old-growth forests. © 2013 Taylor & Francis.
Vayreda J., Gracia M., Martinez-Vilalta J., Retana J. (2013) Patterns and drivers of regeneration of tree species in forests of peninsular Spain. Journal of Biogeography. 40: 1252-1265.EnllaçDoi: 10.1111/jbi.12105
Aim: Our study aimed to identify and explore the main factors that influence tree recruitment of multiple species at a regional scale across peninsular Spain, an understanding of which is essential for predicting future forest species composition in the face of ongoing environmental change. The study focused on the dynamics of the key transition phase from saplings to adult trees. Location: The forests of peninsular Spain. Methods: We used the extensive network of plots sampled in two consecutive Spanish national forest inventories (> 30,000 plots) to identify the factors that determine regeneration patterns of the 10 most abundant forest species of Spain at relatively large temporal (c. 10 years) and spatial scales (across Spain): five coniferous species of Pinus (pines) and five broadleaved species of the genera Fagus and Quercus. We fitted separate generalized linear models for the pine species and the broadleaved species to assess the response of sapling abundance and ingrowth rate to the spatial variability of climate (temperature, water availability and recent warming), forest structure (tree density, understorey and overstorey canopy cover, and basal area change) and disturbances (previous forest logging, wildfires and grazing). Results: Mean sapling abundance was four times higher for broadleaved species than for pines, while mean annual ingrowth was twice as high. Sapling abundance and ingrowth rate were mainly determined by stand structure, both in pines and broadleaved trees. The direct effects of disturbances and climate were comparatively smaller, and there was no detectable effect of recent warming. Main conclusions: The higher values of ingrowth rate of broadleaved species can be explained by their ability to maintain a higher sapling bank due to their greater shade tolerance. This differential response of pines and broadleaved species to canopy closure suggests a probable increase in broadleaved species at the expense of pines. This transition could occur earlier in stands with faster canopy closure dynamics. Spatially explicit, mixed-species demographic models incorporating both the ingrowth and the tree mortality components are needed for predicting the composition of future forests. © 2013 Blackwell Publishing Ltd.
Alvarez A., Gracia M., Retana J. (2012) Fuel types and crown fire potential in Pinus halepensis forests. European Journal of Forest Research. 131: 463-474.EnllaçDoi: 10.1007/s10342-011-0520-6
There is a lack of knowledge to identify and classify forest structures according to the risk of crown fires, especially in Mediterranean regions. In this study, for the first time, we use real information, obtained after a wildfire that burnt under extreme meteorological conditions, to classify forest structures of Pinus halepensis into fuel types as a function of crown fire potential. We identified fourteen forest structures which characterize many forest types in Western Mediterranean areas depending on canopy closure, number of tree layers, percent of each tree layer and overall tree density. By using the pattern of fire types that burnt the most numerous forest structures, we have identified four fire hazard groups of forest structures which are considered different fuel types. The first two had the lowest risk of active crown fires and they differed in the proportion of surface fires and passive crown fires. The third fuel type was the threshold between structures with low and high extreme fire behavior; while the fourth had a high risk of active crown fires. Firefighters and forest managers who are demanding this kind of schema, will test and upgrade this classification of fuel types in function of crown fire potential during future wildfires. © 2011 Springer-Verlag.
Alvarez A., Gracia M., Vayreda J., Retana J. (2012) Patterns of fuel types and crown fire potential in Pinus halepensis forests in the Western Mediterranean Basin. Forest Ecology and Management. 270: 282-290.EnllaçDoi: 10.1016/j.foreco.2011.01.039
Using the databases from the Spanish Forest Inventories, we have classified the forest structures of Pinus halepensis plots across the Iberian Peninsula into different fuel types as a function of the most common fire types that can be supported. The purposes of this study are to determine (i) the proportion of the different fuel types and fire type associated with different disturbance scenarios (undisturbed, after a recent wildfire, after an old wildfire and after thinning), (ii) the effect of climate and soil type on the distribution of fuel types and (iii) the effect of the different disturbance scenarios on the transitions between these fuel types. After a recent wildfire the risk of spreading active crown fires was reduced but the risk increased with time since last fire and in undisturbed areas. Climate and stoniness influenced the spatial distribution of fuel types and the potential crown fire risk. There was a lower risk of active crown fires when there was higher aridity and higher stoniness. Disturbances modify the transitions between fuel types; after a wildfire there was the highest change in fuel types with an increase of fuel type one with open forest structures and the presence of plots without trees that are linked to lower risk of active crown fires. There was also a reduction of fuel types 3 and 4, which burn with high intensity during a wildfire. In the absence of disturbances or after an old wildfire, changes between fuel types were slow, usually leading to increasing canopy closure and higher risk of active crown fires. After thinning there were also important changes in fuel types, with a reduction of active crown fire risk after thinning from below and heavy thinning. Fire plays an important role in maintaining landscape heterogeneity. As a consequence of climate warming, new areas with high structural continuity will increase the risk of extreme fire behavior, and for this reason, small wildfires and specific thinning treatments are the key to reduce crown fire potential. © 2011 Elsevier B.V.
Vayreda J., Gracia M., Canadell J.G., Retana J. (2012) Spatial Patterns and Predictors of Forest Carbon Stocks in Western Mediterranean. Ecosystems. 15: 1258-1270.EnllaçDoi: 10.1007/s10021-012-9582-7
Mediterranean semi-arid forest ecosystems are especially sensitive to external forcing. An understanding of the relationship between forest carbon (C) stock, and environmental conditions and forest structure enable prediction of the impacts of climate change on C stocks and help to define management strategies that maximize the value of forests for C mitigation. Based on the national forest inventory of Spain (1997-2008 with 70,912 plots), we estimated the forest C stock and spatial variability in Peninsular Spain and, we determined the extent to which the observed patterns of stand C stock can be explained by structural and species richness, climate and disturbances. Spain has an average stand C stock of 45.1 Mg C/ha. Total C stock in living biomass is 621 Tg C (7.8% of the C stock of European forests). The statistical models show that structural richness, which is driven by past land use and life forest history including age, development stage, management activities, and disturbance regime, is the main predictor of stand tree C stock with larger C stocks in structurally richer stands. Richness of broadleaf species has a positive effect on both conifer and broadleaf forests, whereas richness of conifer species shows no significant or even a negative effect on C stock. Climate variables have mainly an indirect effect through structural richness but a smaller direct predictive ability when all predictors are considered. To achieve a greater standing C stock, our results suggest promoting high structural richness by managing for uneven-aged stands and favoring broadleaf over conifer species. © 2012 Springer Science+Business Media, LLC.
Vayreda J., Martinez-Vilalta J., Gracia M., Retana J. (2012) Recent climate changes interact with stand structure and management to determine changes in tree carbon stocks in Spanish forests. Global Change Biology. 18: 1028-1041.EnllaçDoi: 10.1111/j.1365-2486.2011.02606.x
Most temperate forests are accumulating carbon (C) and may continue to do so in the near future. However, the situation may be different in water-limited ecosystems, where the potentially positive effects of C and N fertilization and rising temperatures interact with water availability. In this study, we use the extensive network of plots of two consecutive Spanish national forest inventories to identify the factors that determine the spatial variation of the C stock change, growth, and mortality rate of forests in Peninsular Spain (below- and aboveground). We fitted general linear models to assess the response of C stock change and its components to the spatial variability of climate (in terms of water availability), forest structure (tree density and C stock), previous forest management, and the recent warming trend. Our results show that undisturbed forests in Peninsular Spain are accumulating C at a rate of ~1.4 Mg C ha -1 yr -1, and that forest structural variables are the main determinants of forest growth and C stock change. Water availability was positively related to growth and C accumulation. On the other hand, recent warming has reduced growth rate and C accumulation, especially in wet areas. Spatial variation in mortality (in terms of C loss) was mostly driven by differences in growth rate across plots, and was consistent with 'natural', self-thinning dynamics related to the recent abandonment of forest management over large areas of Spain, with the consequent increase in tree density and competition. Interestingly, the negative effect of warming on forest C accumulation disappears if only managed stands are considered, emphasizing the potential of forest management to mitigate the effects of climate change. However, the effect of forest management was weak and, in some cases, not significant, implying the need of further research on its impact. © 2011 Blackwell Publishing Ltd.
Arnan X., Bosch J., Comas L., Gracia M., Retana J. (2011) Habitat determinants of abundance, structure and composition of flying Hymenoptera communities in mountain old-growth forests. Insect Conservation and Diversity. 4: 200-211.EnllaçDoi: 10.1111/j.1752-4598.2010.00123.x
1.Old-growth forests have features that endow them with an extraordinary ecological value. These forests are sources of habitat diversity and, consequently, biodiversity, which makes them a basic objective of conservation programs. Insects have been traditionally used as indicators of forest condition. 2.The aim of this study is to uncover patterns of Hymenoptera abundance and diversity, and their relationship with structural features in old-growth forests. We use pan traps to sample the community of flying Hymenoptera in two old-growth forest types (silver fir and mountain pine) with important structural differences. 3.Compared to other surveys of local Hymenoptera communities, our sampling yielded an extremely high number of species (630). 4.At the plot level, the two forest types showed important differences in family richness and diversity, but not in abundance or in species richness and diversity. However, variability in species richness was higher among pine than silver fir plots, leading to overall higher species richness in the former. 5.Species composition also differed between pine than silver fir forests, and these differences were related to important structural differences between the two forest types. 6.Canonical correspondence and multiple regression analysis yielded contrasting habitat requirements among Hymenoptera families and functional groups (bees, sawflies, parasitic wasps and predatory wasps). 7.We conclude that flying Hymenoptera communities can be used as good indicators of forest structure, habitat complexity and conservation status. © 2010 The Authors. Insect Conservation and Diversity © 2010 The Royal Entomological Society.
Gracia M., Meghelli N., Comas L., Retana J. (2011) Land-cover changes in and around a National Park in a mountain landscape in the Pyrenees. Regional Environmental Change. 11: 349-358.EnllaçDoi: 10.1007/s10113-010-0138-0
The current state of Mediterranean mountain areas has been driven by two main factors: intense traditional human activity and the dynamics of the ecosystem itself. In this study, we examine land-cover changes in a National Park in the Pyrenees mountains (NE Iberian Peninsula), which was designated a protected area 55 years ago. First, we have analyzed spatio-temporal changes in land-cover pattern and forest dynamics from 1957 to 2005. During this period, land-cover dynamics consisted of two main processes: (i) expansion of the forest area and (ii) increasing cover of forests already present in 1957. To analyze the role of the conservation level of the park, we have also compared the results obtained within the park with those of unprotected, peripheral areas. In the two areas with different protection level, dense forests increased throughout the period because of the reduction in forestry activities. The peripheral area showed a higher rate of forest-cover change from 1957 to 2005 compared to the National Park. This higher increase in forest cover in the peripheral area could be related to a higher proportion in the National Park of screes and rocky areas and to the decline and transformation of forest activities in these peripheral, lower elevation areas. © 2010 Springer-Verlag.
Arnan X., Gracia M., Comas L., Retana J. (2009) Forest management conditioning ground ant community structure and composition in temperate conifer forests in the Pyrenees Mountains. Forest Ecology and Management. 258: 51-59.EnllaçDoi: 10.1016/j.foreco.2009.03.029
The search for indicators to monitor management impact on biodiversity is a crucial question because management practices promote changes in community structure and composition of different animal groups. This study explores the effect of widely conducted management practices (forest logging and livestock) in Pinus uncinata forests in the Pyrenees range (NE Spain) on the structure and composition of ground ant communities compared to those of old-growth stands. Forest structure clearly differed in stands with different forest managements. These stands managed for different uses also showed marked differences in structure and composition of ground ant communities. There was a great dominance of a single species, Formica lugubris, which accounted for 99% of ants collected in pitfall traps. Rarefaction curves indicated that species richness was highest in old-growth stands and lowest in even-aged ones, with woodland pasture stands showing an intermediate value. Classification methods allowed us to identify two groups of species: six species related to old-growth plots and three species (including F. lugubris) associated to managed stands. Habitat structure played an important role in determining the structure of ant communities: forests with high tree density but low basal area were the most favourable forest type for F. lugubris, while the abundance of the remaining ant species was negatively affected by the abundance of F. lugubris and by tree cover. © 2009.
Román-Cuesta R.M., Gracia M., Retana J. (2009) Factors influencing the formation of unburned forest islands within the perimeter of a large forest fire. Forest Ecology and Management. 258: 71-80.EnllaçDoi: 10.1016/j.foreco.2009.03.041
Large forest fires have recently increased in frequency and severity in many ecosystems. Due to the heterogeneity in fuels, weather and topography, these large fires tend to form unburned islands of vegetation. This study focuses on a large forest fire that occurred in north-eastern Spain in 1998, which left large areas of unburned vegetation within its perimeter. Based on a satellite post-fire severity map we searched for the relative influence of biotic and abiotic factors leading to unburned island formation. We divided the area of the fire into individual units we called "slopes" which were meant to separate the differential microclimatic effects of contrasted aspects. The number of unburned islands and their areas were related to 12 variables that influence their formation (i.e. land cover composition, aspect, steepness, forest structure, two landscape indices and weather variables). We hypothesized that unburned vegetation islands would concentrate on northern aspects, in less flammable forests (i.e. broadleaf species) and higher fragmentation to interrupt the advance of fire. While north and western aspects did have a higher presence of unburned vegetation islands, our study suggests greater presence of islands in slopes that are larger (i.e. more continuous areas with relatively homogeneous aspect), with greater proportions of forest cover, with higher wood volumes and with lower proportions of broadleaf species. Climate also played a role, with relative humidity and wind speed positively and negatively correlated to island formation, respectively. Unburned vegetation was more frequent on slopes with lower diversity of land covers and higher dominance of one land cover in the slope. Since slopes with only one land cover (i.e. forests) had more islands than slopes with multiple cover types, we infer that under severe meteorological conditions, fragmented forests can be more affected by wind and by water stress, thus burning more readily than forests that are protected from this edge phenomenon. These results would reinforce forest management strategies that avoid linear features (fire-lines and fire-breaks), to enhance fuel treatments that focus on areas and minimize fragmentation. © 2009 Elsevier B.V. All rights reserved.
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