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.
Sardans J., Peñuelas J., Coll M., Vayreda J., Rivas-Ubach A. (2012) Stoichiometry of potassium is largely determined by water availability and growth in Catalonian forests. Functional Ecology. 26: 1077-1089.EnllaçDoi: 10.1111/j.1365-2435.2012.02023.x
The study of the relationships between organisms and environmental elemental stoichiometry and ecosystem structure and function has recently received increasing attention. Some elements, however, have been less studied or have even been neglected. One of these elements is K, despite its critical importance in the water economy of plants. We hypothesized that K concentrations and especially K contents (concentrations × biomass), their stoichiometries with respect to C, N, and P contents, and their relative allocations to foliar and woody compartments would be linked to climatic gradients in the availability of water, forest type and growth. We tested this hypothesis by analysing the data set of the Catalan Forest Inventory. Evergreens, the type of tree with the slowest growth, showed the highest K contents, especially in wood, and the lowest plasticity to change the stoichiometry of K within and between foliar and woody biomasses along climatic gradients. The allocation of K to leaves in relation to the allocation of C, N and P increased with mean annual precipitation (MAP) and was concomitant with decreases in the allocation of K to wood in relation to the allocation of C, N and P (higher K:C L/W, K:N L/W and K:P L/W). In summer, the driest period, higher K:C, K:N and K:P content ratios in leaves were accompanied by lower K:C, K:N and K:P content ratios in wood, mainly in the species at the driest sites. Higher K:C and K:N content ratios in leaves and above-ground biomass in all forest types, and higher K:C L/W and K:N L/W in conifers and deciduous trees were related to higher growth. K contents of leaves were better correlated with tree growth than were K concentrations of leaves in all forest types. These results show that the stoichiometry of K is strongly related to the availability of water and that the uptake of K is more related to the uptake of water than the uptake of N and P. Stoichiometric differences involving K are related to both the response of plants to drought and to plant growth. K thus plays a key role, together with N and P, in the response of plants to climatic gradients for improving the capacities for growth and adaptation to water stress along environmental gradients and through time (seasons). Moreover, these results show that the differences in stoichiometric composition and plasticity involving K contents can underlie the long-term adaptation of trees to different ecological styles of life. K should thus be considered in ecological stoichiometric studies of terrestrial plants. © 2012 The Authors. Functional Ecology © 2012 British Ecological Society.
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.
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