Determinants of drought effects on crown condition and their relationship with depletion of carbon reserves in a Mediterranean holm oak forest

Galiano L., Martínez-Vilalta J., Sabaté S., Lloret F. (2012) Determinants of drought effects on crown condition and their relationship with depletion of carbon reserves in a Mediterranean holm oak forest. Tree Physiology. 32: 478-489.
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Doi: 10.1093/treephys/tps025

Resum:

Severe droughts may increase physiological stress on long-lived woody vegetation, occasionally leading to rapid defoliation and progressive increase in mortality of overstorey trees. Over the last few years, episodes of drought-induced tree dieback have been documented in a variety of woodlands and forests around the world. However, the factors determining tree survival and subsequent recovery are still poorly understood, especially in resprouter species. We have studied the effects of a single drought episode on crown condition in a holm oak (Quercus ilex L.) forest located in NE Spain 7 years after the drought event. Generalized linear models were used to study the environmental correlates of forest crown condition 7 years after the drought event. Additionally, we evaluated the association between crown condition and the carbon and nutrient reserves stored in lignotubers 7 years after the drought. Our study reveals the multifactor nature of a drought-driven forest dieback in which soil depth and the characteristics of individual trees, particularly their number of stems, determined a complex spatial pattern of tree-level responses. This dieback was associated with a depletion of the carbon reserves in lignotubers 7 years after the episode, representing a reduction of up to 60 in highly drought-damaged trees. Interestingly, in the absence of new acute droughts, successive surveys in 2007-11 showed a direct association between carbon reserves depletion and further deterioration of crown condition. More frequent droughts, as predicted by climate change projections, may lead to a progressive depletion of carbon reserves and to a loss of resilience in Mediterranean resprouter species. © 2012 The Author.

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Tree mortality following ENSO-associated fires and drought in lowland rain forests of Eastern Nicaragua

Granzow-de la Cerda Í., Lloret F., Ruiz J.E., Vandermeer J.H. (2012) Tree mortality following ENSO-associated fires and drought in lowland rain forests of Eastern Nicaragua. Forest Ecology and Management. 265: 248-257.
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Doi: 10.1016/j.foreco.2011.10.034

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Occurrence of fires is an unusual event in many tropical terra firma rain forests, in particular those in Mesoamerica. As a result of the unusually intense ENSO event of 1998/1999, these forests were subjected to a prolonged drought that favored the propagation of extensive fires. In the lowland rain forests of the Atlantic Coast of Nicaragua, fires were mostly low-intensity leaf litter fires, but they produced high tree mortality. Data from 5 to 17. years of study of permanent plots/transects from four sites were used to investigate whether survival following the ENSO drought episode and subsequent fires may be affected by factors such as successional status, tree size, taxonomic identity or growth rate. The study included two sites that burned in their entirety, one that had only part of its area burned and a fourth site that was not affected by fire. Overall, tree survival was lower in burned plots, and remained low in those plots for at least 2. years after fires, especially among pioneer and mature forest species. Mortality experience between burnt and unburned plots differed widely among the 15 most abundant species. The 38 most abundant species were grouped into five guilds corresponding to their ecological successional status: (1) pioneers; (2) subcanopy/understory heliophiles; (3) canopy heliophiles; (4) subcanopy mature forests species; (5) canopy mature forest species. All guilds experienced significantly lower survival in plots that burned in mortality among guilds were between significant differences. The only the canopy mature forest species and the pioneers, which had the highest mortality. Two years after fire, survival remained significantly lower in burnt plots than in plots that had not burned, but the pattern of mortality was quite different. Both mature forests species guilds had significantly higher mortality than heliophile guilds. Tree size had an overall significant positive effect on survival after fires particularly among subcanopy and canopy mature forest species guilds. However, in unburned sites, survival was negatively correlated to tree size in mature forest guilds. While relative growth rate had no overall significant effect in plots that burned, faster growth was positively correlated with survival in particular guilds such as in pioneers and canopy heliophiles. Delayed mortality increased with relative growth rate for pioneers but decreased for subcanopy heliophiles. The effect of the ENSO event drought resulted in a significantly reduced overall survival caused by high mortality of pioneers, compared to non-drought years and to other guilds, but non-significant differences in post-drought survival were apparent among the remaining guilds. Individual tree basal area was positively correlated with survival given drought, overall, but particularly in pioneers and mature forest canopy species. Contrary to hurricanes, the delayed mortality and overall damage caused by fires is not conducive to maintaining the structure, diversity and species composition of these forests. © 2011 Elsevier B.V.

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What do we know about the role and regulation of non-structural carbon compounds stored in trees?

Sala A, Martínez-Vilalta J, Lloret F (2012) What do we know about the role and regulation of non-structural carbon compounds stored in trees? AGU Fall Meeting,  Francisco, United States of America, 3-7 December 2012. (comunicació oral).

Las poblaciones ibéricas de pino albar ante el cambio climático: con la muerte en los talones.

Martínez-Vilalta J, Aguadé D, Banqué M, Barba J, Curiel Yuste J, Galiano L, Garcia N, Gómez M, Heres; AM, López BC, Lloret F, Poyatos R, Retana J, Sus O, Vayreda J, Vilà-Cabrera A (2012) Las poblaciones ibéricas de pino albar ante el cambio climático: con la muerte en los talones. Ecosistemas 21: 15-21.

Las poblaciones ibéricas de pino albar ante el cambio climático: con la muerte en los talones.

Martínez-Vilalta J, Aguadé D, Banqué M, Barba J, Yuste JC, Galiano L, Garcia N, Gómez M, Hereş AM, López BC, Lloret F, Poyatos R, Retana J, Sus O, Vayreda J, Vilà-Cabrera A (2012) Las poblaciones ibéricas de pino albar ante el cambio climático: con la muerte en los talones. Revista Ecosistemas 21: 15–21.

Extreme climatic events and vegetation: The role of stabilizing processes

Lloret F., Escudero A., Iriondo J.M., Martínez-Vilalta J., Valladares F. (2012) Extreme climatic events and vegetation: The role of stabilizing processes. Global Change Biology. 18: 797-805.
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Doi: 10.1111/j.1365-2486.2011.02624.x

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Current climatic trends involve both increasing temperatures and climatic variability, with extreme events becoming more frequent. Increasing concern on extreme climatic events has triggered research on vegetation shifts. However, evidences of vegetation shifts resulting from these events are still relatively rare. Empirical evidence supports the existence of stabilizing processes minimizing and counteracting the effects of these events, reinforcing community resilience. We propose a demographic framework to understand this inertia to change based on the balance between adult mortality induced by the event and enhanced recruitment or adult survival after the event. The stabilizing processes potentially contributing to this compensation include attenuation of the adult mortality caused by the event, due to site quality variability, to tolerance, phenotypic variability, and plasticity at population level, and to facilitative interactions. Mortality compensation may also occur by increasing future survival due to beneficial effect on growth and survival of the new conditions derived from global warming and increased climatic variability, to lowered competition resulting from reduced density in affected stands, or to antagonistic release when pathogens or predators are vulnerable to the event or the ongoing climatic conditions. Finally, mortality compensation may appear by enhanced recruitment due to release of competition with established vegetation, for instance as a consequence of gap openings after event-caused mortality, or to the new conditions, which may be more favorable for seedling establishment, or to enhanced mutualistic interactions (pollination, dispersal). There are important challenges imposed by the need of long-term studies, but a research agenda focused on potentially stabilizing processes is well suited to understand the variety of responses, including lack of sudden changes and community inertia that are frequently observed in vegetation under extreme events. This understanding is crucial for the establishment of sound management strategies and actions addressed to improve ecosystem resilience under climate change scenarios. © 2011 Blackwell Publishing Ltd.

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Comparison of burnt area estimates derived from satellite products and national statistics in Europe

Loepfe L., Lloret F., Román-Cuesta R.M. (2012) Comparison of burnt area estimates derived from satellite products and national statistics in Europe. International Journal of Remote Sensing. 33: 3653-3671.
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Doi: 10.1080/01431161.2011.631950

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Burnt area maps based on satellite observations are frequently used in calculations related to fire regime, such as those of carbon dioxide emissions. Nevertheless, burnt area estimates between products vary widely, and validation against independent data is scarce, especially for Europe. Here we compare two active fire maps (the ATSR World Fire Atlas and the Moderate Resolution Imaging Spectroradiometer (MODIS) Active Fire Product) and two fire scars maps (the L3JRC and the MODIS Burned Area Product) to independent national statistics taken from 22 European countries between 1997 and 2008. We also tested the coincidence between satellite products derived by calculation of the fraction of active fires that were confirmed by a subsequent drop in reflectance. As a large proportion of fire pixels (between 40% and 66%, depending on the product) is located on urban land or crop fields, filtering out fires located on these land uses greatly improves the agreement between satellite-based burnt area estimates and national statistics and it also improves the coincidence between satellite products. The MODIS Active Fire Product appears to be most suitable for use as a proxy for burnt area patterns, showing a high correlation to national statistics (R 2 = 0.9), relatively low spatial and temporal heterogeneity and only a slight underestimation of the total burnt area (19 000 ha year -1). Unfiltered products show cases of substantial wildfire overestimation in all products, mainly attributable to anthropogenic activity, in the case of active fire products, and drought-induced vegetation dieback, in that of fire scar maps. Thus, filtering out fires on anthropogenic land uses seems to be essential when analysing patterns of forest fires from satellite observations. However, if agricultural fires are to be included, a combination of MODIS Active Fire and MODIS Burned Area products is recommended. We obtained that such combination shows low temporal and spatial heterogeneity and the highest coincidence between satellite products (25%), although the correlation to national statistics is not very high (R 2 = 0.67) and clearly underestimates the total burnt area (187 000 ha year -1). © 2012 Copyright Taylor and Francis Group, LLC.

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Drought-induced forest decline: Causes, scope and implications

Martínez-Vilalta J., Lloret F., Breshears D.D. (2012) Drought-induced forest decline: Causes, scope and implications. Biology Letters. 8: 689-691.
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Doi: 10.1098/rsbl.2011.1059

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A large number of episodes of forest mortality associated with drought and heat stress have been detected worldwide in recent decades, suggesting that some of the world's forested ecosystems may be already responding to climate change. Here, we summarize a special session titled 'Drought-induced forest decline: causes, scope and implications' within the 12th European Ecological Federation Congress, held in Ávila (Spain) from 25 to 29 September 2011. The session focused on the interacting causes and impacts of die-off episodes at the community and ecosystem levels, and highlighted recent events of drought- and heat-related tree decline, advances in understanding mechanisms and in predicting mortality events, and diverse consequences of forest decline. Talks and subsequent discussion noted a potentially important role of carbon that may be interrelated with plant hydraulics in the multi-faceted process leading to drought-induced mortality; a substantial and yet understudied capacity of many forests to cope with extreme climatic events; and the difficulty of separating climate effects from other anthropogenic changes currently shaping forest dynamics in many regions of the Earth. The need for standard protocols and multi-level monitoring programmes to track the spatiotemporal scope of forest decline globally was emphasized as critical for addressing this emerging environmental issue. © 2011 The Royal Society.

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Stand- and tree-level determinants of the drought response of Scots pine radial growth

Martínez-Vilalta J., López B.C., Loepfe L., Lloret F. (2012) Stand- and tree-level determinants of the drought response of Scots pine radial growth. Oecologia. 168: 877-888.
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Doi: 10.1007/s00442-011-2132-8

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Characterizing the responses of key tree species to extreme climatic events may provide important information for predicting future forest responses to increased climatic variability. Here we aimed at determining which tree- and stand-level attributes were more closely associated with the effect of a severe drought on the radial growth of Scots pine, both in terms of immediate impact and recovery after the drought event. Our dataset included tree-ring series from 393 plots located close to the dry limit of the species range. Time series analysis and mixed-effects models were used to study the growth of each tree and its detailed response to a severe drought event that occurred in 1986. Our results showed that the radial growth responses of Scots pine were determined primarily by tree-level characteristics, such as age and previous growth rate, and secondarily by stand basal area and species richness, whereas local climate had a relatively minor effect. Fast-growing trees were more severely affected by the drought and retained proportionally lower growth rates up to three years after the episode. In absolute terms, however, fast-growing trees performed better both during and after the event. Older trees were found to be less resilient to drought. The effect of stand basal area and species richness indicated that competition for resources worsened the effects of drought, and suggested that the effect of interspecific competition may be particularly detrimental during the drought year. © 2011 Springer-Verlag.

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Effects of climate change on leaf litter decomposition across post-fire plant regenerative groups

Saura-Mas S., Estiarte M., Peñuelas J., Lloret F. (2012) Effects of climate change on leaf litter decomposition across post-fire plant regenerative groups. Environmental and Experimental Botany. 77: 274-282.
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Doi: 10.1016/j.envexpbot.2011.11.014

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Decomposition is a determining factor for the functioning of ecosystems because litter dynamics (litter fall and litter decomposition) constitute a key process in the regulation of the recycling of carbon and nutrients. We studied the litter decomposition properties of a set of 19 Mediterranean-basin woody species with different post-fire regenerative strategies (resprouters and non-resprouters), under experimental climate manipulation (simulating warming and drought) over a 2-year period. We show that climate change modifies litter decomposition of these Mediterranean woody species as litter contributions to the soil (g/year) were lower under drought experimental conditions. Species with different post-fire regeneration performance showed different leaf decomposition patterns, though these patterns were influenced by the taxonomical affiliation of the species. As expected, the mass loss of the non-resprouter litter, after 2 years, was higher than in resprouters. Non-resprouters showed higher nutrient concentration per mass of leaf litter after 2 years of experiment than resprouters, possibly because they have lost more C-rich biomass, allowing high nutrients concentration in the remaining litter. That would apply particularly to P as litter N:P ratio was lower in non-resprouters than in resprouters. This study suggests that, in Mediterranean ecosystems, nutrients' return from leaf litter to the soil will be slower under the projected future drier conditions. Furthermore, changes in fire regimes that lead to modifications in the abundance of post-fire regenerative groups are likely to affect ecosystem's functional properties. Thus, if new fire regimes enhance non-resprouters' abundance, we can expect a greater return of organic matter contributions to the soil and a lower litter N:P. © 2011 Elsevier B.V.

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