Drought-induced tree species replacement is reflected in the spatial variability of soil respiration in a mixed Mediterranean forest

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.
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Doi: 10.1016/j.foreco.2013.06.025

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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on rugged terrain, where Scots pine (Pinus sylvestris) is affected by drought-induced dieback and appears to have been replaced by Holm oak (Quercus ilex) as the dominant tree species.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.

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Seedling emergence and growth of Quercus spp. following severe drought effects on a Pinus sylvestris canopy

Galiano L., Martinez-Vilalta J., Eugenio M., Granzow-de la Cerda I., Lloret F. (2013) Seedling emergence and growth of Quercus spp. following severe drought effects on a Pinus sylvestris canopy. Journal of Vegetation Science. 24: 580-588.
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Doi: 10.1111/j.1654-1103.2012.01485.x

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Questions: We addressed the following questions: (1) did defoliation and die-off of the dominant Pinus sylvestris, induced by an extreme drought episode, favour emergence of other tree species; (2) did the defoliated canopies of P. sylvestris resulting from drought promote radial growth among other pre-existing tree species seedlings under them? Location: P. sylvestris forest in Central Pyrenees (NE Spain) affected by a severe drought in 2004-2005. Methods: Despite increased focus on climate-related forest die-off, studies of the effects on regeneration processes following extreme drought remain scarce. We analysed whether an episode of drought-induced mortality on the dominant P. sylvestris L. may act as a driver of vegetation shift. Seedlings of Quercus humilis Mill. and Q. ilex L. from 27 plots were sampled under P. sylvestris canopies with

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Plant competition and facilitation after extreme drought episodes in Mediterranean shrubland: Does damage to vegetation cover trigger replacement by juniper woodland?

Lloret F., Granzow-de la Cerda I. (2013) Plant competition and facilitation after extreme drought episodes in Mediterranean shrubland: Does damage to vegetation cover trigger replacement by juniper woodland?. Journal of Vegetation Science. 24: 1020-1032.
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Doi: 10.1111/jvs.12030

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Question: We analyse the contribution of plant-plant interactions, particularly the outcome of plant competition and plant facilitation, on vegetation dynamics as a result of extreme drought episodes. These events will likely become more frequent under climate change, can induce vegetation die-off and alter community dynamics. We study succession in a shrubland that tends to be replaced by juniper (Juniperus phoenicea) woodland. Due to drought, shrubland die-off may result in competition release favouring J. phoenicea juveniles, and accelerating shrubland replacement. Alternatively, deleterious abiotic stress may increase after loss of vegetation cover protection. Location: Mediterranean coastal shrublands, South Spain (Doñana National Park). Methods: Field estimates of plant growth, production of needle-like leaves, water-use efficiency (WUE; leaf δ13C) and N leaf content of J. phoenicea juveniles in relation to plant size, drought-induced damage, cover and habit characteristics of surrounding vegetation, and drought-induced defoliation of the surrounding vegetation. Results: Juniperus phoenicea juveniles growing beneath a dense vegetation canopy, particularly trees and large shrubs, were less damaged during the extreme drought episode. Plant size correlated negatively with damage. Post-drought growth was higher in juveniles partially released from the vegetation canopy, supporting the existence of a balance between competition and facilitation. Cover of pines, large shrubs and spiny shrubs favoured growth of juveniles. Needle-like juvenile leaves were more abundant in plants covered by the surrounding vegetation or in moderately damaged plants, but less abundant in plants without damage. Higher leaf δ13C values - indicating water stress - were measured in plants more damaged by drought and in those without canopy protection, or under vegetation strongly affected by drought. Leaf N content was lower in undamaged plants and individuals covered by surrounding vegetation. Conclusion: We did not find evidence that gaps opened by drought promoted growth of the potential replacing J. phoenicea. Thus, drought-induced enhancement of successional replacement of shrublands with woodlands was not supported; instead, our findings foresee shrubland prevalence under future climate change conditions. Plant facilitation will play a relevant role in this process. Thus, we herein extend the relevance of plant-plant interactions to extreme drought episodes related to climate change, highlighting their role as drivers of community dynamics. © 2012 International Association for Vegetation Science.

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Relationship between projected changes in future climatic suitability and demographic and functional traits of forest tree species in Spain

Lloret F., Martinez-Vilalta J., Serra-Diaz J.M., Ninyerola M. (2013) Relationship between projected changes in future climatic suitability and demographic and functional traits of forest tree species in Spain. Climatic Change. 120: 449-462.
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Doi: 10.1007/s10584-013-0820-6

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The response of plant species to future climate conditions is probably dependent on their ecological characteristics, including climatic niche, demographic rates and functional traits. Using forest inventory data from 27 dominant woody species in Spanish forests, we explore the relationships between species characteristics and projected changes in their average climatic suitability (occurrence of suitable climatic conditions for a species in a given territory) obtained by empirical niche-based models, under a business-as-usual climate change scenario (A1, HadCM3, 2001-2100). We hypothesize that most species will suffer a decline in climatic suitability, with a less severe for species (i) currently living in more arid climates or exhibiting a broader current climatic niche; (ii) with higher current growth rates; (iii) with functional traits related to resistance to water deficits. The analysis confirm our hypothesis since apart from a few Mediterranean species, most species decrease their climatic suitability in the region under future climate, characterized by increased aridity. Also, species living in warmer locations or under a wider range of climatic conditions tend to experience less decrease in climatic suitability. As hypothesized, a positive relationship was detected between current relative growth rates and increase in future climatic suitability. Nevertheless, current tree mortality did not correlate with changes in future climatic suitability. In contrast with our hypothesis, functional traits did not show a clear relationship with changes in climate suitability; instead species often presented idiosyncratic responses that, in some cases, could reflect past management. These results suggest that the extrapolation of species performance to future climatic scenarios based on current patterns of dominance is constrained by factors other than species autoecology, particularly human activity. © 2013 Springer Science+Business Media Dordrecht.

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Geographical patterns of congruence and incongruence between correlative species distribution models and a process-based ecophysiological growth model

Serra-Diaz J.M., Keenan T.F., Ninyerola M., Sabate S., Gracia C., Lloret F. (2013) Geographical patterns of congruence and incongruence between correlative species distribution models and a process-based ecophysiological growth model. Journal of Biogeography. 40: 1928-1938.
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Doi: 10.1111/jbi.12142

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Aim: Our aim was to map the climate dependence of tree species distributions (probability of occurrence) and forest growth (net primary productivity) by comparing the congruence and incongruence between correlative and process-based modelling approaches. Location: Iberian Peninsula, south-western Europe. Methods: We used forest inventory data for three widespread tree species (Quercus ilex, Pinus halepensis and Pinus sylvestris) to model climatic suitability with an ensemble of seven correlative species distribution models (using biomod). We then simulated forest net primary productivity (NPP) as a surrogate of forest growth for forests of each species using an ecophysiological process-based model (gotilwa+) along a gradient of climatic suitability. The spatial distribution of the growth estimates was then compared with that of the suitability estimates, and robust regression was used to classify regions in terms of model congruence. Results: Quercus ilex and P. sylvestris both showed a positive relationship between forest NPP and climatic suitability. The main discrepancies were found in the north of the peninsula, where there was high potential forest growth but low climate suitability. Low forest-growth estimates in areas of high suitability only appeared for P. sylvestris in southern montane regions. Pinus halepensis always showed a negative relationship between estimated growth and climatic suitability. The analysis of other ecophysiological parameters (mean leaf life and leaf area index) suggests that this tree species has different physiological strategies that allow differential growth rates in areas of low suitability. Main conclusions: We found that the relationship between estimated growth and distribution varies strongly in different areas and species. Mapping the incongruences between the predicted climatic suitability and growth allowed us to identify regions where other factors (e.g. biotic interactions) may be more significant than the physiological limits on growth. We show that new insights into species distributions can be gained from mapping the differences between correlative and process-based models. © 2013 John Wiley & Sons Ltd.

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Inter-annual variability of seed rain and seedling establishment of two woody Mediterranean species under field-induced drought and warming

del Cacho M., Estiarte M., Peñuelas J., Lloret F. (2013) Inter-annual variability of seed rain and seedling establishment of two woody Mediterranean species under field-induced drought and warming. Population Ecology. 55: 277-289.
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Doi: 10.1007/s10144-013-0365-6

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We aimed to assess the impact of warmer and drier climate change conditions on the seed rain and seedling establishment of Globularia alypum L. and Erica multiflora L., two dominant species in Western coastal Mediterranean shrublands. We performed a non-intrusive field experiment in which we increased the night-time temperatures and excluded spring and autumn rainfall. We monitored the seed rain over 5 years and the seedling recruitment over 9 years on these experimental plots. Seed rain of E. multiflora was enhanced by warming treatment in relation to control, and higher annual rainfall, while seed rain of G. alypum was increased by drought treatment in relation to control, dry years and higher minimum annual temperature. Annual rainfall enhanced the seedling emergence of both species, which also positively correlated with annual mean temperatures. Drought treatment significantly decreased seedling emergence for both species, which was higher in open areas than below vegetation cover. The seedling survival of both species diminished at closer distances to competing neighbours, and in G. alypum seedling survival was higher with lower annual mean temperatures and higher annual rainfall, but also in drought treatment, which have experienced vegetation cover decline. The study confirms that the increasing aridity in Mediterranean ecosystems would constrain the early stages of development in typical co-occurring shrubs. However, there are contrasting responses to climatic conditions between species recruitment, which might favour changes in vegetation through modification of species relative abundance. © 2013 The Society of Population Ecology and Springer Japan.

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Reproductive output in Mediterranean shrubs under climate change experimentally induced by drought and warming

del Cacho M., Penuelas J., Lloret F. (2013) Reproductive output in Mediterranean shrubs under climate change experimentally induced by drought and warming. Perspectives in Plant Ecology, Evolution and Systematics. 15: 319-327.
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Doi: 10.1016/j.ppees.2013.07.001

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The effects of climate change on plant reproductive performance affects the sequence of different plant reproductive stages from flowering to seed production and viability, as well as the network of relationships between them. These effects are expected to respond to different components of climate change, such as temperature and water availability, and may be sensitive to differences in species phenology.We used long-term experimental drought and warming treatments to study the effect of climate change on flower production, fruit and seed-set, seed size and seed germination rate (proportion of germinating seeds) in three Mediterranean shrubs coexisting in a coastal shrubland.Larger plants produced significantly more flowers in all three species, and higher fruit-set in Dorycnium pentaphyllum. Flower production was reduced in drought and warming treatments in the spring-flowering species D. pentaphyllum and Helianthemum syriacum, but not in the autumn-winter species Erica multiflora, which increased flowering in the warming treatment. However, the drought treatment eventually resulted in a decreased seed-set in E. multiflora. Structural equation modelling revealed strong correlations between the sequential reproductive stages. Specifically, flower density in inflorescences determined seed-set in H. syriacum, and seed size and germination rate in E. multiflora. Nevertheless, the relevance of relationships between reproductive traits changed between climatic treatments: in D. pentaphyllum a direct relationship between plant size and seed size only arised in the drought treatment, while in H. syriacum climate treatments resulted in a stronger relationship between the number of flowers and seed-set.This experimental study shows the ability of changing climatic variables to determine the reproductive sequential process of woody species. We show that several parameters of the reproductive performance of some Mediterranean species are affected by drought and warming treatments simulating climate change, highlighting the importance of changes in both water availability and temperature, and the sequential relationship between reproductive stages. Phenological patterns also contribute to species' differential responses to climatic change, due to the relationship of these patterns with resource availability, environmental conditions and plant-pollinator interactions. © 2013.

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