Papers by Josep Maria Espelta
Oecologia, Jan 30, 2018
Variation in seed availability shapes plant communities, and is strongly affected by seed predati... more Variation in seed availability shapes plant communities, and is strongly affected by seed predation. In some plant species, temporal variation in seed production is especially high and synchronized over large areas, which is called 'mast seeding'. One selective advantage of this phenomenon is predator satiation which posits that masting helps plants escape seed predation through starvation of predators in lean years, and satiation in mast years. However, even though seed predation can be predicted to have a strong spatial component and depend on plant densities, whether the effectiveness of predator satiation in masting plants changes according to the Janzen-Connell effect has been barely investigated. We studied, over an 8-year period, the seed production, the spatiotemporal patters of weevil seed predation, and the abundance of adult weevils in a holm oak (Quercus ilex) population that consists of trees interspersed at patches covering a continuum of conspecific density. I...
Integrative zoology, Jan 23, 2017
Synchrony between seed growth and oogenesis are suggested to largely shape trophic breadth of see... more Synchrony between seed growth and oogenesis are suggested to largely shape trophic breadth of seed-feeding insects and ultimately contribute to their co-existence by means of resource partitioning or in the time when infestation occurs. Here we investigated: i) the role of seed phenology and sexual maturation of females in the host specificity of seed-feeding weevils (Curculio spp) predating in hazel and oak mixed forests and ii) the consequences that trophic breadth and host distribution have in the genetic structure of the weevil populations. DNA analyses were used to establish unequivocally host specificity and to determine the population genetic structure. We identified four species with different specificity, namely C. nucum females matured earlier and infested a unique host (hazelnuts) while three species (C. venosus, C. glandium, C. elephas) predated upon the acorns of the two oaks (Q. ilex and Q. humilis). The high specificity of C. nucum coupled with a more discontinuous di...
Proceedings. Biological sciences, Jan 15, 2017
Masting is the highly variable production of synchronized seed crops, and is a common reproductiv... more Masting is the highly variable production of synchronized seed crops, and is a common reproductive strategy in plants. Weather has long been recognized as centrally involved in driving seed production in masting plants. However, the theory behind mechanisms connecting weather and seeding variation has only recently been developed, and still lacks empirical evaluation. We used 12-year long seed production data for 255 holm oaks (Quercus ilex), as well as airborne pollen and meteorological data, and tested whether masting is driven by environmental constraints: phenological synchrony and associated pollination efficiency, and drought-related acorn abscission. We found that warm springs resulted in short pollen seasons, and length of the pollen seasons was negatively related to acorn production, supporting the phenological synchrony hypothesis. Furthermore, the relationship between phenological synchrony and acorn production was modulated by spring drought, and effects of environmental...
Ecosphere, 2017
Escaping seed predation is a classic “economy of scale” hypotheses (predator satiation hypothesis... more Escaping seed predation is a classic “economy of scale” hypotheses (predator satiation hypothesis, Psh) to explain the selection for the synchronous production of massive and nil seed crops (masting) in plants. The Psh postulates that predator satiation occurs through a combination of (1) “functional satiation,” as not all seeds can be consumed during a massive crop, and (2) “numerical satiation,” as predator populations collapse during poor crop years. Many studies advocate for the Psh, but few have investigated the importance of masting compared to other factors for the control of predation extent. Namely, environmental cues prompting masting could also determine predator's success and, ultimately, influence directly and independently seed predation intensity. We explored this question in Mediterranean oaks, as they exhibit strong masting behavior; acorns are heavily predated upon by weevils; and rainfall stochasticity drives masting and the emergence of adult weevils from the...
Forests, 2017
Many studies have outlined the benefits for growth and reproduction resulting from thinning extre... more Many studies have outlined the benefits for growth and reproduction resulting from thinning extremely crowded young forests regenerating after stand replacing wildfires ("thickets"). However, scarce information is available on how thinning may influence fire severity and vegetation regeneration in case a new fire occurs. We investigated the relationship between thinning and fire severity in P. halepensis thickets, and the effects on the establishment of pine seedlings and resprouting vigour in resprouter species the year after the fire. Our results show a positive relationship between forest basal area and fire severity, and thus reserved pines in thinned stands suffered less fire damage than those in un-thinned sites (respectively, 2.02 ± 0.13 vs. 2.93 ± 0.15 in a scale from 0 to 4). Ultimately, differences in fire severity influenced post-fire regeneration. Resprouting vigour varied depending on the species and the size of individuals but it was consistently higher in thinned stands. Concerning P. halepensis, the proportion of cones surviving the fire decreased with fire severity. However, this could not compensate the much lower pine density in thinned stands and thus the overall seed crop was higher in un-thinned areas. Establishment of pine seedlings was negatively affected by the slope and positively driven by the number of cones and thus it was higher in un-thinned than in thinned stands (respectively, 2581 ± 649 vs. 898 ± 325 seedlings•ha −1). Thinning decreases fire intensity, and thus it may facilitate fire suppression tasks, but retaining a higher density of pines would be necessary to ensure P. halepensis regeneration after a new fire event.
Ecology, Jan 27, 2017
Host specialization has long been hypothesized to explain the extraordinary diversity of phytopha... more Host specialization has long been hypothesized to explain the extraordinary diversity of phytophagous insects in the tropics. However, addressing this hypothesis has proved challenging because of the risk of over-looking rare interactions, and hence biasing specialization estimations, and the difficulties to separate the diversity component attributable to insect specialization from that related to host diversity. As a result, the host specialization hypothesis lacks empirical support for important phytophagous insect clades. Here, we test the hypothesis in a radiation of seed-feeding insects, acorn weevils (Curculio spp.), sampled in temperate and tropical regions (California and Nicaragua, respectively) with an equivalent pool of oak host species. Using DNA sequences from three low-copy genes, we delimited to species level 778 weevil larvae extracted from host seeds and assessed their phylogenetic relationships by Maximum Likelihood and Bayesian inference. We then reconstructed th...
Forests, 2016
Drought has been shown to reduce soil respiration (SR) in previous studies. Meanwhile, studies of... more Drought has been shown to reduce soil respiration (SR) in previous studies. Meanwhile, studies of the effect of forest management on SR yielded contrasting results. However, little is known about the combined effect of drought and forest management on SR. To investigate if the drought stress on SR can be mitigated by thinning, we implemented plots of selective thinning and 15% reduced rainfall in a mixed forest consisting of the evergreen Quercus ilex and deciduous Quercus cerrioides; we measured SR seasonally from 2004 to 2007. Our results showed a clear soil moisture threshold of 9%; above this value, SR was strongly dependent on soil temperature, with Q 10 of 3.0-3.8. Below this threshold, the relationship between SR and soil temperature weakened. We observed contrasting responses of SR of target oak species to drought and thinning. Reduced rainfall had a strong negative impact on SR of Q. cerrioides, whereas the effect on SR for Q. ilex was marginal or even positive. Meanwhile, selective thinning increased SR of Q. cerrioides, but reduced that of Q. ilex. Overall, our results showed that the negative effect of drought on SR can be offset through selective thinning, but the effect is attenuated with time.
The New phytologist, Jan 8, 2016
Fruit production (NPPf ), the amount of photosynthates allocated to reproduction (%GPPf ) and the... more Fruit production (NPPf ), the amount of photosynthates allocated to reproduction (%GPPf ) and their controls for spatial and species-specific variability (e.g. nutrient availability, climate) have been poorly studied in forest ecosystems. We characterized fruit production and its temporal behaviour for several tree species and resolved the effects of gross primary production (GPP), climate and foliar nutrient concentrations. We used data for litterfall and foliar nutrient concentration from 126 European forests and related them to climatic data. GPP was estimated for each forest using a regression model. Mean NPPf ranged from c. 10 to 40 g C m(-2) yr(-1) and accounted for 0.5-3% of GPP. Forests with higher GPPs produced larger fruit crops. Foliar zinc (Zn) and phosphorus (P) concentrations were associated positively with NPPf , whereas foliar Zn and potassium (K) were negatively related to its temporal variability. Maximum NPPf and interannual variability of NPPf were higher in Fag...
Ecography, 2016
Weather and its lagged effects have been associated with interannual variability and synchrony of... more Weather and its lagged effects have been associated with interannual variability and synchrony of fruit production for several tree species. Such relationships are used often in hypotheses relating interannual variability in fruit production with tree resources or favourable pollinating conditions and with synchrony in fruit production among sites through the Moran effect (the synchronisation of biological processes among populations driven by meteorological variability) or the local availability of pollen. Climatic teleconnections, such as the North Atlantic Oscillation (NAO), representing weather packages, however, have rarely been correlated with fruit production, despite often being better predictors of ecological processes than is local weather. The aim of this study was to test the utility of seasonal NAO indices for predicting interannual variability and synchrony in fruit production using data from 76 forests of Abies alba, Fagus sylvatica, Picea abies, Pseudotsuga menziesii, Quercus petraea, and Q. robur distributed across central Europe. Interannual variability in fruit production for all species was significantly correlated with seasonal NAO indices, which were more prominently important predictors than local meteorological variables. The relationships identified by these analyses indicated that proximal causes were mostly responsible for the interannual variability in fruit production, supporting the premise that local tree resources and favourable pollinating conditions are needed to produce large fruit crops. Synchrony in fruit production between forests was mainly associated with weather and geographical distance among sites. Also, fruit production for a given year was less variable among sites during warm and dry springs (negative spring NAO phases). Our results identify the Moran effect as the most likely mechanism for synchronisation of fruit production at large geographical scales and the possibility that pollen availability plays a role in synchronising fruit production at local scales. Our results highlight the influence of the NAO on the patterns of fruit production across western Europe.
Royal Society Open Science, 2015
The complexity of animal life histories makes it difficult to predict the consequences of climate... more The complexity of animal life histories makes it difficult to predict the consequences of climate change on their populations. In this paper, we show, for the first time, that longer summer drought episodes, such as those predicted for the dry Mediterranean region under climate change, may bias insect population sex ratio. Many Mediterranean organisms, like the weevil Curculio elephas , become active again after summer drought. This insect depends on late summer rainfall to soften the soil and allow adult emergence from their underground refuges. We found that, as in many protandric species, more C. elephas females emerged later in the season. Male emergence timing was on average earlier and also more dependent on the beginning of late summer rainfall. When these rains were delayed, the observed weevil sex ratio was biased towards females. So far, the effects of global warming on animal sex ratios has been reported for temperature-dependent sex determination in reptiles. Our results...
Perspectives in Plant Ecology, Evolution and Systematics, 2015
Climate change is expected to alter the geographic distribution of many plant species worldwide. ... more Climate change is expected to alter the geographic distribution of many plant species worldwide. However, there is still no clear evidence showing a generalized direction and magnitude of these distribution shifts. Here, we have surveyed, in nine mountainous regions in Spain, an array of tree species along entire elevational ranges, as surrogates of their global climatic ranges, to test for elevational shifts towards cooler locations. We analysed the distribution recruitment patterns of five dominant tree species, recording the abundance and measuring the primary growth of juveniles in 306 plots. Three of the species have a temperate-boreal distribution with populations at their southern edge in the Mediterranean mountain ranges: Pinus sylvestris, P. uncinata and Fagus sylvatica; and the other two species have a Mediterranean distribution: Quercus ilex and P. nigra. Despite the contrasting phylogenies and biogeographies, we identified a similar pattern in recruitment abundance across species, with an asymmetric distribution of juveniles (more recruits in the middle-upper elevation of their range), but higher annual growths at lower elevations. This survival-growth trade-off at the early recruitment stage may potentially counterbalance at population level the negative effect of global warming on recruit survival at the lower edge of species ranges. These findings suggest a demographic stabilisation process at the early recruitment stage of these tree species, and highlight the importance of considering the different demographic stages across the whole climatic range to understand the effects that climate change may exert on species distributions and population dynamics.
Plant Ecology, 2015
The extremely year-to-year variable production of seeds (masting) is an extended plant reproducti... more The extremely year-to-year variable production of seeds (masting) is an extended plant reproductive behaviour important for forest dynamics and food webs. The dependence of these episodes of massive seed production on recently or long-term photosynthesised carbohydrates, however, remains controversial. In this paper, we explore whether vegetation (tree canopy) changes, detected using EVI as a proxy of leaf area and photosynthetic capacity, can provide a reliable estimation of seed production. To complete this analysis, we also explored the effect of weather both in the trends of EVI and in acorn crop size. To this end, we compared the trends of the EVI and acorn production over 10 years (2000-2009) in five stands of Quercus ilex L. in Barcelona (Catalonia, NE Spain). We found that acorn production was mainly driven by a combination of: i) a minimum initial threshold in the EVI values, ii) an increase in EVI in the 9 ± 4 months prior to reproduction, and iii) appropriate weather conditions (low water stress) during spring. These results indicated, apparently for the first time, that reproduction in masting species could be detected and partly predicted by remotely sensed vegetative indices. Our results suggested that this particular reproductive behaviour in Mediterranean oaks was driven by a combination of two factors, i.e. good and improving vegetation conditions, as shown by a minimum initial threshold and the increase in EVI needed for large seed crops, and the need of wet weather conditions during spring. Moreover, our results fully supported recent studies that have associated short-term photosynthate production with seed production.
Plant biology (Stuttgart, Germany), Jan 4, 2014
Understanding the factors determining genetic diversity and structure in peripheral populations i... more Understanding the factors determining genetic diversity and structure in peripheral populations is a long-standing goal of evolutionary biogeography, yet little empirical information is available for tropical species. In this study, we combine information from nuclear microsatellite markers and niche modelling to analyse the factors structuring genetic variation across the southernmost populations of the tropical oak Quercus segoviensis. First, we tested the hypothesis that genetic variability decreases with population isolation and increases with local habitat suitability and stability since the Last Glacial Maximum (LGM). Second, we employed a recently developed multiple matrix regression with randomisation (MMRR) approach to study the factors associated with genetic divergence among the studied populations and test the relative contribution of environmental and geographic isolation to contemporary patterns of genetic differentiation. We found that genetic diversity was negatively...
Modelling, Monitoring and Management of Forest Fires I, 2008
In the Mediterranean Basin, the number of large-scale fires and, consequently, the area of regene... more In the Mediterranean Basin, the number of large-scale fires and, consequently, the area of regenerated forests after a fire have been increasing over the last few decades. Mediterranean pine tree stand regeneration shows differences in growth and reproductive characteristics depending on site quality. Aleppo pine (Pinus halepensis Mill.) is an important obligate seeder species occupying a large surface in this area. In order to study growth and reproductive characteristics depending on different climatic conditions, six sites with similar characteristics (edaphic, exposure, slope) burned in the summer of 1994 were selected along a climate gradient. Different precipitation values determined three different ombroclimates: dry-subhumid, dry and semi-arid. Ten years after the fires, data on growth and cone production were collected from these areas in order to study the pattern of growth and reproduction and the influence of site quality (climate) on them. Results showed significantly higher values for growth and several cone characteristics; furthermore, they were positively related to site quality. Viability and seed germination percentages were higher in sites located in dry-subhumid areas. Serotiny was highest in dry locations (southern sites), and lower percentages were found in low tree density sites. The number of reproductive trees was higher northwards and the reproductive phase was reached in younger stands with lower tree density. In conclusion, results showed a geographical gradient based upon growth and reproductive characteristics, decreasing southwards in contrast to serotiny, which increases in southern sites. Density was found to be a significant factor influencing overall pine stand development.
Plant Ecology, 1999
The post-fire regenerative ability of Pinus halepensis, Pinus nigra and Pinus sylvestris, three o... more The post-fire regenerative ability of Pinus halepensis, Pinus nigra and Pinus sylvestris, three of the most important pine species present in the West Mediterranean basin, has been analyzed in the light of seed tolerance to different temperatures and times of exposure, and of seed position during the fire event (seeds inside cones versus free seeds). The combination of different fire
Oecologia, 2009
The contribution of pre-dispersal seed predation to inter-specific differences in recruitment rem... more The contribution of pre-dispersal seed predation to inter-specific differences in recruitment remains elusive. In species with no resistance mechanisms, differences in pre-dispersal predation may arise from differences in seed abundance (plant satiation) or in the ability of seeds to survive insect infestation (seed satiation). This study aimed to analyse the impact of pre-dispersal acorn predation by weevils in two co-occurring Mediterranean oaks (Quercus ilex and Quercus humilis) and to compare its relevance with other processes involved in recruitment. We monitored the patterns of acorn production and acorn infestation by weevils and we conducted experimental tests of acorn germination after weevil infestation, post-dispersal predation and seedling establishment in mixed forests. Monitoring and experimental data were integrated in a simulation model to test for the effects of pre-dispersal predation in recruitment. In both oaks pre-dispersal acorn infestation decreased with increasing acorn crop size (plant satiation). This benefited Q. ilex which exhibited stronger masting behaviour than Q. humilis, with almost a single and outstanding reproductive event in 6 years. Acorn infestation was more than twice as high in Q. humilis (47.0%) as in Q. ilex (20.0%) irrespective of the number of seeds produced by each species. Although germination of infested acorns (seed satiation) was higher in Q. humilis (60%) than in Q. ilex (21%), this could barely mitigate the higher infestation rate in the former species, to reduce seed loss. Conversely to pre-dispersal predation, no inter-specific differences were observed either in post-dispersal predation or seedling establishment. Our results indicate that pre-dispersal predation may contribute to differences in seed supply, and ultimately in recruitment, between co-existing oaks. Moreover, they suggest that seed satiation can barely offset differences in seed infestation rates. This serves as a warning against overemphasising seed satiation as a mechanism to overcome seed predation by insects.
New Forests, 2004
The use of an enriched CO 2 atmosphere in tree nurseries has been envisaged as a promising techni... more The use of an enriched CO 2 atmosphere in tree nurseries has been envisaged as a promising technique to increase productivity and to obtain seedlings with a higher root/shoot ratio, an essential trait to respond to water stress in Mediterranean-type ecosystems. In that framework, we have analyzed the effects of three levels of atmospheric CO 2 concentration (350, 500 and 700 ppm) on the germination rate, growth and morphology of seedlings of two Mediterranean oaks used in reforestation programs: the evergreen Quercus ilex L. and the deciduous Quercus cerrioides Wilk. et Costa. CO 2 enrichment increased the germination rate of Q. cerrioides (from 70±7 to 81±3 %) while it decreased that of Q. ilex (from 71±10 to 41±12 %). Seedlings of both species increased approximately 60% their total biomass in response to CO 2 enrichment but at two different CO 2 concentrations: 500 ppm for Q. cerrioides and 700 ppm for Q. ilex. This increase in seedlings biomass was entirely due to an augmentation of root biomass. Considering germination and biomass partitioning, an enriched CO 2 atmosphere might not be appropriate for growing Mediterranean evergreen oaks, such as Q. ilex, since it reduces acorn germination and the only gains in root biomass occur at a high concentration (700 ppm). On the other hand, a moderate CO 2 enrichment (500 ppm) appears as a promising nursery technique to stimulate the germination, growth and root/shoot ratio of deciduous oaks, such as Q. cerrioides. Resumen: El uso de una atmósfera enriquecida en CO 2 durante la fase de vivero puede contribuir a aumentar la producción viverística, a la vez que ayudar a conseguir plántulas con una mayor relación biomasa subterránea/biomasa aérea, más adecuadas para hacer frente al severo estrés hídrico que generalmente limita el éxito de las repoblaciones en el clima Mediterráneo. En este estudio hemos analizado el efecto de tres niveles de abonado carbónico atmosférico (350, 500 y 750 ppm) en la germinación y morfología de plántulas de encina (Quercus ilex) y roble cerrioide (Quercus cerrioides). Una atmósfera enriquecida en CO 2 incrementó la germinación de Q. cerrioides (de 70±7 a 81±3 %) mientras que disminuyó la de Q. ilex (de 71±10 a 41±12 %). Las plántulas de ambas especies incrementaron aproximadamente un 60% su biomasa en respuesta a una mayor concentración de CO 2, aunque esta respuesta se produjo a diferentes dosis: 500 ppm en Q. cerrioides y 700 ppm en Q. ilex. El aumento en la biomasa total de las plántulas se debió enteramente a un mayor desarrollo de su sistema radical, Considerando tanto la germinación como los efectos sobre la relación biomasa subterránea/biomasa aérea, una atmósfera enriquecida en CO 2 no parece ser un tratamiento adecuado para la producción en vivero de plántulas de Q.ilex, puesto que diminuye su germinación y solo aumenta su sistema radicular a dosis muy elevadas (700 ppm). Por el contrario, un aumento moderado en la concentración de CO 2 (500 ppm) aparece como una técnica interesante para estimular el crecimiento y obtener plántulas de Q. cerrioides con un sistema radical más desarrollado.
International Journal of Wildland Fire, 2008
Increasing fire recurrence is a major problem threatening Mediterranean-type ecosystems. Moreover... more Increasing fire recurrence is a major problem threatening Mediterranean-type ecosystems. Moreover, this pattern is predicted to increase owing to global change. Although a reduction in the density and growth of post-fire regeneration is usually observed in recurrently burnt areas, the potential effects on reproductive ability have seldom been explored. The aim of the present study is to investigate whether structural changes induced by fire recurrence may constrain reproduction ability of Pinus halepensis forests. We conducted the current study in Catalonia (NE Spain) in 12 study sites, consisting of two adjacent areas differing in the number of fire events suffered throughout the last 16 years (one v. two fires). Twice-burnt areas showed a lower density of pines, lower pine height and a lower reproductive ability, namely (i) a 3-year delay in the onset of pine reproduction; (ii) a reduction of 52% in the number of reproductive pines; and (iii) a 36% lower mean cone crop per tree. T...
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Papers by Josep Maria Espelta