Josep Maria Espelta

Constraints and trade-offs in Mediterranean plant communities: The case of holm oak-Aleppo pine forests

In this paper we review those aspects that are relevant to the development of a mechanistic ecological theory to account for the structure and dynamics of Mediterranean forests, focusing our attention on mixed forests of holm oak (Quercus ilex L.), a shade-tolerant, slowgrowing species that resprouts vigorously after disturbance, and Aleppo pine (Pinus halepensis M.), a fast-growing, nonresprouting, shade-intolerant species. The main objectives of this report are: to introduce some of the primary features of these forests, showing their structural complexity and historical peculiarities; to show that much of this complexity can be conceptually reduced to two main factors of variation, soil-moisture gradients and a complex interaction of historical management and disturbance regimes; and to contrast the unique features of Mediterranean systems with other communities that have inspired generalization in ecology.Plants in Mediterranean-climate regions must face several environmental constraints during their life cycle: water limitation, competition for light, and a complex set of disturbance regimes, mainly fire, herbivory, and human exploitation. The response of co-occurring species to a given set of environmental constraints depends on a combination of physiological and morphological traits. In holm oak-Aleppo pine forests, the lower limit of distribution along a soil-moisture gradient appears to be controlled by dry-season water stress on seedling performance, and the upper limit seems to be controlled by shade tolerance relative to competitors. The processes that generate and maintain these patterns are related to the responses of the two species to the water and light environments that result from interacting gradients of disturbance and resource availability.The dynamics of mixed holm oak-Aleppo pine forests may be represented along two major environmental axes: water availability and light intensity; namely, time since last disturbance. At the regional scale, the presence of holm oak and Aleppo pine is expected to be driven mainly by the precipitation regime, with the proportion of Aleppo pine increasing toward the driest border and with holm oak being the dominant species in areas with higher precipitation. Changes of dominance of holm oak and Aleppo pine also respond to water availability at the local scale. In this case, variations between species depend on different factors in a complex way, because reduced soil-moisture levels may result either from low precipitation or from topography and edaphic features. The dynamics of holm oak-Aleppo pine forests are also determined by temporal changes in canopy closure; that is, forest recovery after disturbance. In this case, the proportion of Aleppo pine would increase in recently disturbed stands (i.e., with high light intensity reaching the forest floor), whereas regeneration of holm oak would be dominant under partially closed canopies.Theories of forest dynamics developed in humid regions may apply only poorly to Mediterranean plant communities, where vegetation change is qualitatively or quantitatively different. Thus, succession in temperate forests appears to be driven by differences in light availability and shade tolerance; but in Mediterranean plant communities, water limitation is of greater importance for the distribution of forest species. In Mediterranean landscapes the interaction of life-history strategies with changing environments is difficult to infer from observational and experimental studies. A mechanistic approach, in which competition or plant performance is measured as a function of resource availability, seems more feasible. The idea should be to develop multispecies models calibrated specifically for Mediterranean forests in a combined program of modeling, field research, and experimentation.ResumenEn este trabajo se revisan los aspectos más relevantes en el desarrollo de una teoría ecológica mecanicista sobre la estructura y dinámica de los bosques mediterráneos, en concreto sobre los bosques mixtos de encina (Quercus ilex L.), una especie tolerante a la sombra, de crecimiento lento, y que rebrota vigorosamente después de una perturbación, y pino carrasco (Pinus halepensis M.), una especie de crecimiento rápido, no rebrotadora, e intolerante a la sombra. Los principales objetivos de este estudio son: introducir algunas de las principales características de estos bosques, mostrando su complejidad estructural y sus peculiaridades históricas; mostrar que gran parte de esta complejidad puede reducirse conceptualmente a dos principales factores de variación, la disponibilidad de agua en el suelo y una compleja interaction de regímenes de gestión y perturbación; y contrastar las características únicas de los sistemas mediterráneos con las de otros sistemas en los que se han basados gran parte de las generalizaciones que se han hecho en ecologia.En las regiones de clima mediterráneo, las plantas deben enfrentarse a diferentes restricciones: limitación hídrica, competencia por la luz, y un conjunto complejo de regímenes de perturbación, principalmente fuego, herbivoría, y explotación humana. La respuesta de las especies a un conjunto de limitaciones ambientales depende de la combinación de caracteres fisiológicos y morfológicos. En los bosques de pino carrasco y encina, el límite inferior de distribución de las especies viene controlado por el estrés hídrico de la estación, mientras que el límite superior viene controlado por la tolerancia a la sombra. Los procesos que generan y mantienen estos patrones están relacionados con las respuestas de ambas especies a lo largo de su ciclo de vida a los ambientes de luz y agua que se generan como resultado de la interacción de los gradientes de perturbación y disponibilidad de recursos.La dinámica de los bosques mixtos de pino carrasco y encina se puede representar a lo largo de dos principales ejes ambientales: disponibilidad de agua e intensidad de luz; o, lo que es lo mismo, tiempo desde la última perturbación. A una escala regional, la presencia de pino carrasco y encina viene determinada por el régimen de precipitación: el pino carrasco aumenta hacia el extremo más seco, mientras que la encina se hace dominante en áreas con mayor precipitación. A escala local, los cambios en la dominancia de ambas especies también responden a la disponibilidad de agua. En este caso las variaciones entre especies dependen de diferentes factores, ya que los niveles bajos de humedad del suelo pueden ser debidos a la baja precipitación, pero también a la topografía o a las características edáficas. La dinámica de los bosques de pino y encina también depende de los cambios temporales en el cierre de la cubierta, es decir, de la recuperación del bosque después de la perturbación. En este caso, la proporción de pino carrasco se incrementa en los rodales recién perturbados (en los cuales llega mucha luz al suelo del bosque), mientras que la regeneración de encina domina bajo cubiertas parcialmente cerradas.Las teorías de dinámica forestal desarrolladas en las regiones húmedas se aplican con dificultad en las comunidades de plantas mediterráneas, donde los cambios de la vegetación suelen ser cualitativamente y cuantitativamente distintos. Así, en los bosques templados la sucesión viene determinada por diferencias en la disponibilidad de luz y la tolerancia a la sombra de las diferentes especies, mientras que en las comunidades mediterráneas la limitación hídrica es más importante a la hora de explicar la distribución de las especies forestales. En los ecosistemas mediterráneos, la interacción de las estrategias del ciclo vital con los cambios en el ambiente dificilmente pueden deducirse a partir de estudios observacionales y experimentales. Se requiere una aproximación mecanicista, en la cual la competencia o el éxito de las plantas se pueda medir en función de la disponibilidad de recursos. La idea sería desarrollar modelos multiespecíficos calibrados específicamente para los bosques mediterráneos, en los que se combinara la modelización con los estudios de campo y la experimentación.

Fire‐related traits for plant species of the Mediterranean Basin

Plant trait information is essential for understanding plant evolution, vegetation dynamics, and vegetation responses to disturbance and management. Furthermore, in Mediterranean ecosystems, changes in fire regime may be more relevant than direct changes in climatic conditions, making the knowledge of fire-related traits especially important. Thus the purpose of this data set was to compile the most updated and comprehensive information on fire-related traits for vascular plant species of the Mediterranean Basin, that is, traits related to plant persistence and regeneration after fire. Data were collected from an extensive literature review and from field and experimental observations. The data source is documented for each value. Since life history traits may vary spatially or with environmental conditions, we did not aggregate them by species; i.e., traits and species are repeated in different records if they were observed by different researchers and/or in different locations. Life history traits inclu...

Regeneration patterns of three Mediterranean pines and forest changes after a large wildfire in northeastern Spain

Abstract Fire has favored pines throughout their natural range in environments subject to continuous disturbances, such as the Mediterranean Basin. However, recovery of pine species after large fires is not always successful. In this study, we analyze the post-fire regeneration pattern of Pinus halepensis, P. nigra and P. sylvestris three years after fire, in an area affected by a large wildfire in 1994. Moreover, we develop a model of succession to predict medium-term changes in forest composition 30 years after fire from the regeneration monitored during the first years after fire. The results show that, although the three pine species regenerate quite well in the absence of fire, their post-fire regeneration is very different: P. halepensis shows high seedling density after fire, but P. nigra and P. sylvestris almost disappear from burned plots. The model simulations of the future forest composition 30 years after fire indicate that 77-93% of plots dominated by these two pines change after fire to communities dominated by oaks (Quercus ilex, Q. cerrioides). There is also a considerable number (7-16%) of these burned pine plots that change to shrublands. Thus, these observational and modelling results suggest that large fire events, which have increased considerably in the Mediterranean region in the last decades, may decrease the overall distribution of these pine species, especially that of P.nigra and P. sylvestris.

Resprouting patterns after fire and response to stool cleaning of two coexisting Mediterranean oaks with contrasting leaf habits on two different sites

The extension of mixed evergreen (Quercus ilex) and winter deciduous (Quercus cerrioides) oak coppices is increasing in NE Spain as a consequence of large wildfires. The best alternative to manage these high-density and low production forests is their conversion into stored coppices (i.e. coppices with few stems per stool). However, in this process, functional differences arising from the contrasted leaf-habit of the two co-occurring oaks should be taken into account. In this study, we explore the resprouting patterns and the response to different intensities of stool cleaning and pruning of these co-occurring Mediterranean oak species in mixed extensive coppices which have appeared after large wildfires. According to our results, both species resprouted vigorously after fire in the different environmental conditions studied, with an important influence of the size of the stool. For a similar stool size, Q. cerrioides exhibited higher number of resprouts, height, basal diameter and crown cover, with specific differences increasing in high-quality sites. Cleaning of stools increased height and basal diameter growth, with low differences between the two cleaning intensities (i.e. one or three resprouts reserved per stool). Pruning did not modify height or diameter but enhanced crown expansion. As a consequence of cleaning, a new wave of basal resprouts appeared. The mean number of these new resprouts was higher in Q. ilex than in Q. cerrioides, while the height they reached increased with cleaning intensity. The reported benefits of cleaning on growth diminished earlier in Q. ilex in comparison with Q. cerrioides, probably due to the larger production of this new wave of basal resprouts in the former species. These results suggest that, despite the differences among evergreen and deciduous species, both oaks may operate successfully in a wide range of environmental conditions. In the two species, moderate cleaning (three reserved stems per stool) appears to be a more suitable practice than intense cleaning (one stem per stool), because these similar growth rates but moderate cleaning favours a lower development of new resprouts.

Regeneration by sprouting of holm-oak (Quercus ilex) stands exploited by selection thinning

Holm-oak forests of the Montseny Massif have traditionally been exploited by selection thinning, which consists of a partial removal of standing trees and biomass. Cutting percentages within each size class indicate that this management technique represents a compromise between forest improvement and harvesting.

An economic and ecological multi-criteria evaluation of reforestation methods to recover burned Pinus nigra forests in NE Spain.

Abstract Recurrence of recent large wildfires is threatening the permanence of Pinus nigra Arnold (Black pine) forests in central Catalonia (NE Spain), due to the almost nil post-fire regeneration of this non-serotinous pine. Potential practices to carry out extensive reforestation programs with P. nigra may differ widely in terms of their final success, economic cost and undesired ecological impact. In this framework, we have analysed different types of vegetation clearing (mechanical, controlled burning or grazing), soil preparation (ripping or planting holes) and reforestation methods (broadcast seeding, spot seeding and planting) to restore P. nigra forests. We have compared these practices in terms of seedling establishment, but also in the light of their economic costs and ecological impact, through a new multi-criteria analysis. Seedling establishment after sowing was very poor and not influenced by vegetation clearing. In plantations, seedling survival was higher in the ripper treatment than in planting holes for all vegetation clearing treatments except the control one. Nevertheless, the higher economic cost of the planting holes treatment and the negative impact of mechanical clearing and burning on the small mammals community made the stated differences in seedling survival irrelevant. Thus, the multi-criteria analysis revealed that the two most preferred options were planting in uncleared or lightly grazed areas with soil preparation through ripping. This study gives some valuable insights about the use of new decision-support tools in restoration programs and provides practical guidelines concerning the restoration of extensive burned P. nigra forests.

Tree density and site quality influence on Pinus halepensis Mill. reproductive characteristics after large fires

In Spain, many Pinus halepensis Mill. forests have been seriously affected by significant forest fires in the past decade, in 1994 alone, more than 100 000 ha were burned in Eastern Spain. In order to study the reproductive characteristics of P. halepensis after forest fires, six locations were selected in four areas affected by serious fires in the summer of 1994, and the following different precipitation zones were studied: dry-subhumid, dry and semi-arid. Ten years after the fires, data relevant to the production of pine cones: serotinous (grey), mature (brown), immature (green) and opened cones, was collected from areas with natural post-fire regeneration. Various cone and seed characteristics, such as pine cone seed number and weight, and germination percentage, were measured. The results showed greater production of cones and strobili in high-density sites. The biggest cone sizes (volume) and seed number per cone were related to site quality (dry-subhumid precipitation zone). Also, viability and germination percentages were higher with better site quality, with significant differences in values for serotinous and non-serotinous cones. Despite these differences, the canopy seed bank was large enough to ensure regeneration in this area for this age stand.RésuméEn Espagne, dans les dix dernières années, de nombreuses forêts de Pin d’Alep ont été gravement touchées par d’importants feux de forêt. Rien qu’en 1994, plus de 100 000 ha ont brûlé dans l’est de l’Espagne. Dans le cadre d’une étude de la dynamique de la reproduction du Pin d’Alep après incendie, six sites, situés dans des zones de feux de forêt survenus lors de l’été 1994, ont été observés. Ces sites présentent des niveaux de précipitations différents : sec à sub-humide, sec et semi-aride. Dix ans après incendie, des données concernant la production de cônes de pin : sérotineux (gris), mûrs (marrons), immatures (vert) et cônes ouverts, ont été collectées dans des zones de régénération naturelle. Diverses caractéristiques des cônes et des graines ont été relevées : nombre et poids des cônes et des graines, taux de germination. Les résultats révèlent une production importante de cônes et de strobiles dans les sites à forte densité de pin. Le volume des cônes et le nombre de graines par cône étaient liés à la fertilité de la station (zone sèche à sub-humide). Les taux de viabilité et de germination étaient aussi plus importants dans les zones présentant le plus de précipitations et des écarts significatifs ont été observés entre les valeurs de cônes sérotineux et non sérotineux. Malgré ces différences, les banques de graines dans les peuplements étaient suffisamment importantes pour assurer la régénération du pin dans cette zone.

Trade-offs between vegetative growth and acorn production in Quercus lobata during a mast year: the relevance of crop size and hierarchical level within the canopy

The concept of trade-offs between reproduction and other fitness traits is a fundamental principle of life history theory. For many plant species, the cost of sexual reproduction affects vegetative growth in years of high seed production through the allocation of resources to reproduction at different hierarchical levels of canopy organization. We have examined these tradeoffs at the shoot and branch level in an endemic California oak, Quercus lobata, during a mast year. To determine whether acorn production caused a reduction in vegetative growth, we studied trees that were high and low acorn producers, respectively. We observed that in both low and high acorn producers, shoots without acorns located adjacent to reproductive shoots showed reduced vegetative growth but that reduced branch-level growth on acorn-bearing branches occurred only in low acorn producers. The availability of local resources, measured as previous year growth, was the main factor determining acorn biomass. These findings show that the costs of reproduction varied among hierarchical levels, suggesting some degree of physiological autonomy of shoots in terms of acorn production. Costs also differed among trees with different acorn crops, suggesting that trees with large acorn crops had more available resources to allocate for growth and acorn production and to compensate for immediate local costs of seed production. These findings provide new insight into the proximate mechanisms for mast-seeding as a reproductive strategy.

Post-fire resprouting ability of 15 non-dominant shrub and tree species in Mediterranean areas of NE Spain

Post-fire resprouting ability of the non-dominant tree and shrub species of the Mediterranean Basin has not yet been experimentally tested, although this group contributes to maintain the richness of Mediterranean plant communities. In this study, we have analyzed the post-fire recovery ability of 15 woody species that occur in relatively low abundance in dry and sub humid Mediterranean areas in NE of Spain. The main goals have been: (i) to determine experimentally the post-fire resprouting ability of these species and (ii) to compare the abundance of these species in areas affected by wildland fires and in unburned areas. We have observed a high resprouting ability after prescribed burning of most species except for Juniperus communis and J. phoenicea which showed a null resprouting. As the species with high resprouting ability showed similar presence in burned and unburned areas, we can conclude that wildfires are not a factor that constrains the presence of these species in Mediterranean woodlands. However, we found a reduction in the abundance of J. communis and J. phoenicea at the regional level after wildland fires.RésuméLa capacité à rejeter de souche après incendie d’espèces d’arbres et d’arbustes non dominants de la région méditerranéenne est peu connue malgré l’importance de la contribution de ce groupe à la richesse des communautés végétales. Dans cette étude, nous avons analysé la capacité à rejeter de souche de 15 espèces ligneuses présentes en faible abondance dans les zones semi arides et subhumides méditerranéennes du nord-est de l’Espagne. Les principaux objectifs étaient : (i) de déterminer expérimentalement la capacité de rejeter après incendie et (ii) de comparer l’abondance de ces espèces dans des zones brûlées et non brûlées. Toutes les espèces testées présentaient une grande capacité à rejeter de souche à l’exception de Juniperus communis et J. phoenicea, qui ont montré une incapacité complète à rejeter de souche après incendie. Les espèces présentant une grande capacité de rejet présentent des abondances similaires dans des zones brûlées et non brûlées. Nous pouvons conclure que les incendies ne sont pas le facteur limitant leur présence dans les forêts méditerranéennes. Au contraire, les espèces sensibles à faible capacité de rejet comme J. communis et J. phoenicea, ont vu leur abondance baisser à l’échelle régionale après incendie.

The ecology of seed dispersal by small rodents: a role for predator and conspecific scents

Summary 1. Seed-caching rodents play a key role in the ecology of seed dispersal by not only consuming but also dispersing seeds. Rodent foraging behaviour is usually framed within optimal models, which predict that their decisions should maximize food intake and minimize foraging costs. Although predation risk and seed pilferage by conspecifics have been envisaged as two potential costs, their relevance for seed-caching behaviour and seed dispersal has barely been addressed. 2. To test the effect of predation and pilferage risk on the patterns of seed predation/dispersal by rodents, we performed a field experiment using a tri-trophic-level model (plant–mice–carnivore; Quercus spp-Apodemus sylvaticus-Genetta genetta) and the scents of the predator and conspecifics as direct cues. The behaviour of mice was analysed with video cameras set for continuous recording on consecutive nights, and we used tagged acorns to assess the patterns of acorn predation and dispersal. 3. Our results revealed that rodents were able to discriminate between the scents of genet and conspecifics and modified their seed dispersal behaviour accordingly. Mice spent more time ‘sniffing’ in rodent cages than in genet cages, where they displayed more ‘vigilance and freezing’ behaviours. In sites with mice scents, acorns were dispersed at shorter distances and were less predated. Conversely, in sites with genet scents acorn removal was delayed. 4. These results show that chemosensory information on predators and conspecifics influences the foraging decisions of seed-caching rodents over short spatial and temporal scales. This might entail cascading effects on the regeneration of plants. In sites where rodents perceive the risk of predation, inefficient foraging behaviour may result in less successful seed dispersal. Conversely, the detection of conspecific scents may increase dispersal efficiency and seedling recruitment. 5. Ultimately, the relationships between two distant levels in trophic webs (plants–carnivores) appear intricate, since carnivores may affect seed dispersal by changing the foraging behaviour of their prey (the seed disperser). This indirect relationship should be considered as a new dimension of the ecology of seed dispersal by small rodents.