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Stand-level drivers of tree-species diversification in Mediterranean pine forests after abandonment of traditional practices

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... The following land-cover classes were delineated and assigned to each 10 Â 10 portion of burned land ( Fig. 2): (i) 'cultivated land', including agricultural fields and other highly altered areas (roads and paths, farms, etc.); (ii) 'nonwooded land', including areas of natural vegetation with no or only scattered trees; (iii) 'open woodland', for woodlands presenting low-to-moderate tree cover (i.e., below 30% of tree canopy cover); and (iv) 'closed woodland', for woodlands presenting moderate-tohigh tree cover (i.e., above 30% of tree canopy cover). In addition, we wanted to assess the relative importance of pre-fire tree canopy cover (TCC), which is known to highly affect the characteristics of the understory vegetation (Coll et al., 2011;Martín-Alcón et al., 2015b). To do this, a new object-oriented semi-automatic image analysis was executed on 50-cm-resolution gray-scale aerial photographs taken three years before the fire (in 1995). ...
... In our case, the vegetation shift from pine forest to shrubland is mostly mediated by the characteristics of the pre-fire vegetation and the topographic attributes of the burnt sites. This vegetation shift mainly occurred on young and relatively open pre-fire pine stands that have recently colonized old fields or open land and that, accordingly, were much less likely to undergone natural processes of species diversification by resprouting tree species prior to the fire (Puerta-Piñero et al., 2012;Navarro-González et al., 2013;Caldeira et al., 2014;Martín-Alcón et al., 2015b). These areas also burned at the lower end of the severity range, likely indicating lower fuel amount and lower abundance of hardwoods in the pre-fire stand composition (Broncano and Retana, 2004), and were far from unburned patches, thereby limiting the availability of pine seeds (Ordóñez et al., 2006). ...
... Around 60% of the burned area changed from pine to oakdominated woods. This reflects the high availability of plant propagules of oaks and points to a fairly widespread presence of them (in the form of young recruits established under the pine canopy or adult individuals sharing the overstory with pines) in the pre-fire forest composition Martín-Alcón et al., 2015b). Site quality (deduced by topographic attributes) was the most important component determining the cover of hardwoods regeneration in the post-fire community. ...
... Estas alteraciones derivaron frecuentemente en la simplificación de sus características estructurales y florísticas (NAVARRO y PEREIRA 2012). Sin embargo, los cambios de uso acontecidos en la región durante el último siglo, han dado lugar, por una parte, a procesos de densificación de las masas forestales (AMÉZTEGUI, et al. 2010), y por otra al progresivo desarrollo del sotobosque dominado por especies arbustivas y arbóreas anteriormente menos abundantes o ausentes (MARTÍN-ALCÓN, et al. 2015, VAYREDA, et al. 2013. Debido a su naturaleza de transición bioclimática entre los territorios genuinamente Mediterráneos y las áreas templadas y alpinas de los Pirineos, estos sistemas se encuentran particularmente amenazados por el cambio climático (GIORGI y LIONELLO 2008, LINDNER, et al. 2010. ...
... y Q. pubescens Wild.), cada una de ellas ampliamente distribuida en las montañas calcáreas del nordeste peninsular. Las tres especies pueden encontrarse creciendo naturalmente en la zona de estudio, no tanto en forma de bosques puros de frondosas, sino más frecuentemente en forma de individuos dispersos o pequeños grupos, siendo algunas de las principales especies protagonistas de la progresiva diversificación espontánea de estos pinares (MARTÍN-ALCÓN, et al. 2015, NAVARRO-GONZÁLEZ, et al. 2013, VAYREDA, et al. 2013. Las tres especies pueden clasificarse en relación a su tolerancia a la sequía como Qi> Qf> Qp, y en el orden opuesto en relación a su tolerancia al frío (MARTÍN-ALCÓN, et al. 2016). ...
... En este estudio el dosel del pinar tuvo un papel remarcable en la amortiguación de los efectos negativos de la temperatura sobre la adaptación de las plántulas. El papel que desempeña el estrato arbóreo en el mantenimiento de unas condiciones microclimáticas adecuadas para la germinación y el establecimiento temprano de especies del género Quercus está ampliamente documentado en la literatura (e. g., BRONCANO, et al. 1998, ESPELTA, et al. 1995, MARTÍN-ALCÓN, et al. 2015. Nuestros resultados tienen implicaciones importantes y sugieren la conveniencia de plantar o sembrar bajo un dosel relativamente cerrado, con el fin de mitigar las respuestas negativas al frío en las actuaciones de migración asistida. ...
... Vidal-Macua et al. (2017) presented the transition of land cover in the Iberian Peninsula from 1987 to 2012 as a primarily successional process towards oak forests, while the rate of transition to pine forests decreased dramatically during the same period. Reduced pine recruitment and oak expansion is supported by changes in management regimes such as decreases in logging and grazing by domestic livestock (Chauchard et al. 2013;DeSoto et al. 2010;Kouba et al. 2012;Martín-Alcón et al. 2015). For example, according to Gea-Izquierdo et al. (2015), an old stand of P. sylvestris L. in central Spain, which is one of the last remnants of an old extensive pine forest in the region, has been colonized by Q. pyrenaica Willd., but similar stands remain dominated by pines. ...
... Analyses of forest inventory data from the last 2-3 decades reveal that the dynamics of regeneration within mixed pine-oak forests select for oak dominance, as oak recruitment increases (e.g. Q. ilex) whereas pine regeneration declines (e.g., P. nigra J.F.Arnold) (Barreda and Doménech 2013;Martín-Alcón et al. 2015). ...
... The regeneration of oaks is also expected to become limited as the density of the forest increases, and therefore forest gaps are necessary for the maintenance of oak forests such as Q. ilex, Q. faginea Lam, Q. cerrioides Willk. & Costa (Barreda and Doménech 2013;Martín-Alcón et al. 2015), Q. ithaburensis Decne. (Cooper et al. 2014), and Q. pyrenaica Willd. ...
Chapter
A long history of human influence, including large-scale afforestation with pines and changes in land-management and conservation priorities, have strongly affected the formation of Mediterranean landscapes with contiguous patches of pine stands and oak woodlands. Consequently, this pine-oak mosaic creates opportunities for cross-colonization of both species and the formation of mixed pine-oak forests. These processes are homologous to classical secondary succession in which abandoned agricultural land is colonized by early-successional pines, which allow the establishment of late-successional oaks. We used the frameworks of forest succession and gap dynamics to explore the evidence for pine and oak regeneration within mixed pine-oak forests, and evaluate the fate of these Mediterranean mixed forests. Our analysis highlights selection towards enhanced oak regeneration and recruitment and lowered survival and recruitment for pines within mixed pine-oak forests, which is expected to drive succession from the intermediate mixed stage towards late-successional oak-dominated forest. However, studies have proposed persistence mechanisms for the maintenance of mixed pine-oak forests as a long-term stable stage, in which gaps form in the mixed forest of pines and oaks, allowing the recruitment of both species. Alternatively, scenarios of novel climates or fire regimes have projected a deterioration of developed forests to vegetation formations dominated by non-tree forms.
... In other regions, however, studies have reported increases in forest biomass linked with land use changes and/or CO 2 fertilization. For instance, several remote sensing studies in Spain have reported that forests are greening (Domingo, Javier, Paruelo Jose, & Miguel, 2008;Eastman et al., 2013;González-Alonso, Merino-De-Miguel, Roldán-Zamarrón, García-Gigorro, & Cuevas, 2006;Khorchani et al., 2018;Peñuelas, Filella, & Comas, 2002), despite warming (Gouveia, Bastos, Trigo, & Dacamara, 2012;Vicente-Serrano, Lopez-Moreno, et al., 2014), probably because successional processes following land abandonment are currently more influential than climate change in driving forest canopy dynamics (Carnicer et al., 2014;Martín-Alcón, Coll, & Salekin, 2015;Vayreda, Gracia, Martinez-Vilalta, & Retana, 2013). To our knowledge, nobody has yet attempted to disentangle the effects of forest succession and climate change on canopy greenness. ...
... Understanding differences in the resilience of forest types is key to improving resilience to drought (Carnicer et al., 2011;Gavinet, Prévosto, & Fernandez, 2016;Machar et al., 2017;Martín-Alcón et al., 2015;Mochida, Saisho, & Hirayama, 2015;Ruiz-Benito, Lines, Gómez-Aparicio, Zavala, & Coomes, 2013;Yin & Bauerle, 2017). ...
... Spanish forests are currently dominated by pines, which occupy 67% of forested lands, including P. pinaster and P. sylvestris in big proportions ( Figure S2; Brus et al., 2012). The second most dominant species groups are oaks (22% including Q. robur and Q. petraea), secondary successional species, which are favoured by managers and are reported to be advancing on pines (Brus et al., 2012;Martín-Alcón et al., 2015;Pausas et al., 2004). Understanding the resilience to drought of these two groups is therefore paramount for Spanish ecosystems. ...
Article
Full-text available
A widespread increase in forest cover is underway in northern Mediterranean forests because of land abandonment and decreased wood demand, but the resilience of these successional forests to climate change remains unresolved. Here we use 18-year time series of canopy greenness derived from satellite imagery (NDVI) to evaluate the impacts of climate change on Spain's forests. Specifically, we analyzed how NDVI was influenced by the climatic water balance (i.e. Standardized Precipitation-Evapotranspiration Index, SPEI), using monthly time-series extracted from 3,100 pixels of forest, categorized into ten forest types. The forests increased in leaf area index by 0.01 per year on average (from 1.7 in 2000 to 1.9 in 2017) but there was enormous variation among years related to climatic water balance. Forest types varied in response to drought events: those dominated by drought-avoiding species showed strong covariance between greenness and SPEI, while those dominated by drought-tolerant species showed weak covariance. Native forests usually recovered more than 80% of greenness within the 18 months and the remainder within 5 years, but plantations of Eucalyptus were less resilient. Management to increase the resilience of forests-a key goal of forestry in the Mediterranean region-appears to have had a positive effect: canopy greenness within protected forests was more resilient to drought than within non-protected forests. In conclusion, many of Spain's successional forests have been resilient to drought over the past 18 years, from the perspective of space. Future studies will need to combine remote sensing with field-based analyses of physiological tolerances and mortality processes to understand how Mediterranean forests will respond to the rapid climate change predicted for this region in the coming decades.
... Mölder et al. 2014;Martín-Alcón et al. 2015;Kirby et al. 2017;Rolecek et al. 2017;Scolastri et al. 2017) and stand densification (e.g.Vogt et al. 2006;Manetti et al. 2016;Kirby et al. 2017;Salekl et al. 2017;Serrada et al. 2017) were the major causes of stand homogenisation(Vogt et al. 2006;Durak 2012;MacColl et al. 2014;Sitzia et al. 2017) and consequent loss of mosaic landscapes(Agnoletti 2005;Gondard et al. 2006) in Japan and COJ(Fig. 5). ...
... Low-light conditions pushed out the inhabiting species and increased prospects for shade-tolerant species or mesic plants, which can grow in altered environments (e.g.Hédl et al. 2010;Szymura et al. 2014;Kirby et al. 2017;Rolecek et al. 2017;Scolastri et al. 2017). These structural changes occurred because of natural succession; thus, the consequences of forest structural changes were similar between Japan and COJ.Interspecies competition for sunlight (e.g.Mölder et al. 2014;Martín-Alcón et al. 2015;Kirby et al. 2017;Rolecek et al. 2017;Scolastri et al. 2017) and water (e.g.Rodriguez- Calcerrada et al. 2011;Vayreda et al. 2012;Franklin and Johnson 2014;De Caceres et al. 2015;Giuggiola et al. 2018) increased among plant species(Fig. 5). ...
... 5). Such competition often subsequently led to lowered seed production(Rodriguez-Calcerrada et al. 2011;Salomon et al. 2016Salomon et al. , 2017Serrada et al. 2017), regeneration failure(Coppini and Hermanin 2007;Raybuck et al. 2012;Altman et al. 2013;Martín-Alcón et al. 2015;Serrada et al. 2017) and replacement or invasion of vegetation(Martín-Alcón et al. 2015;Chakraborty et al. 2016;Takeuchi et al. 2016;Yamamoto and Hirano 2016;Salekl et al. 2017) (Fig. 5). Furthermore, weakened stands became susceptible to infectious disease and pest outbreaks (e.g.Franklin and Johnson 2014;Yahr et al. 2014;Yamamoto and Hirano 2016;Dieler et al. 2017;Imamura et al. 2017) and to erosion and landslides during severe weather events(Binder et al. 2004;Agnoletti 2005;Sasakawa et al. 2007) (Fig. 5). ...
Article
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In Japan, abandonment of forest management and underuse of forest resources are substantial challenges to environmental protection and sustainable wood production, while in many other countries, leaving a forest untouched may be perceived as positive for conserving the forest. This study thus analyses the scientific knowledge landscape on the causes and consequences of reduced human intervention in formerly managed forests and compares differences between Japan and other countries. Advanced search rules in the bibliographic databases returned 188 publications [35 for Japan and 153 for other countries] related to reduced human intervention in formerly managed forests. Reduced human intervention in formerly managed forests occurred in developed countries in the mid-twentieth century because of socio-economic changes and was primarily directed by small-scale private landowners, causing structural homogenisation, stand ageing, and canopy closure, often followed by the consequences of decreasing biodiversity and ecosystem services in Japan and other countries. In Japan, heavy dependency on wood imports at the expense of biodiversity and ecosystem services and the impending need for harvesting abundant mature forests due to the unbalanced tree age distribution possibly enhanced the recognition and urgency of underuse as a threat to biodiversity and wood security, while in other countries, multiple management objectives of diverse landowners probably dispersed the recognition of underuse. Although less recognised in other countries, the absence of forest management can pose a risk to biodiversity and ecosystem services. Hence, reactivating forest management in underused private forests can be a cost-effective precautionary approach against possible damage from underuse-induced ecosystem disservices. Policy implications for sustainable forest management in underused private forests are discussed from a perspective beyond conventional private ownership.
... positive effect of understory shrub cover on oak regeneration, either through observation of natural regeneration (Martín-Alcón et al., 2015;Vayreda et al., 2013) or experimental introduction of acorns as was the case here. Positive effects of understory shrubs have even been found for the regeneration of highly lightdemanding species like pine (Rodríguez-García et al., 2011), although negative effects on pine saplings have also been reported (Martín-Alcón et al., 2015). ...
... positive effect of understory shrub cover on oak regeneration, either through observation of natural regeneration (Martín-Alcón et al., 2015;Vayreda et al., 2013) or experimental introduction of acorns as was the case here. Positive effects of understory shrubs have even been found for the regeneration of highly lightdemanding species like pine (Rodríguez-García et al., 2011), although negative effects on pine saplings have also been reported (Martín-Alcón et al., 2015). However, the degree of canopy opening in studied stands can also influence understory shrub effects (Martín-Alcón et al., 2015;Muhamed et al., 2013aMuhamed et al., , 2013b because it creates different microclimatic conditions in the understory (Gavinet et al., 2015). ...
... Positive effects of understory shrubs have even been found for the regeneration of highly lightdemanding species like pine (Rodríguez-García et al., 2011), although negative effects on pine saplings have also been reported (Martín-Alcón et al., 2015). However, the degree of canopy opening in studied stands can also influence understory shrub effects (Martín-Alcón et al., 2015;Muhamed et al., 2013aMuhamed et al., , 2013b because it creates different microclimatic conditions in the understory (Gavinet et al., 2015). For instance, in the understory of Pinus nigra stands in north-east Spain, Martín-Alcón et al. (2015) found that positive shrub effects on marcescent oak seedling density Table 3. Left: Barbentane site, no species differences, grey area represents 95% confidence intervals, R 2 of the whole model = 9.5%. ...
Article
Shrubs have been shown to facilitate tree seedling establishment in open Mediterranean ecosystem habitats, but their effects in forests have been much less explored. We investigated the role played by shrubs in seedling emergence, survival and growth for two co-occurring oak species – Quercusilex and Quercuspubescens – in the understory of clear Aleppo pine stands (10 m²/ha). Acorns of both species were sown in two sites in South-East France that contrasted in terms of former land-use (pastoral vs agricultural), soil thickness (shallow vs deep) and type of understory (shrubs + grass vs only shrubs). Oak seedlings had a higher survival and growth on the former agricultural site with deeper soil. In general, the more stress-tolerant Q. ilex performed better than Q. pubescens. In the harsher site, seedling survival decreased with grass cover but increased with shrub cover. Shrub and grass cover decreased seedling diameter and had no effect on seedling height. In the more fertile site, shrub cover had no influence on seedling survival but had a species-specific effect on seedling growth: shrubs mostly competed with Q. ilex but ameliorated Q. pubescens growth, leading to changes in the two species performance ranking at high shrub cover. We conclude that shrubs can act as nurses for oak seedling establishment in pine forest understory, particularly in harsh conditions and for stress-intolerant species. In harsh conditions, shrub cover should be factored in as a way to promote pine forest diversification toward mixed pine–oak stands.
... Based on the results, by 2015 there was a high increase in the number of recruits compared with 2005 (up to 5.9 times). A comparison of these results with those of Martín-Alcón et al. (2015) shows an opposite trend because a decreasing number of recruits is reported in that paper. Such a trend is observed in other studies (Urbieta et al. 2011;Carnier et al. 2014). ...
... Such a trend is observed in other studies (Urbieta et al. 2011;Carnier et al. 2014). Martín-Alcón et al. (2015) explain a decreasing number of recruits by the worse availability of light in forest stands. In other works a negative influence of the competition of herbaceous vegetation is mentioned (Lucas-Borja et al. 2011;Prévosto et al. 2012). ...
Article
Changes in the structure and development of managed Scots pine (Pinus sylvestris L.) stands with respect to changing environmental conditions were set for the period 1979-2015. The study was conducted in conditions of natural pinewoods and pine-oak sites on five permanent research plots (0.25 ha) in Eastern Bohemia, Czech Republic (CR). Studied forest stands showed positive development of stand structural characteristics related to their diversity, number of regeneration individuals and growth characteristics. The standing volume of regularly distributed tree layer in 2015 was in the range of 320-434 m³ ha⁻¹, which indicates an increase by 5.9-20.0% over 10 years. Correlation between pine radial increment and the amount of precipitation was generally the strongest one. Positive statistically significant correlation between diameter increment and temperature was demonstrated only for the average March temperature of the current year. Within the CR, study site can be characterised as a medium polluted area both for sulphur and nitrogen, despite this SO2 concentrations and N deposition in combination with extreme climate events caused severe defoliation in pine stands. Conversely, radial growth was positively significantly correlated with mean NOx concentrations. Drought mainly in combination with even medium environmental pollution can further worsen the health status of pine stands in lowland areas of Central Europe. Thus, formulation of silvicultural techniques able to mitigate the impact of these stress factors is needed. © 2016, Finnish Society of Forest Science. all rights reserved.
... Mixed forests have recently become a significant research topic (Coll et al. 2017), as tree diversity has emerged as an important forest characteristic that enhances forest functions and services and guarantees stability through species complementarity (Del Río et al. 2016). Consequently, promoting tree diversity in both natural and planted forests is becoming a widely recognized practice in forest management (Martín-Alcón et al. 2015). However, in Europe further efforts are still needed regarding the definition and classification of mixed forests (Bravo-Oviedo et al. 2014). ...
... Most reforestations during the second half of the 20 th century in Spain led to monospecific forests composed of Pinus species (Valdell et al. 2016). The decrease in silvicultural interventions through time together with the reduction in livestock grazing is now leading to a progressive process of colonization by other tree species, mainly Quercus (Vayreda et al. 2016, Martín-Alcón et al. 2015. Consequently, the 5 types of pine-oak mixed forests described in this study might also be interpreted as transitional stages leading to oak dominated forests. ...
... These land-use changes, together with extensive reforestation programmes, have triggered extended encroachment and densification processes in forests, mainly pinewoods (Am eztegui et al. 2010;Navarro & Pereira 2012). More recently, the activity of seed dispersers like jays or mice (G omez 2003), and the increasing cover of facilitating shrubs in the understorey of pine forestsfavoured by the sharp decrease in livestock grazing pressureare enabling a gradual entry of late successional broad-leaved species in their understorey (Gracia et al. 2007;Martin-Alcon et al. 2012), enhancing natural processes of tree species diversification (G omez 2003;G omez-Aparicio et al. 2005;Navarro-Gonz alez et al. 2013;Mart ın-Alc on et al. 2015b). An increase in tree species diversity is generally thought to enhance the forest resilience to environmental changes, including variations in disturbance regimes (e.g. ...
Article
How do thermal migration distance and extreme cold events affect seedling emergence and survival in assisted migration schemes in the sub-Mediterranean context? What role does plant provenance play? Can biotic interactions such as nurse effect of the overstorey and shrub layer buffer the negative responses to plant translocation? Are any of these effects species-specific? Three pinewoods in the Catalan Pre-Pyrenees, northeast Iberian Peninsula. We used a replicated field trial to test the early years establishment of two contrasted provenances of four Quercus species (Q. coccifera, Q. ilex, Q. faginea and Q. pubescens) that were sown and planted along gradients of elevation and understorey microsite conditions in sub-Mediterranean pinewoods. Seedling responses to translocation were evaluated through seedling emergence, seedling survival and re-sprouting after dieback events according to seedling provenance, thermal migration distance, extreme cold events and microenvironment. The study reports high success of both the planting (with an overall 76.3% of initial 3-yr survival) and sowing (with an overall 50% of seedling emergence) experiments. The results show that: (1) the thermal migration distance and the occurrence of extreme cold events have strong effects on the responses of the translocated species (particularly the evergreen oaks); (2) the forest overstorey plays an important role in attenuating the negative effects of thermal migration distance on seedling survival; and (3) these responses are species-specific. The evergreen Quercus species showed more evidence of high ecotypic differentiation in terms of cold tolerance, enabling local provenances to respond better to translocation. In contrast, marcescent species, showed high phenotypic plasticity that led to a better overall establishment success. The implementation of assisted migration is a feasible option to increase the diversity and resilience of the sub-Mediterranean pinewoods. Assisted migration programmes should manage risks by thoroughly considering thermal migration distances and the occurrence of extreme cold events when selecting species and seed sources, since Mediterranean tree species show different strategies regarding adaptation to cold. Programme managers should also consider the advantage of planting/sowing under relatively closed canopy to buffer some of the negative responses associated with translocation.
... Our study was focused on a comparison of pine stands without active forest management and intensively managed stands using near-natural silvicultural practices. Related to this topic, more information is available from the area of southern and northern Europe (Angelstam, Kuuluvainen 2004;Montes et al. 2005;Poyatos et al. 2013;Martín-Alcón et al. 2015), but in conditions of central Europe researchers paid attention mainly to stands being composed of shade-tolerant species such as spruce, beech and fir. ...
Article
Full-text available
Structural parameters of Scots pine stands (129-191 years) on their natural sites (270-600 m a.s.l.) are described on 6 permanent research plots (PRP; 3 in managed stands using near-natural silvicultural practices and 3 in stands without active forest management for 3 decades at least) in areas of western, central and eastern Bohemia and in the Polish part of the Krkonoše Mts. In the framework of the study structural and growth parameters, horizontal and vertical structure and biodiversity were evaluated on the plots. A comparison of the plots, and of managed and unmanaged plots showed a relatively high variability in different parameters. Nevertheless, the results document that managed stands, compared to forest stands without management, mostly have significantly higher standing volumes (1.5 times in total and 1.7 times in pine), which is caused by more extreme sites. An opposite trend was found out in dead wood volume, which is distinctly higher in unmanaged stands. Differences in the other parameters are not so pronounced, probably because small-scale management is used and because a relatively short time since the stands were left to spontaneous development has elapsed (30-52 years).
... This nurse effect of the canopy layer is frequently observed in Mediterranean areas, where, e.g., different oak species find appropriate conditions to germinate and establish underneath the more or less closed canopy of pinewoods spontaneously developed on abandoned agricultural land ( Fig. 4.2a). Once the new species have established and reached a certain size, the relatively low light transmitted to the understory hampers their growth, and thus canopy openings of a certain extent are required for promoting them to more advanced developmental stages (Martín-Alcón et al. 2015). ...
Chapter
Mixed forests have been proposed as a tool for more flexible wood production that simultaneously improves conditions for biodiversity and various social demands. Therefore, regeneration of mixed forests has become an important topic of practical concern throughout the world. Here, we briefly review important ecological processes in the early phases of stand development. In addition, we review the various regeneration techniques that can be used, i.e., natural and artificial regeneration of mixtures. Our paper highlights some important knowledge gaps for improved management of young mixed-species stands in Europe. For example, few studies have addressed the specific seed production conditions in mixed forests. Thus, even if some management recommendations can be given for mixed-species regeneration, predicting natural regeneration in mixed stands is problematic. Generally, it is more complicated to formulate rules for young mixed stand development than for monocultures. Much species-specific knowledge is still lacking regarding responses to interactions, although from a management perspective, it seems easier to manage mixtures groupwise rather than stem-wise. Finally, we highlight high deer populations as perhaps the greatest challenge for mixed forest regeneration. More knowledge in the field and greater cooperation between researchers and different stakeholder groups are needed to solve this problem.
... This nurse effect of the canopy layer is frequently observed in Mediterranean areas, where, e.g., different oak species find appropriate conditions to germinate and establish underneath the more or less closed canopy of pinewoods spontaneously developed on abandoned agricultural land ( Fig. 4.2a). Once the new species have established and reached a certain size, the relatively low light transmitted to the understory hampers their growth, and thus canopy openings of a certain extent are required for promoting them to more advanced developmental stages (Martín-Alcón et al. 2015). ...
Chapter
Full-text available
Mixed forests have been proposed as a tool for more flexible wood production that simultaneously improves conditions for biodiversity and various social demands. Therefore, regeneration of mixed forests has become an important topic of practical concern throughout the world. Here, we briefly review important ecological processes in the early phases of stand development. In addition, we review the various regeneration techniques that can be used, i.e. natural and artificial regeneration of mixtures. Our paper highlights some important knowledge gaps for improved management of young mixed-species stands in Europe. For example, few studies have addressed the specific seed production conditions in mixed forests. Thus, even if some management recommendations can be given for mixed-species regeneration, predicting natural regeneration in mixed stands is problematic. Generally, it is more complicated to formulate rules for young mixed stand development than for monocultures. Much species-specific knowledge is still lacking regarding responses to interactions, although from a management perspective it seems easier to manage mixtures group-wise rather than stem-wise. Finally, we highlight high deer populations as perhaps the greatest challenge for mixed forest regeneration. More knowledge in the field and greater cooperation between researchers and different stakeholder groups is needed to solve this problem.
... It seems likely that Quercus spp. seedlings indirectly benefit from the negative feedback of diseased canopy trees on P. nigra regeneration, as seen in other studies (Gómez-Aparicio et al., 2017;Mangan et al., 2010); however, other causes cannot be discounted (see Martín-Alcón et al., 2015). Nevertheless, we did not find an association between Quercus regeneration and stand density, indicating that we can at least discount the idea that the same light conditions that could favour disease could also favour Quercus spp. ...
Article
Previous investigations of the role of pathogens as drivers of community dynamics in forests have mostly focused on interspecific interactions between canopy trees and seedlings via soil feedbacks. However, feedbacks can also occur directly between canopy trees and seedlings when spores of foliar and stem pathogens fall onto seedlings regenerating underneath the canopy. We studied pathogen spillover between canopy trees and con‐ and heterospecific regeneration in the Pinus nigra–Diplodia sapinea pathosystem. We sampled 70 pine stands distributed across a temperature gradient of 9.5°C to 13.7°C (mean annual temperature). In each stand, we linked spore load and spore survival in the canopy with pathogen biomass and disease severity in regenerating seedlings. The density of regenerating seedlings and the health status of con‐ and heterospecific seedlings were also measured. The strength of canopy–understory interactions was correlated with both climatic and stand variables. The most severe symptoms of disease in regenerating seedlings were found in the warmest stands. Structural equation models suggested that disease severity in seedlings was mediated by an increased spore spillover from the canopy. Temperature also increased the pathogen survival rate within seedlings, further contributing to disease severity. Interestingly, disease severity in the canopy did not correlate with spore load in the canopy, suggesting that adult trees were spilling pathogens onto the understory regardless of their health status. Pathogen spillover increased in more open stands. Greater disease severity in seedlings correlated with stands with a higher density of oak seedlings. Synthesis. These results suggest an understudied mechanism of the Janzen–Connell theory, according to which, canopy–understory negative feedbacks could be driven by canopy pathogens. Our data also suggest that temperature shapes canopy–understory interactions, which implies that under a warming climate scenario, an acceleration of conspecific negative feedbacks could be expected, with implications for forest regeneration dynamics.
... Particularly, we observed Corylus avellana, as dominant tree species, and Crataegus monogyna, as accompanying tree species, in mixed broadleaf forests plots presenting selective felling practices. The latter finding is in line with published studies analysing the effects of selective felling in coniferous and deciduous species, which also consider selective felling as a management practice that favours biological diversity for those forest habitats (Atlegrim and Sjöberg, 2004;Martín-Alcón et al., 2015). ...
Article
The establishment and maintenance of protected areas is the backbone of global conservation strategies to halt biodiversity loss. However, despite the more than 200,000 legally designated protected sites worldwide, the rate of species extinction has not decreased, for which some debate the real effectiveness of protected areas to preserve biodiversity. Using data from tropical areas, many studies have attempted to test the effectiveness of protected areas by comparing species richness in protected and neighbouring unprotected sites, without reaching a consensus. Here, we extend this line of research with data from temperate deciduous forests inside and outside Picos de Europa National Park and Biosphere Reserve (N Spain). Specifically we compare data from mixed broadleaved woodlands, beech forests (Fagus sylvatica L.) and Pyrenean oak (Quercus pyrenaica Willd.) forests. We conducted botanical inventories and recorded ecological data from 25 0.2-ha concentric plots distributed in forest commons inside the reserve and from other 25 similar plots established in neighbouring not protected forest commons. Data were used to construct a set of ecological indicators and evaluated using modelling methods. We found no significant differences in species composition between plots in protected and non-protected forest commons, likely due to the similar management criteria applied in both land uses. We found less active management outside the protected area, which helps to maintain stands in a semi-natural state. In contrast, we observed the presence of silvicultural treatments inside the protected area, although these treatments were non-intensive, promoting vegetation composition associated to late-successional ecosystems. We only detected significant differences between plots inside and outside the protected area when relation between species richness was analysed with reference to forest habitat type. Precisely, plots of beech forests inside Picos de Europa were more homogenous than plots outside the protected area, which may indicate that management practices inside the protected area do not favour tree species diversity. Non-intensive silviculture management in beech forests inside Picos de Europa seems to promote the presence of the dominant tree species Fagus sylvatica L., which in the absence of perturbations is characterized by conforming monospecific vegetation communities. Overall, our results do not support the idea that protected areas hold more biodiversity than surrounding forest commons. Conservation treatments applied in protected areas should promote the presence of species associated to disturbances, particularly in stands tending to homogeneous species composition at late-successional stages, as this may enhance their resilience under the current rapid global changes.
... Quercus whenever seed sources are available (Lookingbill and Zavala, 2000;Martín-Alcón et al., 2015). ...
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Understanding ecosystem vulnerability is essential in risk management to anticipate disasters. While valuable efforts have been made to characterize vulnerability components (exposure, sensitivity, and response capacity) at particular ecosystem stages, there is still a lack of context-specific studies accounting for the temporal dimension of vulnerability. In this study, we developed a procedure to identify the main natural dynamics of monospecific and mixed forests and to assess the variations of sensitivity and response capacity to fire along successional dynamics. In the procedure, we generated forest chronosequences by summarizing the dynamics between consecutive surveys of permanent plots into a set of longer successional trajectories represented in a multidimensional space. Then, we calculated several variables of sensitivity and response capacity to fire of forest stages associated with each trajectory and we assessed their variation along succession. The procedure was applied to Mediterranean forests in Spain dominated by a pine species poorly adapted to severe crown fires. We found that forest vulnerability components varied differently among successional trajectories, which depended on the composition and structure of their initial stages and the environmental context in which they occurred. Autosuccessional dynamics of pine forests showed relatively low sensitivity to fire along trajectories. However, their response capacity was related to the changes in shrub cover. In contrast, diversifying dynamics showed an increasing sensitivity to fire, but also a higher response capacity the greater the functional diversity along succession. These results highlight the need for considering the temporal dimension of vulnerability in risk management and the importance of assessing sensitivity and response capacity as independent components of vulnerability that can be modified through management at critical forest stages.
... This density is substantially higher in comparison with another relict pine forest in Czech Republic, . During the monitored period, a rather significant increase (by 55.0%) in the number of natural regeneration was recorded, with the exception of one PRP, but Martín-Alcón et al. (2015) presents the opposite trend. The works by Štícha et al. (2010) and Prévosto et al. (2012) then present the competition of grass vegetation as a possible negative impact on natural regeneration. ...
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The paper deals with the dynamics of structure, diversity and growth of natural pine stands without direct human impact during the ten-year period in Nature Reserve (NR) Kostelecké bory, Czech Republic. The objective was to determine the main characteristics of the spontaneous development of Scots pine (Pinus sylvestris L.) forest stands in relation to their naturalness, ecological stability and adaptation to climate change and air pollution stress. Horizontal and vertical structure and species diversity of the tree layer, dead wood and natural regeneration of each permanent research plot (PRP) were evaluated (n = 6, 50 × 50 m /0.25 ha/, Northern Bohemia, 410–425 m above sea level). The average ring series of PRPs were correlated with the climatic data (precipitation, temperature) according to individual years from the Doksy climatic station, and the air pollution data (SO2 in 1988–2015, NOX and AOT40F – ozone exposure in 1992–2015) from Radimovice station. In 2016, the stand volume increased by 26.0% to 136 m3 ha−1 (108 m3 ha−1 in 2006) and the volume of dead wood increase by 127.2% to 27 m3 ha−1 (12 m3 ha−1 in 2006). The horizontal structure of tree layer and natural regeneration was predominantly aggregated to random. More distinct changes in biodiversity and structural characteristics occurred in the natural regeneration (21.5%) compared to tree layer (2.8%). The precipitation had a significantly higher effect on radial growth compared to temperature. The lack of precipitation in growing season and high temperature in previous autumn and winter were limiting factors for growth. Climatic factors had significant effect on diameter increment in July of the current year (P < 0.01) and June of the current and previous year (P < 0.05). Radial growth was negatively correlated with SO2 concentrations (P < 0.01) and ozone exposure (P < 0.05). NOX concentrations had low effect on radial growth. The natural stand dynamics had positive effect on biodiversity and functional integrity of natural pine ecosystems.
... In addition, small scale variation has particular roles in forest productivity (Kuuluvainen & Juntunen, 1998). However, in natural forest, various disturbances and practises within sites create diversity, which is much more complex and dynamic and has been explored in a rigorous way (Martín-Alcón et al., 2015;Peterson & Pickett, 1990;Runkle, 1981;Runkle & Yetter, 1987). For example, gap phase dynamics Yamamoto, 2000) and gap models (Bugmann, 2001) are used to study those complex micro-site characteristics for old growth forest. ...
... Small scale, or microsite, is defined as locations with microclimates that differ from their surroundings [15]. Variation due to microsite has been recently explored in both mature natural forests [16][17][18][19][20] and plantation forests [21][22][23]. However, the effects of microsite variation on juvenile plantations merit further attention. ...
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The effect of microsite on juvenile forest plantation yield is rarely explored. This is because juvenile plantation growth is considered to be reasonably homogenous due to a lack of resource competition between trees prior to canopy closure. However, models of juvenile plantation height growth and survival that are sensitive to microsite variation could aid decisions relating to site preparation, plantation establishment and early silvicultural treatments. In this study, juvenile Eucalyptus bosistoana and E. globoidea height growth and survival proportion were modelled against topographic and environmental microsite characteristics as independent variables. The experiment included three different sites situated in a sub-humid region of New Zealand. A total of 540 plots were planted with 18,540 trees in regular rows and columns. Micro-topographical variables significantly influenced height growth and survival proportion of both E. bosistoana and E. globoidea, but species differed in their responses. More sheltered microsites yielded greater height growth and survival for both species. The height of both species was influenced by wind exposure, morphometric protection, and distance from the nearest ridge. E. bosistoana height was also influenced by topographic position and surface plan curvature. Survival was affected by surface profile curvature for both species, while E. globoidea survival was also impacted by surface plan curvature and distance from the top ridge. This study identified microsite factors influencing juvenile height and survival of two Eucalyptus species.F
Chapter
Pine species are dominant in a large part of the western Mediterranean landscapes where they provide a wide range of goods and services. At present, the different components of global change threaten the delicate equilibrium between the current structure and functioning of most of these pinewoods and the future provision of ecosystem services required by human societies. In this chapter, we briefly review the origin of these systems and the main biotic and abiotic drivers of their current dynamics. We put particular emphasis on the factors modulating survival and growth at early growth stages, which are the most vulnerable to environmental uncertainty. In addition, we highlight the importance of natural diversification processes for the resilience of these systems to natural disturbances and we revise the main silvicultural models and treatments that are applied for the execution of different management objectives. Finally, we underline the need to move towards management schemes oriented to the enhancement of the adaptive capacity of these forests to climate change.
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Installation d’espèces feuillues en forêt de pin d’Alep : interactions avec les strates arborées et arbustives. L’installation de plantules est une phase critique fortement influencée par les interactions avec la végétation établie - compétition ou facilitation. Dans cette thèse, nous montrons que l’effet du pin d’Alep ou d'arbustes sur l’installation d’espèces feuillues dépend de la densité du couvert, de la stratégie des espèces cibles et nurses et des conditions locales. Sous couverts très denses, la survie et la croissance des plantules sont drastiquement limitées par compétition lumineuse et hydrique, le stress hydrique des plantules étant renforcé par une faible allocation de biomasse vers les racines. À l’autre extrémité du gradient, en milieu ouvert, la photoinhibition et la concurrence avec les herbacées peuvent limiter l'installation des plantules. Les espèces sclérophylles sempervirentes sont peu sensibles aux fortes irradiances, températures et demandes évaporatives et peuvent profiter de conditions favorables momentanées par polycyclisme. L’effet d’un couvert végétal sur ces espèces est donc principalement compétitif, tandis que les espèces décidues à fort SLA bénéficient d’un couvert modéré qui tamponne les extrêmes. En pépinière, la litière des principales espèces ligneuses modifie les propriétés physico-chimiques et microbiologiques du sol sous-jacent mais sans effet sur des plantules de chêne au bout de 2 ans, montrant un faible effet allélopathique. L’éclaircie des peuplements de pin d’Alep denses est une stratégie pour faciliter l’installation d’espèces feuillues et ainsi améliorer la diversité et la résilience au feu des forêts. L’optimum de couvert semble se déplacer vers des couverts plus denses dans des sites à conditions climatiques ou édaphiques plus sévères et pour les espèces décidues.
Chapter
Two key processes constitute the dynamics of planted pine forests (Pinus halepensis, P. brutia) in Mediterranean Israel: pine regeneration and reestablishment of native broad-leaved tree species (broad-leafs). These two processes are regulated mainly by aridity, overstory cover and grazing. Pine forests in Israel have been planted throughout the semiarid to dry-subhumid climate zones (Wetness Index = 0.13–0.50) and over variable geology and topography. They exhibit a range of pine overstory cover levels (Leaf Area Index = 1–3.5) and are subjected to variable grazing by cattle, sheep and goats. Pine regeneration is mainly observed in P. halepensis stands with annual rainfall >350–400 mm. Above this threshold, the extent of pine regeneration is highly unpredictable and strongly influenced by bedrock type, being considerably higher on soft than on hard calcareous bedrock. Pine regeneration is negatively affected by overstory cover and grazing, which limit survival and growth of recruits. Pine regeneration in P. brutia stands is currently limited, though overstory thinning appears effective in promoting this process. Broad-leafs reestablishment occurs where annual rainfall exceeds 450 mm; it increases sharply with rainfall and is higher on north- than on south-facing slopes. Broad-leafs recruitment is positively influenced by overstory cover while the effect of grazing is minor. Aridity, overstory cover and grazing all restrict the growth rate of broad-leaf recruits. Based on our understanding of these processes, management guidelines are proposed for converting the manmade pine monocultures into complex, sustainable mixed-forest ecosystems.
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Tree recruitment is a key process underlying stand dynamics and sustainability in managed forests. Woody plant cover is known to affect the regeneration success of Pinus nigra, suggesting the existence of facilitative plant-plant interactions. The regeneration patterns of this Mediterranean pine were analyzed across its distribution area, using data from 3226 plots of the Spanish National Forest Inventory. We aimed to test the hypothesis that seedlings establishment occurs under higher values of either canopy or shrub cover in the driest populations, as predicted by the stress-gradient hypothesis. Data were analyzed by means of Generalized Linear Models and multivariate methods. Results revealed that regeneration failure occurs on a regional scale, and that regeneration is facilitated by tree canopy cover of 55%–80%. A non-linear pattern of interaction along an aridity gradient was identified, with competition at the wettest site, high facilitation at the mid-dry sites, and low facilitation at the driest site. Evidence suggests that some shrub species may facilitate recruitment in the harsher areas. Collectively, our results reduce the possibilities of adapting forest management to drying climates by the application of alternative silvicultural prescriptions involving canopy cover
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Most of the world's plantations were established on previously disturbed sites with an intensive land-use history. Our general hypothesis was that native forest regeneration within forest plantations depends largely on in situ biological legacies as a source of propagules. To test this hypothesis, we analyzed native oak regeneration in 168 pine plantation plots in southern Spain in relation to land use in 1956, oak patch proximity, and pine tree density. Historical land-use patterns were determined from aerial photography from 1956, and these were compared with inventory data from 2004-2005 and additional orthophoto images. Our results indicate that oak forest regeneration in pine plantations depends largely on land-use legacies, although nearby, well-conserved areas can provide propagules for colonization from outside the plantation, and pine tree density also affected oak recruit density. More intense land uses in the past meant fewer biological legacies and, therefore, lower likelihood of regenerating native forest. That is, oak recruit density was lower when land use in 1956 was croplands (0.004 +/- 0.002 recruits/m2 [mean +/- SE]) or pasture (0.081 +/- 0.054 recruits/m2) instead of shrubland (0.098 +/- 0.031 recruits/m2) or oak formations (0.314 +/- 0.080 recruits/m2). Our study shows that land use in the past was more important than propagule source distance or pine tree density in explaining levels of native forest regeneration in plantations. Thus, strategies for restoring native oak forests in pine plantations may benefit from considering land-use legacies as well as distance to propagule sources and pine density.
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Quercus forest regeneration is limited by different factors, post-dispersal predation being one of the most critical stages. We analysed seed removal of four coexisting Quercus species (Q. ilex, Q. suber, Q. faginea and Q. pyrenaica) in a Mediterranean forest located in Southern Spain. Marked and weighed acorns from each of the species were placed in experimental units with or without exclusion of large herbivores and in two microhabitat types (Q. ilex shade or open). Acorn removal was monitored for 120 days in order to test the effect of exclusion of large herbivore and microhabitat type on seed removal rates and species selection. Interestingly, the results of microhabitat and species selection differed depending on the presence of large herbivores. Removal was faster in sites without exclusion, where most seeds (≈85%) disappeared during the first 9 days. In these sites, no differences in seed removal were found between different microhabitats, but seeds of two species, those with higher seed mass (Q. suber and Q. pyrenaica) were most preferred. However, under exclusion of large herbivores, seed removal was affected by the microhabitat, this being greater in Q. ilex shaded microhabitats, which showed a higher structural diversity. Also, species selection was completely different under exclusion of large herbivores, and seeds of Q. ilex and Q. faginea were removed faster. These results highlight the importance of large herbivore activity on seed removal and its effect on microhabitat and species selection. Therefore, specific selection by seed consumers may modify seedling recruitment and may have an important influence on the relative abundance of coexisting Quercus species.
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For millennia, mankind has shaped landscapes, particularly through agriculture. In Europe, the age-old interaction between humans and ecosystems strongly influenced the cultural heritage. Yet European farmland is now being abandoned, especially in remote areas. The loss of the traditional agricultural landscapes and its consequences for biodiversity and ecosystem services is generating concerns in both the scientific community and the public. Here we ask to what extent farmland abandonment can be considered as an opportunity for rewilding ecosystems. We analyze the perceptions of traditional agriculture in Europe and their influence in land management policies. We argue that, contrary to the common perception, traditional agriculture practices were not environmentally friendly and that the standards of living of rural populations were low. We suggest that current policies to maintain extensive farming landscapes underestimate the human labor needed to sustain these landscapes and the recent and future dynamics of the socio-economic drivers behind abandonment. We examine the potential benefits for ecosystems and people from rewilding. We identify species that could benefit from land abandonment and forest regeneration and the ecosystem services that could be provided such as carbon sequestration and recreation. Finally, we discuss the challenges associated with rewilding, including the need to maintain open areas, the fire risks, and the conflicts between people and wildlife. Despite these challenges, we argue that rewilding should be recognized by policy-makers as one of the possible land management options in Europe, particularly on marginal areas.
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Aim of the study: The aim of the research is to analyse the role of Quercus ilex advance regeneration in the stand regeneration of pine plantations after small-sized canopy openings, and to assess the influence of the forest stand and the canopy opening. The performance of the advance regeneration under the pine plantation is also examined. Area of study: A Pinus nigra plantation in dry Continental Mediterranean climate in eastern Spain. Materials and Methods: The tree regeneration of ten canopy openings of 0.17-0.43 ha was monitored during five years after treatment. It was also sampled in 0.12 ha-plots in the non-treated pine plantation surrounding the openings. Main results: An important increase in the height of Q. ilex regeneration was observed in the openings, unlike what was found in the intact pine plantation. In the pine plantation, stand density showed a moderate positive influence on the density of Q. ilex regeneration, whereas in the canopy gaps Q. ilex height was negatively influenced by stand density before the opening. Research highlights: The canopy opening triggered a response in Q. ilex advance regeneration, although height growth rates seemed to reduce over time. The results support the view that promoting Q. ilex in pine plantations may require different management strategies depending on the characteristics of the pine overstorey and on the density and size of the advance regeneration.
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Land cover mapping obtained from photointerpretation of aerial photographs and orthophotographs was used to quantify land cover changes between 1957 and 1996 in a Mediterranean middle mountain area. Expansion of forested area is clearly the main land cover change caused by the abandonment of traditional agricultural activities and by the use of other materials and energy sources instead of forest resources. As a result, about 64% of the area was covered by forest by 1996, whereas in 1957 forests accounted for only 40% of the land cover. Spontaneous afforestation of abandoned fields with Scots pine (Pinus sylvestris L.) in terraced areas and areas of sparse scrub vegetation, coupled with an increase in the density of forest canopies, has been responsible for this expansion of woodland. The influence of physiographic factors in land cover change processes in the terraced areas of the catchment was also considered. The results demonstrate that within the terraced areas, north-facing and more elevated steeper slopes are more intensely afforested. However, an accurate analysis of the role played by these factors in land cover change cannot be carried out because the pattern of land abandonment is not independent of these physiographic characteristics. Furthermore, field observations at the terrace scale are evidence of the relevant influence of local topography in afforestation dynamics.
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The relative abundance of two codominant Mediterranean tree species, shade-tolerant Quercus ilex L. and shade-intolerant Pinus halepensis Mill., is inversely correlated along aridity gradients, but this pattern is not explained by seedling responses to water or light availability, suggesting that subsequent life history stages may explain forest composition. To test this hypothesis, we calibrated statistical models of sapling growth and height–diameter allometry as functions of light availability and climatic variation as well as models of sapling mortality as a function of growth history. Contrary to the expectation of a sun–shade growth trade-off, P. halepensis grew faster than Q. ilex saplings at both low and high light levels. Low precipitation and aridity suppressed sapling growth rates, but no evidence of a shade–drought growth trade-off was found either. Pinus halepensis sapling mortality was strongly growth dependent, exhibiting high mortality rates at low growth, but the mortality of Q. ilex saplings was not. Height–diameter allometric variation was higher in low- than in high-light environments and was more pronounced with respect to changes in light than climatic water. Our results suggest that interspecific differences in sapling mortality and plasticity, rather than growth, may control species distributions at the mesic end of the aridity gradient.
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Encroachment of trees into low-productivity grasslands is a serious threat to grassland biodiversity throughout Europe. Although the rate of encroachment of trees into grassland is variable and poorly understood, it is thought to result from variation in tree seedling recruitment, which is limited mainly by the availability of safe sites and the dispersal distances of propagules. In this study, we established spatially explicit models of seedling recruitment for two major sub-Mediterranean trees: Quercus pubescens Willd. and Fagus sylvatica L. We quantified the spatial distribution of Q. pubescens and F. sylvatica seedlings up to 20 years old at three grassland sites. We also quantified the spatial distribution and size of mature trees and of the two dominant species of shrubs, Buxus sempervirens L. and Juniperus communis L., at each of the three sites. Ninety-eight percent of the regeneration took place under shrub canopy. Quercus pubescens seedlings showed higher seedling production per unit of canopy area and longer mean effective dispersal distances than did F. sylvatica seedlings. Quercus pubescens seedlings also had a large advantage over F. sylvatica seedlings for establishment in open areas. Juniperus shrubs were better safe sites for the establishment of Q. pubescens and F. sylvatica seedlings than were Buxus shrubs. We calculated indices of seedling dispersal limitation and safe site availability for recruitment for four dates. In the dolomitic Causse grasslands examined in this study, the availability of safe sites for germination and survival was far more important than seedling dispersal limitation for recruitment of both tree species.
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Canopy gaps play an important role in the dynamics of old-growth forests, although it is not well known how gap dynamics differ among regions. To further our understanding of natural gap dynamics in mixedwood forests, this study compares mixed stands located in eastern (Gaspésie region) and western (Témiscamingue region) Quebec. We tested whether the gap fraction in mixedwood stands was similar in these two regions. Data from field transects were used to characterize current canopy gaps, and aerial photos were used to contrast gap characteristics before and after the most recent spruce budworm (Choristoneura fumiferana Clem.) outbreak, which occurred from 1973 to 1991 in Gaspésie and from 1972 to 1984 in Témiscamingue. The current gap fraction was found to differ between the two regions: it varied from 2% to 48% with an average of 25.6% for the Gaspésie region and from 24% to 52% with an average of 36.6% for the Témiscamingue region. While the last spruce budworm outbreak coincided with a significant increase in canopy openings in the Témiscamingue region (p = 0.047), no such effect was observed in Gaspésie. These results suggest that the temporal pattern of small-scale disturbances can vary among regions, even when similar forest types are compared.
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A major challenge for foresters in the future will be issues related to global change. Global change expresses itself in a variety of ways, depending on regional vegetation and climate patterns, small-scale topographic differences, tree species, and stand development stages. Using silviculture as an example, the variety of steps linking global change—as a general concept—and actual management decisions is explored. The first task is to relate global change aspects to silviculturally relevant scales. Second, silvicultural responses must reflect the wide variety of changes, including their interactions. A number of management recommendations have been proposed from the global scale to the application of specific silvicultural treatments. These recommendations are mostly focused on increasing the resistance of forests to perturbations. Increasing ecosystem adaptability and resilience through silvicultural practices may benefit from developments in other scientific fields. Recent advances in the complexity and ecosystem sciences may provide approaches that are better suited for a future with increased variability and uncertainty in ecological and social conditions. Specifically, managing forests as complex adaptive systems may provide a conceptual framework that can be useful for silviculture, even though much work still must be done to fully explore the implications of such a new framework for silvicultural decisionmaking.
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Land cover mapping obtained from photointerpretation of aerial photographs and orthophotographs was used to quantify land cover changes between 1957 and 1996 in a Mediterranean middle mountain area. Expansion of forested area is clearly the main land cover change caused by the abandonment of traditional agricultural activities and by the use of other materials and energy sources instead of forest resources. As a result, about 64% of the area was covered by forest by 1996, whereas in 1957 forests accounted for only 40% of the land cover. Spontaneous afforestation of abandoned fields with Scots pine (Pinus sylvestris L.) in terraced areas and areas of sparse scrub vegetation, coupled with an increase in the density of forest canopies, has been responsible for this expansion of woodland. The influence of physiographic factors in land cover change processes in the terraced areas of the catchment was also considered. The results demonstrate that within the terraced areas, north-facing and more elevated steeper slopes are more intensely afforested. However, an accurate analysis of the role played by these factors in land cover change cannot be carried out because the pattern of land abandonment is not independent of these physiographic characteristics. Furthermore, field observations at the terrace scale are evidence of the relevant influence of local topography in afforestation dynamics.
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Ecological processes within forests provide vital ecosystem services to society, most of which depend on the persistence of tree cover that can be altered after the impact of a disturbance. The aim of the present study was to examine the role of seed dispersal and resprouting that mediate resilience to large fires and evaluate the economic costs that these ecological functions provide. We used field data from 412 plots of the Spanish National Forest Inventory providing information on pre- and post-fire conditions of Mediterranean Pinus spp. and Quercus spp.-dominated forests. Then, we determined the need for restoration (N Rest) and estimated the minimum pre-fire densities needed to ensure adequate post-fire cover. Economic valuations were assessed through three different scenarios (Sc) of possible human-management actions aimed at ensuring proper post-fire tree cover: Sc. 1) a pre-fire management scenario evaluating the costs of planting Quercus spp. seedlings in the understory, mimicking the whole dispersal function; Sc. 2) a pre-fire scenario in which enrichment plantations increased the densities of natural oaks; and Sc. 3) a post-fire scenario where the restoration is done through planting pines within the burned area. Approximately 90% of the burned area (371 out of 412 plots) was able to recover after fire supporting the view that Mediterranean forests are resilient to fire. This resilience was primarily mediated by biotic seed dispersal and posterior resprouting of tree species. These ecological functions saved between 626 and 1,326 €/ha compared to the human-management actions. Ensuring key ecological processes within forests increases forest resilience and recovery after fire leading to a generally significant saving of economic resources. In a perspective of increased future impact of disturbances and decrease availability of economic resources for forest management, the implications of the present study can be far reaching and extended to other forest planning exercises.
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Spatial heterogeneity of abiotic factors influences patterns of seedling establishment at different scales. In stress-prone ecosystems such as Mediterranean ones, heterogeneity generated by shrubs has been shown to facilitate the establishment of tree species. However, how this facilitation is affected by spatial scale remains poorly understood. We have experimentally analysed the consequences of the abiotic heterogeneity generated by pioneer shrubs on survival, growth and physiology of seedlings of three important tree species from Mediterranean mountains (Acer opalus ssp. granatense, Quercus pyrenaica and Quercus ilex). Patterns of abiotic heterogeneity and seedling performance were studied at two scales differing in grain: 1) the microhabitat scale, by using open interspaces as controls of the effect of different shrub species, and 2) the microsite scale, analysing the effects of fine-grain heterogeneity (within-microhabitat heterogeneity). Results showed that, at the microhabitat scale, seedling establishment of the three tree species significantly benefited from the modification of the abiotic environment by nurse shrubs. However, we found shrub-seedling interactions to be species-specific, due to differential modification of both aboveground (light availability) and belowground (soil compaction, water content, and fertility) abiotic factors by nurse shrub species. Heterogeneity at the within-microhabitat scale was rather high, although it did not significantly affect seedling performance of any of the tree species. The study demonstrates that the effects of the abiotic heterogeneity generated by shrubs are not consistent across the range of spatial scales considered. The regeneration niche of tree species becomes very complex at fine spatial scales, and thereby estimators of abiotic heterogeneity are valuable descriptors of spatial patterns of seedling establishment only when microsite "noise" is averaged out at greater scales.
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Mediterranean forest plantations are currently under an intense debate related to their ecological function, sustainability and future performance. In several Mediterranean countries, efforts are directed to convert pine plantations into mixed and more diverse forests. This research aims to evaluate the effect of the spatial configuration of pine plantations on regeneration and plant diversity in order to facilitate plantation management towards more diversified stands. Spatial characteristics of plantations (proximity to different vegetation types, fragmentation and internal patch structure) were related to abundance of seedlings of an ecologically important broadleaved species, Holm Oak (Quercus ilex L.), and the Shannon diversity index of the community. Q. ilex seedling abundance and plant diversity in pine plantation patches are favoured by the proximity to oak patches located uphill. Fragmentation affected only plant diversity, with smaller patches having more diversity. The internal structure of the pine patch influenced both regeneration of Q. ilex and diversity. Pine patches with lower pine tree density were characterized by higher diversity and less Q. ilex regeneration confirming that internal structure affects species differently. From a management perspective, the process of conversion of Mediterranean pine plantations to mixed oak–pine forests could be facilitated by (1) having the seed source uphill from the plantation, (2) increasing the fragmentation of plantations and (3) promoting the internal heterogeneity of plantations to create a diverse range of light environments matching the different requirements of species.
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A broad-scale analysis of the structure and understory composition of Pyrenean mountain pine (Pinus uncinata Ram.) stands was performed using data from the Spanish National Forest Inventory. Twelve structure-based forest typologies were defined from variables related to tree size, stand density, vertical structure and standing deadwood, using cluster analysis techniques. These typologies were adequately classified (accuracy >75 per cent) by a dichotomous key obtained from classification and regression trees. Multiple regression models were then used to analyse relationships between the main stand structural variables and a set of climatic and physiographic factors. The models showed significant correlations between winter temperature, slope and continentality (among other variables) and the current structure of mountain pine stands. The relationships between the understory composition of mountain pine forests and different environmental and structural overstory factors were found to be driven by an elevation-pH gradient and a stand density-soil stoniness gradient. The results of this study can be directly used for forest planning at different scales and could help forest managers to establish strategies designed to facilitate a given habitat for species of conservation interest.
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Statistical approaches to overdispersion, correlated errors, shrinkage estimation, and smoothing of regression relationships may be encompassed within the framework of the generalized linear mixed model (GLMM). Given an unobserved vector of random effects, observations are assumed to be conditionally independent with means that depend on the linear predictor through a specified link function and conditional variances that are specified by a variance function, known prior weights and a scale factor. The random effects are assumed to be normally distributed with mean zero and dispersion matrix depending on unknown variance components. For problems involving time series, spatial aggregation and smoothing, the dispersion may be specified in terms of a rank deficient inverse covariance matrix. Approximation of the marginal quasi-likelihood using Laplace's method leads eventually to estimating equations based on penalized quasilikelihood or PQL for the mean parameters and pseudo-likelihood for the variances. Im...
Chapter
Processes operating during the seed, seedling and juvenile phases are crucial for understanding patterns, dynamics and succession in plant communities (Schupp 1990; Grime and Hillier 1992). Many recent studies in temperate forests have emphasized the difficulties seedlings encounter in establishing themselves and the problems hardwood tree species face for survival, in particular the species of the genus Quercus (Lorimer 1984; Ross et al. 1986; Crow 1988, 1992; Johnson 1992; Keeley 1992; Ward 1992). This apparent paradox presented by late-successional species which, while dominant in the landscape, do not regenerate easily has long been noticed (Crow 1988), but the reasons for this are still poorly understood. The reasons for failure to recruit may include many factors, such as adequacy of seed dispersal (McClanahan 1986; Willson 1992), seed predation by animals (Crawley 1992), abiotic stresses such as drought or light limitation (Pons 1992; Espelta et al. 1995), and availability of suitable microsites for seed germination and seedling establishment (Crawley 1990). Furthermore, the regeneration failure of recent oak forests has also been attributed to the fact that the distribution of oaks overlaps to a large extent zones of intense human activity and thus oak forests have suffered from human impact (Matsuda et al. 1989).
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This study presents a silvogenetic typology of single-species stands of Pinus nigra in Catalonia (NE of Spain) based on the information supplied by the Second National Forest Inventory. Results of factor analysis applied to dendrometric variables show nine different stand types, identified mainly from variables related to diameter distribution and stand stocking. The character of uneven-aged stands, present in five of the groups though with different features, was analyzed using a truncated Weibull distribution. The typology, which is managed by computing regeneration levels and diametric classes, encompasses different developmental stages and allows the diagnosis of compromised situations in terms of stand persistence caused by low or high stocking.
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A major challenge for foresters in the future will be issues related to global change. Global change expresses itself in a variety of ways, depending on regional vegetation and climate patterns, small-scale topographic differences, tree species, and stand development stages. Using silviculture as an example, the variety of steps linking global change-as a general concept-and actual management decisions is explored. The first task is to relate global change aspects to silviculturally relevant scales. Second, silvicultural responses must reflect the wide variety of changes, including their interactions. A number of management recommendations have been proposed from the global scale to the application of specific silvicultural treatments. These recommendations are mostly focused on increasing the resistance of forests to perturbations. Increasing ecosystem adaptability and resilience through silvicultural practices may benefit from developments in other scientific fields. Recent advances in the complexity and ecosystem sciences may provide approaches that are better suited for a future with increased variability and uncertainty in ecological and social conditions. Specifically, managing forests as complex adaptive systems may provide a conceptual framework that can be useful for silviculture, even though much work still must be done to fully explore the implications of such a new framework for silvicultural decisionmaking.
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AimLarge-scale patterns of limitations in tree recruitment remain poorly described in the Mediterranean Basin, and this information is required to assess the impacts of global warming on forests. Here, we unveil the existence of opposite trends of recruitment limitation between the dominant genera Quercus and Pinus on a large scale and identify the key ecological drivers of these diverging trends. LocationSpain Methods We gathered data from the Spanish National Forest inventory to assess recruitment trends for the dominant species (Pinus halepensis, Pinus pinea, Pinus pinaster, Pinus nigra, Pinus sylvestris, Pinus uncinata, Quercus suber, Quercus ilex, Quercus petraea, Quercus robur, Quercus faginea and Quercus pyrenaica). We assessed the direct and indirect drivers of recruitment by applying Bayesian structural equation modelling techniques. ResultsSevere limitations in recruitment were observed across extensive areas for all Pinus species studied, with recruitment failure affecting 54-71% of the surveyed plots. In striking contrast, Quercus species expanded into 41% of the plots surveyed compared to only 10% for Pinus and had a lower local recruitment failure (29% of Quercus localities compared to 63% for Pinus species). Bayesian structural equation models highlighted the key role of the presence of Q.ilex saplings and the increase in the basal area of Q.ilex in limiting recruitment in five Pinus species. The recruitment of P.sylvestris and P.nigra showed the most negative trends and was negatively associated with the impacts of fire. Main conclusionsThis study identified Q.ilex, the most widespread species in this area, as a key driver of recruitment shifts on a large scale, negatively affecting most pine species with the advance of forest succession. These results highlight that the future expansion/contraction of Q.ilex stands with ongoing climate change will be a key process indirectly controlling the demographic responses of Pinus species in the Mediterranean Basin.
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Standard statistical practice ignores model uncertainty. Data analysts typically select a model from some class of models and then proceed as if the selected model had generated the data. This approach ignores the uncertainty in model selection, leading to over-confident inferences and decisions that are more risky than one thinks they are. Bayesian model averaging (BMA) provides a coherent mechanism for accounting for this model uncertainty. Several methods for implementing BMA have recently emerged. We discuss these methods and present a number of examples. In these examples, BMA provides improved out-of-sample predictive performance. We also provide a catalogue of currently available BMA software. KEYWORDS: Bayesian model averaging; Bayesian graphical models; Learning; Model uncertainty; Markov chain Monte Carlo Research supported in part by the U.S. National Science Foundation and the U.S. Office of Naval Research (N00014-91-J-1014). The authors are grateful to David Lewis and Ro...
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In 1976-1977, 284 gaps (canopy-opening sizes 1-1490 m^2) were sampled (age, size, species composition) from old-growth mesic forests in Great Smoky Mountains National Park, Joyce Kilmer Wilderness Area and Walker Cove Research Natural Area. In 1983, the woody vegetation (stems @>1 cm dbh) of 273 of these gaps was resampled, rates of gap closure by canopy tree branch growth and sapling height growth were estimated, and incidences of disturbances occurring since 1976-1977 were noted. The average yearly crown extension growth rate was 18 cm/yr, with much variation among species and individuals. Some individual crowns grew into the canopy opening as much as 4 m in the 7 yr. Saplings grew an average of 30 cm/yr in height, again with much variation. Overall, taller saplings grew somewhat faster than smaller ones and saplings in large gaps grew faster than those in small gaps. These two rates of gap closure together suggest that most saplings will require two or more gap episodes to reach the forest canopy. For woody vegetation, basal area per unit gap area was originally highest in small gaps, though it increased between sampling dates most in large gaps. Stem density had been highest in small old gaps, but decreased the most in old gaps. Tsuga canadensis, Fagus grandifolia, Acer saccharum, and Halesia carolina were the most important species in the gaps studied. Most species did not change in relative density or dominance between the two sampling dates and showed no significant correlations between those parameters and gap size and age. Overall, Tsuga and Fagus decreased and Acer saccharum increased in importance. High rates of repeat disturbance favor species able to grow in intermediate light levels and to survive several periods of suppression before reaching the canopy.
Article
Statistical approaches to overdispersion, correlated errors, shrinkage estimation, and smoothing of regression relationships may be encompassed within the framework of the generalized linear mixed model (GLMM). Given an unobserved vector of random effects, observations are assumed to be conditionally independent with means that depend on the linear predictor through a specified link function and conditional variances that are specified by a variance function, known prior weights and a scale factor. The random effects are assumed to be normally distributed with mean zero and dispersion matrix depending on unknown variance components. For problems involving time series, spatial aggregation and smoothing, the dispersion may be specified in terms of a rank deficient inverse covariance matrix. Approximation of the marginal quasi-likelihood using Laplace's method leads eventually to estimating equations based on penalized quasilikelihood or PQL for the mean parameters and pseudo-likelihood for the variances. Implementation involves repeated calls to normal theory procedures for REML estimation in variance components problems. By means of informal mathematical arguments, simulations and a series of worked examples, we conclude that PQL is of practical value for approximate inference on parameters and realizations of random effects in the hierarchical model. The applications cover overdispersion in binomial proportions of seed germination; longitudinal analysis of attack rates in epilepsy patients; smoothing of birth cohort effects in an age-cohort model of breast cancer incidence; evaluation of curvature of birth cohort effects in a case-control study of childhood cancer and obstetric radiation; spatial aggregation of lip cancer rates in Scottish counties; and the success of salamander matings in a complicated experiment involving crossing of male and female effects. PQL tends to underestimate somewhat the variance components and (in absolute value) fixed effects when applied to clustered binary data, but the situation improves rapidly for binomial observations having denominators greater than one.
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Our study aimed to identify and explore the main factors that influence tree recruitment of multiple species at a regional scale across peninsular Spain, an understanding of which is essential for predicting future forest species composition in the face of ongoing environmental change. The study focused on the dynamics of the key transition phase from saplings to adult trees. The forests of peninsular Spain. We used the extensive network of plots sampled in two consecutive Spanish national forest inventories (> 30,000 plots) to identify the factors that determine regeneration patterns of the 10 most abundant forest species of Spain at relatively large temporal (c. 10 years) and spatial scales (across Spain): five coniferous species of Pinus (pines) and five broadleaved species of the genera Fagus and Quercus. We fitted separate generalized linear models for the pine species and the broadleaved species to assess the response of sapling abundance and ingrowth rate to the spatial variability of climate (temperature, water availability and recent warming), forest structure (tree density, understorey and overstorey canopy cover, and basal area change) and disturbances (previous forest logging, wildfires and grazing). Mean sapling abundance was four times higher for broadleaved species than for pines, while mean annual ingrowth was twice as high. Sapling abundance and ingrowth rate were mainly determined by stand structure, both in pines and broadleaved trees. The direct effects of disturbances and climate were comparatively smaller, and there was no detectable effect of recent warming. The higher values of ingrowth rate of broadleaved species can be explained by their ability to maintain a higher sapling bank due to their greater shade tolerance. This differential response of pines and broadleaved species to canopy closure suggests a probable increase in broadleaved species at the expense of pines. This transition could occur earlier in stands with faster canopy closure dynamics. Spatially explicit, mixed-species demographic models incorporating both the ingrowth and the tree mortality components are needed for predicting the composition of future forests.
Chapter
In this chapter, we show that Mediterranean forests can be considered complex adaptive systems and thus be understood and managed according to the principles of complexity science. First, we examine the profound impact of the long history of human use on Mediterranean forest ecosystems and how this human impact has created a unique set of conditions not commonly found elsewhere in the world. Then we define management strategies that increase the resilience and adaptive capacity of Mediterranean forests facing rapidly changing conditions.
Article
Summary 1. Tree recruitment in Mediterranean ecosystems is strongly limited at the seedling stage by drought. Increasing evidence shows the critical positive role of the canopy nurse effect on seedling survival which results from direct and indirect, positive and negative, interactions between species. 2. Most studies, however, have only focused on the effects of tree canopy on water and light, ignoring other critical factors affecting seedling regeneration, such as canopy effects on high temperatures and the competing herb biomass. 3. Here, we evaluate how tree canopy cover and removal of herbs affect the survival and growth of seedlings of two dominant Mediterranean Quercus species during a three-year study. We use an integrated model that combines several data sets to quantify and predict regeneration dynamics along environmental gradients of soil moisture, temperature and light. 4. Low soil moisture, increased soil temperature and herb biomass negatively affected seedling survival of both Quercus species. Seedling growth was positively associated with increasing soil moisture and light. 5. Although tree canopy cover directly facilitated seedling survival in both Quercus species, it also negatively affected herb biomass and thus indirectly facilitated the survival of Q. suber, but not of Q. ilex seedlings at low levels of soil moisture. 6. Overall, tree canopies increased seedling survival but not growth during the establishment phase, mainly by ameliorating the effects of low soil moisture and high temperatures. Tree canopy indirectly facilitated survival of Quercus suber seedlings by negatively affecting the competing herb layer. 7. Synthesis and applications. To improve tree recruitment and conserve Mediterranean Quercus woodlands, the removal of herbs should be integrated into management plans for dry habitats. Interactions between abiotic and biotic factors may also effect the regeneration of these tree species. In particular, a healthy tree canopy will become important for providing conditions to facilitate seedling establishment if these habitats become drier and warmer, as predicted by some climate change scenarios.
Article
Aim There is increasing concern regarding sustainable management and restoration of planted forests, particularly in the Mediterranean Basin where pine species have been widely used. The aim of this study was to analyse the environmental and structural characteristics of Mediterranean planted pine forests in relation to natural pine forests. Specifically, we assessed recruitment and woody species richness along climatic, structural and perturbation gradients to aid in developing restoration guidelines.
Article
A precise knowledge of forest demographic gradients in the Mediterranean area is essential to assess future impacts of climate change and extreme drought events. Here we studied the geographical patterns of forest demography variables (tree recruitment, growth and mortality) of the main species in Spain and assessed their multiple ecological drivers (climate, topography, soil, forest stand attributes and tree-specific traits) as well as the geographical variability of their effects and interactions. Quantile modeling analyses allowed a synthetic description of the gradients of multiple covariates influencing forest demography in this area. These multivariate effect gradients showed significantly stronger interactions at the extremes of the rainfall gradient. Remarkably, in all demographic variables, qualitatively different levels of effects and interactions were observed across tree-size classes. In addition, significant differences in demographic responses and effect gradients were also evident between the dominant genus Quercus and Pinus. Quercus species presented significantly higher percentage of plots colonized by new recruits, whereas in Pinus recruitment limitation was significantly higher. Contrasting positive and negative growth responses to temperature were also observed in Quercus and Pinus, respectively. Overall, our results synthesize forest demographic responses across climatic gradients in Spain, and unveil the interactions between driving factors operating in the drier and wetter edges.
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Plant colonization studies usually address density-dependent processes in the narrow sense of recruitment constraints due to negative density-dependent seed and seedling mortality. However, complex density-dependent effects may be involved in additional stages of the recruitment process. We hypothesized that seed arrival and seedling establishment are influenced by density dependence acting at small scales at the site of colonization, and at larger scales as a function of the colonizing species' landscape abundance. These hypotheses were tested in a study of colonization of pine forests by oaks in a heterogeneous Mediterranean landscape. Maximum-likelihood models show that density effects switch from positive to negative along the range of landscape-scale oak seed source abundance. Contrary to expectations, high seed source densities limited oak recruitment, suggesting a landscape-scale Janzen-Connell effect. We propose a range of mechanisms that generate positive or negative density dependence during colonization, resulting in nonlinear density-dependent feedbacks that can generate unexpected colonization patterns.
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Changes in climate may reduce the success of natural regeneration and hence require adjustments to silvicultural practices. Special attention is required for species such as Spanish black pine (Pinus nigra Arn. ssp salzmannii) because of impediments to achieve successful natural regeneration. Spanish black pine seed germination was studied under field conditions at a normal altitude for the species and at the upper altitude limit of its distribution in the Cuenca Mountains (southeastern Spain). The aim was to assess the effect of location, overstory density, soil treatment, sowing date and climatic characteristics on regeneration success. ANOVA results indicated a significant interaction of location by overstory density on germination rates. A logistic model containing the temperature variable TemminAc, the light radiation effect (Rad variable) and the interaction term between soil treatment (Soil Treat) and radiation (Soil Treat×Rad) correctly predicted the germination success in 94% of cases. Moreover, two Poisson regression models (one for each experimental site) showed that the number of germinated seeds depends on stand basal area (G), soil treatment (Soil Treat) and sowing date (Seed Season). Conservation management could increase initial seedling recruitment by promoting soil preparation and higher basal area levels. The populations at higher altitudes are particularly endangered due to the unfavourable environmental conditions for the development of this species, which seriously affect seed rain density and germination rates.
Article
A two-level multifactor experimental approach was used to compare seed germination and seedling performance of two Mediterranean tree species: the early successional Aleppo pine (Pinus halepensis Mill.) and the late successional holm oak (Quercus ilex L.). In a first experiment germination rate was evaluated under the combined effects of shade, nitrogen availability, and pine or holm oak leaf litter. In a second experiment we tested for the effects of shade, nutrient availability, and litter type on seedling survival, growth and biomass allocation. Holm oak showed higher germination rates under shaded than under unshaded conditions, while Aleppo pine showed no differences between shaded and unshaded conditions. Nitrogen availability and litter type had no significant effect on germination of either species. Both species showed increased RGR, but also higher mortality rates, when grown in an enriched nutrient environment. While Aleppo pine showed no differences in RGR and mortality rate under different shading levels, RGR decreased and mortality increased for holm oak in full light. Increased radiation decreased LAR, SLA and height:diameter ratio, and increased RWR and R/S in both species, although Aleppo pine showed more pronounced changes. Unlike Aleppo pine, holm oak responded to increased nutrient availability by decreasing R/S and increasing LAR. From these results, no seed-seedling conflicts were found in either species, but a trade-off does seem to exist for holm oak between biomass allocation traits deployed in response to increased nutrient availability and radiation. Aleppo pine outperformed holm oak under most environmental conditions tested and showed a wider regeneration niche.
Article
This paper analyses changes in plant cover in a valley of the Spanish Central Pyrenees during the second half of the 20th century. The role played by the extensification of farming activities in this process has been examined. The method used was based on evolutionary mapping of the plant cover, application of landscape indices and spatial analysis of livestock pressure. Throughout the century the stocking rate in the valley has decreased dramatically and sheep have been replaced by cattle. Most crop areas have been abandoned, and these areas have gone through a revegetation process (involving both natural vegetation and forest plantation). Nowadays the forest is the most abundant element. The study shows that both agricultural set-aside and the stocking rate decrease have led to a more natural landscape. Finally, the positive and negative effects of extensification and the changes in vegetation patterns associated with it are debated.
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Recent, rapid and often underestimated landscape changes have occurred over large areas in Mediterranean Europe. They are the result of major rural depopulation. Old photographs of landscapes taken at the beginning of the twentieth century (i.e. old postcards) and present-day photographs taken at the same places were compared in a 2500-km2 area of southern France. Vegetation changes were analysed using transition matrices. During the 80-year study period, land uses and vegetation changed dramatically. Woodland cover and tree height increased; but in contrast, the extent of cropped lands and rangelands decreased. Forest spread was heterogeneous, depending on initial composition of the vegetation, and locally dominant ecological and socio-economic conditions. Our data show that a Mediterranean forest can re-establish under humid climatic conditions and spread within a century, despite severe prior exploitation over several decades. These dramatic changes are liable to have biological and ecological consequences (e.g. spread of woodland species, threat against open habitat species, fire regime modification, deterioration in water resources), some of them being already perceptible.
Article
Tree replacement in gaps was studied in old-growth mesic forest stands in western Pennsylvania, Ohio, and the southern Appalachian Mountains. Predictions of future overstory composition, based on sapling composition in small gaps (average 200 m^2), were compared to current canopy composition. Both Markov analyses and simple average sapling composition of gaps support the hypotheses that regeneration in small gaps was sufficient to perpetuate the current canopy species composition of the stands studied. In some cases the saplings most likely to replace a dead canopy tree were of the same species. In other cases, especially low-diversity beech-sugar maple stands, each species seemed to enhance significantly the success of the other species.
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This second edition contains ten chapters dealing with oak ( Quercus )-related literature published since the first edition. Included are the relatively new subject areas of forest biomass use for fuel, the importance of carbon sequestration by forests, and how climate change is expected to affect the distribution of oaks and associated tree species. The chapters are grouped into three parts. The first part contains three chapters covering the ecological characteristics and distribution of oak species, the various kinds of oak forests in the USA and how they have been classified, and their history of human use. The second part comprises three chapters covering site productivity and stand development. An understanding of the productive capacity of oak forests is central to a broad spectrum of issues related to their management and potentialities, not only for timber but also for wildlife and other values, including carbon sequestration. The third part comprises four chapters on silvicultural methods and the growth and yield of oak forests.
Article
a b s t r a c t Aleppo pine is the most widespread pine species around the Mediterranean Basin. Its post-fire recruit-ment has been studied in depth, but regeneration of mature stands in fire-free conditions has received considerably less attention. This study examines the impact of different site preparation treatments on pine recruitment using three experimental mature stands along a gradient of site fertility in southeastern France. The stands were partially felled and subjected to the following treatments replicated four times on each site: mechanical chopping (all sites), chopping followed by single soil scarification (all sites) or double scarification (2 sites), controlled fire of low intensity (2 sites) or of high intensity (1 site) and con-trol (all sites). In addition, the influence of slash, either left on the soil or removed before treatments, was tested for the single scarification treatment on two of the sites. Pine regeneration was counted and soil cover conditions described at different time intervals: 1–6 years after the end of the treatments for two sites and 1–16 years for one site. Seedling dimensions were determined during the last count. Mean seed-ling densities after 6–9 years (0.57–1.06 pines/m 2) were comparable to those found in post-fire condi-tions, although with a narrower range. Pine density was negligible in the control, while chopping followed by a single soil scarification emerged as the most favourable treatment tested in the three sites on seedling density (0.74–1.54 pines/m 2 after 6–9 years) and seedling growth. For this treatment, the amount of slash had a contrasting influence on pine density according to site conditions. Double scarifi-cation did not affect pine density. Controlled high intensity fire, due to slash presence, was very favour-able for pine regeneration (2.35 pines/m 2), although this treatment was only tested at one site. Lastly, we found low pine densities in the chopping and low-intensity controlled fire treatments (0.20–0.56 pines/ m 2). Variation in herb cover was a major factor influencing pine recruitment. This study emphasises the need for adapted site preparation treatments to regenerate mature pine stands in southern Europe. Ó 2012 Elsevier B.V. All rights reserved.
Article
Questions: What is the current distribution of pine and oak species along environmental gradients in southern Spain? Do pine and oak regeneration niches differ from the environmental niches of adults? Is oak species regeneration favored under the canopy of pine forests? Location: Forest areas of Andalusia (87 600 km2, southern Spain). Methods: We compiled extensive forest inventory data to explore differences in abundance (basal area, m2.ha-1), patterns of adults (dbh > 7.4 cm) and regeneration (dbh<7.4 cm) of five pine and five oak species. Canonical correspondence analysis (CCA) and generalized linear models were applied to explore species–environment relationships along climatic, edaphic, topographic and fire-frequency gradients. Results: Both pines and oaks segregated along complex environmental gradients, with pines generally dominating in more severe (colder and drier) environments, while oaks dominated in milder, wetter winter areas. In 40-55% of mature pine stands there was a lack of regeneration in the understorey, while in two oak species (Q. suber and Q. canariensis) 70% of stands did not show regeneration. Pine recruits were found at a higher frequency and abundance under the canopy of their congeners, whereas some oaks (Q. ilex) had greater regeneration under mixed pine–oak canopies. Conclusions: Climatic limitations and soil properties partly explained the regional distribution of pines and oaks. We found evidence for an upward shift of Q. ilex recruits towards areas with colder conditions in pine forests, which could be explained by a possible facilitative effect of the pine canopy on seedling establishment.
Article
In southern France, the natural invasion by Quercus humilis of calcareous grassland takes place in a mosaic of herbaceous and scrubby patches. We hypothesized that the presence of the shrubs Buxus sempervirens and Juniperus communis alter the rate and the pathway of the succession by facilitating the regeneration of Q. humilis. To infer the process of facilitation at a large scale, the spatial distribution of Q. humilis was studied in relation to acorn sources and the type of plant cover in grazed and ungrazed sites.Abundant recruitment up to 80 m from the wood margins and from isolated oak trees in grassland shows that acorns are dispersed effectively. At the three study sites, the density of Q. humilis individuals was higher under shrubs than in grassland, suggesting that facilitation may occur. This density difference was much higher in the grazed sites than in the ungrazed site. Moreover, before grazing by livestock, the distribution of first-year seedlings is independent of vegetation cover. Thus, shrubs improve Q. humilis regeneration by protecting individuals from grazing. The high density of individuals at the northern edge of shrubs suggests that a second facilitation mechanism may exist, probably related to improved germination conditions. Facilitation by shrubs appears to be very important for Q. humilis dynamics.
Article
Lack of information on ecological characteristics of species across different continents hinders development of general world-scale quantitative vegetation dynamic models. We constructed common scales of shade, drought, and waterlogging tolerance for 806 North American, European/West Asian, and East Asian temperate shrubs and trees representing about 40% of the extant natural Northern Hemisphere species pool. These scales were used to test the hypotheses that shade tolerance is negatively related to drought and waterlogging tolerances, and that these correlations vary among continents and plant functional types. We observed significant negative correlations among shade and drought tolerance rankings for all data pooled, and separately for every continent and plant functional type, except for evergreen angiosperms. Another significant trade-off was found for drought and waterlogging tolerance for all continents, and for evergreen and deciduous angiosperms, but not for gymnosperms. For all data pooled, for Europe and East Asia, and for evergreen and deciduous angiosperms, shade tolerance was also negatively associated with waterlogging tolerance. Quantile regressions revealed that the negative relationship between shade and drought tolerance was significant for species growing in deep to moderate shade and that the negative relationship between shade and waterlogging tolerance was significant for species growing in moderate shade to high light, explaining why all relationships between different tolerances were negative according to general regression analyses. Phylogenetic signal in the tolerance to any one of the three environmental factors studied was significant but low, with only 21–24% of cladogram nodes exhibiting significant conservatism. The inverse relationships between different tolerances were significant in phylogenetically independent analyses both for the overall pool of species and for two multispecies genera (Pinus and Quercus) for which reliable molecular phylogenies were available. Only 2.6–10.3% of the species were relatively tolerant to two environmental stresses simultaneously (tolerance value !3), and only three species were tolerant to all three stresses, supporting the existence of functional trade-offs in adjusting to multiple environmental limitations. These trade-offs represent a constraint for niche differentiation, reducing the diversity of plant responses to the many combinations of irradiance and water supply that are found in natural ecosystems.
Article
Land use changes and shifts in disturbance regimes (e.g. wildfires) are recognized worldwide as two of the major drivers of the current global change in terrestrial ecosystems. We expect that, in areas with large-scale land use changes, legacies from previous land uses persist and affect current ecosystem responses to climate-associated disturbances like fire. This study analyses whether post-fire vegetation dynamics may differ according to specific historical land use histories in a Mediterranean forest land-scape of about 60,000 ha that was burnt by extensive fires. For that, we assessed land use history of the whole area through the second half of the XXth century, and evaluated the post-fire regeneration suc-cess in terms of: (i) forest cover and (ii) tree species composition (biotic-dispersed, resprouter species, Quercus spp. vs. wind-dispersed species with or without fire-resistant seed bank, Pinus spp.). Results showed that stable forest areas exhibited a higher post-fire recovery than younger forests. Furthermore, the longer since crop abandonment translates into a faster post-fire recovery. Results highlight that to anticipate the impacts of disturbances on ecosystems, historical land trajectories should be taken into account.
Article
European Mediterranean landscapes have undergone changes in structure in recent years as a result of widespread agricultural land abandonment and cessation of silvicultural regimes. Studies concerning the regeneration dynamics of dominant forest species have become critical to the prediction of future landscape trends in these changing forest stands. Quercus ilex (holm oak) and Q. pubescens (downy oak) are considered to be the terminal point of secondary succession in extensive areas of the Mediterranean region. Recent studies, however, have suggested the existence of recruitment bottlenecks in oak genet populations as a result of current management regimes. In this study, we present evidence of the successful establishment of Q. ilex and Q. pubescens in Pinus halepensis (Aleppo pine) woodlands. We investigate the distribution patterns and spatial relationships among oak recruits and resident pines. Established P. halepensis is randomly distributed throughout the study area. Oak seedlings are positively associated with pine trees, suggesting that P. halepensis individuals provide safe sites for oak genet recruitment. We show that spatial patterns of recruitment are in agreement with the general model of spatial segregation described for other Mediterranean plant communities, with seeder species colonizing large openings after disturbance, followed by a more aggregated recruitment of resprouter species.