Flammability of some companion species in cork oak (<Emphasis Type="Italic">Quercus suber</Emphasis> L.) forests

Key message

The high flammability of some companion species in Quercus suber forests, estimated in laboratory tests, could potentially generate an increase in fire vulnerability and in fire risk.

Context

Recurrent wildfire is one of the main causes of forest degradation, especially in the Mediterranean region. Increased fire frequency and severity due to global change could reduce the natural resilience of cork oak to wildfire in the future. Hence, it is important to evaluate the flammability of companion species in cork oak forests in the particularly dry bioclimatic conditions of North Africa.

Aims

This study aimed to assess and compare flammability parameters at laboratory scale among ten companion frequent species in cork oak forests.

Methods

Fuel samples were collected in a cork oak (Quercus suber L) forest in the southern part of the mountains of Tlemcen (Western Algeria). A series of flammability tests were carried out using a Mass Loss Calorimeter device (FTT ®). A cluster analysis to classify flammability of the selected species was conducted using the K-means algorithm.

Results

The results revealed differences in the four flammability parameters (ignitability, sustainability, combustibility and consumability), in both fresh and dried fine fuel samples from Quercus suber, Pinus halepensis, Quercus ilex, Quercus faginea, Erica arborea, Arbutus unedo, Pistacia lentiscus, Calicotome spinosa, Juniperus oxycedrus and Tetraclinis articulata. Application of the K-means clustering algorithm showed that C. spinosa, T. articulata, J. oxycedrus and P. halepensis are highly flammable because of their high combustibility and sustainability.

Conclusion

The findings identify species that could potentially increase the vulnerability of cork oak forests to forest fires.

     

Agrobacterium-mediated transformation of cork oak (<Emphasis Type="Italic">Quercus suber</Emphasis> L.) somatic embryos

A simple method is presented for the unreported genetic transformation of cork oak (Quercus suber L.). Pro-embryo masses (PEMs) were induced on immature zygotic embryos applied to medium supplemented with 2.3 M 2,4-dichlorophenoxyacetic acid. The established PEMs were inoculated with Agrobacterium tumefaciens LBA4404/p35S GUS INT/pCAMBIA 1301 strain. Transformants were selected on hygromycin (HYG) 94 M-supplemented medium. Viable embryos constituted 13% of those selected on HYG during 4 months. Expression of -glucuronidase at 4 months following co-cultivation confirmed transformation in 5.8% embryos selected on HYG. This method forms a basis for genetic transformation of cork oak somatic embryos.     

Base-age invariant cork growth model for Spanish cork oak (<Emphasis Type="Italic">Quercus suber</Emphasis> L.) forests

A cork growth model for Spanish cork oak forests was developed using data from 432 cork samples. Ten dynamic equations derived from the generalised algebraic difference approach (GADA) were considered for analysis, and both numerical and graphical methods were used to compare alternative models. All of the equations are base-age invariant and directly estimate accumulated cork thickness in complete years at any moment of the cork rotation. The fittings were done using the nested iterative method of the stochastic regression approach. The GADA formulation derived by Krumland and Eng (2005) from the Richards model by considering a ₁ and a ₃ as related to site productivity was finally selected. The cork growth model developed in this study allows the total accumulated cork thickness in complete years to be estimated in the fourth year of cork rotation with close to 80% reliability for any debarking period, increasing to nearly 90% from the eighth year onwards.     

Nutrients return from leaves and litterfall in a mediterranean cork oak (<Emphasis Type="Italic">Quercus suber</Emphasis> L.) forest in southwestern Spain

The knowledge of the cycle of nutrients is fundamental for the correct comprehension of the tree–soil relationship and for an adequate forest management. In order to analyse the nutrients return from leaves and litterfall in a Mediterranean cork oak forest in southwestern Spain, 12 trees were randomly selected and litterfall collected for 2 years. Samples were taken monthly and separated in different fractions (leaves, twigs, catkins, acorns and miscellaneous), then leaves nutrients were analyzed. Simultaneously, we analyzed the nutrient content of living leaves from the same trees in each season during 1 year. The analyzed nutrients were C, N, P, K, Ca, Mg, Fe, Mn, S, Cu, Zn and Mo. Annual patterns of each nutrient in fallen leaves were characterized and compared with seasonal values of these nutrients in living leaves. Leaves fall has two annual maximum, first and most important in spring around April coinciding with renewal of foliar cover and second around October. Main concentration patterns of N, P and K are related with phenological patterns, in consequence minimum concentration in leaves fall were obtained in periods of growing and maximum litterfall. Concentrations of Ca, Fe and Mn increase with the age of the leaves and maximum concentrations were obtained before periods of maximum litterfall while concentrations of Cu, Mo and Mg stay stable. Seasonal analysis of nutrients in living leaves collected from the same trees in four different periods of the year allowed to corroborate the patterns of leaves fall and the probable osmotic function of K.     

Cork oak (<Emphasis Type="Italic">Quercus suber</Emphasis> L.) wood growth and vessel characteristics variations in relation to climate and cork harvesting

Variations in tree ring growth of Quercus suber L. were analysed using dendrochronological techniques on cork oak discs from trees harvested in the cork producing region of Alentejo, Portugal. A tree-ring chronology containing a strong common signal and covering the period from 1970 to 1995 was build for ca. 30-year-old cork oaks never submitted to cork harvesting using 14 trees that crossdated satisfactorily out of 30 sampled trees. The tree ring indices correlated positively with September temperature (r = 0.48, P < 0.05) and very strongly with precipitation totals from previous October until current February (r = 0.82, P < 0.001) showing that the water stored in the soil during the autumn and winter months prior to the growing season has a primordial effect on the growth of the given season. The effects of cork harvesting were analysed by comparing mean ring width, mean annual vessel area, vessel density (n°vessels/mm²), and vessel coverage (percentage of transverse surface occupied by vessels) between three mature cork oak trees and three young trees, for the period from 1987 to 1996, corresponding to the growth between two consecutive cork removals in the case of mature trees. In 1988, 1989 and 1996 (corresponding to the first and second years after cork removal, and 1996 to a year of cork removal), the ratios between ring widths of young versus mature trees was twice that for the rest of the period. However, an effect of cork removal indicated by eventual alterations in vessel size and distribution in the wood rings corresponding to the years 1988, 1989 and 1996 in the mature cork oaks was not observed.     

Radial variation of vessel size and distribution in cork oak wood (<Emphasis Type="Italic">Quercus suber</Emphasis> L.)

Quercus suber L. is an important species producing cork whose wood characteristics have not been investigated a lot. Cork oak wood vessels are a striking feature and the most abundant wood tissue largely influencing density and permeability. Vessel size and distribution were studied in approximately 40 year-old and never debarked cork oaks by continuously measuring along the radial direction in the transverse section of wood discs taken at 1.3 m of height using image analysis techniques. The vessel size increases with age from 7660 ± 2286 to 21136 ± 6119 μm², the conductive area from 5.4 ± 2.2 to 11.6 ± 3.9%, and the vessel density remains approximately constant between 5.2 ± 1.5 and 7.3 ± 3.5 vessels/mm². In comparison with ring-porous and some evergreen oaks, cork oaks show a similar conductive area but smaller vessels. Vessel architecture is known to play an important role on oaks tolerance to hydric stress, and these cork oak trees were growing under very harsh edaphoclimatic conditions, not tolerated by other oaks. The well-developed and deep root system allowing access to constant water supply may contribute to the cork oak’s relatively high conductive area.     

<Emphasis Type="Italic">Quercus suber</Emphasis> forest and <Emphasis Type="Italic">Pinus</Emphasis> plantations show different post-fire resilience in Mediterranean north-western Africa

Key message

In the African rim of the Western Mediterranean Basin, cork oak forests and pine plantations coexist. Under similar fire regimes, cork oak forest is more resilient in terms of habitat structure (canopy, understory, and complexity of vegetation strata) than pine plantation. By contrast, both woodland types show similar resilience in plant species composition. Resilience in habitat structure varies between the two woodland types because of the resprouting and seeding strategies of cork oak and pine species, respectively. These differences can be relevant for the conservation of biodiversity of forested ecosystems in a future scenario of increased fire frequency and scale in the Mediterranean basin.

Context

Wildfires have major impacts on ecosystems globally. In fire-prone regions, plant species have developed adaptive traits (resprouting and seeding) to survive and persist due to long evolutionary coexistence with fire. In the African rim of the Western Mediterranean Basin, cork oak forest and pine plantation are the most frequently burnt woodlands. Both species have different strategies to respond fire: cork oak is a resprouter while pines are mostly seeders.

Aims

We have examined the hypothesis that pine plantations are less resilient in habitat structure (canopy, understory, diversity of vegetation strata) and plant composition than cork oak woodlands.

Methods

The habitat structure and plant species composition were measured in 30 burnt and 30 unburnt 700-m transects at 12 burnt sites from north-western Africa, where the two forest types can coexist. Habitat structure and plant species composition were compared between burnt and unburnt transects from cork oak and pine plantation woodlands with generalized linear mixed models and general linear models.

Results

The results showed significant interaction effect of fire and forest type, since cork oak forest was more resilient to fire than was pine plantation in habitat structure. By contrast, both forest types were resilient to fire in the composition of the plant communities, i.e., plant composition prior to fire did not change afterwards.

Conclusion

The higher structural resilience of cork oak forest compared to pine plantation is related to the resprouting and seeding strategies, respectively, of the dominant tree species. Differences in the responses to fire need to be considered in conservation planning for the maintenance of the Mediterranean biodiversity in a future scenario of changes in fire regime.

     

Spatial pattern of beech (<Emphasis Type="Italic">Fagus sylvatica</Emphasis> L.) and oak (<Emphasis Type="Italic">Quercus pubescens</Emphasis> Mill.) seedlings in natural pine (<Emphasis Type="Italic">Pinus sylvestris</Emphasis> L.) woodlands

Spatial pattern of recruitment is an important factor influencing population dynamics of plant communities. The spatial pattern is determined by seed dispersal and by the spatial variability of germination and initial survival. In the process of forest expansion following farmland abandonment, mid- and late-successional species are often dispersed in pioneer forests by birds. This could result in an aggregated spatial pattern of seeds that could influence the dynamics of these species in mixed pioneer forests. In the sub-Mediterranean area, mid- and late-successional species such as Quercus pubescens (downy oak) and Fagus sylvatica (European beech) are expected to replace pioneer Pinus species. Using a point sampling method we demonstrated that beech and oak seedlings (height <2 m) have a clumped distribution in the understorey of pine. This could result from an aggregated dispersal by jays (dispersal effect) or from preferential recruitment in particular habitats (habitat effect). To test these hypotheses we proposed a statistical analysis of spatial patterns of regeneration of beech and oak. Ground cover variables (i.e. cover by rock outcrops, herbs, box shrubs, mosses or pine) did not differ significantly around seedlings as compared with random sample plots. Likewise, clumped seedlings had growth similar to isolated seedlings, thus refuting the hypothesis of preferential location in the most favourable microsites. Aggregated dispersal seems to be involved in the process of regeneration. Since beech and oak seedlings have contrasting ecological demands, we discuss the implication of this pattern for the replacement dynamics of pine by these species.     

Artificial regeneration with <Emphasis Type="BoldItalic">Quercus ilex</Emphasis> L. and <Emphasis Type="BoldItalic">Quercus suber</Emphasis> L. by direct seeding and planting in southern Spain

 Introduction   

The limited ability of Quercus species to regenerate naturally in Mediterranean forests has led to the development of various artificial regeneration methods; however, there is no general consensus as to what specific method is the best one for this purpose.     

Leaf-level responses to light in two co-occurring <Emphasis Type="Italic">Quercus</Emphasis> (<Emphasis Type="Italic">Quercus ilex</Emphasis> and <Emphasis Type="Italic">Quercus suber</Emphasis>): leaf structure,chemical composition and photosynthesis

We studied morphological, biochemical and physiological leaf acclimation to incident Photon-Photosynthetic-Flux-Density (PPFD) in Quercus ilex (holm oak) and Quercus suber (cork oak) at Mediterranean evergreen oak woodlands of southern Portugal. Specific leaf area (SLA) decreased exponentially with increasing PPFD in both species. Q. ilex had lower SLA values than Q. suber. Leaf nitrogen, cellulose and lignin concentration (leaf area-based) scaled positively with PPFD. Maximum rate of carboxylation (V_cmax), capacity for maximum photosynthetic electron transport (J_max), rate of triose-P utilization (V_TPU) and the rate of nonphotorespiratory light respiration (Rd) were also positively correlated with PPFD in both Quercus species, when expressed in leaf area but not on leaf mass basis. Q suber showed to have higher photosynthetic potential (V_cmax, J_max^m and V_TPU^m) and a higher nitrogen efficient nitrogen use than Q.ilex. Leaf chlorophyll concentration increased with decreasing PPFD, improving apparent quantum use efficiency (Φ) in both Quercus species. We concluded that, in Q.ilex and Q.suber, leaf structural plasticity is a stronger determinant for leaf acclimation to PPFD than biochemical and physiological plasticity.     

Site quality of pedunculate oak (<Emphasis Type="Italic">Quercus robur</Emphasis> L.) stands in Galicia (northwest Spain)

Accurate estimates of forest productivity are required for sustainable forest management. Height of dominant and codominant trees at a reference age is often used as a measure of site productivity. Eight algebraic difference equations based on the models proposed by Sloboda, Bertalanffy-Richards, Korf, Hossfeld, and McDill-Amateis were tested on the transformed, longitudinal data structure that considers all possible growth intervals. Autocorrelation was modelled by expanding the error term as an autoregressive process according to this data structure. Generalized nonlinear least squares methods were used for model fitting. Several numerical and graphic analyses were used to compare the different candidate models. The relative error in dominant height prediction was used to select 60 years as the best reference age. An algebraic difference equation based on the model proposed by Korf provided the best compromise between biological and statistical aspects and produced the most adequate site index curves. This model is therefore recommended for height growth prediction and site classification of pedunculate oak stands in Galicia. The model is polymorphic and base-age invariant, having only one asymptote. Predictions of height for age intervals between t₁ and t₂ of more than 15 years should be considered with caution because of the associated critical error.     

Influence of forest structure and environmental variables on recruit survival and performance of two Mediterranean tree species (<Emphasis Type="Italic">Quercus faginea</Emphasis> L. and <Emphasis Type="Italic">Q. suber</Emphasis> Lam.)

We investigated the regeneration requirements of the two dominant tree species in a mixed-oak forest of SW Portugal: Quercus suber (cork oak, evergreen) and Q. faginea (Portuguese oak, winter-deciduous). We hypothesized that (1) recruits of different oak species are differentially influenced by soil and overstory variables and (2) different factors explain the recruitment occurrence and performance of the same species. We sampled the recruits’ height and diameter, and several environmental and forest structure variables of their microsites. Both recruitment occurrence and performance were modeled using generalized linear models. Our final models predicted the probability of occurrence of recruits of Q. faginea and Q. suber with 74 and 82% of accuracy, respectively, and explained about 50% of the variance of their recruitment performance. The recruits of Q. faginea tended to occur in microsites with higher canopy height, canopy density and litter cover, and closer to both conspecific and heterospecific adults, while the opposite was true for recruits of Q. suber. The performance of recruits of Q. suber was favored by the higher litter cover (a good surrogate for N and P availability), but negatively affected by the higher litter depth. We concluded the following: (1) there were significantly different regeneration niches for each species (Q. faginea and Q. suber); (2) the factors explaining the recruitment occurrence differed from those explaining the recruitment performance; (3) the overstory plays a complex and important role in the regeneration process; (4) different variables apparently related with the same environmental factor (e.g. litter cover and litter depth) could affect recruits in an opposite way; (5) sensitive trade-offs must be considered for delineating management actions, since they could favor the regeneration of Q. suber, but, at the same time, negatively affect the regeneration of Q. faginea.     

Introducing the <Emphasis Type="Italic">montado</Emphasis>, the cork and holm oak agroforestry system of Southern Portugal

The Portuguese montado is an agro-silvo pastoral system quite similar to the dehesa in Spain, and covering in Portugal most of the Southern region of the country, Alentejo. The trees in the montado are cork oh holm oak, and the system is mostly acknowledged due to the cork production, but also due to its singular savanna like land cover pattern, its multiple and complementary productions, the support of a diversity of ecosystems services and its biodiversity. The present special issue covers the diversity of components of the montado and of perspectives required to understand and assess the functioning of the system. This Editorial introduces the montado system, describes its extension and similarities to the dehesa in Spain, and addresses the multiple productions and externalities of the montado. It also refers to the several components of the system. It focus on the uniqueness and values of this system, stressing its potential and threats, and the requirements for integrated knowledge production. In the end of the Editorial, each one of the papers that compose the special issue are presented.     

Pathogenic effects of <Emphasis Type="Italic">Xenorhabdus nematophilus</Emphasis> and <Emphasis Type="Italic">Photorhabdus luminescens</Emphasis> (Enterobacteriaceae) against pupae of the Diamondback Moth, <Emphasis Type="Italic">Plutella xylostella</Emphasis> (L.)

The relationship between Xenorhabdus nematophilus and Photorhabdus luminescens, the bacterial symbionts of the entomopathogenic nematodes Steinernema carpocapsae and Heterorhabditis bacteriophora, against the diamondback moth, P. xylostella (L.), pupae showed the pathogenic capability of P. luminescens to be over that of X. nematophilus. They gave 60 and 40?% mortality with LC₅₀ values of 5?×?10⁴ and 5.5?×?10⁵ cells/ml, respectively.The number of bacterial cells influences the rate of killing the pupae of P. xylostella and a direct correlation between mortality of the pupae, percentage of deformed adults and the spraying dose with significant differences were observed. An inverse correlation found between the no. of eggs laid/female, percentage of hatching, adult survival and the spraying dose. These bacterial effects have been attributed to different toxic bacterial enzymes which damage the haemocytes and inhibit activation of the humoral immune system of the insect.     

Influence of water-stress acclimation and <Emphasis Type="Italic">Tuber melanosporum</Emphasis> mycorrhization on <Emphasis Type="Italic">Quercus ilex</Emphasis> seedlings

Mycorrhizal and non-mycorrhizal holm oak (Quercus ilex L.) seedlings inoculated with black truffle (Tuber melanosporum) were grown under nursery conditions and subjected to drought hardening for 4 months in autumn and winter followed by irrigation for 10 days. Leaf water potential and stomatal conductance were monitored during the 4 months of drought. When the test was completed (March), measurements were made for each treatment (inoculated or non-inoculated), and watering regime (watered and water-stressed). Pressure–volume curves, osmotic potential at full turgor, osmotic potential at zero turgor and the tissue modulus of elasticity near full turgor were calculated. Mycorrhizal colonization and growth, and the content of the main mineral nutrients N, P, K, Ca and Mg were measured. Water stress affected plant growth, caused an elastic adjustment of the plant tissues, and decreased the P and K content, and inoculation improved the nitrogen content. Drought acclimation apparently achieved the goal of improving the drought tolerance of holm oak seedlings, without depressing ectomycorrhizal root colonization by T. melanosporum. José Antonio Rodríguez Barreal—Deceased     

Successful <Emphasis Type="Italic">Agrobacterium</Emphasis>-mediated transformation of <Emphasis Type="Italic">Populus tomentosa</Emphasis> with apple <Emphasis Type="Italic">SPDS</Emphasis> gene

The problem of salinized soils has become one of the most serious constraints to agricultural and forest productivity. With the purpose of enhancing salt stress tolerance of Populus tomentosa, we transformed this tree species with spermidine synthase （SPDS） genes derived from an apple by an Agrobacterium-mediated method. Four transgenic clones were confu＇med by PCR and Southern blot analysis. As well, the expression of introduced SPDS genes was analyzed by real-time quantitative PCR.     

Direct and admixture toxicity of diatomaceous earth and monoterpenoids against the storage pests <Emphasis Type="Italic">Callosobruchus</Emphasis><Emphasis Type="Italic">maculatus</Emphasis> (F.) and <Emphasis Type="Italic">Sitophilus oryzae</Emphasis> (L.)

We investigated the effects of two commercial diatomaceous earth based insecticides (DE), Protect-It^® and SilicoSec^®, the nano-structured silica product AL06, developed by the section for Urban Plant Ecophysiology at Humboldt University Berlin, and the monoterpenoids, eugenol, and cinnamaldehyde on two stored product pests, Callosobruchus maculatus and Sitophilus oryzae. Protect-It^® was more effective than SilicoSec^® against C. maculatus while the reverse was true for S. oryzae. Generally C. maculatus was more sensitive towards DE and silica treatment than S. oryzae. Mortality rate of both pest species increased when DE’s were applied to food commodities previously treated with a monoterpenoid. In admixture experiments, the toxicity of SilicoSec^® + cinnamaldehyde (LD₅₀ = 42.73 ppm), SilicoSec^® + eugenol (LD₅₀ = 24.30 ppm), and Protect-It^® + eugenol (LD₅₀ = 2.60 ppm) was increased over DE alone against S. oryzae. Both substances showed a synergistic effect considering their co-toxicity coefficient relative to the LD₅₀-value. In contrast, we could not find any synergistic effects in experiments with C. maculatus. Here only Protect-It^® + cinnamaldehyde (LD₅₀ = 20.84 ppm) showed an additive effect while all other combinations of monoterpenoid and DE indicated antagonistic effects. In addition to contact insecticidal effects both monoterpenoids showed a strong fumigant action. The presented results indicate that the natural product DE has great potential to replace synthetic pesticides commonly used in stored product pest management. Efficacy of DE can be improved by adding certain monoterpenoids against certain insect pests.     

Leaf morphological and genetic differentiation between <Emphasis Type="Italic">Quercus robur</Emphasis> L. and its closest relative,the drought-tolerant <Emphasis Type="Italic">Quercus pedunculiflora</Emphasis> K. Koch

• Introduction  

The study of phenotypic and genetic differentiation between incipient species or species that have recently diverged provides insights into the evolutionary history of species complexes and may contribute to our understanding of how species will evolve in contrasting environmental conditions.