Cork Oak Forests Research Articles

Robust Trend Analysis in Environmental Remote Sensing: A Case Study of Cork Oak Forest Decline

We introduce a novel methodological framework for robust trend analysis (RTA) using remote sensing data to enhance the accuracy and reliability of detecting significant environmental trends. Our approach sequentially integrates the Theil–Sen (TS) slope estimator, the Contextual Mann–Kendall (CMK) test, and the false discovery rate (FDR) control. This comprehensive method addresses common challenges in trend analysis, such as handling small, noisy datasets with outliers and issues related to spatial autocorrelation, cross-correlation, and multiple testing. We applied this RTA workflow to study tree cover trends in Los Alcornocales Natural Park (Southern Spain), Europe’s largest cork oak forest, analysing interannual changes in tree cover from 2000 to 2022 using Terra MODIS MOD44B data. Our results reveal that the TS estimator provides a robust measure of trend direction and magnitude, but its effectiveness is dramatically enhanced when combined with the CMK test. This combination highlights significant trends and effectively corrects for spatial autocorrelation and cross-correlation, ensuring that genuine environmental signals are distinguished from statistical noise. Unlike previous workflows, our approach incorporates the FDR control, which successfully filtered out 29.6% of false discoveries in the case study, resulting in a more stringent assessment of true environmental trends captured by multi-temporal remotely sensed data. In the case study, we found that approximately one-third of the area exhibits significant and statistically robust declines in tree cover, with these declines being geographically clustered. Importantly, these trends correspond with relevant changes in tree cover, emphasising the ability of RTA to detect relevant environmental changes. Overall, our findings underscore the crucial importance of combining these methods, as their synergy is essential for accurately identifying and confirming robust environmental trends. The proposed RTA framework has significant implications for environmental monitoring, modelling, and management.

Monitoring ink disease epidemics in chestnut and cork oak forests in central Italy with remote sensing

Forests provide multiple ecosystem services including water and soil protection, biodiversity conservation, carbon sequestration, and recreation, which are crucial in sustaining human health and wellbeing. Global changes represent a serious threat to Mediterranean forests, and among known impacts, there is the spread of invasive pests and pathogens, often boosted by climate change and human pressure. Remote sensing can provide support to forest health monitoring, which is crucial to contrast degradation and adopt mitigation strategies. Here, different multispectral and SAR data are used to detect the incidence of ink disease driven by Phytophthora cinnamomi in forest sites in central Italy, dominated by chestnut and cork oak respectively. Sentinel 1, Sentinel 2, and PlanetScope data, together with ground information, served as input in Random Forests to model healthy and disease classes in the two sites. The results indicate that healthy and symptomatic trees are clearly distinguished, whereas the discrimination among disease classes of different severity (moderate and severe damage) is less accurate. Crown dimension and sampled spectral regions are a critical factors in the selection of the sensor; better results are obtained for the larger chestnut crowns with Sentinel 2 data. In both sites, the red and near infra-red bands from multispectral data resulted well suited to monitor the spread of the ink disease.

Modeling and spatialization of biomass and carbon stock using unmanned Aerial Vehicle Lidar (Lidar-UAV) metrics and forest inventory in cork oak forest of Maamora

Recently, the concerns about climate change have heightened the need for effective methods for estimating and mapping Biomass and Carbon stock at local, national, continental, and global scales. Reliable Biomass and Carbon stock quantification and spatialization is a challenge, especially in degraded Mediterranean Cork oak forest. To estimate and map Biomass (Btree−Total) and Carbon stock (Cst−total), we explored an improved approach using extracted metrics collected by Lidar-UAV (unmanned aerial vehicles Lidar), combined with forest inventory data. We approach three types of models for data analysis: Simple linear regression, multiple linear regressions, and stepwise multiple linear regression. The best Biomass and Carbon stock model fit is the Stepwise multiple linear regressions, involving the following metrics: maximum elevation, canopy cover and point cloud density and intensity. Our finding provides a quantification and spatialization Biomass and Carbon stock model based on Lidar-UAV metrics in Cork Oak Mediterranen forest and the results confirm the degraded state of Maamora Forest with a Biomass and Carbon stock relatively medium to low.

Natural regeneration of cork oak forests under climate change: a case study in Portugal

The sustainability of forest species is directly related to the success of stand regeneration. Assuring success is particularly critical in stands where perpetuity relies on natural regeneration, as is often the case with cork oak forests. However, 59% of the stand in Portugal have no natural regeneration, and climate change could further worsen the sustainability of the system. The study summarizes the factors that affect the natural regeneration of cork oak (Quercus suber L.) based on current knowledge and presents a case study on a forest in Northeast Portugal, where the natural regeneration of Quercus suber under the effect of climate change have been monitored and analyzed. The present work focuses on the effect of stand density, i.e., tree cover, on the production of acorns, the establishment and survival of seedlings, and the impact of the summer season on seedling mortality. The monitoring was carried out in February, June, September 2022, and January 2023 in two stands with distinct stand canopy cover, when the region was under extreme drought. Data analysis was performed using the analysis of variance for repeated measures and the Mann–Whitney-Wilcoxon test. The study showed that cork oak regeneration is influenced by stand density, which promoted the establishment success and survival of natural regeneration in a period of reduced precipitation, despite possible competition for water resources. The mean number of seedlings differed significantly between the two stands. However, there were no significant differences in the mean number of seedlings throughout the field measurements. Additionally, the percentage of dead seedlings was low even after the summer season (9.5% of the total seedlings) in the denser stand. These results indicate that high canopy cover can have a protective effect for extreme climatic events and should be considered in forestry management to promote regeneration of the cork oak forests.

Insights into haplotype distribution and population dynamics of Biscogniauxia mediterranea in cork oak forests in Tunisia

Biscogniauxia mediterranea is a pathogenic fungus associated with Quercus species causing charcoal canker disease in forests. Its phenotypic and genetic variability has been well studied in Tunisia but little is known about its demographic history. Population dynamic of B. mediterranea was evaluated among 30 isolates from different cork oak forests in Northwestern Tunisia. The genetic structure analyzed with two DNA markers (ITS and tef-1α) showed low genetic differentiation associated with high genetic flow among populations. The polymorphism analysis showed low nucleotide diversity but high haplotypic diversity among and within B. mediterranea populations which could be due to rapid growth from a small ancestral population. This hints at recent population growth after a period of decline likely due to a catastrophic event involving fires and human-induced tree cutting. The resulting genetic diversity within populations primarily arises from the emergence of new haplotypes. Overall, this study provides insights into the polymorphism of B. mediterranea and its potential adaptive strategy.

Post-fire dynamics of recolonization by Cistus plants in the Aleppo pine and Cork oak forests in Bejaia region, central north Algeria

Abstract In August 2021, very intense fires swept through various forested areas in northern Algeria, particularly in Kabylia. The population dynamics of Cistus salviifolius and Cistus monspeliensis were studied along a post-fire successional gradient during the first nine months (September 2021 – May 2022), in two forest communities: the Cork oak (Quercus suber) and the Aleppo pine (Pinus halepensis) forests in the coastal zone of Bejaia province. The Cistus spp. germinations were counted monthly in each forest on 10 plots of 1 m2, evenly distributed along 2 transects of 45 m each. The results showed that the majority of germinations occur in December with 207.5 ± 24.2 seedlings in the Cork oak forest and 171.3 ± 18.0 seedlings in the Aleppo pine forest. Student's t-test does not reveal significant differences for December between the two communities. During the first months following the fire (September and October 2021), germinations were almost absent. The monthly rainfall in September 2021 was estimated at 16.25 mm, an insufficient amount for effective germination. Late germination results in an extension of its period, showed a high rate in January 2022 with 42.2 ± 2.1 and 11.5 ± 0.8 seedlings, respectively, in the Cork oak forest and the pine forest. Beyond January, germination was practically nonexistent in both communities. Therefore, the majority of the seed germination was concentrated in November, December, and January with a unimodal germination pattern.

Assessing Carbon Storage and Biomass in Moroccan Cork Oak Forests

Assessing Carbon Storage and Biomass in Moroccan Cork Oak Forests

Resilience of Bird Communities After Wildfires in Algerian Cork Oak Forests

Resilience of Bird Communities After Wildfires in Algerian Cork Oak Forests

The relationships between modern pollen rain assemblages and vegetation from Sougna Mountain (Rif Mountains-Northern Morocco)

This study discovers the distribution of vegetation in the Sougna Mountain and its modern pollen representation and investigates the relationships with environment and land use variables. A total of 31 moss polsters were collected between 138 and 1364 m above sea-level from wooded to open areas (e.g. cork oak forest, pine reforestation, and shrubland). At each sampling site land use and environment variables were collected. The vegetation in the Sougna Mountain is scattered in a mosaic along three vegetation belts which are displayed in distinct pollen assemblages dominated by the pollen equivalents of the vegetation types (Quercus suber forest, matorrals of Erica spp. and Cistus spp. and Pinus pinaster reforestation). The variation between the variables and pollen data was performed using CCA analysis. The anthropogenic variables such as grazing, and cultivation, contribute to discrimination of the degraded and grazed areas. High precipitation and altitude have a rather good correlation to wooded communities and consequently a good relation to the tree cover. Our results show that the pollen types of Poaceae, Cannabis, Plantago, Nicotiana, Asteraceae, and Urtica are related mostly to human practice and distinguish the disrupted communities. The presence of those taxa in the pollen diagram must be interpreted carefully. A high taxonomic resolution is recommended to prevent the overestimation of the abundance of some taxa. Our results contribute to the advancement of palynological reconstructions of past vegetation and land use in Sougna Mountain and generally in the Rif landscape.

An Integrating Framework for Biomass and Carbon Stock Spatialization and Dynamics Assessment Using Unmanned Aerial Vehicle LiDAR (LiDAR UAV) Data, Landsat Imagery, and Forest Survey Data in the Mediterranean Cork Oak Forest of Maamora

Spatialization of biomass and carbon stocks is essential for a good understanding of the forest stand and its characteristics, especially in degraded Mediterranean cork oak forests. Furthermore, the analysis of biomass and carbon stock changes and dynamics is essential for understanding the carbon cycle, in particular carbon emissions and stocks, in order to make projections, especially in the context of climate change. In this research, we use a multidimensional framework integrating forest survey data, LiDAR UAV data, and extracted vegetation indices from Landsat imagery (NDVI, ARVI, CIG, etc.) to model and spatialize cork oak biomass and carbon stocks on a large scale. For this purpose, we explore the use of univariate and multivariate regression modeling and examine several types of regression, namely, multiple linear regression, stepwise linear regression, random forest regression, simple linear regression, logarithmic regression, and quadratic and cubic regression. The results show that for multivariate regression, stepwise regression gives good results, with R2 equal to 80% and 65% and RMSE equal to 2.59 and 1.52 Mg/ha for biomass and carbon stock, respectively. Random forest regression, chosen as the ML algorithm, gives acceptable results, explaining 80% and 60% of the variation in biomass and carbon stock, respectively, and an RMSE of 2.74 and 1.72 Mg/ha for biomass and carbon stock, respectively. For the univariate regression, the simple linear regression is chosen because it gives satisfactory results, close to those of the quadratic and cubic regressions, but with a simpler equation. The vegetation index chosen is ARVI, which shows good performance indices, close to those of the NDVI and CIG. The assessment of biomass and carbon stock changes in the study area over 35 years (1985–2020) showed a slight increase of less than 10 Mg/ha and a decrease in biomass and carbon stock over a large area.

Do leaf traits affect insect herbivory in a Chinese cork oak forest?

Background: It is widely accepted that certain leaf traits indicating leaf quality play an important role in regulating insect herbivory. Numerous studies have attempted to find a clear relationship between insect herbivory and leaf traits. However, the results are inconsistent. In particular, it is still unclear whether leaf traits of a tree species affect insect herbivory in the field. Methods: We examined the effects of leaf traits including structural defensive traits (specific leaf area), nutritional traits (nitrogen content, water content, and soluble sugar content), and chemical defensive traits (tannin content and carbon content) on variation of insect herbivory among three forest strata (vertical variability) and 18 locations (horizontal variability) in a Chinese cork oak (Quercus variabilis) forest landscape. Results: Vertically, insect herbivory in the low-canopy stratum was significantly higher than in the other strata, but variation of leaf traits among strata had little explanatory power for the vertical pattern of insect herbivory. Horizontally, leaf carbon content had weak negative effects on insect herbivory while leaf tannin content had weak and divergent effects on insect herbivory in different strata. Conclusions: Leaf traits selected in this study have weak effects on insect herbivory in the Chinese cork oak forests we studied. These effects may be masked by other abiotic and biotic factors, but further examination is needed.

Revising the role of belowground fungi in pentachlorophenol pollution management: insights from Tunisian cork oak forests

This summarizes the doctoral thesis “Belowground fungi are key sentinels in forest soils vulnerable to pentachlorophenol pollution: a mechanistic study in Quercus suber forests”. The PhD degree was awarded by ITQB NOVA in March 2018. It comprises a revision of the major issues, objectives, methodology and results, as well as a few possible recommendations for future work on bioremediation of halogenated aromatic pollutants. The working hypothesis of the thesis proposed that soils in Tunisian cork oak forests are likely contaminated with pentachlorophenol (PCP), a persistent organic pollutant (POP). This hypothesis is based on frequent reports of cork contamination with pentachloroanisole (PCA), which is likely formed through microbial conversion of PCP. Furthermore, PCP was considered a suitable model for investigating the role of soil mycobiota in pollution management due to its ability to travel long distances in the atmosphere, partition favourably into the soil layer, and moderate abiotic resistance, besides its life-threatening toxicity. Finally, fungi play a key role in colonizing soil in cork oak forests. These concepts have been applied in the "Preventive and remediation strategies for continuous elimination of polychlorinated phenols from forest soil" project (SfP-NATO 981674, 2006-2011). Overall the major findings of the study include the demonstration of the prevalence of PCP in the soils of Tunisian cork oak forests. Additionally, the study elucidated both the significance and impact of fungal activity in the mitigation and dispersion of PCP.

A review analysis of the degradation of cork oak forests in North Atlantic, Morocco

Like Mediterranean forests, Morocco's forest formations are highly diverse and comprise many species. Among the noble species, the cork oak (Quercus suber L.) is important in the Moroccan forest landscape. However, anthropogenic and natural pressures have almost completely degraded cork oak ecosystems. This study was conducted on the Maamora cork oak forest in north-west Morocco. The methodology adopted was based on a bibliographic review, GIS cartographic and teledetection analysis of the forest area, in order to assess the state of degradation of the forest areas and their evolution and to investigate the causes about anthropic pressure and the climate change factor. Progressive degradation results from a combination of climatic and anthropogenic factors that continue to intensify. The lack of natural regeneration is one of the most worrying problems, requiring advanced reforestation and sustainable development.

Climatic drivers of cork growth depend on site aridity

Cork is one of the main non-timber forest products in the world. Most of its production is concentrated in the Iberian Peninsula, a climate change hotspot. Climate warming may lead to increased aridification and reduce cork production in that region. However, we still lack assessments of climate-cork relationships across ample geographical and climatic gradients explicitly considering site aridity. We quantified cork growth by measuring cork ring width and related it to climate variables and a drought index using dendrochronology. Four cork oak (Quercus suber) forests located from north eastern Spain to south western Morocco (31.5–41.5° N) and subjected to different aridity levels were sampled. Warm conditions in spring to early summer, when cork is formed, reduced cork width, whereas high precipitation in winter and spring enhanced it. The response of cork to increased water availability in summer peaked (r = 0.89, p = 0.00002) in the most arid and continental site considering 14-month long droughts. A severe drought caused a disproportionate loss of cork production in this site, where for every five-fold decrease in the drought index, the cork-width index declined by a factor of thirteen. Therefore, site aridity determines the responses of cork growth to the soil water availability resulting from accumulated precipitation during winter and spring previous to cork growth and until summer. In general, this cumulative water balance, which is very dependent on temperature and evapotranspiration rate, is critical for cork production, especially in continental, dry sites. The precipitation during the hydrological year can be used as a proxy of cork production in similar sites. Assessments of climate-cork relationships in the western Mediterranean basin could be used as analogues to forecast the impacts of aridification on future cork production.

Genotypic and genetic diversity of Maâmora’s cork oak forest using simple sequence repeats (SSR) and inter simple sequence repeats (ISSR) markers for conservation

Genotypic and genetic diversity of Maâmora’s cork oak forest using simple sequence repeats (SSR) and inter simple sequence repeats (ISSR) markers for conservation

Cotula australis Hook.f. (Asteraceae), a species newly introduced in Morocco

We report the discovery of Cotula australis Hook.f. for the first time in Morocco and North Africa. We found it living in the cork-oak forest of Maamora (Northwest). This species, a member of the family Asteraceae, is introduced to Morocco and has potential to be highly invasive and destructive to the native environment. We describe the distinctive taxonomic and morphological characters of this species and provide notes on its distribution, ecology, and habitat.

A Contingent Valuation-Based Method to Valuate Ecosystem Services for a Proactive Planning and Management of Cork Oak Forests in Sardinia (Italy)

The paper develops a model through a contingent valuation approach to support public authorities in the exploration and assessment of ecosystem services (ESs) generated by forest and woodlands (FOWLs). This approach is employed to the cork oak forests of the Sardinia region (Italy) due to their ability in the provision and regulation of cultural and recreational values to society. The paper describes the economic valuation of cultural ESs through the contingent valuation method (CVM) with the purpose to explore residents and tourists’ willingness to pay (WTP) preferences towards conservation, valorisation, and the management of Goceano’s cork oak forests in Sardinia. The approach may help retain suitable support for DMs, planners, technicians, and operators for a better understanding of the ESs’ role in policy decisions, leading FOWLs towards a learning process between the environment, human beings, and landscape to promote and develop a proactive landscape and forest planning and management within the region.

A Metabolome Analysis and the Immunity of Phlomis purpurea against Phytophthora cinnamomi.

Phlomis purpurea grows spontaneously in the southern Iberian Peninsula, namely in cork oak (Quercus suber) forests. In a previous transcriptome analysis, we reported on its immunity against Phytophthora cinnamomi. However, little is known about the involvement of secondary metabolites in the P. purpurea defense response. It is known, though, that root exudates are toxic to this pathogen. To understand the involvement of secondary metabolites in the defense of P. purpurea, a metabolome analysis was performed using the leaves and roots of plants challenged with the pathogen for over 72 h. The putatively identified compounds were constitutively produced. Alkaloids, fatty acids, flavonoids, glucosinolates, polyketides, prenol lipids, phenylpropanoids, sterols, and terpenoids were differentially produced in these leaves and roots along the experiment timescale. It must be emphasized that the constitutive production of taurine in leaves and its increase soon after challenging suggests its role in P. purpurea immunity against the stress imposed by the oomycete. The rapid increase in secondary metabolite production by this plant species accounts for a concerted action of multiple compounds and genes on the innate protection of Phlomis purpurea against Phytophthora cinnamomi. The combination of the metabolome with the transcriptome data previously disclosed confirms the mentioned innate immunity of this plant against a devastating pathogen. It suggests its potential as an antagonist in phytopathogens' biological control. Its application in green forestry/agriculture is therefore possible.

Analysis of cork quality and cork tree health in stands of western Spain

Good knowledge of the factors that influence cork quality and the health of forests is essential when making decisions regarding the management of productive cork oak forests. The quality of cork planks depends on: plank thickness, plank porosity and the presence of anomalies. So far, no studies have evaluated cork quality based on cork anomalies, partly due to the difficulty of obtaining adequate data. In 1985, it was developed the first Cork Quality Field Assessment Plan (CQFAP) to provide information about the quality of harvested cork by collecting data of very high quality. This work analyzes the anomalies present in 2049 cork sampling units collected from ten stands under the CQFAP for the period 1986–2012 in three consecutive cork stripping cycles. Data on diseases, pests and stripping damages are also analyzed in the 654 trees where cork sampling units were collected in the third stripping cycle. Additionally, mean monthly temperature and precipitation values for the 35 years covering the three stripping periods were calculated. A progressive deterioration of cork quality was observed in the sampled cork oak stands. It was found a positive relationship between cork thickness and temperature indicating that the cork growing period could be extended when temperatures are moderate. Sampled cork oaks were more affected by Cerambycidae attacks and Coraebus undatus indicating their prevalent nature. The presence of Coraebus undatus were higher in stands with good health status. The studied diseases were Biscogniauxia mediterranea, Phytophthora sp. and fungi of the family Botryosphaeriaceae, finding a not previously reported correlation between the presence of Crematogaster scutellaris and the three studied diseases. Stripping damages increase with smaller cork thickness but climate is not a decisive factor for a proper stripping. Although having room for improvement, the CQFAP is proving to be an effective tool for assessing the quality of cork, the health status of cork oak forests and the evolution of these forests over time. The information provided could be also used to identify places worthy of further studying. The main recommendation for improving the CQFAP would be to monitor the same trees in each consecutive sampling for properly monitoring cork thickness and the presence of defects to be used as a proxy for tree health. Regarding forest management, the main recommendations would be to extend the length of stripping cycles to enhance cork production, and to use the new tools and systems for cork stripping to reduce damage.

Forest raptor nest selection traits in Mediterranean forest (Maamora, North Africa): conservation implications

SummaryForest raptor nest-site selection is mostly influenced by the quality of the habitat, food resources, and competition. Here, we identified common targets of trees selected as breeding sites and differences in selection traits, i.e. prey availability and intra- and interspecific competition, among Booted Eagle, Long-legged Buzzard, Black Kite, and Common Kestrel in a Mediterranean Cork Oak forest (private protected reserve of 25 km2). Using generalised linear mixed models we developed species-specific models describing nesting habitat selection. We tested the overlap in nesting habitat selection among species using environmental principal component analysis. The densities of forest raptor breeding pairs were high (3.1 pairs/km2) and the distance between occupied territories was short, strongly connected with food availability and competition. The results showed that all the species, with the exception of Common Kestrel, selected for nesting areas characterised by higher conspecific distance, highlighting the importance of conspecific competition. Booted Eagle and Black Kite selected areas with a high abundance of rabbits. The height of the nesting tree, the size and distance between surrounding trees, and the scrub cover were significant habitat characteristics for Booted Eagle and Long-legged Buzzard. Indeed, the environmental analyses showed a moderate nest site overlap between Black Kite and both Booted Eagle and Long-legged Buzzard, and a high overlap between Common Kestrel and both Booted Eagle and Long-legged Buzzard. Our study improves knowledge of the habitat requirements for nest selection and the potential competitive interactions between these raptor species in Mediterranean forests, and highlights the need for implementation of habitat management and conservation strategies.