The Renaissance of Mixed Forests? New Insights Into Shifts in Tree Dominance and Composition Following Centuries of Human-induced Simplification of Iberian Forests

Anthropic activities have modelled and simplified southern European forest landscapes for centuries. Over recent decades, new drivers related to human-mediated global change have induced the redistribution of tree species and an increase in more complex forests. However, the current large-scale patterns and drivers of these changes are yet to be fully described for the Mediterranean Basin. In this frame, this work identifies and examines changes in dominance and composition from pure to mixed forests across bioclimatic gradients and forest types in Iberian forests over recent decades based on data from the Spanish National Forest Inventory from 1960 to 2020. Then, considering different environmental, anthropic, and disturbance variables we also identify some of the most important drivers associated with the shifts observed from 1986 to 2020. Our results confirm an ongoing increase in mixed forests involving the replacement of conifers by broadleaved species. These shifts are greater in the Atlantic biogeoregion and in pure broadleaved deciduous forests. Climate warming-associated disturbances such as drought severity together with land use legacies and forest types showed the strongest relationships with the observed changes in the studied forests. Our results support the premise put forward by palaeoecologists which states that the increase in tree mixtures is a natural process reversing the historical human-induced simplification of Iberian forests. The increasing importance of mixed forest in southern Europe makes decisive the revision of forest classifications as well as forest management and conservation plans in order to include these increasingly abundant novel stands in forest policies.

Impact of climate change on potential distribution of Quercus suber in the conditions of North Africa

Climate change, which is expected to continue in the future, is increasingly becoming a major concern affecting many components of the biodiversity and human society. Understanding its impacts on forest ecosystems is essential for undertaking long-term management and conservation strategies. This study was focused on modeling the potential distribution of Quercus suber in the Maamora Forest, the world’s largest lowland cork oak forest, under actual and future climate conditions and identifying the environmental factors associated with this distribution. Maximum Entropy approach was used to train a Species Distribution Model and future predictions were based on different greenhouse gas emission scenarios (Representative Concentration Pathway RCPs). The results showed that the trained model was highly reliable and reflected the actual and future distributions of Maamora’s cork oak. It showed that the precipitation of the coldest and wettest quarter and the annual temperature range are the environmental factors that provide the most useful information for Q. suber distribution in the study area. The computed results of cork oak’s habitat suitability showed that predicted suitable areas are site-specific and seem to be highly dependent on climate change. The predicted changes are significant and expected to vary (decline of habitat suitability) in the future under the different emissions pathways. It indicates that climate change may reduce the suitable area for Q. suber under all the climate scenarios and the severity of projected impacts is closely linked to the magnitude of the climate change. The percent variation in habitat suitability indicates negative values for all the scenarios, ranging –23% to –100%. These regressions are projected to be more important under pessimist scenario RCP8.5. Given these results, we recommend including the future climate scenarios in the existing management strategies and highlight the usefulness of the produced predictive suitability maps under actual and future climate for the protection of this sensitive forest and its key species – cork oak, as well as for other forest species.

Linking Cork to Cork Oak Landscapes: Mapping the Value Chain of Cork Production in Portugal

Traditional farming landscapes in South and Central Portugal, known as montados, are affected by global socio-economic and biophysical pressures, putting the sustainability of the systems in jeopardy. Cork oak trees (Quercus suber L.) are characteristic features of these complex agro-silvo-pastoral agroforestry systems, delivering a globally important product, cork. The increasingly distant, global scale of decision making and trade can consequently be observed on the local, landscape, scale. In this study, we use a value chain approach to test the concept that landscape products can ensure sustainable management of the landscape of origin. We interviewed agents—cork producers, intermediaries, industrial transformers, and winemakers—about the challenges they perceived in the business and how these were connected to the landscape of origin. We illustrate the network of agents and sub-actors involved in the sector and highlight the most prominent concerns. We conclude that this approach can reveal the major points for determining the future of the montado, and we suggest that collaboration amongst value chain agents can be a pathway to landscape sustainability.

Contrasting nuclear and cytoplasmic exchanges between phylogenetically distant oak species (Quercus suber L. and Q. ilex L.) in Southern France: inferring crosses and dynamics

Gene flow is particularly frequent in the genus Quercus (oaks), especially between closely related species. We focus here on Quercus ilex and the cork-producing Quercus suber, which occasionally hybridize although they are phylogenetically markedly separated. Morphological observations were combined with both allozymic and chloroplastic diagnostic markers to characterize hybridization and introgression and to infer their dynamics in two French regions (French Catalonia and Provence), which are separated by several hundred kilometres. Some hybrids were found in both regions, indicating recent hybridization events. As expected from previous studies, most hybrids resulted from female symbol Q. ilex x male symbol Q. suber crosses, but our data showed that the reciprocal cross is also possible. Partial independence between nuclear and chloroplastic introgression was observed in the two species. Nuclear introgression was limited in both species and both regions, with no preferred direction. In Provence, chloroplastic introgression was very rare in both species. Conversely, all Q. suber individuals from French Catalonia were introgressed by Q. ilex chlorotypes. This might be explained by introgression in the Iberian Peninsula antedating the first occurrence of the two species in French Catalonia. We also observed a new chlorotype that was created locally, and was exchanged between the two species. However, the two species still remain genetically differentiated. The dynamics and complexity of exchanges and the factors determining them (including human management of Q. suber) are discussed.

Summer and winter sensitivity of leaves and xylem to minimum freezing temperatures: a comparison of co-occurring Mediterranean oaks that differ in leaf lifespan

Freezing sensitivity of leaves and xylem was examined in four co-occurring Mediterranean oaks (Quercus spp.) grown in a common garden to determine whether freezing responses of leaves and xylem were coordinated and could be predicted by leaf lifespan. Freezing-induced embolism and loss of photosynthetic function were measured after overnight exposure to a range of subzero temperatures in both summer and winter. Both measures were found to be dependent on minimum freezing temperature and were correlated with leaf lifespan and vessel diameter. The dependence of xylem embolism on minimum freezing temperature may result from the decline in water potential with ice temperature that influences the redistribution of water during freezing and leads to an increase in xylem tension. Winter acclimatization had a relatively small effect on the vulnerability to freezing-induced embolism, although leaf photosynthetic function showed a strong acclimatization response, particularly in the two evergreen species. Quercus ilex, the species with the longest leaf lifespan and narrowest vessel diameters, showed the highest freezing tolerance. This helps explain its ability to inhabit a broad range throughout the Mediterranean region. By contrast, the inability of the deciduous oaks to maintain photosynthetic and vascular function throughout the winter indicates a competitive disadvantage that may prevent them from expanding their ranges.

Phylogeographical variation of chloroplast DNA in holm oak (Quercus ilex L.)

Variation in the lengths of restriction fragments (RFLPs) of the whole chloroplast DNA molecule was studied in 174 populations of Quercus ilex L. sampled over the entire distribution of this evergreen and mainly Mediterranean oak species. By using five endonucleases, 323 distinct fragments were obtained. From the 29 and 17 cpDNA changes identified as site and length mutations, respectively, 25 distinct chlorotypes were distinguished, mapped and treated cladistically with a parsimony analysis, using as an outgroup Q. alnifolia Poech, a closely related evergreen oak species endemic to Cyprus where Q. ilex does not grow. The predominant role of Q. ilex as maternal parent in hybridization with other species was reflected by the occurrence of a single very specific lineage of related chlorotypes, the most ancestral and recent ones being located in the southeastern and in the northwestern parts of the species' geographical distribution, respectively. The lineage was constituted of two clusters of chlorotypes observed in the 'ilex' morphotyped populations of the Balkan and Italian Peninsulas (including the contiguous French Riviera), respectively. A third cluster was divided into two subclusters identified in the 'rotundifolia' morphotyped populations of North Africa, and of Iberia and the adjacent French regions, respectively. Postglacial colonization probably started from three distinct southerly refugia located in each of the three European peninsulas, and a contact area between the Italian and the Iberian migration routes was identified in the Rhône valley (France). Chlorotypes identical or related to those of the Iberian cluster were identified in the populations from Catalonia and the French Languedoc region, which showed intermediate morphotypes, and in the French Atlantic populations which possessed the 'ilex' morphotype, suggesting the occurrence of adaptive morphological changes in the northern part of the species' distribution.

Frequent cytoplasmic exchanges between oak species that are not closely related: Quercus suber and Q. ilex in Morocco

Chloroplast (cp) and mitochondrial (mt) DNA variation were studied in 97 populations of cork oak (Quercus suber) in Morocco; in 31 of these populations, holm oak (Quercus ilex), a clearly distinct species, also occurred and was compared with Q. suber. Three cpDNA and one mtDNA primer pairs were used in the survey, each in combination with one restriction enzyme. Six haplotypes belonging to two very divergent lineages were detected; one lineage predominates in each species, and is probably ancestral, as inferred from comparisons with other oak species. In the mixed-species populations, cytoplasmic genomes were frequently shared across species, as indicated by an introgression ratio of 0.63. This index is a new measure of the propensity of species to share locally genetic markers, varying from zero (complete differentiation) to one (no differentiation). By contrast, more closely related deciduous oak species (Q. robur, Q. petraea and Q. pubescens) have introgression ratios varying from 0.82 to 0.97. The introgression events appear to have been more frequent in the direction Q. ilex (female) x Q. suber (male), a finding which seems attributable to the flowering phenology of these two species. This asymmetry may have favoured immigration of Q. suber beyond its main range, in regions already colonized by Q. ilex. There, rare hybridization and further introgression through long distance pollen flow have established populations that are morphologically indistinguishable from Q. suber but that have cytoplasmic genomes originating from the local Q. ilex populations.