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Genetic diversity is essential to evolution and a recognized target of conservation. When threats are high and populations are small, in-situ gene conservation needs to be reinforced with an ex-situ approach, where a genetically representative sample of the target taxon is safeguarded in a favorable environment. The fragmented habitat of Pinus nigra subsp. salzmannii (Dunal) Franco 1943 (Salzmann’s pine) is threatened by wildfires. In France, gene flow from massive plantations of other subspecies of black pines threatens its genetic diversity. Here, using individual tree genotypic data at thirteen microsatellite loci, we identified differentiated lineages for in-situ gene conservation. Discriminating between autochthonous and hybrid trees, we proposed a method for the creation of an ex-situ core collection. We confirmed that Salzmann’s pine is an original genetic lineage within the western European and Mediterranean black pine subspecies. We identified five genetic groups in France that can serve as the basis for in-situ gene conservation. Maximizing overall genetic diversity while maintaining among population diversity, we identified 80 native and non-hybridized trees that can form the basis of a representative ex-situ core collection. Our cost-effective methods combining in-situ and ex-situ conservation can be easily applied to many forest tree species.

Questions: What are the differences in tree growth and resilience in response to extreme droughts between three co-existing pine tree species? Are growth–resilience components more influenced by intrinsic (age, tree height) or extrinsic factors (mass effects or functional diversity of the neighbourhood)? Is the tree's vulnerability to drought buffered by the functional diversity of the surrounding plant community? Location: Mediterranean forest, northeast Spain. Methods: Trees from three co-existing pine species with different drought tolerances (Aleppo pine – Pinus halepensis Mill.; Black pine – Pinus nigra subsp. salzmannii (Dunal) Franco; Scots pine – Pinus sylvestris L.) were sampled. The average age of the studed pines was ∼30 years. Radial growth from 1980 to 2015 was measured and the pines' responses to the most severe recent droughts (2005 and 2012) were studied by relating different growth resilience components (Rt, resistance; Rc, recovery; RRs, relative resilience) with intrinsic (age and height) and extrinsic factors of the neighbouring community around each tree. The considered extrinsic factors were CWM and niche complementarity of tree (height), leaf (specific leaf area, leaf dry matter content) and wood (wood specific gravity) traits. The way in which these factors affected growth resilience to drought was assessed. Results: For the whole study period we found lower growth of Scots pine than Aleppo pine, but similar resilience components (except for higher Rt in 2005 for the former species). Despite each drought year resulting in distinct responses, the growth resilience to drought was mostly dependent on tree age for Aleppo pine, while functional diversity modulated a high percentage of the resilience variance of Scots pine. Black pine had an intermediate strategy. CWM explained, in general, less variance of the resilience components than functional diversity. Conclusions: The positive effect of surrounding functional diversity on Scots pine could be related to a greater resistance to drought, likely due to improved resource acquisition favouring its performance at the dry limit of its distribution area. Intrinsic tree features, mass effects and functional diversity differently shape growth resilience to different droughts for co-existing tree species.

The Ministry of Science and Innovation of Spain funded this research through the projects UMBRACLIM (PID2019-111781RB-100), VULNIFOR (PID2022-142108OB-I00), and the predoctoral grant PRE2020-094807. Ana Lucia Mendez-Cartin benefited from the Marie Skodowska-Curie Research and Innovation Staff Exchange (RISE)\\u2014grant agreement Nr: 101007950. Aitor Ameztegui is supported by a Serra-H\\u00FAnter fellowship. The authors are grateful to Gil Torn\\u00E9 for his support in fieldwork and to the personnel of the Sant Lloren\\u00E7 del Munt National Park for all their help during our fieldwork. We are also grateful to everyone in the USFS in Corvallis, who helped achieve Ana Lucia\\u2019s internship\\u2014crucial to this paper\\u2014with special thanks to David Bell, who hosted and assisted Ana Lucia while she was in Corvallis. We also like to thank Dustin Gannon for all his aid in statistics.

Key messageSpanish black pine showed greater resilience and resistance, but generally lower recovery to drought events in managed than in unmanaged forest stands under Mediterranean humid climate.Drought negative effects on forest ecosystems are projected to increase under global warming all over the world. In this context, forest management can be an effective option for reducing drought impacts and increasing tree growth stability to extreme drought events. Here, we aim to evaluate black pine (Pinus nigra subsp. salzmannii) growth response to climatic variability and drought events in managed and unmanaged stands under similar Mediterranean climatic conditions. Drought events were identified using long-term climatic data, and basal area increments were calculated for 100–120-year old trees cored in managed and unmanaged plots. Results showed that tree size, temperature, and the interaction between management treatment and water availability significantly influenced tree growth. Basal area increment was reduced in response to the 1983, 1991, 1994‒1995, 1999–2000 and 2005 drought events. Trees in managed plots showed lower growth reductions in response to drought than those located in unmanaged plots, probably experiencing higher competition for soil water, whereas the reverse happened under wet climate conditions. Black pines showed greater resilience and resistance, but generally lower recovery to drought events in managed than in unmanaged stands. Our results suggest that forest management enhances drought tolerance in black pine stands, which may help to ameliorate the negative impacts of global warming across Mediterranean forest ecosystems.

The protection and conservation of old-growth forests (OGFs) are becoming a global concern due to their irreplaceability and high biodiversity. Nonetheless, there has been little research into the identification and characterization of OGFs of the oldest tree species in Mediterranean areas. We used forest inventory data, low-density airborne laser scanning (ALS) metrics, and geostatistical analysis to estimate old-growth indices (OGIs) as indicators of old-growth forest conditions. We selected a pilot area in European black pine (Pinus nigra subsp. salzmannii) ecosystems where the oldest known living trees in the Iberian Peninsula are found. A total of 756 inventory plots were established to characterize standard live tree and stand attributes. We estimated several structural attributes that discriminate old growth from younger age classes and calculated different types of OGI for each plot. The best OGI was based on mean tree diameter, standard deviation of tree diameter, and stand density of large trees (diameter > 50 cm). This index is useful for assessing old-growthness at different successional stages (young and OGFs) in Mediterranean black pine forests. Our results confirm that the estimation of OGIs based on a combination of forest inventory data, geostatistical analysis, and ALS is useful for identifying OGFs.

Stress caused by parasitic plants, e.g. mistletoes, alters certain host-plant traits as a response. While several physical implications of the parasite-host relation have been well studied, shifts in the host chemical profile remain poorly understood. Here we compare the chemical profiles of mistletoe (Viscum album subsp. austriacum) leaves and host pine (Pinus nigra subsp. salzmannii) needles and we investigate chemical changes in host needles of trees with different parasite loads (control, low, medium, and high). Our results reveal that despite the intimate contact between mistletoe and host pine, their chemical profiles differed significantly, revealing extremely low concentrations of defense compounds (including a complete lack of terpenes) and high levels of N concentrations in mistletoe leaves. On the other hand, parasitized pines showed unique chemical responses depending on parasite loads. Overall, the content in monoterpenes increased with parasitism. Higher parasitized pines produced higher amounts of defense compounds (phenols and condensed tannins) than less parasitized trees, but amounts in samples of the same year did not significantly differ between parasitized and unparasitized pines. Highly parasitized pines accumulated less N than pines with other parasite loads. The strongest response was found in sesqui- and diterpenes, which were at lower levels in pines under medium and high parasitism. Chemical responses of pines to mistletoe parasitism resembled reactions to other kinds of stress. Low levels induced reactions resembling those against drought stress, while medium and high parasitism elicited responses comparable to those against burning and defoliation.

Post-fire forest resilience must be quantified in a long-term perspective considering changes in land-use related to fire dynamics. Historical land-use changes leading to increased wildfire severity may produce no analogue regime shifts including a loss in post-fire growth recovery. Here we reconstruct the historical fire dynamics by combining paleoecological proxies, historical fire records and tree-ring width data of relict Pinus nigra subsp. salzmannii forests in the Sierra de Gredos (central Spain). A high incidence of historical fires was recorded in the 1890s, coinciding with a peak in charcoal accumulation rates and a sharp decrease in pollen of P. nigra/Pinus sylvestris with a rapid increase of pollen of more flammable Pinus pinaster and shrubs. The shift observed in pollen assemblages, coupled with a peak in charcoal influx, support the occurrence of high-severity fires during the 1890s, when abrupt growth suppressions were observed. Trees took 2 years to recover to their pre-fire growth rates. Lasting growth-recovery periods or no growth suppression were observed in the 1920s and 1980s, when fire frequency was also high but the study sites were fragmented or protected. We documented an abrupt regime shift in the fire record during the 1890s affecting pine forests, which rapidly recovered pre-fire growth rates.

The aim of this study was to investigate the chemical diversity of Pinus nigra Arn. essential oils. The research was carried out on the needles collected from eighteen provenances of black pine grown in common garden located in West-Northern Tunisia and belonging to four different subspecies (Pinus nigra subsp. nigra, Pinus nigra subsp. salzmannii, Pinus nigra subsp. pallasiana, and Pinus nigra subsp. laricio). Essential oil yields ranged from 0.19% to 0.68%. The obtained essential oils have been analyzed by GC-FID and GC-MS apparatus. Twenty-three constituents accounting about 98% of total essential oil composition were identified. The essential oil compositions appeared to be very different according to their origin. Thus, five main essential oil chemotypes were identified in Pinus nigra plants: caryophyllene oxide, camphene, ß-caryophyllene, α-amorphene, and germacrene D. The chemotaxonomic value of the essential oil compositions was discussed in relation to the results of the multivariate statistical test, including a detailed survey of the available literature data.

Indirect interactions emerge among a wide range of herbivores sharing the same plant resource. Consumers usually belong to different trophic guilds, from folivores and sapsuckers to parasitic plants. We propose that mistletoes parasitizing pines could play a key role acting as herbivores on host pines and coming indirectly into competition with other herbivores feeding on the same host. Changes caused by mistletoes on its host have been well studied, but its effects running across trophic webs remain unrevealed. In this study, we investigate the effect of European mistletoe (Viscum album subsp. austriacum) on the host‐feeding herbivores via trait‐mediated indirect interactions (TMIIs) across their shared pine host (Pinus nigra subsp. salzmannii). We performed field and laboratory experiments, and analyzed the net effect of different mistletoe parasite loads on three host‐phytophagous species: the sapsucker Cinara pini (Aphididae), the winter folivore Thaumetopoea pityocampa (Thaumetopoeidae), and the summer folivore Brachyderes sp. (Curculionidae), all being members of different functional feeding groups (FFGs). We summarize the mistletoe–host–herbivore interactions by means of a TMII, where mistletoe parasitism causes non‐trophic links and detrimental indirect interactions on pine‐feeding herbivores across its shared host, suggesting a worsening of host quality as food. These indirect interactions vary according to three parameters. First, the intensity has a non‐proportional relation with parasite load, showing an impact threshold on highly parasitized pines. Second, the movement capacity of insect herbivores determines their response, by decreasing the abundance of herbivores with low movement ability (aphids and pine processionary caterpillars) while altering the behavior (plant selection) of more mobile herbivores (pine weevils). Finally, FFG determines the intensity of mistletoe parasitism effects, folivores being more responsive than sapsuckers. Overall, mistletoe generates non‐trophic interaction linkages in the forest able to modify community structure by becoming a nexus of the entire herbivore community of the pine canopy.

Pinus nigra subsp. salzmannii is found in the east and centre of the Iberian Peninsula, in the south of France and in North Africa. This subspecies occupies the westernmost position of the species’ general range. The persistence on the Iberian Peninsula of very long-lived specimens of Pinus nigra subsp. salzmannii , along with their sensitivity to climate, has drawn the attention of many researchers, but to date the importance of dendroecological studies relating to conservation of biodiversity or the genetic resources of this taxon had not been stressed. In the present paper we use dendroecological methods to analyse the relict pine forest in Navalacruz, an interesting and endangered genetic forestry resource on the northern slopes of the Gredos mountains (in Spain’s Central System Range) at the subspecies’ south-western global limit. This forest provides a prime example for demonstrating the potential application of dendroecology for studying the origin, dynamics, local variability, relationships with climate and anthropogenic disturbances of relict forest populations. We dated 93 growth sequences from 47 trees ranging from 1809 to 2006 and we have determined that interspecific competition is the most relevant factor as regards differences in the diameter growth of these trees. Moreover, we detected great variability and numerous common growth disturbances unrelated to climatic oscillations. These quasi-periodic disturbances alternate between suppression and release suggesting continuous management cycles of different intensities. Despite its high level of disturbance, the pine forest presents a certain degree of climatic sensitivity. Comparing with others Pinus nigra subsp. salzmannii populations, we denoted a temporal grading of the growth response to precipitation that is indicative of differences in the start and length of the vegetative period. Furthermore, we compiled different dendroecological and palaeobiogeographical data to demonstrate that this dense, homogeneous and relatively younger P. nigra population is of an indigenous nature. This study aims to provide data for improved management and conservation of this exceptional and highly endangered bastion of biodiversity.