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Eight Pinus nigra Arn. populations from Southern Spain and Northern Morocco were examined using inter-simple sequence repeat markers to characterize the genetic variability amongst populations. Pair-wise population genetic distance ranged from 0.031 to 0.283, with a mean of 0.150 between populations. The highest inter-population average distance was between PaCU from Cuenca and YeCA from Cazorla, while the lowest distance was between TaMO from Morocco and MA Sierra Mágina populations. Analysis of molecular variance (AMOVA) and Nei’s genetic diversity analyses revealed higher genetic variation within the same population than among different populations. Genetic differentiation (Gst) was 0.233. Cuenca showed the highest Nei’s genetic diversity followed by the Moroccan region, Sierra Mágina, and Cazorla region. However, clustering of populations was not in accordance with their geographical locations. Principal component analysis showed the presence of two major groups—Group 1 contained all populations from Cuenca while Group 2 contained populations from Cazorla, Sierra Mágina and Morocco—while Bayesian analysis revealed the presence of three clusters. The low genetic diversity observed in PaCU and YeCA is probably a consequence of inappropriate management since no estimation of genetic variability was performed before the silvicultural treatments. Data indicates that the inter-simple sequence repeat (ISSR) method is sufficiently informative and powerful to assess genetic variability among populations of P. nigra.

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Abstract& Key message Shrub cover has visible effects on Pinus nigra Arn ssp. salzmannii seedling emergence, but only in drier years under moderate basal area (25-30 m 2 ha −1 ). In the wetter year, shrub cover favours seedling survival without basal area influence. & Context Shrubs are known to favour tree seedling recruitment in difficult climate environments, but facilitation may prove optimal in intermediate-level rather than harsh conditions, although such an effect remains to be specified. & Aims The main aim of this work was to evaluate the influence of stand basal area (15-20, 25-30 and 35-40 m 2 ha −1 ) × with/without shrub facilitation on seedling recruitment in Spanish black pine (P. nigra Arn. ssp. salzmannii), which has been struggling to regenerate natural forests since the late nineteenth century, sometimes jeopardizing stand persistence. & Methods In spring 2012 and 2013, 72 subplots of 25 × 25 cm were established in Cuenca Mountains, centraleastern Spain (four replicates, each counting three subplots × 2 shrub conditions, both under and outside shrub cover, in three different stand basal areas). All plots were randomly distributed at least 500 m apart within each stand basal area. Each subplot comprised 20 seeds previously collected at the experimental forest site. Seedling emergence and early survival were monitored every month in 2012 and 2013. Handling Editor: Laurent BergèsContribution of the co-authors ME Lucas-Borja and D Candel-Pérez conceived the research, obtained the field data, run the data analysis and wrote the manuscript. PA Tíscar, T Onkelinx and P Balandier run the data analysis and wrote the manuscript. FA García Morote obtained the field data and wrote the manuscript.

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The aim of this work was to examine the effect of pine forests on the soil microbial community along an altitudinal gradient in the Cuenca Mountains, Spain. Six experimental forest areas and two tree diversity levels (monospecific and mixed pine forest) were selected according to the following types: (i) at lower altitudes (up to 960 m above sea level), a monospecific Spanish black pine (Pinus nigra Arn. ssp salzmannii) forest stand and a mixed forest stand (maritime pine (Pinus pinaster Ait.) and Spanish black pine); (ii) at medium altitudes (up to 1350 m), a monospecific Spanish black pine forest stand and a mixed forest stand (Scots pine (Pinus sylvestris L.) and Spanish black pine); and (iii) above 1670 m, a monospecific Spanish black pine forest stand and a mixed forest stand (Scots pine and Spanish black pine). Soil moisture and temperature and different forest stand variables such as tree species composition, tree height, basal area and shrub cover were measured. Physicochemical soil properties including texture, pH, carbonates, total organic carbon, organic matter, electrical conductivity, N, P, soil enzyme activities, soil respiration and soil microbial biomass carbon were analysed at the selected forest areas and tree diversity levels. Results showed that soil moisture and temperature differed significantly across the altitudinal gradient. Carbon:nitrogen ratio, total carbonates, total organic carbon, phosphorus and total N values also differed at each experimental area but tree diversity level was not an influential factor. Soil respiration, microbial biomass carbon and enzyme activities tended to be less at low altitudes whereas no differences were found between monospecific and mixed pine forests across the altitudinal gradient. This work suggests that the soils of the Cuenca Mountains may be more sensitive to some physical and chemical site‐specific aspects such as soil temperature and moisture than changes in tree stand composition (when considering only pine species). In addition, differences in soil physicochemical properties found at each experimental area may also act as additional factors for regulating enzymatic activities.

Density is an important wood mechanical property and an indicator of xylem architecture and hydraulic conductivity. It can be influenced by forest management and climate. We studied the impact of thinning and climate variables on annual stem radial growth (ring width and ring density, and their earlywood and latewood components) in two contrasting Scots pine (Pinus sylvestris L.) stands in northern Spain (one continental, one Mediterranean). At each site, three thinning regimes (control or T0, removing 20% basal area or T20, and removing 30% or T30) were randomly applied to nine plots per site (three plots per treatment) in 1999. Thinning was repeated at the Mediterranean site in 2009 (increasing thinning intensity in T30 to 40%). Eight trees per plot were cored in spring 2014. Second thinning at the Mediterranean site and first thinning at the continental site generally caused significantly wider ring (RW), earlywood (EW) and latewood (LW) widths, although no differences between T20 and T30/40 were found, supporting in part the common observation that radial growth is enhanced following thinning as competition for water and nutrients is reduced. At the Mediterranean site, values of latewood density (LD) and maximum density (Dmax) relative to pre-thinning conditions were significantly lower in T0 than in T30. However, at the continental site, relative changes of ring density (RD) and LD were significantly higher in T0 than in T20 and T30. Climate significantly affected not only RW but also RD, with significant RD drops during or right after unusually warm-dry years (e.g., 2003, 2011), which were characterized by LD reductions between 5.4 and 8.0%. Such RD decreases were quickly followed by recovery of pre-drought density values. These results indicate trees temporarily reduce LD as a way to enhance hydraulic conductivity during dry summers. However, climate effects on wood density were site-dependent. We also detected that the thinning effect was not intense enough to prevent drought-induced changes in wood density by altering water availability, but it could help to reduce wood properties fluctuations and therefore maintain more homogeneous wood mechanic features.