Pinus nigra Arn. ssp. salzmannii seedling recruitment is affected by stand basal area, shrub cover and climate interactions

Abstract

Key message: Shrub cover has visible effects onPinus nigraArn ssp.salzmanniiseedling emergence, but only in drier years under moderate basal area (25–30 m2 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 m2 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, central-eastern 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. Results: Seedling recruitment was strongly influenced by year. The year 2012 was drier and warmer than 2013. In 2012, seedling emergence was higher under the 25–30-m2 ha−1 stand basal area and favoured by shrub cover, whereas in 2013, neither basal area nor shrub cover had significant effects. No seedling survived the summer in 2012. Higher seedling survival was found outside shrub cover at every basal area interval in 2013 with living seedlings until the end of the year. Conclusion: Climate is the main factor controlling seedling emergence and survival. Shrub facilitation has visible effects on seedling emergence, but only in drier years under moderate light (moderate basal area). Facilitation collapses at extreme stress levels. In wetter years, shrub cover does not promote seedling survival and basal area is not an influential factor.