Twelve-year oak seedling survival and growth in post-coal mining pastures of northern Spain: Combined effects of nurse shrubs and grazing exclusion

Abstract

Facilitative interactions among plants can support vegetation recovery in degraded environments, yet their medium-term effectiveness in restoration remains insufficiently understood. We conducted a 12-year field experiment in reclaimed open-cast coal mines in northern Spain to assess whether native colonizer legume shrubs enhance the survival and growth of seedlings of two Quercus species. A total of 800 seedlings were planted under four treatments combining the presence/absence of shrub and grazing, allowing us to disentangle biotic and abiotic facilitation mechanisms. Shrubs enhanced seedling survival, particularly for Q. pyrenaica, by buffering early mortality during summer droughts. Medium-term seedling survival under shrubs was markedly higher than in shrub-free areas (13–25 % vs. 1–4 %). Herbivory had a limited effect on survival, with fencing providing marginal benefits in specific years. Slow increments of Quercus seedling height and diameter over time were found, being more pronounced for Q. petraea, and varying as the combined effect of shrubs and grazing exclu- sion. Annual growth varied over time for both Quercus species, being greater under than outside shrubs for Q. petraea, while for Q. pyrenaica, the shrub effect depended on the year. Q. pyrenaica seedlings exhibited higher annual growth than Q. petraea. Differences in seedling annual growth with and without shrubs were smaller in the driest years. We conclude that native shrubs play a key facilitative role in the medium-term restoration of degraded oak ecosystems under sub-Mediterranean conditions. Their effectiveness varies by species and response variable (survival vs. growth), and the underlying facilitation mechanisms differ over time.