Understanding tree recruitment is crucial to assess species turnover in mixed forests and woodlands, particularly in light of current anthropogenic impact. Here, we investigate whether oak recruitment (density of young oaks) in co-existing deciduous and evergreen oaks is disproportional to the abundance of their corresponding reproductive trees, favoring regeneration of some species over others, which may anticipate a future species turnover. We also examined whether browsing damage and microsite location play a differential role in such recruitment process. We uncovered an overall underrepresentation of young oaks (<7.5 cm DBH) in both deciduous species (Q, lobata and Q, douglasii), compared to the evergreen oak (Q, agrifolia), even in areas where deciduous species were the locally predominant mature trees. Such differential recruitment was accentuated in areas where Q, lobata was predominant, with young evergreen oaks reaching densities 3-times higher than those of the deciduous oak. Recruitment of all three species was significantly associated with shrub cover, suggesting a facilitative effect, via reduction of both occurrence and intensity of herbivory. However, evergreen oaks were much less vulnerable to herbivory (lower occurrence and intensity) than deciduous oaks and, thus, evergreen recruits were able to thrive even in uncovered areas. Overall, our results provide evidence of differential recruitment at the juvenile stage in co-occurring oak species, which may lead to a replacement of deciduous species with evergreen oaks. The disproportionate effect of shrubs as safe microsites for regeneration of both deciduous and evergreen species suggests that herbivory pressure is an important factor reducing the abundance and heterogeneity of favorable regeneration niches, particularly for the most herbivore-sensitive tree species. Further studies should address to what extent the recruitment patterns described here translate into future differences in adult stages, and whether the actual biased regeneration is mostly due to differential herbivory on deciduous vs. evergreen species. We posit that, for the management and conservation of multi-specific oak savannas, regeneration of deciduous oaks should be prioritized over that of live oaks in order to maintain greater levels of biodiversity and reduce biotic homogenization.

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Mediterranean dehesas are an archetypal example of high nature value farmland in Europe. To better understand the interactions between trees and grasslands in the conservation and management of the dehesa, we measured, over 3 years, the contribution of scattered trees to the diversity and biomass dry-matter (DM) yield of the herb layer and the relationship between the herb layer (biomass and diversity interaction) and the trees. The study was based on 135 subplots representing four aspects and three distances from fifteen trees selected randomly in a plot within a dehesa in central Spain. The specific questions were the following: (i) To what extent do trees affect biomass of the herbaceous layer? (ii) What is the relationship between herbaceous layer biomass and species diversity under the influence of trees? and (iii) What are the effects of trees and small-scale topography on the herbaceous diversity? Results differed between years depending on water availability. Legume and forb DM yields were highest at the edge of the tree crown and beyond the canopy. Species richness was positively related to legume DM yield, and both were higher at the bottom of the hillside, especially in dry conditions. Given the importance of tree influence on the biomass and diversity of grasslands, maintaining proper agroforestry management practices for trees and grasslands is important for conserving dehesa diversity and productivity under the conditions of high climatic and soil variability in Mediterranean dehesas.

Questions Oak savanna‐like systems (dehesas) are highly valuable and protected systems that are strongly compromised by a continuous failure of oak recruitment. Our question is whether different long‐term management types involving cattle, sheep and wild ungulates affect the abundance and diversity of safe microsites (shrubs) for oak recruitment. We also assess microsite location and species‐specific differences among the nurse shrubs in order to provide management guidance. Location Dehesas of holm oak (Quercus ilex) in central Spain. Methods We examined oak regeneration in four different microsites (tree canopy, shrub cover, open and tree–shrub) and across three distinctive and representative management types for a period of, at least, 30 yr: (1) traditional management (i.e. extensive sheep rearing); (2) high commercially competitive management (extensive cattle); and (3) wildlife management (wild ungulate grazing, mostly deer). Results Wildlife management showed higher abundance and diversity of shrubs. Predicted seedling density under wildlife management was three‐fold higher than sheep, and almost nine‐fold higher than under cattle management. Highest oak density was found under shrubs that grew beneath trees, twice as high than under trees, ten‐fold higher than under shrubs outside the tree canopy, and almost 40‐fold higher than in open microsites. However, the proportional facilitative effect of shrubs was higher in open areas than under trees. Three shrub species (Erica australis, Genista hirsuta and Rosmarinus officinalis) showed significantly higher probability of facilitating oak seedlings in comparison to other shrub species, indicating species‐specific differences in plant facilitation. Most young oak trees (saplings and juveniles) exceeded the average shrub height, suggesting that facilitation occurs mostly at the seedling stage. Conclusions Only dehesas under reasonable wildlife management showed a clear three‐layered structure with a significant representation of the shrub layer, which increased seedling density. Current cattle management was the least sustainable in terms of oak regeneration. Even though shrub cover, in general, facilitated oak seedlings, not all species were equivalent facilitators. Only three low‐palatable and large‐sized shrub species were keystone facilitators that should be especially considered in restoration and management practices. Further agriculture policies should promote reduction of cattle densities, rotational grazing and mixed species management to enhance the regeneration of these systems.

Oak woodlands of Mediterranean ecosystems, a major component of biodiversity hotspots in Europe and North America, have undergone significant land-use change in recent centuries, including an increase in grazing intensity due to the widespread presence of cattle. Simultaneously, a decrease in oak regeneration has been observed, suggesting a link between cattle grazing intensity and limited oak regeneration. In this study we examined the effect of cattle grazing on coast live oak (Quercus agrifolia Née) regeneration in San Francisco Bay Area, California. We studied seedling, sapling and adult density of coast live oak as well as vertebrate herbivory at 8 independent sites under two grazing conditions: with cattle and wildlife presence (n = 4) and only with wildlife (n = 4). The specific questions we addressed are: i) to what extent cattle management practices affect oak density, and ii) what is the effect of rangeland management on herbivory and size of young oak plants. In areas with cattle present, we found a 50% reduction in young oak density, and plant size was smaller, suggesting that survival and growth young plants in those areas are significantly limited. In addition, the presence of cattle raised the probability and intensity of herbivory (a 1.5 and 1.8-fold difference, respectively). These results strongly suggest that the presence of cattle significantly reduced the success of young Q. agrifolia through elevated herbivory. Given the potential impact of reduced recruitment on adult populations, modifying rangeland management practices to reduce cattle grazing pressure seems to be an important intervention to maintain Mediterranean oak woodlands.