The Effects of Native Shrub, Fencing, and Acorn Size on the Emergence of Contrasting Co-Occurring Oak in Mediterranean Grazed Areas

Research Highlights: The regeneration of Quercus species is usually very difficult in many oak woodlands transformed by livestock farming. Some studies have reported that shrubs can facilitate regeneration. However, the strength of interaction may vary depending on, among other factors, the shrub species and the stress tolerance of the oak species. Moreover, further studies are necessary to clarify the relative importance of the two facilitation mechanisms in the same community. Background and Objectives: Cytisus multiflorus (L’Her.) Sweet is a predominant shrub species in the Mediterranean grazed open-oak-woodlands found in the central west of the Iberian Peninsula (bioclimatic limit) and is present with Quercus pyrenaica Willd and Quercus ilex subsp. ballota Samp trees. Thus, we assessed the effect of these native shrubs and acorn size, and the effect of excluding large herbivores, on the seedling emergence of two contrasting co-occurring Quercus species under a bioclimatic limit. Materials and Methods: A manipulative field experiment was carried out considering four treatments as a combination of shrubs (shrub/no-shrub) and fence (fenced/open) factors. A total of twenty plots, five replicates for each treatment were available. In each plot, 20 acorns were sown: 10 acorns (5 small and 5 large) for each Quercus species. Acorn emergence was recorded during the first four years following the sowing. Results: Seedling emergence took place mostly in the spring of the first year after sowing. The presence of shrub was the main significant factor and incremented the emergence of both Q. ilex and Q. pyrenaica. The effect of the fence depended on the Quercus species considered, improving only the emergence of Q. pyrenaica. A negative effect with the small acorns was detected but only for Q. pyrenaica. In all treatments, Q. ilex emerged more than Q. pyrenaica. Conclusions: C. multiflorus had a clear facilitative effect on the seedling emergence of Q. ilex and Q. pyrenaica, which was much greater than the physical effect that acorn size and excluding large herbivores had. As such, this native shrub may have a key role in oak regeneration in Mediterranean grazed areas. Furthermore, in these areas of contact between marcescent and sclerophyllous Quercus species, Q. ilex currently emerges more than Q. pyrenaica. This could be indicative of a shift towards more xeric climatic conditions, which could lead to a change in the dominant tree species in the future. However, this change could be modulated by the effects of native shrub and large herbivores.

Influence of acorn size on morphological characteristics of one-year-old Northern red oak (Quercus rubra L.) seedlings

The aim of this research was to examine the influence of acorn size on morphological characteristics of one-year-old Northern red oak (Quercus rubra L.) seedlings. The quality of seedlings correlates with the quality of the seed they are derived from. In species with large seeds, as in the case of Northern red oak, the seedling growth in the first growing season is closely related to seed size or the amount of reserve nutrients that are stored in the seed. The height and root collar diameter of one-year-old Northern red oak seedlings increases with increase in acorn size. The results obtained in this research can be used as a guideline for acorn grading, because they show that improved Northern red oak seedlings quality can be achieved using seeds of appropriate size.

Ecological Implications of Acorn Size at the Individual Tree Level in Quercus suber L.

Few studies have determined the influence of acorn size on germination and predation percentage at tree level. To evaluate the seed size influence at individual tree level, trees producing two different sizes of acorn were chosen. Our results show that smaller acorns were significantly more infested (49.6–75.3%) than larger ones (11.0–27.33%). About germination, big acorns achieved the best germination percentage compared to the smaller ones (18% in infested and 76% in sound acorns for the small acorn group versus 69.3% in infested and 93.3% in sound acorns belonging to the big acorn group). We also found that there was a difference in behaviour between big and small seeds at tree level. The same size belonging to different functional groups presented a difference at the behavioural level per tree. Infested small acorns from trees 8 and 10 had only 33 and 13% germination, while big acorns from trees 2, 3, and 6 (there was no difference between both sizes) presented 67, 97, and 83%, respectively. These results indicate that the production of acorns with two different sizes could be a strategy for species regeneration, producing each size for a different purpose.